Veronika Bolshakova
email: bolshakova@crai.archi.fr

Annie Guerriero

Hugo Carvalho

Gilles Halin

Collective decision-making with 4D BIM: Collaboration group persona study

Keywords: 4D BIM, 4D BIM uses, Decision-making, Digital collaboration, Collaboration persona, Collaboration group, Collective decision support

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

AEC industry transformation, influenced by the latest information and communication technologies, is due to a major shift of this economic sector towards the new digital globalized economy [START_REF] Weippert | AEC industry culture: a need for change[END_REF]. The changes brought in the last decades by digital tools and by the progressive adoption of Building Information Modeling/Management (BIM) have had an impact on the common practices of design, management and construction of projects, and subsequently, have brought in new business models [START_REF] Schultz | The Economics of BIM and added Value of BIM to Construction Sector and Society[END_REF]. The advanced digital project management is one of the key features for an effective BIM project management [START_REF] Jadhav | Application of Building Information Modeling in Construction Projects -A Critical Review[END_REF]. Over the project development, collective competencies are implemented into both the project design development and value engineering processes [START_REF] Tory | Physical and digital artifact-mediated coordination in building design[END_REF]. In the literature and in practice 4D BIM is mostly used as a process visualization support [START_REF] Boton | Designing adapted visualization for collaborative 4D applications[END_REF], whereas we focus on 4D BIM uses for decision-making [START_REF] Guerriero | BIM-enhanced collaborative smart technologies for LEAN construction processes[END_REF]. Decision-making sessions unite the stakeholders' competencies to benefit from better communication and from collective intelligence as a decision assistance tools. But how does a group organize decision-making? Which competencies are required to efficiently support 4D? How should the collaboration persona adjust to 4D BIM uses?

Firstly, this paper introduces collaboration and decision-making in the scope of the BIM era, in order to understand collaboration types and their place in the project workflow. The next part introduces 4D BIM uses, and proposes a collaboration personae summary study. The final part describes a methodology of future research.

4D collective decision-making support development steps

4D BIM offers more detailed cost and error control, whilst also improving scheduling and coordination. It relies on interoperability and high levels of collaboration, which have increased the value of collective competencies. Thus it is important to understand which competences are relevant for which decisions, and how should the collective decision-making be organized in correlation with project phases, scale, contracts, etc.

4D Collab project research stages

This paper is a part of research conducted for the 4D Collab (http://www.4dcollabproject.eu) international project which aims to develop a proposition of decisionmaking support configuration and collaboration scenarios for 4D BIM. Fig. 1 shows the research stages, where firstly, a series of experiments with existing digital synchronous collaboration tools ("Shariiing" software https://www.shariiing.com by Immersion and multi-touch screens table and wall) have already been conducted.

The experiment scenario required AEC professionals to collaborate and to resolve some design development and value engineering tasks with the assistance of 3D models, and it summarizes the limits of present collaboration solutions and the users demand for more natural and AEC customized interactions [START_REF] Bolshakova | Digital Synchronous Collaboration Workspace and 3D Interactions for an AEC Project. Decision-Making Scenario Evaluation[END_REF]. As the next stage, this paper summarizes collaboration groups and also personae study and 4D BIM uses, Decision-making tools limits evaluation for AEC collective decision-making which will be evaluated and improved with the further research (4.4). Subsequently, a decision-making tools evaluation and specification will complete the tool proposition. Finally, AEC professionals decision-making sessions with the use of 4D, a touch table and Shariiing software, altered to 4D interactions, conclude the research.

Synchonous collaboration decision-making tool configuration proposition

Decision-making session experiments with AEC professionals

Proposition validation

Digital interactive interfaces for 4D BIM collaborative context

Digital project development and management rely on specific technical tools, with BIM the tools are in the core of a sociotechnical system, and they are operated by social elements: synchronous collaboration, coordinated work practices, institutional and cultural frameworks [8] [9]. Various systems provide users with necessary options and interactions. Yet, there is a gap in solutions for synchronous co-located collaboration and decision-making. Thus, we propose to develop a multi-user digital documents interaction interface, which will complete a digital continuity of project documents at decision-making sessions, with an aim to avoid printing documents and primarily to keep the same BIM model in the center of discussions as in the center of design development. The solution aims to avoid incorrect interpretation and losses of information. For users' comfort it is important to offer the best usability and relevant interactions.

Natural user interfaces (NUI) offer the ease of access and low appropriation time with a tool [START_REF] Steinberg | Natural User Interfaces. Human-Computer Interact[END_REF], therefore we propose to implement a multi-touch collaborative table and wall within a NUI as a technical support for decision-making. The interface fosters 4D BIM uses with project documents convergence with the ease of interactions.

AEC project collaboration, decision-making and 4D BIM uses

Any project development includes decision-making activities, with a collaborative BIM model and a range of project participants' competencies (e.g. knowledge, skills, experience) assisting decision-making. This part introduces AEC decision-making essentials with an objective to outline potential 4D BIM collaboration process requirements.

AEC project collaboration

AEC projects are highly dependent on local conditions, they usually involve stakeholders who have not been working together before [START_REF] Hill | SmartMarket Report The business value of BIM for construction in major global markets: How Contractors Around the World Are Driving Innovation With Building Information Modeling[END_REF]. The project team is almost always temporary, and has specific tasks and responsibilities [START_REF] Kubicki | Assister la coordination flexible de l'activité de construction de bâtiments. Une approche par les modèles pour la proposition d'outils de visualisation du contexte de coopération[END_REF], these factors do not foster stakeholders to integrate completely into the BIM collaboration process and network.

Naturally, different ways of working together are implied in project development, with networking at a basic level, followed by coordination, then cooperation and the highest level is collaboration, where actors have information exchanges, alternate activities, share resources and have a mutual benefit (collaborative continuum) [START_REF] Himmelman | Collaboration for a change[END_REF]. In addition, "the integration" is a concept where the parts are no longer apparent. A custom collaborative BIM decision framework implementation, suggested by Gu and London [START_REF] Gu | Understanding and facilitating BIM adoption in the AEC industry[END_REF], offers to provide the actors with the understanding and implementations of BIM technologies. Even if BIM relies on technology, as a core of BIM sociotechnical system, collaboration still stays a human activity. Collaboration can be synchronous or asynchronous, collocated or distant. Collocated synchronous project sessions are the main subject of our study on collective decision-making with 4D.

AEC project decision-making

According to Hughes and Murdoch, an AEC project starts and ends with decision points, the decision classes are: Policy, Strategic, Tactical and Operational [START_REF] Hughes | Roles in Construction Projects: Analysis and Terminology[END_REF]. Also any project requires various types of decision roles systems summarized in Table 1, where three systems have various action types involved. Depending on project lifecycle, and on decision types there will be relevant decision-making constitutes involved.

AEC project lifecycle and 4D BIM uses

The first concepts of time management are usually introduced even earlier than the design development starts, and with the design progress the time management evolves from a simple programming, to a planning and then to a detailed scheduling [START_REF]The Chartered Institute of Building: Guide to good practice in the managment of time in major projects. Dynamic time modelling[END_REF]. This evolution also involves the BIM model development with an increase of level of detail (LOD) (Fig. 2). The phase of Operation & Maintenance does not enter into the scope of current study due to the specifics and phases' relevance mostly to activity management. When the "3D" dimension of the enriched virtual model of a project is completed with the notion of "Time", introduced as the "fourth dimension", the information from time management (time dimension & related data) enriches the 3D Model (x,y,z geometry & related spatial model elements data) and creates a 4D model with 4D relationships of attending data. Thus 4D is a model, but also a simulation, as "conceptually 4D CAD represents a type of graphic simulation of a process" [START_REF] Mckinney | Interactive 4D-CAD[END_REF]. One of 4D major benefits is information accessibility and clear visualization -all actors have access to a 3D model and an attached schedule, moreover they can access to simulations and analysis.

Kreider and Messner studied BIM uses and proposed a definition that: "BIM Use: A method or strategy of applying Building Information Modeling during a facility's lifecycle to achieve one or more specific objectives." [START_REF] Kreider | The Uses of BIM: Classifying and Selecting BIM Uses[END_REF]. Based on this uses study Guerriero et al. [START_REF] Guerriero | BIM-enhanced collaborative smart technologies for LEAN construction processes[END_REF] summarized 4D BIM uses: 4D Scheduling, 4D Clash Detection, 4D Safety Management, 4D Site Lay-out & Environment Management, 4D Constructability Management, 4D Monitoring and 4D Visual communication is core use. These uses can be introduced into the project on specific phases (Fig. 2).

For example it is possible to forecast construction phases and sequencing as 4D Scheduling, once the first "3D" and "time" concepts are present, when the construction is started the scheduling is followed by 4D management, monitoring. All the uses involve stakeholders' expertise and collaboration. Digital documents, such as 4D BIM simulations, foster new collaborative project workflow implementation and develop new requirements for synchronous collaborative decision-making methods and supports (e.g. tools, documents, interactions).

4

Designing a collective decision-making support for 4D BIM

AEC project collaboration groups and decision-making

BIM engages the collective decision-making for design development or value engineering. The project stakeholders have many roles, influences & interactions [START_REF] Halin | A relational approach of cooperation in building design[END_REF] (Responsible Validator, Responsible Coordinator, Participant Production, Participant Expert, Participant Reader [START_REF] Bolshakova | Identification of relevant project documents to 4D BIM uses for a synchronous collaborative decision support[END_REF]), but the current scope focuses on a group and not individuals. Table 2 summarizes a proposition of main AEC project collaboration groups. The summary is general, it may vary from a project type, stakeholders contracts, etc. The groups are united by a General group's goal, which is related to a project development phase. The stakeholders collaborate on the project in various groups, carrying their project roles and bringing their expertise as a member of various groups in order to complete the goals. Group collaboration typology proposed by Mathews [START_REF] Matthews | Collaboration Personas : A New Approach to Designing Workplace Collaboration Tools[END_REF], summarizes the types and their descriptions (personnel, workstyle, leadership), thus in the table below Group collaboration style proposes to apply the types to AEC project teams.

The proposed summary (Table 2) suggests that collaboration type and decision class are related. Project development starts with a client-supplier collaboration type, engages a stable deliverable oriented professional design team with a democratic leadership to produce a design and then a democratic Committee with a BIM manager brings an order and control to the design collaboration. From a Pre-construction phase the project teams become more dynamic with a designated leader, also they continue the decision order from Management to Operation and then Control.

For example, on an early stage of project development a Project program team establishes the program and project requirements, approves design production, thus this group will have a collaboration type: client supplier, with the same core members of the team focusing on communication, with a designated leader. This group will make Policy class decisions, with a focus on management system roles, however other roles are possible as well, as the secondary ones. In addition, all the collaboration groups are the long life length groups, have an objective as a constant element and a motivation factor, except the BIM project team with the shared interest as a motivation. Also, most of the groups contain more than three members. However, for a BIM project team BIM is a core element, thus members receive guidance and coordination from BIM management. Moreover the teams must have technology and BIM basic skills and knowledge. professional relationship. Every type is described with dimensions: goals (group tasks and motivation), group members (if the members are stable or dynamic), work style and leadership style (designated or democratic), etc. By the approach, CP are built from real work observations and guided interviews with a focus on group members (actors) experiences. CP aims to describe the needs and collaboration of group, interactions among actors and their goals. CP approach allows equipment designers to consider different actor roles throughout the collaboration process, and to be more focused on the group needs and interactions, in addition to the common user persona benefits [START_REF] Bornet | The effects of personas on creative codesign of work equipment: an exploratory study in a real setting[END_REF].

Design methodology of collaboration personae proposition

This study suggests CP approach within an alteration to BIM specifics, and with a correlation to different problems encountered by the professionals' workgroups. Further research will run a series of BIM familiar professionals interviews, which will gather a real practices vision on the relevance of personae proposition and of specific 4D uses. Methods: the non-directive interviews will gather personal experiences of participants about collaborative group work, and will focus on following themes: Who must be participating at the collaboration? Who participates on a collaboration in relevance to the project phase? Who participates in relevance to the 4D BIM use? Who is the role and responsibility of every member of the collaboration group and his contribution to the collective intelligence? How does a collaboration group organize decisionmaking? How are the collaboration sessions managed and held? Which information on the project was necessary for a decision-making and which competences? How does 4D BIM influence a decision-making at collaboration sessions?

The collected data will be studied with thematic analysis which is a qualitative method for analyzing and identifying themes or patterns [START_REF] Braun | Using Thematic Analysis in Psychology[END_REF]. The data sets gathered from the interviews will be classified, according to the research question, into the following themes: group and member goals; members and roles; group tasks; group work style; current 4D BIM collaboration tools and methods gaps. In addition, to test and to improve the CP, we conduct a group interview with persona experienced designers.

Population: the interviews participants will be BIM experienced AEC project professionals: architects, structure, civic and MEP engineers, construction firms, construction managers, BIM mangers, safety, site and quantity managers and clients. The interviews provide a complementary information about the collaboration and project development process, also they will test our proposition relevance, additionally, they allow us to adjust the model of Matthews to the complex 4D BIM collaboration situations.

4D BIM uses and collaboration persona study

With this research we continue to progress towards a new collective decision-making support proposition, which will be adapted to AEC specific needs and 4D BIM uses.

Thus we need to study the support users and their needs. Every 4D BIM use requires specific decisions, in addition a relevant digital collaboration support for project documents must assist as a decision-making session equipment with relevant and easy interactions, and provide the digital documents continuum.

Conclusion

4D becomes a part of common practices for the professionals who have adopted high BIM and collaboration levels. Collaboration and precise project management are very important for the complex projects. The design, time, BIM process and construction managements play their role in development, and also require a collective decisionmaking, with collective skills and competences brought in by project stakeholders.

There are many collaboration groups on a construction project, and in correlation with the project goals we summarize general collaboration groups descriptions, identify their collaboration type and decision-making class.

Groups are highly dependent on a project, however, with a collaboration persona approach for such a group description it is possible to design a new configuration for the methodology and equipment which will be relevant for a collaborative decisionmaking scenario on BIM projects, and for 4D BIM uses in particular. The new configuration aims to provide an efficient collocated synchronous digital collaboration and preserve the digital continuum. Thus, it is important to understand how the project groups collaborate and interact at decision-making to optimize the configuration.

Fig. 1 .

 1 Fig. 1. Summary of methodology steps for decision-making support proposition

Fig.

  Fig. 2. Project phases and model LOD with 4D BIM use potential introduction

Table 1 .

 1 Decision roles systems (by Hughes and Murdoch[START_REF] Hughes | Roles in Construction Projects: Analysis and Terminology[END_REF])

	Operating	Control	Managing
	Operating	Monitoring	Coordinating
	Cooperating	Supervising	Directing
	Advising	Resourcing	Recommending
	Receiving		Approving

2 .

 2 Project phases and model LOD with 4D BIM use potential introduction

	PROJECT DEVELOPMENT PHASE
	Project		Pre-design	Pre-construction	Construction	Operation &
	identification	Schematic design	Construction	Fabrication	Maintenance
	Program		Design development	documentation
	BIM MODEL LOD
	Conceptual Approximate	Precise	Detailed	Recorded
	TIME MANAGEMENT
	4D BIM uses	4D Scheduling 4D Clash Detection	4D Safety Management 4D Site Lay-out, Environment Management 4D Constructability Management
					4D Monitoring

Table 2 .

 2 Main characteristics, collaboration styles and decision-making classes study

Phases:

Program -Design Phases: Pre-construction -Construction Group main characteristics Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Group 7 Group 8 Group 9 Group Name

	Project	Project	Design	Pre-	Construc-	Construc-	Construc-	Construc-	Safety
	program	design	process	construc-	tion design	tion docu-	tion team	tion pro-	manage-
	team	team	team	tion man-	team	ment		cess	ment team
				agement		manage-		team
				team		ment team
	Group members roles
	Client	Architects	BIM	Construc-	Contractors	BIM	Contractors	Construc-	Safety
	Architects Engineers	Engineers	manager Architects Engineers	tion manager Architects Engineers Client	Architects Engineers	manager Contractors Architects Engineers	Subcon-tractors	tion manager Contractors Architects Engineers	manager Construction manager Contractors Architects Engineers
								Client	Client
	General goal
	Conceive	Design the	Control	Prepare,	Design the	Manage the	Construct	Prepare,	Manage the
	the project	project and	and man-	coordinate	project	project	the quality	coordinate	project
	program	the pro-	age the	and survey	construc-	construc-	project by	and survey	construc-
	and re-	ject's BIM	project	the project	tion docu-	tion docu-	the pro-	the project	tion site
	quirements,	model	design	pre-	mentation	ments and	gram and	construc-	security
	approve		collabora-	construc-		efficient	require-	tion phase	over the
	design		tion and	tion phase		teams	ments		construc-
	production		BIM mod-			collabora-			tion pro-
			els quality			tion			cess
	Group collaboration style
	Collaboration type
	Client-	Stable	Committee Client-	Dynamic	Committee Dynamic	Dynamic	Dynamic
	supplier	team		supplier	team		team	team	team
	Personnel
	Stable	Stable	Stable	Dynamic Dynamic Stable	Dynamic Dynamic Dynamic
	Workstyle

  To study AEC collaboration groups we propose to apply the CP approach, since the single user persona description does not encompass collaborative aspects. The study proposition is based on common practices of project development and BIM implementation guides and protocols. The CP example for one of stakeholders group synopsizes main persona dimensions[START_REF] Matthews | Collaboration Personas : A New Approach to Designing Workplace Collaboration Tools[END_REF]: Pre-construction, construction. Workstyle: based on communication with a BIM model is in the core of collaboration, it evolves with the project, being more of a client-designers dialogue in the beginning, and changing to interdisciplinary collaboration. Tasks: Identification of project limits; Establish the basis and progressive development of time management; Offer order to the project development process; Assure project deadlines and the basis for construction monitoring; etc. 4D BIM model with a detailed schedule information, as a collaboration and decision-making support for 4D Scheduling, Clash Detection, Safety Management, Constructability Management, Site Lay-out & Environment Management.

	Context: a large public building project "A-project" is in development with BIM.
	Collaboration Persona Name: "A-project" Pre-construction management team.
	General goal: Prepare, coordinate and survey the project pre-construction phase.
	Roles: Client, Construction manager, Architects, Engineers (sometimes BIM manag-
	er, contractors, construction site, safety, quantities managers).
	Project phase:

Needs: strict aims and objectives of the program, an access to a digital workflow, work protocol, BIM methodology, technological core, interactions system, etc. Skills and knowledge: related to roles, basic and advanced BIM and technology understanding, and collective intelligence and professional experience. Dependences: Inside the group a Design team of architects and engineers supports the design basis of work and decision supports, and the construction manager is in strong dialogue with them, the client has a strong interaction with design team. Decision-making: Policy and Strategic classes, focused on management: cooperating. 4D BIM place:

Acknowledgments. The authors acknowledge financial support from Fonds Nationale de la Recherche (FNR) Luxembourg, and Agence Nationale de la Recherche (ANR) France, to 4DCollab grant reference: 11237662 (LU) / ANR-16-CE10-0006-01(FR).

4D BIM uses AEC project collaboration groups

Collaboration groups may imply various 4D BIM uses according to the stage of progress of the project, as illustrated in Fig. 1. Thus the proposed Project design team and Design process team on a design phase may use 4D Scheduling and Clash detection. Further, these uses are required for Pre-construction management, Construction design and Construction document management teams, who also benefit from 4D Safety, Site lay-out, Environment, Constructability management. The groups encompass a large specter of 4D uses since the project model is well developed and there is a high concentration of management and coordination tasks on a pre-construction phase.

The Safety management team mostly benefits from 4D Safety management, however, keeps the connection with other 4D uses. For a Construction process team the 4D Monitoring and Construction management are the main uses. The Construction team may benefit from 4D Visualization for a better communication. The relevant uses for collaboration groups will be defined within the series of interviews (4.4) and 4D Collab research project experiments and survey results.

Collaboration personas approach

When project stakeholders collaborate in groups, the knowledge and competencies of every individual team member are important. Furthermore, BIM allows the team to use the entire set of collective knowledge, skills and competencies as parts of collective intelligence, which is the key feature for advanced digital management of the project.

In order to describe a project group collective activity, we propose to implement a collaboration persona (CP) approach [START_REF] Judge | Comparing collaboration and individual personas for the design and evaluation of collaboration software[END_REF], which is derived from the original user persona method, also called Individual Persona. CP describes a new type of persona which focuses on group goals and needs, instead of personal needs, and includes collaborative work aspects. According to Judge et al., "[c]ollaboration personae are empirically derived descriptions of hypothetical groups of people with specific qualities, goals and needs." [START_REF] Judge | Comparing collaboration and individual personas for the design and evaluation of collaboration software[END_REF]. In addition to the competencies, the collaboration group internal influences and subordination must be considered as well, because "[c]ollaboration personas include individual personas (playing the roles), who enact scenarios at different phases of the collaboration" [START_REF] Matthews | Collaboration personas: A framework for understanding & designing collaborative workplace tools[END_REF]. In AEC project case the phases relate to development phases.

Matthews et al. [START_REF] Matthews | Collaboration Personas : A New Approach to Designing Workplace Collaboration Tools[END_REF] developed a CP based on collaboration types: dynamic project team, stable project team, customer-supplier relationship, committee, community and


Keywords: ATLAS, CMOS pixel module, serial powering, shunt regulator

A shunt regulator was designed to meet the specifications for the serial powering of the CMOS pixel detector modules in compatibility with the next upgrade of the ATLAS detector. Serial powering greatly increases the system's power efficiency when compared to a parallel powering scheme and allows for significant material budget savings in the power cabling. In such a scheme, each pixel detector chip is powered by a shunt regulator that takes in a constant current and produces a regulated output voltage relative to the module's potential ground. The proposed regulator has a modular structure. Each regulator module consists of a shunt regulation submodule followed by a low drop-out voltage regulation submodule, and is designed to deliver a nominal output current of 10 mA. The regulator module's schematic is presented along with a theoretical study and stability analysis. A test chip was designed in CMOS 0.18 µm technology containing one main shunt regulator composed of 126 modules, as well as two separate regulators each composed of one single module. The characterization measurements show a correct DC startup for various load conditions, as expected by simulations. The output voltage of a single module is regulated with a precision < 1%. Moreover, the regulator module works with a low voltage drop-out of 200 mV for a large range of input current from 3 mA to 18 mA. The equivalent series resistance of a 40-module regulator is measured to be 15 mΩ, including the wire bonding and test bench parasitics. The test chip is successfully tested in serial mode and in parallel mode. In the latter mode, the current mismatch between parallel chips is measured to be less than 3.4% for an input current of 1 A. Moreover, transient measurements performed with an active load show a proper functioning with no undershoots or overshoots. Finally, the test chip was irradiated with an X-ray source up to 125 Mrad. Measurements show a stable response of the regulator with an intrinsic output voltage variation less than 1%.

I. INTRODUCTION

ATLAS Inner Tracker (ITk) is scheduled for upgrade to cope with the High Luminosity Large Hadron Collider (HL-LHC) operating in 2026 [START_REF] Flick | The phase II ATLAS Pixel upgrade: the Inner Tracker (ITk)[END_REF]. One motivation is to increase power efficiency and decrease material budget in pixel detector modules, which adopt a parallel powering scheme today as described in Fig. 1(a). In such a scheme, high current densities circulate in long cables up to 100 m with a limited cross section leading to significant power losses. In the present pixel detector, the power efficiency is around 20% [START_REF] Gonella | A serial powering scheme for the ATLAS pixel detector at sLHC[END_REF].

To resolve this problem, serial powering has been proposed as shown in Fig. 1(b). In a serial powering scheme, a relatively small current flows through the cables, and through the modules connected in series. This current corresponds to one module consumption plus some headroom for proper operation. If the total current is reduced by a factor N, the power losses due to cable resistance will decrease by N 2 . Such increase in efficiency allows the reduction of cables volume, reducing thus the material budget.

In order to benefit from serial powering, the module electronics need to be adapted accordingly, as they are designed to work under a constant voltage and not under a constant current. The proposed solution consists in adding shunt regulators in each pixel detector chip, which takes in a constant current, and produces a regulated output voltage with respect to the module's ground.

The serial powering solution has been investigated in previous front end chips FE-I3 [START_REF] Peric | The FE-I3 Readout Chip for the ATLAS Pixel Detector[END_REF] and FE-I4 [START_REF] Karagounis | Development of the ATLAS FE-I4 Pixel Readout IC for b-layer Upgrade and Super-LHC[END_REF] designed for ATLAS. Nevertheless, it has been decided to adopt a classical parallel powering approach for the first IBL upgrade [START_REF] Gonella | The Shunt-LDO Regulator to Power The Upgraded ATLAS Pixel Detector[END_REF]. Today, the serial powering scheme is chosen as the baseline for the next ITk upgrade. Consequently, the shunt regulator concept previously developed [START_REF] Karagounis | An Integrated Shunt-LDO Regulator for Serial Powered Systems[END_REF] is adapted to match the specifications for the hybrid pixel modules designed by the RD53 collaboration [START_REF] Orfanelli | Serial Powering Optimization for CMS and ATLAS Pixel Detectors within RD53 Collaboration for HL-LHC: System Level Simulations and Testing[END_REF].

Alternatively, monolithic CMOS pixel modules [START_REF] Snoeys | Monolithic Pixel Detectors for High Energy Physics[END_REF] [9] constitute a high potential technology for future applications. In a monolithic module, the sensor is implemented in the same substrate as the readout electronics, reducing thus the time and cost of modules production when compared to hybrid modules. This solution still needs to be fully compatible with the serial powering system.

In this paper, a novel shunt regulator circuit is proposed to be integrated in the CMOS pixel modules. In the next section, shunt regulator architecture will be described followed by a stability analysis. In section III, the test chip architecture will be illustrated with emphasis on power management. Finally, characterization measurements will be presented along with irradiation test results. The shunt regulator has a modular structure. Each module consists of two submodules: a shunt submodule and a voltage lowdropout (VLDO) submodule as shown in Fig. 2. The shunt submodule is composed of an amplifier controlling the gate voltage of a shunt transistor Ms. The amplifier compares a fraction of the input voltage Vin to a reference voltage Vref generated by a bandgap circuit and controls the shunt transistor MS in order to stabilize the input voltage at a value Vin = Vref /α1, where α1 is the resistive ratio R1/(R1+R2). Accordingly, Ms is controlled to shunt the fraction of input current that is not consumed by the load.

The voltage LDO submodule performs a second round of regulation, where its amplifier senses the input voltage Vin, and controls a pass transistor Mp in order to keep the output voltage regulated at Vout = Vref/α2, where α2 is the resistive ratio R4/(R3+R4). This regulation requires a certain voltage drop across Mp that should be kept to a minimum value, around 200 mV.

Combining the shunt and VLDO submodules, the output voltage is regulated two times providing thus a good noise immunity. Circuit stability is, however, crucial for proper operation. The circuit in Fig. 2 can be studied as two independent feedback regulation loops, one constituted by the shunt submodule and the other constituted by the VLDO submodule. In fact, the two loops have the same transfer function; they are both composed of a differential amplifier controlling the gate of a transistor in common source configuration. That said, the position of the poles and zeroes are different in each loop. One particular problem is that the output impedance of the VLDO can become very high in some load conditions. This problem will be addressed further in the article. This section will focus on the transfer function of the shunt submodule.

The shunt regulation loop consists of two low frequency poles (or zeros depending on the studied transfer function), one at the amplifier output and the other at the regulator output. A compensation is thus needed to stabilize the circuit. Most regulators use an off-chip capacitor for stability and noise filtering [START_REF] Rincon-Mora | A Low-Voltage, Low Quiescent Current, Low Drop-out Regulator[END_REF]. Other work proposes a fully on-chip solution [START_REF] Milliken | Full On-Chip CMOS Low-Dropout Voltage Regulator[END_REF], however, there is an upper limit on the load capacitance beyond which the system becomes unstable. In the present work, the compensation is mainly performed by two capacitors connected between Vin and the sources of cascode transistor M9 and M11 as it is classically done in operational amplifiers. This compensation technique guarantees a low impedance feedback path for the current at high frequencies. For symmetry, both capacitors have the same value CC1/2. The closed loop behavior of the circuit can be modeled, with some approximation, as a second order system: [START_REF] Flick | The phase II ATLAS Pixel upgrade: the Inner Tracker (ITk)[END_REF] Where R0 is the equivalent series resistance:

𝑅 0 = 1 𝛼 1 𝑔𝑚 𝑠 𝐵 𝑔𝑚 1 𝑅 𝐴 (2)
And where gms is the effective transconductance of MS taking into account the degeneration by Rd, B is the mirroring ratio between the first and second stage of the amplifier, and gm1 denotes the transconductance of M1 which is equivalent to that of M2, and RA is the output resistance of the amplifier. The Zero 'z1' is expressed as:

𝑧 1 = 1 𝑅 𝐴 𝐶 𝐴 (3)
CA being the total capacitance at the output node of the amplifier. For a load capacitance CL, the natural frequency ω0 is written as:

𝜔 0 = √ 𝛼 1 𝑔𝑚 𝑠 𝐵 𝑔𝑚 1 𝐶 𝐴 (𝐶 𝐿1 + 𝐶 𝑐1 ) (4)
While the expression for the damping factor ζ is:

𝜁 = 𝐶 𝑐1 𝑔𝑚 𝑠 𝑅 𝐴 + 𝐶 𝐿1 2 𝑅 𝐴 √ 1 𝛼 1 𝑔𝑚 𝑠 𝐵 𝑔𝑚 1 𝐶 𝐴 (𝐶 𝐿1 + 𝐶 𝑐1 ) (5)
According to control theory, the damping factor must be greater than zero in all cases for the system to be stable. Therefore, the expression (5) must be strictly positive. In the absence of load capacitor, the damping factor has the following form:

𝑉 𝑜𝑢𝑡 𝐼 𝑙𝑜𝑎𝑑 = -𝑅 0 [1 + 𝑠 𝑧 1 ] 1 + 2𝜁 𝑠 𝜔 0 + 𝑠 2 𝜔 0 Fig. 2. Shunt regulator module architecture 𝜁 ≈ 1 2 √ 𝐶 𝑐1 𝑔𝑚 𝑠 𝛼 1 𝐵 𝑔𝑚 1 𝐶 𝐴 (6)
This expression can be designed to have a positive value by choosing an appropriate Cc1. If the load capacitor is sufficiently large, that is CL1 >> Cc1 gms RA, then the damping factor takes the following form:

𝜁 ≈ 1 2 𝑅 𝐴 √ 𝐶 𝐿1 𝛼 1 𝑔𝑚 𝑠 𝐵 𝑔𝑚 1 𝐶 𝐴 (7)
In that case, the damping factor increases linearly with √𝐶 𝐿1 .By studying the derivative of equation ( 5), it can be proved that the function has one global minimum at CL1 = CL_critical = Cc1 gms RA, where the minimum damping factor is:

𝜁 𝑚𝑖𝑛 = √ 𝐶 𝑐1 𝛼 1 𝐵 𝑔𝑚 1 𝑅 𝐴 𝐶 𝐴 (8)
By choosing Cc1 as to set ζmin > 0, one can guarantee the stability for all values of CL, as shown in in Fig. 3.

Finally, the phase margin can be calculated directly from the damping factor [START_REF] Dorf | Modern Control Systems[END_REF]:

∅ 𝑀 = 𝑡𝑎𝑛 -1 2 [(4 + 1/ 4 ) 1/2 -2] 1/2 (9)
Fig. 4 shows a simulation where a current pulse of 1 mA is drawn as a load current, for three different values of CL1. For CL1 = 0, the system is stable with a damping factor of 15, but the transient regulation is relatively poor. If CL1 is at its critical value CL_critical = CC1 (= 100 nF in that case) the system exhibits some damped oscillations with a damping factor of 0.2. If CL1 has a relatively large value of 10 µF, the system is stable with a damping factor of 0.9. In that case, the system response approaches a first order system with good regulation and transient response. For that reason, it was chosen to operate the circuit in that last mode; with on-chip compensation capacitors and an off-chip load capacitor. It is worth noting that the circuit would probably be stable with such a large value of CL1 even without compensation. However, the present calculations did not take into consideration wire bonding parasitics which could affect the circuit stability by introducing a high impedance in series with CL1. Therefore, the used compensation guarantees stability, even in the absence of a load capacitor, which gives robustness to the design.

A single shunt module is designed to deliver an output current of 10 mA. To build a bigger regulator, as many modules as needed are connected in parallel to deliver the required output current. The concept works well in case all the modules are identical. However, mismatches are to be expected within the modules of the same chip, as well as between different chips. In that case, the current may not be evenly distributed between parallel regulators. To compensate for this problem, as many signals as possible are put in common between parallel modules; that is Vin, Vref and VG_shunt. By connecting Vref in common, any bandgap voltage offset is corrected and by connecting VG_shunt in common, the amplifier offset is corrected. There will still be the shunt transistor offset. For that matter, the shunt transistor Ms is operated in strong inversion which decreases the transconductance of MS when compared to the weak inversion mode. Current mismatch due to threshold voltage offset is thus reduced. Moreover, a degeneration resistance Rd is added to the source of Ms which acts negative local feedback. Any increase of current in one regulator will be counterbalanced by the increase of the source potential of MS due to Rd. Furthermore, Rd is implemented as many resistances in parallel with a careful layout in order to minimize the mismatch. Another advantage of Rd is the reduction of the drainsource voltage of MS to be compliant with technology's specifications.

As mentioned earlier, the VLDO submodule has the same transfer function as the shunt submodule therefore it can follow the same stability study approach. It is worth noting however that the VLDO's output impedance may become very high if the load current is low or absent. This could make the compensation very difficult, as gms approaches zero. To get around this problem, a fictive load current IF is created via transistor MF. This transistor guarantees that there is always a minimum current flowing through the pass transistor MP. As for high load currents, MF will be automatically switched off and will not cost the circuit any additional power. With such a configuration, a reasonable value of CC2 is calculated to stabilize the circuit for all values of CL2. For best performance, a CL2 of 10 µF is chosen. Furthermore, an enable transistor ME is added giving the possibility to cut off the current from the load. This option ensures proper startup in case of an unknown load behavior.

III. TEST CHIP ARCHITECTURE

A test chip was designed and fabricated in a TowerJazz 0.18 µm CMOS technology. The standard epitaxial process has been modified as described in [START_REF] Snoeys | A Process Modification for CMOS Monolithic Active Pixel Sensors for Enhanced Depletion, Timing Performance and Radiation Tolerance[END_REF] to increase pixel performance and radiation tolerance. The chip has a total dimension of 3.97 mm × 1.88 mm as illustrated in Fig. 5. The shunt regulator is designed to power a monolithic CMOS pixel chip that requires a total current up to 1.26 A. Therefore, the regulator consists of 126 elementary modules, divided into four domains. Domains separation means that while all the shunt submodules are in parallel, the VLDO submodules are connected commonly only within the same domain. This allows for better noise immunity and reduces noise coupling between different domains. On the right side of the chip, two independent regulators, namely Pwell and Psub, are implemented each with its own bandgap circuit, and each consisting of only one regulator module. These regulators are designed to polarize the Pwell and Psubstrate of the CMOS pixels, to address a sensor bias problem in the serial powering scheme. The Pwell and the Psubstrate constitute the anode of the pixel sensing diode, and they must be kept at a constant potential with respect to the module's ground potential. Such a problem does not exist in a parallel powering scheme as a fixed voltage source can be used for all anodes, as all modules share the same ground potential. This constant polarization solution is not adapted to a serially powered system, as each module has its own relative ground potential, and therefore a different potential difference will be applied to each sensor, depending on its location in the serial chain. In the case of hybrid pixel modules, a fixed polarization solution is tolerated as the sensor bias can be as high as several hundreds of volts. In that case, a ground potential shift by a few volts will constitute a relatively small difference in the sensor bias. However, in the case of a monolithic CMOS pixel modules, this solution is not tolerated as the sensor bias can be as low as a few volts, which needs to be kept constant for all modules. Therefore, an adapted solution is needed to produce a sensor bias that shifts with the module's relative ground.

The proposed solution is to shift the ground potentials of the Pwell and Psub regulators with respect to the Main Domain regulator using two negative voltage sources 'V1' and 'V2' as shown in Fig. 6. In that case, the output of each Pwell and Psub regulator is shifted by a value of V1 and V2 respectively, with respect to the module's ground potential. Consequently, the sensor bias remains constant for all serially powered modules.

IV. CHARACTERIZATION RESULTS

A. Measurements on a single module

First, a single module was tested independently by testing the Pwell and the Psub regulators as they are identical in structure. The DC input current was swept between 0 and 20 mA. This measurement was done in several load conditions going from no load up to Iload =18 mA by connecting the output with variable resistors (passive load). The measurements and simulations are shown in Fig. 7. The measurements agree well with the simulation, with the exception that the bandgap value is different. Various measurement on different bandgaps of different chips showed that the reference voltage Vref measurements are consistent among themselves with a mean value of 870 mV, which is less by around 100 mV than the simulated value. This offset can be easily corrected in future submissions. The nominal operation current for one module is 10 mA, but it can operate efficiently for a large range of input currents between 3 mA and 18 mA with a constant voltage drop of 200 mV between Vin and Vout. Therefore, the module's consumption can be easily adapted to the load. To measure the regulation, the input current was fixed to Iin = 12 mA, and the output voltage was measured for variable load currents up to 10.6 mA. The measurement results, shown in Fig. 8 indicate that the output voltage is stable with respect to the input current, with a relatively small variation of 14 mV which represents 0.9% of the DC value. This regulation corresponds to an equivalent series resistance of 1.32 Ω. In simulations, the series resistance of one module R0 is in a range between 81 mΩ and 281 mΩ depending on the load current. The measured difference could be attributed to wire bonding resistance and other parasitic resistances coming from the board and the test setup. A single wire bond resistance is estimated to be 200 mΩ, therefore it is difficult to measure the intrinsic series resistance of the regulator with a good accuracy.

B. Measurement on the Main Domain

The first Main Domain tests showed unexpected behavior at high current levels. Investigation with a thermal camera showed that there was an overheat problem due to the used setup; the chip temperatures exceeds 230 °C at Iin = 2 A without cooling. Indeed, at this level of input current, the power density is 54 W/cm 2 , which largely exceeds specifications for the ATLAS ITk modules of 0.7 W/cm 2 . This high-power density is mainly due to the fact that all of the regulator modules are placed in a relatively small area; that of the test chip. When the regulator will be implemented in a full-size CMOS chip of approximately 2 cm × 2 cm, the regulator modules should be distributed alongside the periphery to reduce the power density. In addition, this strategy allows better supply distribution with less voltage drops with respect to the solution where only one full sized regulator is placed in the periphery. It is also worth mentioning that the case where the regulator shunts the entire input current is a rare case, as most of the power should be dissipated in the pixel matrix. It is however a case to be foreseen and must be addressed in the final chip. For test purposes, the chip was cooled with a cooling spray to perform the DC tests at high currents. Other tests were performed at relatively low currents to avoid chip damage. A new test setup is being prepared to allow cooling with a Peltier device.

DC startup of the Main Domain is shown in Fig. 9. The test shows that the Main Domain can operate properly with an input current ranging from 0.2 A to 2 A, with a low drop-out. The regulator performs as expected by simulation with the exception that the input voltage increases linearly with the input current with a slope corresponding to a parasitic resistance of 82 mΩ.

The chip startup was also tested successfully with various load currents as shown in Fig. 10. In this test the load was applied to the output of the Analog Domain regulator 'Vout_ANA'. Fig. 11 shows the regulation performance when applying a load current up to 320 mA, knowing that the total input current is 450 mA. The output voltage is measured to be stable with a variation of 4 mV, corresponding to less than 0.3% variation of the output DC value. The output slope corresponds to a series resistance R0_ANA = 15 mΩ. In fact, when combining N regulator modules in parallel, their total series resistance Rtot is equivalent to R0/N, R0 being the single module's series resistance equivalent to 100 mΩ. Given the very low series resistance of the Analog Domain regulator (N = 40), the measurement is once again limited by the setup parasitic resistances. It is worth mentioning that multiple input and output pads were used for the Main Domain (80 pads in total) in order to reduce resistive as well as inductive parasitics.

C. Serial powering

Two test boards were powered serially with an input current up to 500 mA. The voltages were measured with respect to the absolute ground, i. e. the ground of the second test board B2 as shown in Fig. 12. The measurements show that the two test boards start up correctly, as expected by simulations.

D. Parallel powering

In a serially powered chain, if one element fails, the functionality of the whole chain is at risk. Therefore, in ATLAS Inner Tracker, it has been decided that each pixel detector module must contain four CMOS chips in parallel, each having its own shunt regulator. This ensures that if a chip fails, the other three chips should remain functional and relay the current to the next module. This significantly reduces the system failure risk. The technical difficulty of this solution is to guarantee the even distribution of input current between the four parallel shunt regulators.

As discussed in section II, some design measures have been adopted to reduce mismatch effects between parallel regulator modules. Corner simulations predicted a current mismatch within ±10% for an input current of 1 A distributed between 2 parallel Main Domain regulators as shown Fig. 13. In order to confirm simulation predictions, characterization measurements were performed in a parallel powering scheme. Two boards were powered with the same current source, and the current in each circuit branch was measured using two similar ammeters. 'Vref' and 'VG_shunt' signals were put in common as previously described in Section II. Measurements for parallel powering of two test boards named B1 and B2, consuming currents I_B1 and I_B2 respectively, are shown in Fig. 14. These measurements indicate a proper startup and an even distribution of current between the two test boards. It is worth mentioning that the test setup must be as symmetrical as possible for this kind of test.

These measurements were repeated for other test boards and the current mismatch was found to be less than ± 17 mA for an input current of 1 A representing 3.4%. More measurements will be carried out in the future to obtain better statistics. Up till now, all measurements indicate a proper startup and a proper functioning with a current distribution within the corner simulation limits.

E. Transient test

The transient response of the Digital Domain regulator within the Main Domain was tested using an active load. A current pulse was drawn from the circuit output with an amplitude of 100 mA for duration of 500 ms, knowing that the input current was set to 400 mA and the slew rate was 5 A/µs. Off-chip capacitors of 10 µF were used to filter the input voltage, output voltage and the bandgap reference voltage. The output transient response is illustrated in Fig. 15, where the signal was averaged to suppress the setup noise. A voltage variation of 2.8 mV is measured in response to the current pulse. No overshoot or undershoot are observed in the averaged signal.

Iin [A] Vin_B1 Vout_PUMP_B1 Vout_LVDS_B1 Vout_DIGI_B1 Vout_ANA_B1 Vref_B1 Vout_PUMP_B2 Vout_LVDS_B2 Vout_DIGI_B2 Vout_ANA_B2 Vref_B2 V in V out V ref I_B1 I_B2
V. IRRADIATION TESTS

A. Test Setup

The irradiation tests were performed at IM2NP laboratory in Marseille using an X-ray machine powered at 20 kV and 20 mA. The shunt regulator test board was placed inside the X-ray chamber and fixed on a table whose coordinates were controlled by computer. The device under test (DUT) was centered at a distance of 20 mm under the X-ray beam. At these settings, the expected dose rate is 15 kRad/min. The positioning and the dose rate were previously calibrated using a photodiode. The DUT was powered using two current sources to independently test the Pwell regulator (1 module) and the Main Domain regulator (126 modules). The two current sources shared the same ground potential, which was connected to the common substrate. A Labview program controlled the current sources, as well as a multi-channel voltmeter that was placed inside the X-ray chamber. Both regulator domains were powered at a nominal current of 10 mA for Pwell domain, and 200 mA for the Main Domain. Without cooling, the chip temperature was about 40 ˚C as measured by a thermal camera.

During irradiation, an automatic follow-up of the input voltages was set for both domains with a reading every 5 minutes. At several doses, the irradiation was paused to perform DC scans. These doses are: 0.1, 1, 3, 18, 38, 61 and 125 MRad. While scanning one domain, the other domain was kept at its nominal operation point. The DUT was irradiated up to 125 MRad.

B. Test Results

The input voltages follow up is shown in Fig. 16. Both the Main Domain and the Pwell input voltages increase slightly with irradiation at an average rate of 0.7 mV/MRad and 0.6 mV/MRad respectively.

1) Results for a single module

For the Pwell regulator (1 module), the output voltage 'Vout_Pwell', and bandgap reference voltage 'Pwell_Vref' scans at various irradiation doses are presented in Fig. 17 and Fig. 18 respectively. In all curves, the voltage levels tend to increase within the limit of 8% after 125 MRad in comparison to the corresponding value before irradiation (pre-Rad). Fig. 19 plots the variation of the input and output voltages as a function of the bandgap reference voltage as it increases with the irradiation dose. A strong correlation is observed, as the input and output voltages follow the increase of Vref_Pwell linearly. The slope of the linear fit corresponds to the resistive ratios α1 and α2.

Indeed, the input and output voltages are directly proportional to the bandgap reference potential as described in section II. In order to assess the irradiation impact on the shunt regulator itself, the contribution of the reference voltage was removed in postprocessing, i.e. the calculated contribution of Vref is substracted from Vin and Vout. The results presented in Fig. 20 show that the regulator's response is stable with respect to the dose with an intrinsic variation of the input and output voltages less than 0.9% and 0.4% respectively.

2) Results for the Main Domain

Fig. 21 shows the variation of the input, output and bandgap reference voltages with respect to the irradiation dose. The input and output voltages increase with dose by less than 8% at 125 MRad. Following the same analysis as in the previous section, the bandgap voltage contribution is removed in post-processing, and the intrinsic Main Domain regulator performance is proven to be stable with irradiation with a variation less than 1.9 % for the input voltage and less than 0.6% for the output voltage.

VI. CONCLUSION

This work presents a solution for serially powering the pixel CMOS modules in ATLAS ITk or in other future applications, thus allowing a significant reduction in power losses and material budget. The shunt regulator is based on a modular structure, which gives flexibility in design, and is advantageous for heat power dissipation. The module circuit design was presented with a stability analysis that leads to a design choice including on-chip compensation capacitors and off-chip decoupling capacitors. The elementary module was designed to deliver a nominal output current of 10 mA. Each module can operate with an input current between 3 mA and 18 mA with voltage drop-out of 200 mV. The input power can thus be adjusted to the load in order to minimize losses. A test chip was designed in CMOS 0.18 µm technology and includes a total of 128 modules and 3 bandgaps, connected in different manners to construct separate domains. Characterization measurements show that all domains start up correctly as expected by simulations. The bandgap value is however different by 100 mV and will be corrected in future submissions. For a regulator formed by 40 modules, the equivalent series resistance is measured to be 15 mΩ. The test chips were successfully tested in serial powering mode. In parallel powering mode, the current mismatch between two test chips was measured to be less than 3.4 % for an input current of 1 A. Moreover, transient tests showed the system to be stable and performing as expected by the theory. An overheat problem was identified which will be addressed in the next PCB design. Finally, irradiation tests confirm the proper functioning of the regulator after a total dose of 125 Mrad. A variation of less than 8% was measured in the regulator output voltage. This variation was found to be mainly due to variation in the bandgap reference voltage. After signal processing, the intrinsic variation of the shunt regulator output voltage is found to be less than 0.6%, once the bandgap contribution is removed.

Fig. 1 .

 1 Fig. 1. Parallel powering scheme (a) versus serial powering scheme (b)

Fig. 4 .Fig. 3 .

 43 Fig. 4. Input voltage response to a step excitation of 1 mA load current.

Fig. 5 .

 5 Fig. 5. Microscope photo of the shunt regulator test chip The four domains are: the 'Digital Domain' (50 modules) the 'Analog Domain' (40 modules), the 'LVDS Domain' (30 modules), and the 'Charge Pump Domain' (6 modules). These domains together form what is called the 'Main Domain' and they have one common bandgap that provides the reference potential. The center of the test chip is filled with decoupling capacitors.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Illustration of the concept of the sensor bias in a serially powered system

Fig. 10 .Fig. 11 .Fig. 9 .Fig. 8 .

 101198 Fig. 10. DC startup of Main Domain regulator with various load currents

Fig. 12 .Fig. 13 .

 1213 Fig. 12. Serial powering of two test boards

Fig. 15 .

 15 Fig. 15. Transient response of the Digital Domain regulator to a current pulse of 100 mA.

Fig. 19 .Fig. 20 .Fig. 21 .

 192021 Fig. 19. Input and output voltages as a function of the bandgap voltage

  A. Habib 1 , M. Barbero 1 , P. Barrillon 1 , S. Bhat 1 , T. Kugathasan 2 , P. Pangaud 1 , H. Pernegger 2 , W.

	Shunt Regulator for the Serial Powering of the
	ATLAS CMOS Pixel Detector Modules
	Snoeys 2
	1 Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
	2 CERN, 1211 Geneva 23, Switzerland

II. SHUNT REGULATOR ARCHITECTURE


V D'ambrosi
email: veronica.d'ambrosi@cea.fr

C Destouches

G Ricciardi

S Breaud

F Lebon

J M Gatt

J Julien

were with DEN/ D Parrat

Experimental characterization of PCMI impact on vibrating fuel rod representative of JHR irradiation device ADELINE

Keywords: Nuclear, Instrumentation, Measurement, Test Bench, fuel rod, PCMI, dynamics, vibrations, LDV I

Pellet Clad Mechanical Interaction (PCMI) has been recognized as one of the major concerns to guarantee clad integrity while attempting at increasing the flexibility of PWR nuclear reactor operations to follow grid demand. This has motivated through the years the realization of power ramp tests in dedicated irradiation devices in "Material Testing Reactors" MTR as OSIRIS and SILOE. ADELINE, an irradiation device in the next future "Jules Horowitz Reactor" (JHR), is being designed for in-pile experiments of this kind. To improve present knowledge and increase the validation capability of simulation codes, a complex multi-physics measurement method is needed, to characterize all relevant physical quantities involved and impacted by PCMI. Researches are also being carried out to identify innovative methodologies. In this frame, we investigate the technological feasibility of measuring the effects of PCMI on the vibrations of the nuclear fuel rod externally excited by a turbulent axial flowrate. Analytical and numerical models cannot predictively describe the system due to the complexity of non linear interactions occurring in the fuel rod: pellet-pellet friction and pellet-clad shock. The IMPIGRITIA experimental set-up has been thus developed to reproduce the mechanical interaction between pellets and clad in a controlled environment: low pressure, room temperature and out of neutron flux. The designed test bench presents a clamped free single short rod, centered in the test section by means of four centering elements. Different rod configurations are implemented and localized closure of the gap is remotely realized by means of a dilatation system. Laser Doppler Vibrometry (LDV) is used to measure the transverse displacement of the sample rod in three different conditions: in air, in stagnant water and under turbulent axial flow rate. In this paper we detail the design criteria of the experience and measurement principle to then focus on the experimental program and its results are discussed.

margins. Consequently, research has attempted to improve the knowledge of this complex phenomenon by numerical simulations, developing multi-physic models, and by experiments in Material Testing Reactors (MTR) [START_REF] Michel | Simulation of Pellet-Cladding Interaction with the PLEIADES Fuel Performance Software Environment[END_REF]. In the latter, environmental constraints have strongly limited the possibility to obtain real time measurements and thus to allow the characterization of the kinetics phenomenology. At present days, a need for highly instrumented experiences in MTRs and for on-line measurements to gain real time data during irradiation has been expressed. ADELINE is a specific irradiation device designed for the next future JHR. PWR temperature and pressure conditions are reproduced in the device and it will be partially dedicated at studying PMCI.

In this frame we have identified relevant and measurable parameters associated to PCMI during normal irradiation and incidental conditions. We also proposed a model to characterize fuel material behavior undergoing initiating central melting in the attempt at evaluating its macroscopic impact on PMCI [START_REF] D'ambrosi | Proceedings of MECAMAT 2018[END_REF]. We are working to propose innovative and efficient measurement methods for detecting and characterizing this complex multi-physic phenomenology. Among the identified relevant parameters, vibrations distinguished as a possible application case. Flow Induced Vibrations (FIV) of nuclear fuel rods in axial turbulent flow rate have been concerned by a large variety of scientific studies. According to Païdoussis [START_REF] Païdoussis | Fluid-Structure Interactions, Slender Structures and Axial Flow[END_REF] [4] and Axisa [START_REF] Axisa | Modélisation des systèmes mécaniques : Vibrations sous écoulements[END_REF] flow induced vibrations come from two mechanisms: the drag force in the turbulent boundary layer near the structure (nearfield component) and from the wall pressure fluctuations and far field noise. They generally induce transversal vibrations of small amplitudes, typically the vibration amplitude y/D c does not exceed 10 -3 , y being the r.m.s. amplitude of vibration and D c the cylinder diameter. In the nuclear field, works are mainly motivated by fretting concern, phenomenon appearing at the grid-rod interface. Few researches have been carried out on the effects of non linear mechanisms arising from the presence of pellets within the fuel rod. It is generally argued that modeling those effects is rather difficult [START_REF] Ferrari | Non-linear vibrations of nuclear fuel rods[END_REF]. Benhamadouche in [START_REF] Benhamadouche | CFD estimation of the flow induced vibrations of fuel rod downstream a mixing grid[END_REF] proposed to evaluate an equivalent mass density for the rod which accounts for the presence of pellets. This density is then used in the evaluation of the dynamic behavior of the fuel rod taken as a linear Euler Bernoulli beam. Park and al. in [START_REF] Park | Indirect estimation method of the turbulence induced fluid force spectrum acting on a fuel rod[END_REF] make the point on the fact that since a gap exists between pellets and gap, the contribution of the pellet mass to the density for dynamic evaluation cannot be clearly determined. They propose to compare the natural frequencies of empty and fulfilled rods to estimate an appropriate density. The fuel rod vibration is then evaluated, still assuming a linear behavior. The non linear vibration of fuel rod has been recently studied by Ferrari and al. [START_REF] Ferrari | Non-linear vibrations of nuclear fuel rods[END_REF]: his study concerned a 900 mm zircaloy clad, clamped at both extremities, eventually fulfilled with cylindrical tungsten carbide pellets modeling uranium fuel pellets in terms of dimensions and density. They compared the dynamics of empty rod, fulfilled rod freely moving (no spring at one bottom of the fuel column) and fulfilled rod axially constrained in air and in quiescent water. They observed that pellets decrease natural frequencies of the rod and that axial constriction increases natural frequencies and reduced damping with respect to freely moving configuration, both in air and in water. Moreover, they stated that non linear mechanisms were experimentally observed during tests. No studies have been retrieved discussing the effect on the dynamics of the fuel rod of local pellet-clad contact due to gap closure. The unknown, non linear behavior of the system implies that predictively model the system cannot lead to a reliable evaluation of the feasibility of characterizing PCMI through its impact on rod dynamics. This justified the design of the experimental set up IMPIGRITIA1 . The system has been designed to be representative of the geometry of ADELINE irradiation device and to recreate the turbulent axial excitation. The experimental program has been carried out in two phases: a first test phase in air leads to state the feasibility of measuring the phenomenon under laboratory conditions and to identify a correlation between the axial extension of PCMI and the effect on rod dynamic response. The second phase, under axial turbulent excitation allows to define whether FIV are sufficient to have a passive measurement and what is the impact of flow rate on the feasibility of the measure. This paper is structured as follows: we briefly proceed in the next section II with the numerical approach developed to describe the system, highlighting the empirical parameters to be measured and proposing a linear approximation of the problem as a support for the design phase. Section III is then dedicated to the measurement principle. We proceed to section IV which is devoted to the design criteria of the experimental set-up. Section V then briefly describes the set up, details the instrumentation used and rod configurations. In section VI we first present the comparison between experimental measures results by linear approximation. Then we discuss the results of air tests meant at demonstrating the feasibility of the measure in controlled conditions. The experimental evidence of a modification in the dynamic behavior of the rod depending on PCMI shall motivate the realization of the second phase of the experimental campaign to prove the feasibility of the measure under passive FIV representative of ADELINE irradiation device in JHR. Conclusions and further work are presented in section VII. and the axial direction (u y ) and can rotate (θ z ). Pellets are considered as discrete masses m p interacting along the radial direction with corresponding clad nodes through the non linear mechanism of dissipative shock and with each other through Coulomb friction. The equation of motion to be solved is [START_REF] Gradin | Mechanical Vibrations, Theory and Application to Structural Dynamics[END_REF]:

[M ]{ẍ} + [C]{ ẋ} + [K]{x} + {f nl} = {F e } (1)
Where [M ] N,N is the total mass matrix, [C] N,N the total damping matrix, [K] N,N the total stiffness one. They are obtained as follows:

[M ] =   [M c ] 0 0 [M p ]   [C] =   [C c ] 0 0 [C p ]   [K] =   [K c ] 0 0 [K p ]   (2)
where [M c ] n,n is the tridiagonal mass matrix for the clad and [M p ] m,m is the diagonal mass matrix for the pellets, with N = m + n. In the same way the total stiffness matrix and the total damping one depends on clad and pellets ones.

Rayleigh damping is used to model the damping matrix [C]:

[C] = α[M ] + β[K] (3)
Assuming proportional damping, the dissipation in the structure can be described by two parameters α and β. Those parameters, which are not a priori known, are experimentally determined measuring the first two eigen-frequencies of the system in air. Indeed it can be shown that [START_REF] Alarcon | Code aster: Modeling of damping in dynamics linear[END_REF]:

ζ = 1 2 α ω i + βω i ( 4
)
where ζ is the modal damping, which can be recovered by logarithmic decrement from free damped vibrations of the system and ω i are the eigen-frequencies.

The non-linear forces acting on the system are expressed by the {fnl} term in equation [START_REF] Michel | Simulation of Pellet-Cladding Interaction with the PLEIADES Fuel Performance Software Environment[END_REF]. Those forces are the dry friction Fig. 2: Reduced PSD of wall pressure fluctuations [START_REF] Axisa | Modélisation des systèmes mécaniques : Vibrations sous écoulements[END_REF] between discretized pellet nodes and the shock interaction between pellet nodes and clad ones. Gap size is a parameter in the formulation of pellet-clad shock interaction. The presence of external water plays a role in the external solicitation {F e } but it also induces an additional viscous damping to the structure: according to De Pauw in [START_REF] De Pauw | Operational modal analysis of flow-induced vibration of nuclear fuel rods in a turbulent axial flow[END_REF] the effects of flowing fluid on the spectral response of the structure is described by an added mass m a , corresponding to the mass of displaced fluid, and a dynamic amplification factor C m such that:

f = 1 2π K M + C m m a (5)
where K and M are the structural stiffness and mass of the system. The dynamics amplification factor is equal to 1 for quiescent water thus the two parameters can be experimentally obtained by comparing the oscillation frequencies of the system in air, in quiescent water and in fluid flow. The external force {F e } corresponds to the drag force and wall pressure fluctuations present in the fluid flow rate. According to the statistic approach proposed by Axisa [START_REF] Axisa | Modélisation des systèmes mécaniques : Vibrations sous écoulements[END_REF] on the basis of the study done by Clinch [START_REF] Clinch | Measurements of the wall pressure field at the surface of a smooth-walled pipe containing turbolent water flow[END_REF], the reduced "Power Spectral Density PSD" of the {F e } is in the form:

P SD(f r ) = P SD(f r0 f r f r0 m i (6)
where P SD is obtained by dividing the PSD by ρ 2 0 v 3 0 D with ρ 0 the fluid density, v 0 the fluid velocity and D the characteristic dimension of the problem (in this case the hydraulic diameter). f r = f D v is the reduced frequency while m i is detailed in figure 2.

To solve the equation, explicit Newmark scheme has been implemented. As reported in [START_REF] Gradin | Mechanical Vibrations, Theory and Application to Structural Dynamics[END_REF] direct time integration is necessary when the problem is non linear in order to avoid strong dependencies of results on integration time step.

B. Investigation of parameters: an empirical model

The displacement {x} of the structure, obtained when solving motion equation ( 1) by mean of explicit Newmark integration scheme, can be expressed as a function of the set of parameters P, resulting from the previous discussion:

{x} = f {P} = f M p , M c , m a , C m , K p , K c , α, β, K co , C co , µ, j, v 2 (7)
where M i , K i , α and β have already been defined, m a and C m are the added mass and the dynamic amplification factor due to the fluid presence and fluid flow. K co and C co are the parameters characterising the pellet-clad shock interaction, modelled through a contact stiffness K co and a viscous dissipation C co [START_REF] Wriggers | Computational Contact Mechanics[END_REF]; µ is the friction coefficient to model the pellet-pellet interaction. j is the width of the gap and c is the fluid velocity. While some of those parameters are completely defined by the geometry of the structure (i.e. the mass matrix of pellets M p and sample holder M c , the stiffness matrix of the sample holder), the others have to be experimentally determined.

C. The linear approximation

A simplified approach can be used to dimension the heterogeneous system behavior in terms of expected frequencies and modes of vibration by neglecting the non linear interactions {fnl}.

[M eq ]{ẍ} + [C eq ]{ ẋ} + [K eq ]{x} = {F e } (8)
In this case we assume the bending stiffness to be given only by the geometry and inertia of the clad and the sample holder

[K eq ] = [K c ].
Pellets contribute instead to the linear mass of the system [M eq ]. The damping matrix of the simplified system is a function of the equivalent mass, of sample holder and clad stiffness through Rayleigh (eq. 3):

[C eq ] = α[M eq ] + β[K eq ] (9)
Validation of this approach is presented in section VI-B III. THE MEASUREMENT PRINCIPLE Mechanical dissipations, as already mentioned, arise from the presence of pellets inside the clad [START_REF] Ferrari | Non-linear vibrations of nuclear fuel rods[END_REF]. The progressive closure of the radial gap along rod axis limits those effects and changes the behavior of the system: a reduction in structural damping as a consequence of the decrease in dissipative mechanisms is expected. A slight decrease in its eigenfrequencies could arise. Those variations can be retrieved by measuring the displacement amplitude of the system x(t).

A. Free vibration tests

During free vibration tests, viscous damping ratio is obtained by an adaptation of the logarithmic decrement method, the "Consequtive Logarithmic Decrement Method" C-LDM as described by Heitz in [START_REF] Heitz | Nonlinear local behaviours and numerical modeling of damping in civil engineering structures in dynamic[END_REF]. We recall that the viscous damping for an under-damped linear "Single Degree Of Freedom" SDOF system can be obtained by "Logarithmic Decrement Method" LDM measuring its displacement along time. The displacement can be written in the following form:

x(t) = exp(-ζω 0 t) (Acos(ω d t) + Bsin(ω d t)) ( 10
)
where ω 0 = 2πf 0 is the natural frequency of the SDOF system, ω d = ω 0 1 -ζ 2 is the pseudo-angular frequency which is directly measured by the pseudo-period T d , A and B depend on the initial conditions of the problem, as presented in figure 3. The term exp(-ζω 0 t) describes the decay of free oscillations. In this case, to obtain the viscous damping ratio ζ it is sufficient to consider two different local maxima: considering the peak displacement x i occurring at time t and the peak displacement x i+n at time t + nT d :

x i x i+n = exp(-ζω 0 t) exp(-ζω 0 (t + nT d )) = exp(ζω 0 nT d ) (11)
For non linear viscous damping phenomenon the "Consecutive LDM" C-LDM adapted method can be applied: two successive local maxima are considered and the viscous damping ratio is evaluated N times. The viscous damping ration is a function of displacement amplitude due to non linearities in the system.

B. Forced vibrations tests

Under the turbulent fluid flow rate excitation the system undergoes forced vibrations and the characterization is to be carried out in the frequency domain through Fourier transform.

X(ω) = P[x(t)] = ∞ -∞ x(t)e -iωt dt (12)
Making the hypothesis that the low-level dynamic excitation lead us to assume a linear behavior of the system, the "Halfpower bandwidth" method HBM can be applied: taking the transfer function of the measured displacement with respect to the external applied force as taken from Axisa [START_REF] Axisa | Modélisation des systèmes mécaniques : Vibrations sous écoulements[END_REF], we are able to locally fit a dynamic displacement amplification factor in the following form:

|H d (ω)| = 1 1 -ω ω0 2 + 2ζ ω ω0 2 (13)
Fig. 3: Principle for the LDM for under-damped free vibrations of a SDOF system Fig. 4: Principle for the HBM for under-damped free vibrations of a SDOF system From this the viscous damping ratio can be estimated as

ζ ∼ = ω 2 -ω 1 2ω 0 ( 14
)
where ω 2 and ω 1 are the frequencies for which

|H d (ω 1 )| = |H d (ω 2 )| = |H d,max | √ 2
. The general method is represented in figure 4.

IV. DESIGN CRITERIA: THE REPRESENTATIVENESS ISSUE ADELINE [START_REF] Gonnier | Proceedings of IGORR 2017[END_REF] is a 4m high irradiation device and constitutes the evolution of ISABELLE 1 system as it was in OSIRIS reactor [START_REF] Alberman | Technique for power ramp tests in the isabelle 1 loop of osiris reactor[END_REF]. As its predecessor, it presents a single rod test section where temperature and pressure conditions characteristics of a PWR are reproduced (155 bar and 275 • C inlet water temperature). The tested rod has the geometry of a FABRICE type fuel-rod [START_REF] Alberman | Technique for power ramp tests in the isabelle 1 loop of osiris reactor[END_REF]: a short element (about 60 cm in length) obtained cutting a previously irradiated 4m long fuel rod to be adapted to MTR geometry. The rod is cooled by an up-streaming turbulent flow rate. It is maintained in the section by a sample holder and centered by four centering elements. In order to be representative of ADELINE configuration, we accounted for the real environmental conditions, for the geometrical design, material characteristics and hydraulic solicitation.

A. The environment

Thermal and mechanical properties are obviously impacted by the environmental conditions but, through the years, several correlations have been developed to account for these phenomena [START_REF] Fink | Thermophysical properties of uranium dioxide[END_REF]. Those correlations will be used during posttreatment of results to establish the attended behavior in PWR conditions. The pressurization of the circuit has no impact on the vibration of the component, thus leading to design a low pressure system.

B. The geometry

Mechanical similarity and scale reduction cannot simply be applied and a full scale system implied non negligible conception, exploitation and cost issues. Still, geometry and fixation conditions strictly impact the dynamics of a structure thus the clamped free conditions of ADELINE sample is maintained in the experimental set-up. ADELINE sample holder is made of an upper part and a lower part, the sample rod is screwed on the lower part of the sample holder and centered in the experimental section by mean of four centering elements. The upper and lower parts are coupled by a mechanical system. We decided to limit our study to the lower part of sample holder. The decision lead to an experimental system of about 1.5 m and the choice is expected to strengthen the impact of a local pellet-clad interaction on the global dynamics of the structure. The shortening of axial extension induces a reduction in the stiffness of the system, leading to an increase in the eigenfrequencies of the system. Table II resumes the first six expected eigenfrequencies f i,l for ADELINE and IMPIGRI-TIA, obtained according to the linear approximation described in section II-C.

C. The hydraulic similarity for FIV

Fuel rod to be installed in the ADELINE device will be submitted to a turbulent flow rate of approximately 1 m/s at about 275 • C and 155 bar along the axial direction, from the bottom to the top. Hydraulics is dimensioned in order to reproduce the external solicitation in terms of the axial turbulent flow and pressure fluctuation in the flow. We identified a range of representativeness first by imposing the conservation of wall pressure fluctuations and secondly applying the Burgreen principle [START_REF] Prakash | Experimental qualification of subassembly design for prototype fast breeder reactor[END_REF]. The conservation of wall pressure fluctuation p rms was imposed according to the definition proposed by Axisa in [START_REF] Axisa | Modélisation des systèmes mécaniques : Vibrations sous écoulements[END_REF] on the basis of the envelop PSD derived from Clinch data [START_REF] Clinch | Measurements of the wall pressure field at the surface of a smooth-walled pipe containing turbolent water flow[END_REF] and reported in equation [START_REF] Gonnier | Proceedings of IGORR 2017[END_REF]. The simulation criterion established by Burgreen instead has been recently used by De Paw et al. in [START_REF] De Pauw | Operational modal analysis of flow-induced vibration of nuclear fuel rods in a turbulent axial flow[END_REF] to study flow induced vibrations in a prototype representative of the MHYRRA system and it is reported in equation [START_REF] Alberman | Technique for power ramp tests in the isabelle 1 loop of osiris reactor[END_REF].

p rms = 0.003(ρv 2 ) ( 15
)
where p rms is the root mean square of pressure fluctuations, ρ is fluid density and v is average flow velocity.

v exp = v ρ 0.5 µ -0.33 E -0.33 M 0.165 ρ 0.5 exp µ -0.33 exp E -0.33 exp M 0.165 exp (16)
Subscript 'exp' stands for prototype, µ the fluid dynamic viscosity, E elastic modulus and M the mass of the sample rod at operating temperature.

Applying the described criteria in our test conditions we found a range of representative flow rate. Fluid flow velocity ranges between 0.9 to 2.6 the fluid velocity at the sample rod in ADELINE.

v exp v ∈ [0.9 : 2.6] (17)
V. THE EXPERIMENTAL SET-UP For the experience, several representative hollow zircaloy clads were supplied. As previously mentioned, the geometrical configuration of the sample rods was defined on the basis of the FABRICE type fuel rods used in ADELINE: the zirconium alloy clad has an external diameter of 9.5 mm and its material properties are reported in table III. We defined a length coherent with the FABRICE characteristics, thus 550 mm long. Each side is welded to a plug which is screwed on a prolongation tube in stainless steel.

The prolongation tube constitutes the sample holder and it is fixed at the top of the experimental section. The other side is instead free to move. A simplified scheme of the sample holder is presented in figure 5.

Due to environmental and realization issues, the UO 2 constituting the real fuel material was replaced by cylindrical Molybdenum pellets. A comparison of room temperature material properties of UO 2 and Mo are resumed in table III. Pellets diameter was defined according to standard radial dimension of UO 2 fuel pellets, the height instead was deduced by mass conservation of UO 2 fuel pellets.

In order to fulfill the fabrication gap existing between pellets and clad and remotely control PCMI, a dilatation system capable of local expansion is used. It has been developed for stent implantation in the medical domain and it is made of a balloon which expands once inflated. The system is remotely controlled by a connection tube. A general assembly scheme is presented in figure 6: the component is introduced from the top of the sample holder, through centrally drilled pellets and the balloon is placed in the region where gap closure wants to be induced. There, pellets are replaced by two half pellets, which are displaced by mean of the previously described dilatation of the system, leading to pellet-clad contact. Different rod configurations were tested to identify the minimum number of contacting pellets leading to a measurable effects. For this we used balloons having different lengths. Rod configurations are resumed in table IV. To calibrate the model and characterize system dynamics response, two other rods were manufactured: an empty rod C0 to characterize modal parameters governing the linear dynamic response of the sample rod and C6, where Mo pellets and clad are designed to stick together. This last configuration allows to measure the effect due to a complete closure of the gap on sample rod dynamics. We also manufactured C1 fuel rod with standard Mo pellets which allows to highlight possible bias due to the presence of dilatation system. We wont present this last study, rather focusing on the first objective of the experience: to demonstrate the possibility of measuring a first pellet-clad contact through a modification in system dynamics.

Vibration data of the sample rod are collcted through a Laser Doppler Vibrometer (LDV) system: the laser beam is pointed on the vibrating object and scattered back from it. The velocity and displacement amplitudes of the studied object generate a frequency or a phase modulation respectively, due to Doppler effect. The laser used is a class 2 Helium-neon type, having a wavelength of 633 nm and a cavity length of 204 +/1 mm. Located at about 0.5 m from the moving rod, the spot on the rod is about 50 µm. The experimental section is completely designed in optical quality plexiglass. It has been demonstrated by De Pauw et al in [START_REF] Pauw | Benchmarking of deformation and vibration measurement techniques for nuclear fuel pins[END_REF] that LDV technique leads to a higher signal-to-noise ratio than other measurement techniques as grid method, fiber Bragg grating sensors, electrical strain gauges and accelerometers.

To produce and control flow conditions in the experimental section we have designed a dedicated independent water loop, equipped with a circulation pump, an expansion tank and a cooling system to keep temperature constant in the system during tests. All conditioning components are located on a separate frame to limit noise effect on LDV measures. To monitor conditions in the circuit we have installed a flow meter at the experimental section inlet, three thermocouples located at different positions to verify temperature evolution in the loop and three pressure gauges to check pressure drops. A summary of instrumentation characteristics and accuracy is reported in table V.

VI. FEASIBILITY IN AIR

During the experience we were not able to assess the evolution of C4 and C5 due to unforeseen experimental events. Water fulfilled the rods leading to the impossibility to observe any modification depending on gap closure. As a consequence we cannot identify a clear correlation between the number of attached pellets to the clad and the modification in system behavior. Anyway the main object of the experience is to state the feasibility of measuring first contacts between pellets and clad, which is possible analyzing the behavior of the other rod configurations. Free vibration tests were carried out in air, maintaining the same fixation conditions of flow rate tests and imposing an initial displacement to the fixed-free system, exciting the first vibration mode. Oscillations were measured at the bottom end of the rods, where displacements amplitudes were the highest.

A. Data Processing

We collected the displacement and velocity of the moving rod at a sampling frequency of 10 kHz. Test was repeated N times for each rod configuration. The dilatation system was inflated at 8 bars, internal pressure which guarantees the local disappearance of relative motion of pellets. We apply to the measured signal a low pass Butterworth filter with a cutting frequency of 5 kHz. As previously described, the C-LDM is used to evaluate the decay of viscous damping ratio of the i th measurement. We then evaluate the average evolution and data dispersion for each rod. To obtain the frequency, DFT is applied, evaluating it according to Hanning type windowing, with a superposition of 0.5.

B. Validation of the linear approximation method

We compare collected data for the C0 and C6 rod with the linear approximation of the system as described in section II-C. The absence of the dissipative mechanisms coming from the gap presence and relative motions of the pellets with respect to the clad is expected to lead to a linear behavior of the two rod configurations.

1) The empty rod (C0): For the specific case we have:

[M eq ] = [M c ] [C eq ] = [C c ] [K eq ] = [K c ] (18)
We impose viscous damping ratio ζ by evaluating the average value from air tests and assuming a linear behavior of the empty rod. Figures 7 shows the comparison between calculated and measured displacement of the moving rod in time and frequency domain. The experimental and calculated first mode eigen-frequencies of the system correspond, with a relative variation smaller than the accuracy of the LDV system. On the other hand we can observe that the empty rod does not present a perfectly linear behavior. This is better highlighted in figure 8. Here we have in light blue the ζ obtained by C-LDM method and in dark blue the mean value obtained by moving average. Comparing it with the global mean value imposed in the model, we have that relative variation can locally reach 50% in the region from x RM S = [10 -4 : 7.10 -4 ] mm to increase to 80% at smaller displacement amplitudes. This increase in the relative variation of the estimation comes from an experimental bias of the measure characteristics of all air measurements. It results that the linear approximated method is not adapted to model the dynamics of the rod, especially in the region x RM S < 2.10 -4 which corresponds to the range of transversal displacement expected in ADELINE, when submitted to FIV. The non linear model is thus necessarily to correctly model system behavior.

2) The closed gap rod (C6): In this case we assumed only pellets to contribute to the mass of the rod while the stiffness of the clad is the one of a full rod having the external diameter of the original clad, thus 9.5 mm. As done for the empty rod, the viscous damping ratio ζ is determined by taking the global mean value experimentally obtained. As shown in figure 9 the frequency of the first mode is evaluated with an accuracy comparable to the one of the measurement system. Figure 10 presents the result of C-LDM for the full rod: in this case the much more linear behavior of the rod leads to keep the relative error smaller than 30% along the examined region.

C. Technological feasibility of the measure 1) Modification of the frequency of the first mode: Table VI resumes the experimental results about first mode eigenfrequency for the rod configurations under analysis and depending on PCMI interaction for rods allowing it (C2 and C3). No variation in frequency was observed during the experience for both fuel rod configurations C2 and C3 depending on the local closure of the gap. Difference in frequencies for the C1, C2 and C3 configurations lies within the precision of the LDV system (+/-0.1 Hz). C6 shows a slightly higher frequency for the first mode, coming from an increase in the global stiffness of the geometry. Indeed by linear approximation we obtained a first mode eigen-frequency with a precision of 0.1 Hz, as mentioned in the previous section VI-B. 2) Modification of the decay of free vibrations: Figure 11 shows the decay of the viscous damping ratio for the C3 rod configuration and corresponding data dispersion. We recall that C3 presents 3 couples of half pellets leading to the possibility of inducing gap closure over 45 out of 495 mm of the axial length of the "fuel" column. According to what we expected, the local closure of the gap leads to a decrease in the viscous damping ratio: the system vibrates more due to a decrease in the internal dissipation mechanisms: pellet-clad shocks are reduced as a consequence of the disappearance of the radial gap. The relative variation ranges from a maximum decrease of 35% to a minimum one of about 20% and is clearly measurable even in the region of representative displacement of ADELINE device x RM S < 2.10 -4 . To better characterize the non linear interactions leading to this reduction, further work is being done to study system behavior by the non linear model presented in section II. This experimental result positively states on the possibility to measure the impact on rod dynamics due to a local closure of pellet-clad gap and motivates the further study under FIV.

VII. CONCLUSIONS AND FURTHER WORK

The IMPIGRITIA test bench was designed to investigate PCMI and innovative measurement methods. The experiment was designed to verify whether a modification can be measured in the frequency or in the vibration damping of the system as a function of a local gap closure. The relative motion of pellets against the clad depends on the relative motion between the two, which is a function of the gap extension. When gap disappears due to pellet-clad attachment, the dissipation is meant to disappear, thus leading the system to longer oscillations. Different rod configurations were manufactured to verify the possibility of observing a progressive reduction in damping and modification in frequency depending on the axial extension of pellet-clad contact. This was obtained in controlled conditions by remotely inflating a dilatation system which leads to gap closure. Experimental unforeseen events lead to the impossibility to characterize the dynamic behavior of rod configurations having the highest number of pellets attached to the clad (C4 and C5). The assessment of the other rods still gave us the possibility to verify the impact of a first contact between pellets and clad on system dynamics, which is the main objective of this work. The experimental program presented a first phase of air characterization of the structure, whose geometry is designed to be representative of the ADELINE sample holder. Air tests were carried out in free vibrations conditions, imposing an initial displacement to the structure and measuring the decay of oscillations as a function of displacement amplitude. Experimental results highlighted the limits of linear approximation method. We are working for the further development of the non linear modeling. This phase allowed to positively state on the feasibility of measuring PCMI through vibrations under controlled conditions. Even though no variations in frequency were observed, the analysis of the decay of the free vibrations of C3 lead us to observe that a modification is induced by the local closure of the gap. The gap closure over 45/495 mm induced a clear reduction in the viscous damping ratio of the structure, ranging from a reduction of 20% to 35%. This difference was clearly visible also in the region of displacement amplitude representative of the FIV in the ADELINE device. Following the results of air tests, we are currently analyzing measures obtained in the second test phase. In this phase, different rod configurations were tested under turbulent fluid flow rate to prove measure feasibility under passive FIV representative of the real application case. Depending on the results of such analysis, we shall proceed to propose a technological solution adapted to the environmental and geometrical burdens of the real irradiation device ADELINE inside the JHR nuclear research reactor.

Fig. 1 :

 1 Fig. 1: Numerical model of IMPIGRITIA: schematic view of the configuration

Fig. 5 :Fig. 6 :

 56 Fig. 5: Sample holder scheme: the dashed region indicats the fictive fuel rod position

Fig. 7 :Fig. 8 :

 78 Fig. 7: Comparison of calculated and measured behavior for the C0 rod configuration: time evolution of the displacement (left) and first eigen-frequency (right)

Fig. 9 :

 9 Fig. 9: Comparison of calculated and measured behavior for the C6 rod configuration: time evolution of the displacement (left) and first eigen-frequency (right)

Fig. 11 :

 11 Fig. 11: ζ evolution along the amplitude of displacement for C3 rod, in blue the open gap configuration, in green after inflation of the dilatation system leading to local closure of the gap. In red the relative variation

TABLE I :

 I Model parameters

	Parameter Description
	Kp	Pellet Stiffness
	ζ(α, β)	Modal damping
	ma	Added mass
	Cm	Dynamic amplification factor
	Kc	Contact Stiffness
	Cc	Contact viscous dissipation
	µ	Pellet-Pellet Friction Coefficient
	j	Pellet-clad gap
	v	Fluid velocity

TABLE II :

 II Eigen-frequencies of ADELINE and IMPIGRITIA devices by linear approximation. Frequencies are reported in Hz

	Device	f 1,l	f 2,l	f 3,l	f 4,l	f 5,l	f 6,l
	ADELINE	1.5	7.6	13.0 21.6	42.6	65.1
	IMPIGRITIA 6.2	29.0 79.9 170.2 261.9 343.0

TABLE III :

 III Material properties of interest at T AM B

	Material	Density	Young's Modulus
		[kg/m 3 ]	[GPa]
	Zircaloy	6570	95
	UO 2	10963	194.3
	Mo	10200	320

TABLE IV :

 IV Rod configurationsRod name N • of displaced pellets Axial extension of gap closure

	C0	No pellets	none
	C1	0	0 mm
	C2	1	15 mm
	C3	3	45 mm
	C4	5	75 mm
	C5	9	135 mm
	C6	0	495 mm

TABLE V :

 V Instrumentation

	Parameter	f [Hz]	Instrument	Accuracy (1σ)
	frequency	10 4	Laser Doppler Interfermeter	0.1 Hz
	Temperature	1	K type thermocouples	+/-1.5 K
	Pressure	1	pressure gauges	+/-0.3 bars
	Flow rate	imposed	Vortex effect flow meter	+/-2 l/min

TABLE VI :

 VI Experimental frequencies for the different rod configurationsRod configuration f 1 ,open [Hz] f 1 ,closed [Hz]

	C0	8.2	none
	C1	5.7	none
	C2	5.8	5.8
	C3	5.7	5.7
	C6	6.7	none

Acronym form "Interaction Mécanique PastIlle Gaine :RéalIsation du Test vIbrAtoire", from latin impigritia, impigritiae; means "zeal, dedication"