Dounia Idorane

Jack Suddaby

ENERGY, CARBON AND ENVIRONMENTAL IMPACTS OF PEDESTRIAN FLOW MANAGEMENT CONFIGURATIONS IN TRAIN STATIONS: WHICH LINK TO MODAL SHIFT?

INTRODUCTION

For a long time, public transportation was rather considered for its technical aspects. In 2019 in France, the mobility orientation law has been voted and brought into law new thoughts about transportation: each travel is as dependant on the existing system as on passengers' personal experience and needs. In addition, this regulation underlines the environmental emergency to change our mobility habits. According to the French prime minister, modal shift towards public transports may permit a greenhouse gas decrease of 14% by 2030. Thus, the future of mobility leans on train station's ability to challenge the growing use of cars [START_REF] Baron | Les gares au miroir de l'urbain[END_REF]. The latter is, literally and figuratively, the entrance gate towards a more carbon-free mobility. At the same time, its functioning is allowed by multiple devices (escalators, lifts, informational boards…), and these devices consume themselves carbon and energy. As the SNCF group has launched a ten-year program to reduce its operating buildings' carbon footprint (SNCF Réseau, 2017), one question that comes in mind is: should we reduce train stations' carbon footprint by limiting the number of these devices?

At first glance, the answer could be "yes": having less comfort won't stop the stations to fulfil their role. But we argue that this question must be integrated into a wider consideration. There are no useful train stations without passengers, and this is a reason to focus on their most important needs. Thus, we must find a way to bring together both users and environmental goals in the train station conception. Indeed, we can't decide on whether we should add or remove a device only by measuring its carbon footprint if this structure gives the user a better experience encouraging them to prefer public transport to cars on the long term. In this paper we will use a new indicator to assess a device's relevance (tCO₂eq/pass) and then wonder if this tool is sufficient to make a decision by measuring their perceived importance level for passengers. We'll then provide first elements from our methodology and give a first comparison between the carbon cost of a pedestrian overpass and its potential impact, showing the importance of evaluating both sides of the equation.

STATE OF THE ART

Very different carbon investments

In 2020, an environmental regulation ("Réglementation environnementale -RE2020") was introduced into French law and made obligatory the use of the Life Cycle Assessment (LCA) method to measure environmental impacts of buildings. For now, only office areas within train stations need to comply with these obligations: the method is not mandatory for train stations and infrastructure projects. However, the method itself is interesting to assess the environmental impact of such projects and may be mandatory in the future. We then decide to use a similar method to assess stations conception. We draw inspiration from the "RE2020" reglementation methodology and assess the carbon footprint derived from materials consumption used in a train stations refurbishment projects. We presented this approach in a different paper to be published in 2023 [START_REF] Suddaby | Energy, carbon and environmental impacts of pedestrian flow management configurations in train stations[END_REF]. The Figure 1 shows differences in terms of carbon footprint but this information needs to be contextualized: train stations are also characterized by their attendance level and geographic localisation. Nevertheless, we can wonder whether we should find a reasonable and common carbon footprint per passenger.

Carbon footprint of pedestrian flow equipments

With this analysis (over 50 years) we can identify the most carbon consuming device. Results show that flow systems such as escalators, lifts and stairs in a train station have the main environmental impact: they represent 15% of the whole station's carbon footprint in France (Figure 2). With the aim of a greater sobriety, we first wonder if these devices are relevant based on the attendance level criteria. The escalator is greater in terms of capacity per minute (it can bear 90 passengers per minute). The idea is to observe which of these three devices has the best ratio carbon footprint/ capacity efficiency. To move 90 passengers per minute, a train station should add 17 elevators which is both carbon and space consuming. Whereas, for stairs the resulting additional carbon footprint to reach 90 pass/min is very low and mostly, stays under the escalator's global one. It is important to underline the fact that these three devices have different traffic objectives. Elevators are not made to absorb a high traffic but rather to offer a suitable accessible level. However, the selecting process can, in theory, be made between stairs and escalators. Thus, the tCO₂/pass metric can assess the device's level of relevance: we suppose the frequency level is a good indicator of usefulness. We choose the "Versailles Chantiers" train station case to illustrate how can this unit become a decision-making tool. In 2016, they planned to implement a new crossing infrastructure to create a juncture between the passenger building and platforms. The low frequency level observed in this structure was responsible for a relatively high ratio "carbon footprint/ frequency level" which led to the abandonment of this project. Given that, the tCO₂eq/pass has been a rather great indicator to integrate in the decisionmaking process as it was pertinent before the carbon-based materials consumption.

To sum up, this indicator can be used to compare stations' equipment between themselves to make the most sober choices. To come back to traffic systems, the difference between escalators and stairs lies on the comfort level: how important is this additional comfort in train stations for passengers? It is important to understand if the reduction of the carbon footprint of train stations does not have a negative impact on their attractivity to passengers, and by consequence decreases train travel, this would, at some point, have a global negative effect on the carbon footprint of the transport system.

Link between comfort, mode choice, and modal shift

Station comfort in train station is a polysemous and subjective notion that researchers and operators are trying to standardize. According to the French operator "SNCF Gares et Connexions", it is also a component of user satisfaction i which depends on "information, travel to the station, cleanliness and safety, time spent at the station (i.e. comfort) as well as services and shops" (Décret n° 2012-70, 2012). Again, the term "comfort" is not clearly defined as we can suppose there is an interdependence between cleanliness or safety and perceived comfort. In fact, comfort is not really a satisfaction trait but rather a witness of the user satisfaction: comfort is the perceived evaluation of all the attributes cited above. For this study, we decide to focus on train station as we consider they are an inevitable crossing point before taking the train. The literature about comfort in train stations is rare whereas train stations' facilities are likely to have a greater importance if spending time in it takes a large portion of the total travel time [START_REF] Hansson | Preferences in regional public transport: a literature review[END_REF].

Modal choice studies are various and have evolved over time. Since the 1970's, modal choice is the result of cost, time and comfort [START_REF] Brisbois | Le processus de décision dans le choix modal: importance des déterminants individuels, symboliques et cognitifs[END_REF]. On one hand, many actual research confirm these affirmations and show that time travel is the most important attribute impacting modal choice [START_REF] Brisbois | Le processus de décision dans le choix modal: importance des déterminants individuels, symboliques et cognitifs[END_REF] [START_REF] Pearce | ENQUÊTE NATIONALE MOBILITÉ ET MODES DE VIE[END_REF]) [START_REF] Hansson | Preferences in regional public transport: a literature review[END_REF]. This attribute can be objectively measured (effective time travel) but also subjectively (perceived time travel): 50% of public transportation users perceive the time spent as longer than reality [START_REF] Brisbois | Le processus de décision dans le choix modal: importance des déterminants individuels, symboliques et cognitifs[END_REF]. The perceived time is directly influenced by the user satisfaction as "the value of comfort increases as travel time increases" [START_REF] Hansson | Preferences in regional public transport: a literature review[END_REF]. Adopting a time travel approach could be a potential indicator of perceived comfort in train stations. On the other, the cost attribute is also defined as one of the main factor choices. Nevertheless, it is shown that the solution is not to lower public transport prices but rather to increase the level of comfort allowing an offer aligned with the indicated price [START_REF] Redman | Quality attributes of public transport that attract car users: A research review[END_REF]. In fact, the comfort attribute could be a relevant indicator to explain or enhance train use attractivity. Actual studies, still consider both physical and individual characteristics as main role player in the chosen mode but have added attributes. If physical traits in the mobility system (such as price, accessibility, time travel…) can easily be identified and valued, individual ones are more complex as they nowadays consider social and economic criteria as well as personal experiences, goals or preferences which are difficult to list.

Even though, improving comfort leads to a better user experience but not to an increase in attendance [START_REF] Redman | Quality attributes of public transport that attract car users: A research review[END_REF], a better user experience can "determine a retention behaviour" [START_REF] Mugion | Does the service quality of urban public transport enhance sustainable mobility?[END_REF]. This explains our choice to measure the risk of loss rather than the modal shift. Indeed, we see comfort as a long-term investment allowing cars modal share reduction in the future. Furthermore, by doing so, we neutralize the possible habits effect on using cars [START_REF] Javaid | Determinants of low-carbon transport mode adoption: systematic review of reviews[END_REF]. Comfort is not the only word used to describe passengers' experiences. Indeed, what is being studied is actually discomfort because it is a more objective notion [START_REF] Loriquet | Apport d'une réflexion anthropocentrée pour la conception d'appuis dans les transports publics urbains : vers une nouvelle représentation du confort pour le passager[END_REF]. Discomfort is a co-existing attribute that needs to be considered to create a pleasant environment: adding comfort without reducing discomfort is a counterproductive strategy.

Effect of comfort on modal shift or, in this case, retention behaviour is supposed but rarely assessed. One of the purposes of this study will be to establish a potential causal link between added comfort (or reduced discomfort) in train stations and retention power.

METHODOLOGY AND FIRST RESULTS

The state of art has permitted to establish a list of keywords related to comfort. These words can be divided into two categories: comfort linked to railway operation (frequency, speed, security, availability) and comfort linked to the well-being in the travel (safety, accessibility, cleanliness, information, fluidity). Our work focuses on users' perceived comfort unrelated to the mobility functioning system. The first step of our research would be to make a general analysis of comfort impact in passengers' experience. The SNCF group gathers several information made available in public databases. Since 2019, it possesses a customer satisfaction barometer at the station which goal is to question clients about the existing level of service. Other databases, contain data which make the inventory of comfort apparatus in stations. Given that, the objective is first to realize a statistical study on different train stations to know if there is a correlation between the given note and the availability of comfort equipment and then to classify this equipment in order of importance.

The station can be observed on two scales, sometimes functioning in contradictory ways. On the macroscopic scale we observe movements guided by the layout of the infrastructure while on the microscopic level, random and individual behaviours are practiced. The station is a lively space which can be characterized by movements it accommodates. The latter are the sum of several explanatory factors: the reason for travel, its speed, its rhythm, its experience, its route, its origin/destination [START_REF] Cresswell | Ne pas dépasser la ligne ![END_REF]. What happens when the macroscopic level doesn't meet the microscopic one?

We chose to work on the French train station "Saint-Cyr" (in Saint-Cyr-l'École, near Versailles) to observe effects of flow discomfort in passengers' mobility behaviour. This station has been served since mid-2022 by Tramway T13 Express generating additional passenger flows and creating congestion in the underground passage. The questions are: what is the losing risk level created by this source of discomfort in the station? Is it positively impacting to reduce this discomfort?

Reducing this inconvenience comes down to invest in carbon by constructing a new crossing way. Our carbon study shows that a footbridge emits 1,500 kgCO₂eq/m², so for an average structure of 495 m², the total carbon footprint would represent 742,500 kgCO₂eq. By taking a basic itinerary from Saint-Cyr to Paris Montparnasse, a thermal car would cross 26 km and emit 10 kgCO₂eq for the round-trip (French Government, 2023) and 0,104 kgCO₂eq by train. By doing a small calculation, the overpass impact divided by the corrected travel impact (car travel emissions minus train travel's ones) divided by the LCA duration of 50 years, we can affirm to write off the footbridge carbon investment, we should be able to attract at least 5 car travellers per day for 50 years. For this research, we wonder if this new bridge could have a long-term positive effect on actual users, so it deters them from choosing car in the future and permits to enhance the whole mobility system by reducing its environmental impacts.

CONCLUSION

Train station are the gateways to a sober mobility. However, the role of their level of comfort to the modal shift are insufficiently known. This makes difficult to know the carbon impact of station improvement projects: on one hand, we're able to determine the carbon consumption of the improvements and to compare different projects; on the other hand, it is complicated to know the positive or negative impact on train use, and therefore the carbon consumption linked to the use of other modes, particularly the car. In this paper, we propose a first calculation, and we show, that considering a lifespan of 50 years, the carbon consumption of a new pedestrian overpass in one particular station is equivalent to the car-related emissions of 5 people using their car for a typical journey from the station. In other terms, if the implantation of the overpass prevents at least 5 persons doing the journey daily to switch to car, it is a good carbon investment within 50 years. In future research, we will try to quantify the potential modal shift to determine if the overpass (or similar structures such as escalators) are actually good carbon investments. We plan to use a declarative approach by the travellers to assess the potential for modal shift. We will also apply this methodology more systematically for stations, to gather more complete data on this matter.

Figure 1 :

 1 Figure 1 : Train stations' carbon footprint

Figure 2 :

 2 Figure 2: Carbon footprint (in tCO₂eq) comparison for 90 passengers per minute


Student Member, IEEE Mawuena R Degnon
email: rdegnon@itopp-alcen.com

Member, IEEE Anton Gusev
email: anton.gusev@univ-pau.fr

Antoine Silvestre De Ferron

Aleksandr Baranov
email: abaranov@itopp-alcen.com

Christophe Mielot

Sébastien Boisne

Michael Barnes
email: mike.barnes@cern.ch

Viliam Senaj

Thomas Kramer

Senior Member, IEEE Bucur M Novac
email: b.m.novac@lboro.ac.uk

L Pecastaing

A Saturable Pulse Transformer Based on Nanocrystalline Magnetic Cores for an Adjustable Nanosecond High-Voltage Generator

Keywords: Nanocrystalline magnetic core, pulsed power systems, saturable pulse transformer, semiconductor opening switch. I

This paper is devoted to saturable pulse transformers (SPTs), combining the functions of a pulse transformer and a magnetic switch. Two nanocrystalline magnetic cores are investigated in the SPT of an inductive energy storage (IES) pulsed power system based on a semiconductor opening switch (SOS). The first magnetic core has a square hysteresis loop (Br/Bsat > 90%), while the second core has a flat hysteresis loop (Br/Bsat ~4%). A test bench with an initially stored energy of 10 J is developed. The circuit design is discussed, and the magnetic materials are compared. Based on the features of the hysteresis loops, two nanosecond high-voltage SOS generators are tested, with the output voltage adjusted by tuning the input voltage and controlling the bias magnetic field. The influence of the optimal core saturation on the operation of the SOS diode is studied. An adjustable output voltage impulse of more than 200 kV amplitude with a 16 ns rise time is demonstrated on a 1 kΩ resistive load.

industrial applications such as high-power microwaves, pulsed HV gas, vacuum and underwater plasma electrical discharges, particle acceleration, X-ray devices, and many others [START_REF] Rukin | Pulsed power technology based on semiconductor opening switches: A review[END_REF]- [START_REF] Wang | A semiconductor opening switch based generator with pulse repetitive frequency of 4 MHz[END_REF].

From a structural viewpoint, the SOS generators usually involve magnetic elements, used in voltage-boosting pulse transformers and magnetic pulse compressors (MPCs) [START_REF] Rukin | Pulsed power technology based on semiconductor opening switches: A review[END_REF], [START_REF] Rim | Repetitive Nanosecond All-Solid-State Pulse Generator Using Magnetic Switch and SOS Diodes[END_REF]- [START_REF] Gusev | Semiconductor opening switch generator with a primary thyristor switch triggered in impact-ionization wave mode[END_REF]. The robustness, stability, high-frequency operation, and long lifetime of magnetic elements [START_REF] Mesyats | Pulsed Power[END_REF]- [START_REF] Holma | Prototype Inductive Adders With Extremely Flat-Top Output Pulses for the Compact Linear Collider at CERN[END_REF], make these items suitable for use in solid-state generators. A noteworthy illustration is their utilization in the injection and extraction systems utilized in state-of-the-art particle accelerator facilities [START_REF] Holma | Prototype Inductive Adders With Extremely Flat-Top Output Pulses for the Compact Linear Collider at CERN[END_REF], [START_REF] Barnes | Future circular collider injection and extraction kicker topologies and solid state generators[END_REF]. In addition, the use of multiple MPC stages enables reducing the stress on the primary solid-state switch while keeping a high voltage output, although the efficiency might be compromised [START_REF] Sakugawa | An all-solid-state pulsed power generator using a high-speed gate-turn-off thyristor and a saturable transformer[END_REF]- [START_REF] Zhang | A compact, high repetitionrate, nanosecond pulse generator based on magnetic pulse compression system[END_REF], [START_REF] Rong | Research on pulse power based on saturable pulse transformer and magnetic switch[END_REF]. It is well known that the material of the magnetic element strongly affects the efficiency of the generator, especially at the fast magnetization that is particularly required for SOS generators.

Furthermore, adjustment of the output voltage level is often required in pulsed power applications including x-ray, e-beam, excimer lasers, generation of ozone, and others [START_REF] Sakugawa | An all-solid-state pulsed power generator using a high-speed gate-turn-off thyristor and a saturable transformer[END_REF], [START_REF] Korzhenevskiy | Output voltage adjustment of a pulsed highvoltage nanosecond generator with inductive energy storage and a solidstate switching system[END_REF]. In [START_REF] Pecastaing | Development of a 0.6-MV Ultracompact Magnetic Core Pulsed Transformer for High-Power Applications[END_REF] and [START_REF] Korzhenevskiy | Output voltage adjustment of a pulsed highvoltage nanosecond generator with inductive energy storage and a solidstate switching system[END_REF], the output voltage was adjusted by varying the input voltage, while using either a Fe-based amorphous or a permalloy magnetic core pulse transformer, respectively. However, the absence of a pre-magnetizing circuit introduces limits on the voltage regulation range and energy efficiency. The work [START_REF] Li | Investigation on Adjustable Magnetic Pulse Compressor in Power Supply System[END_REF] proposed a method to regulate the peak voltage, the saturation time, and the pulse compression ratio, using different connection points of the magnetic switch winding to adjust the number of turns. However, this approach appears to be complex when a large number of connection points is required. A novel topology was suggested by [START_REF] Driessen | Compact Pulse Topology for Adjustable High-Voltage Pulse Generation Using an SOS Diode[END_REF] in which an additional primary circuit connected to the primary winding is IEEE TRANSACTIONS ON PLASMA SCIENCE, ACCEPTED FOR PUBLICATION, OCTOBER 2023 used to reset the core. In this configuration though, the input voltage remains unchanged, which may lower the efficiency.

The present work studies a saturable pulse transformer (SPT) with a controlled time to saturation, as part of a highefficiency adjustable SOS generator. A circuit containing a single magnetic element, an SPT, is used to ensure high efficiency.

In a single magnetic element SOS generator, an SPT combines two functions: (i) a step-up transformer with an energy transfer time of about 500 ns, and (ii) a magnetic switch with a current compression ratio of about 2.5. Nanocrystalline soft magnetic materials, presenting a high saturation induction, low magnetic losses, high permeability, and high resistivity, meet all the requirements for a highly efficient SPT [START_REF] Yuan | New advances in solidstate pulse generator based on magnetic switches[END_REF], [START_REF] Burdt | Evaluation of nanocrystalline materials, amorphous metal alloys[END_REF], [START_REF] Filchev | High voltage high frequency power transformer for pulsed power application[END_REF]. Nanocrystalline MPCs were used in our recently published study [START_REF] Degnon | Off-the-Shelf Diodes as High-Voltage Opening Switches[END_REF], where off-the-shelf diodes were examined as high-voltage solid-state opening switches. The present paper is centered on the SPT-based system in general, while also using an SOS diode. It is therefore focused on two types of nanocrystalline magnetic cores with different shapes of the hysteresis loop (i.e., B-H curve). The first type has a flat loop (i.e., a low remanent induction), which is traditionally used in transformers. The second type has a square hysteresis loop (i.e., a high remanent induction), which is a well-known characteristic of magnetic materials used in pulsed power magnetic switches. These magnetic materials are investigated when used in a single-core transformer topology, to compare their efficiency and suitability for the SPT of the SOS generator. To this end, a test bench with an initially stored energy of 10 J, is developed.

The paper is structured as follows. In Section II, the design of the pulse transformer, the operating principle of the SOS generator, and the characteristics of the nanocrystalline magnetic cores are described. In Section III, the results obtained using the two different magnetic materials are compared and discussed. In the same section, the high-voltage SOS generators are presented. The paper ends with the Conclusion and the suggested way ahead.

II. CIRCUIT PRINCIPLE AND SYSTEM DESIGN

A. General Layout and Operating Principle of the Circuit

The general layout of the test bench is presented in Fig. 1. The arrangement consists of a triggered spark gap switch SG and a film capacitor C1, connected to the primary winding of the SPT. The secondary winding of the transformer is connected to the low inductive ceramic capacitor C2, followed by the SOS diode and the resistive load R in parallel. The current viewing resistors RSh1 and RSh2, made in-house from carbon composition resistors, are used for the SOS current and load voltage measurements. The measurement of VC1 and VC2 is performed using North Star high-voltage probes type PVM-12 and PVM-100, respectively. A pre-magnetizing circuit (magnetic bias circuit) is used to reset the core before each pulse. This circuit comprises a direct current (DC) source (IPS 303A, 0-3 A, 0-30 V) and an in-house made choke, which protects this current source from the main high-voltage pulse.

The specifications of the circuit components are described in TABLE I.

The operating principle of the experimental setup (Fig. 1) is as follows. C1 is charged by an external power supply (for simplicity this is not shown). When SG closes, the energy stored in C1 is transferred to the capacitor C2 through the transformer SPT. The current of amplitude I + and duration t + , termed "forward pumping current", flows through the SOS diode in a forward direction, charging C2. Subsequently, when the voltage across C2 reaches its maximum value, the SPT is designed to saturate. As a result, the secondary winding is decoupled from the primary winding. In addition, the inductance of the secondary winding of the saturated core significantly decreases due to the low permeability of the core in its saturated state. Hence, the discharge current of C2 of higher amplitude I -, termed "reverse pumping current", flows through the SOS for the duration t -. In order to reach the reversed current density of several to tens of kA/cm² that is required for the SOS-mode current cut-off [START_REF] Rukin | Pulsed power technology based on semiconductor opening switches: A review[END_REF], a high value of I -is necessary to be generated. Finally, the high-value reverse current is interrupted by the SOS within the current cut-off time t0, then the energy is transferred to the load R, generating a nanosecond rise time high-voltage pulse. The typical waveforms of the SOS current, the charging voltages VC1 of C1 and VC2 of C2, and the load voltage VR are presented in Fig. 2. A non-zero resistance of the SOS in the conductive state causes negative and positive pre-pulses on the load voltage (VR). This can be suppressed by two oppositely connected diodes in series with the load as described in [START_REF] Alichkin | Formation of Short Pulses with a Subnanosecond Rise Time and a Peak Power of Up to 1 GW by a Semiconductor Avalanche Sharpener[END_REF]. However, this topic is beyond the scope of the present work.

TABLE I SPECIFICATIONS OF CIRCUIT COMPONENTS

B. Design of the 100 kV Saturable Pulse Transformer

The 10 kV capacitor C1 set the input energy to 10 J maximum.

A toroidal shape of the core is adopted, and the transformation ratio n is fixed to 10. To improve energy transfer efficiency, the primary and secondary capacitors are matched as follows:

𝐶1 = 𝑛 2 × 𝐶2 (1)
According to the requirement for an SOS diode pumping as stated in [START_REF] Rukin | Pulsed power technology based on semiconductor opening switches: A review[END_REF] and based on preliminary studies, the forward and reverse pumping durations of the diode were defined respectively as t + = 500 ns and t -= 100 ns. Therefore, the equivalent capacitance Ceq and self-inductance Leq of the transformer, when referring to the primary side, can be estimated as:

𝐶 𝑒𝑞 = 𝐶 1 × 𝑛 2 𝐶 2 𝐶 1 + 𝑛 2 𝐶 2 = 𝐶 1 2 (2) 𝐿 𝑒𝑞 = (𝑡 + ) 2 𝜋 2 × 𝐶 𝑒𝑞 ( 3
)
where t + is the forward pumping time of the SOS. For an ideal energy transfer, t + must correspond to the time required for the peak voltage of C2 to be reached and to the saturation time Tsat of the transformer (see Fig. 2). In this condition, the size of the core is determined using the simplified form of the Faraday law termed the volt-second product:

2∆𝐵 × 𝑆 𝑐 × 𝑤 2 = 𝑉 𝑚𝑎𝑥 × 𝑇 𝑠𝑎𝑡 ( 4
)
where ∆B is the magnetic induction swing, Sc is the effective cross-section area of the material without the interlaminar insulating layers, w2 is the number of turns of the secondary winding and Vmax is the peak voltage of C2. Sc is determined by multiplying the geometrical cross-section of the core Sg (see Fig. 3, (1)) by the packing factor k = 0.73, as indicated by the manufacturer. The packing factor represents the ratio of the effective to the geometrical cross-section area of the core, and due to the interlaminar insulation of the wound cores it is always less than 1.

The magnetic induction swing ∆B, representing the change in the magnetic induction, is one of the most crucial parameters to consider when choosing the core material. Indeed, for the same core size, a material with a higher induction swing will lead to a higher energy transfer. In other words, a high saturation flux density implies a low magnetic core volume for the same voltage amplitude across the secondary winding.

As the transformer is operating in air, proper attention should be paid to the electrical insulation. Hence, to prevent an electrical breakdown between the winding and the core, plastic insulating supports are required (see Fig. 3,(2)). The plastic supports were 3D printed using polylactic acid (PLA) filaments. These supports, designed by considering the air dielectric strength as 30 kV/cm, ensure a minimum distance between the winding and the core of 12 mm in all directions. To extend the current path and prevent any surface breakdown, ripples are applied at the two ends of the support sleeves (see Fig. 3,(3)). The transformer windings comprise a single-turn primary (w1=1) and a 10-turn secondary (w2=10) windings. The primary winding is made up of a metallic cylinder with a diameter of 100 mm, which is placed in the center of the transformer. The top part of the cylinder is connected to the primary capacitor C1 through a metallic plate. The base of the cylinder is directly connected to the grounded plate underneath the transformer. The secondary winding consists of two parallel sections covering the entire surface of the core. To regulate the induction swing, an additional 5-turn winding is applied, as a part of the pre-magnetizing circuit.

The link between the pre-magnetizing current and the induction is expressed, respectively by (5) and (6):

𝐻 = 𝐼 𝑏 × 𝑤 𝑏 𝑙 𝑚 (5) 𝐵 = 𝜇𝐻 ( 6
)
where H is the bias magnetic field, Ib is the pre-magnetizing current, wb is the number of turns of the pre-magnetizing winding, lm is the mean magnetic path length of the core, B is the induction flux and µ is the magnetic permeability.

C. Overview of the Tested Cores

Two nanocrystalline Fe-based soft magnetic materials were investigated as SPTs. The nanocrystalline cores provide high saturation flux density, high relative permeability, low core loss and excellent temperature characteristics for a wide frequency range [START_REF]Nanocrystalline Soft Magnetic Material FINEMET®[END_REF]. The characteristics of the cores, including the saturation induction Bsat, the remanent induction Br, the coercive force Hc, the magnetic material cross-section Sc and the core size are all provided in TABLE II, and their corresponding B-H loops are presented in Fig. 4.

TABLE II PARAMETERS OF

A. Investigation of the Nanocrystalline Magnetic Cores

As described in the previous sections, the parameters of the magnetic core, and particularly the effective induction swing, are critical to minimize the size of the core. However, the static characteristics provided in the manufacturer's datasheet may differ from dynamic characteristics, especially under fast pulse magnetization [START_REF] Yuan | New advances in solidstate pulse generator based on magnetic switches[END_REF], [START_REF] Nakajima | Fe-based nanocrystalline FINEMET cores for induction accelerators[END_REF]. Hence, in these experiments, the effective induction swings of the two cores are determined and compared.

To this end, at a fixed input voltage, the bias magnetic field H0 is continuously applied using the pre-magnetizing circuit, to set the initial induction to B0. Thus, the total induction swing ΔB can be defined by: ∆𝐵 = 𝐵 𝑠𝑎𝑡 -𝐵 0 ≤ 2𝐵 𝑠𝑎𝑡 [START_REF] Rim | Repetitive Nanosecond All-Solid-State Pulse Generator Using Magnetic Switch and SOS Diodes[END_REF] where the induction swing is obtained using the volt-second product defined in (4). The peak voltage Vmax of C2 is measured and the saturation time Tsat is graphically determined using the SOS current and the VC2 waveforms (see Fig. 2).

In these experiments, the input voltage is fixed at 6 kV and the magnetic field H0 is varied from 0 to 23 A/m, to regulate the induction swing. H0 is applied in the opposite direction compared to the main pulse, to increase the effective induction swing. The results obtained are plotted in Fig. 5. The error bar corresponds to the inaccuracy in the measurement of the saturation time Tsat.

The induction swings obtained when operating the materials under fast pulse magnetization are in good agreement with the datasheet specifications. For the square B-H loop core (i.e., Finemet FT-3H), a maximum induction swing of 2 ± 0.2 T is achieved by applying a low magnetic field of absolute value H0 about 5 A/m. The further increase in the bias magnetic field does not lead to the increase of the ∆B, which confirms the operation at the maximum induction swing. Hence, the maximum flux density, inducing the highest voltage across the secondary of the transformer (VC2), can be achieved by applying a relatively low bias magnetic field using the square B-H loop core. However, the steep slope of the induction in the area between 0 and 2 A/m (Fig. 5, (1)) seriously reduces the possibility of tuning the ∆B.

On contrary, at the maximum bias magnetic field of 23 A/m, the maximum induction swing obtained while using the flat B-H loop core (i.e., Finemet FT-3L) is 1.75 ± 0.2 T. Even though the remanent induction of the Finemet FT-3L is low (~0.05 T), the continuously applied H0 makes it possible to achieve a substantial induction swing, close to 2Bsat. Due to the limit of the pre-magnetizing circuitlimited current source and number of turnsthe induction swing could not be further increased for this arrangement. However, the smooth slope of the induction curve (Fig. 5,(2)) provides the excellent advantage of regulating the induction swing.

B. Adjustable Nanosecond High-Voltage Generator Based on SOS-180-4 and Finemet FT-3L

The advantage of the flat B-H loop core is harnessed in the development of an adjustable high-voltage SOS generator. The adjustment of the output voltage is performed by changing the input voltage (i.e., the C1 charging voltage VC1). However, to maintain a high efficiency, an optimal pre-magnetizing H0 should be applied to achieve an optimum saturation. The optimum saturation corresponds to the situation where the core is saturated at the precise moment when the charging voltage of the secondary capacitor VC2 reaches its peak amplitude, and the SOS current is zero (Fig. 2). If saturation occurs too early, it prevents the transmission of all the energy stored in the primary capacitor C1. If saturation occurs too late, a part of the energy transmitted to the secondary capacitor C2 is transferred back to the primary circuit. Therefore, a transformer designed to saturate at the optimum moment ensures an efficient and complete energy transfer from C1 to C2.

A preliminary study was carried out to determine the optimal pre-magnetizing H0 for each VC1. In this preliminary study, the diode is short-circuited and the secondary of the transformer consists of the secondary winding in series with the capacitor C2. The input voltage VC1 is varied from 3 to 10 kV. For each input voltage, the magnetic field is tuned to obtain the optimum saturation. Fig. 6 presents the three different states of early, optimum, and late saturation. It is shown that the reverse current is at its maximum when the saturation time is optimal. Consequently, the reverse pumping current density required for the SOS effect is improved when operating at the optimum saturation. For the input voltage of 7 kV, the optimum magnetic field applied is 7.5 A/m for which a maximum reversed current amplitude of 1.9 kA is obtained (see Fig. 6,(2)). Based on this preliminary study, an adjustable SOS generator, having an electrical circuit shown in Fig. 1, was designed and manufactured. The generator components are the same as described in TABLE I. A single core of Finemet FT-3L is used for the saturable pulse transformer, whose design parameters are as described in Section II.B. The resistive load is set to 50 Ω as an example. For insulation considerations, both the diode and the load are immersed in oil.

The typical waveforms of VC2 and VR are shown in Fig. 7 for different values of VC1 and H0. Fig. 8 presents the voltage amplitudes of the VC2 and VR as a function of VC1. By varying the input voltage from 3 kV to 10 kV, VC2 is changed from 24 kV to 80 kV and the output load voltage is adjusted almost linearly from 10 kV to 60 kV. The proper bias magnetic field is also applied, to ensure the optimum saturation is reached at each operation point. Although the rise time and duration of the load voltage are shortened with the voltage increase, these parameters are hardly tunable in this circuit. Therefore, the adjustment described in this paper is only related to the amplitude of the output voltage.

The main characteristics of the adjustable high-voltage generator working on 50 Ω load are summarized in TABLE III, including the load voltage pulse parameters: peak amplitude VR, rise time tR(0.1-0.9), and full width at half maximum (FWHM). The transformer efficiency ɳPT is evaluated as the ratio of primary and secondary stored energy (8):

ɳ 𝑃𝑇 = 𝐸 𝐶2 𝐸 𝐶1 (8)
It is worth noting that this efficiency includes spark gap losses. Therefore, the ultimate efficiency of the transformer itself is likely to be higher than the calculated ɳPT. Thanks to the regulation of the induction swing, the transformer efficiency remains almost constant for all the output voltages (see TABLE III); its value of up to 70% was calculated using [START_REF] Driessen | Compact Pulse Topology for Adjustable High-Voltage Pulse Generation Using an SOS Diode[END_REF].

For an input voltage in the range from 3 kV to 5 kV, no magnetic field H0 is applied since the induction swing is higher than required for obtaining an efficient transfer of the energy from the primary to the secondary capacitor. This causes the SPT to operate in the late saturation state and, consequently, part of the energy is transferred back to the primary. A magnetic field of the same direction as the main pulse could have been applied to regulate the induction swing in order to optimize the overall energy efficiency of the generator at these operation points (from 3 kV to 5 kV). The range of adjustment can also be extended by increasing the voltage capability of the primary capacitor C1.

C. Nanosecond High-Voltage Generator Based on SOS-180-4 and Finemet FT-3H

For this generator, the electrical circuit and parameters are the same as presented in Fig. 1 andTABLE I. The SPT of transformation ratio 10 is based on the Finemet FT-3H core (see TABLE II). The generator is designed to operate at a fixed input voltage VC1 of 10 kV. The bias field H0 of 1.5 A/m is required for an operation at the optimum saturation of the core. This applied magnetic field is only 2.5 times higher than the coercive force but is strong enough to properly bias the square B-H loop core. In such conditions, the effective induction swing of the core is evaluated as ~2.2 T. Fig. 9 illustrates the SOS current and the load voltage impulse obtained for a load of 1 kΩ. To prevent breakdown, both the diode and the load are immersed in oil. The amplitude and duration of the forward current are 620 A and 430 ns, respectively. The reverse current has a duration of 86 ns and an amplitude of 2 kA. A diode cut-off time of 22 ns is measured. The transformer compression ratio, defined as the ratio of the SOS forward pumping current duration to twice the reverse pumping time (t + /2t -), is evaluated as 2.5. The ratio of the forward to reverse current amplitude (I + /I -) is more than 3. A load voltage pulse is generated, having an amplitude of 210 kV, a rise time of 16 ns and an FWHM of 32 ns. The charging voltage VC2 of the secondary capacitor is measured as 85 kV. The overvoltage coefficient kov of the SOS, defined as the ratio of the output voltage VR to the charging voltage of C2, is 2.5.

The influence of the load upon the operation of the generator was also studied for resistive loads varied between 50 Ω and 1 kΩ. In what follows, the energy switching efficiency of the SOS diode is defined as the ratio of the energy transmitted to the load R to the energy stored in the secondary capacitor C2 (ɳSOS = ER/EC2). The load peak power is calculated from the load voltage VR measured experimentally.

The main results are presented in Fig. 10. A peak output power PR of ~100 MW is obtained with a load of 250 Ω while the maximum voltage of ~210 kV is obtained for a 1 kΩ load. The change in the load does not influence the transformer efficiency ɳPT which is superior to 70% for each load (up to 77% at 1 kΩ). The diode energy switching efficiency reaches its maximum of 54% with the load of 50 Ω and its minimum of only 16% for the load of 1 kΩ (Fig. 10). Considering that the energy efficiency is highest when the source and load impedances match, a higher efficiency should be reached by matching the source impedance, estimated as 32 Ω. On the contrary, the overvoltage coefficient is maximum (kov = 2.5) for 1 kΩ and minimum (kov = 0.8) for the 50 Ω load. This result is in agreement with [START_REF] Rukin | Pulsed power technology based on semiconductor opening switches: A review[END_REF], which reports that the switching efficiency of SOS decreases with the increase of the overvoltage coefficient.

IV. CONCLUSION

The present paper describes the studies related to optimizing the operation of a saturable pulse transformer when used as part of a pulsed-power high-voltage SOS generator. It was found that the saturation flux density is one of the most critical parameters when designing an SPT. The SPT was studied using nanocrystalline magnetic cores of both flat and square hysteresis loops. The comparison of results obtained with these cores reveals the most important features of a hysteresis loop. Thus, it was found that the core presenting the square hysteresis loop, due to the low magnetic field required for the maximum induction swing, is suitable for an operation at a fixed voltage. On the other hand, when using a flat hysteresis loop core, the generator output voltage can be adjusted almost linearly, by tuning the input voltage and by applying the adequate bias magnetic field. It was also demonstrated that a relatively high DC current amplitude is necessary to generate the required magnetic field for higher voltages while using the flat hysteresis loop core. In such a case, proper cooling of the choke must be considered. Two SOS generators based on the aforementioned cores were designed, manufactured and tested. The control of the bias magnetic field enabled these generators to be operated with a high transformer efficiency of more than 70%. Future research will focus on using a solid-state switch in the primary circuit to improve the overall energy efficiency and address the high pulse repetition frequency operation of these generators. Applying the proposed voltage regulation method to more energetic (>10 J) SOS generators will also be considered.

  kV/4 kA 0.05-1 kΩ 0.15 Ω 0.5 Ω Type STP-02QI UHV-9A SOS-180-4 TVO-20 RCC050

Fig. 1 .

 1 Fig. 1. Circuit diagram of the experimental arrangement.

Fig. 2 .

 2 Fig. 2. Typical waveforms of (a) the current flowing through the SOS diode, and (b) the voltages VC1, VC2 (reversed for clarity), and VR.

Fig. 3 .

 3 Fig. 3. Sectional view of the SPT showing the cross-section Sg, (1) the core, (2) the PLA supports and (3) the rippled sleeves.

  1.23 > 90% 0.6 8.29 256 x 165 x 25 Finemet FT-3L 1.23 4% 0.6 8.29 256 x 165 x 25 III. TEST AND RESULTS

Fig. 4 .

 4 Fig. 4. B-H loops of (a) Finemet FT-3H, and (b) Finemet FT-3L reproduced from [27].

Fig. 5 .

 5 Fig. 5. Effective induction swing of the SPT cores as a function of the bias magnetizing field H0 for (1) Finemet FT-3H and (2) Finemet FT-3L.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Waveforms of the current through C2 at different saturation states of the SPT, depending on the bias magnetic fields H0: (1) early saturation, (2) optimum saturation, and (3) late saturation.

Fig. 8 . 9 .

 89 Fig. 8. Evolution of the amplitude of (1) secondary charging voltage VC2 and (2) output load voltage VR, both as a function of the input voltage VC1. Fig. 9. Waveforms of (1) the current flowing through the SOS diode ISOS and (2) the load voltage VR for R = 1 kΩ, for the generator based on the Finemet FT-3H core.

Fig. 10 .

 10 Fig. 10. Load characteristic of the generator based on the Finemet FT-3H core: (1) load voltage VR, (2) peak power PR, and (3) diode energy switching efficiency ɳSOS for different values of resistive load ranging from 50 Ω to 1 kΩ.

This work was supported in part by the French "Investissements d'Avenir" Program within the Framework of the Energy and Environment Solutions-Université de Pau et des Pays de l'Adour (E2S UPPA) Project [Solid-State Pulsed Power (S2P2) and Pulsed Power Applications (PULPA) Chairs] managed by the Agence Nationale de la Recherche (ANR) under Grant ANR-16-IDEX-0002 and in part by

In 2002, he joined Thomson CSF Linac, Orsay, France, a Leader in linear electron accelerators, where he was in charge of the development of electron beam range of products, intended to be employed in high-demand industrial uses. He is currently with the Head of BETA BEAMS, ITOPP ALCEN Group, Donzenac, France, specializing in the design and supply of electron beam accelerators, installed on end-users' production sites, and mainly used in surface and core sterilization of pharmaceutical products.

Michael


Nabil Hosni
email: nabil.hosni@fst.utm.tn

Wafa Selmi

Karim Zehani

Frédéric Mazaleyrat

Hager

H Maghraoui-Meherzi

Investigation of Rechtschaffner and Doehlert approches to elaborate the NiFe2O4 thin film

Keywords: NiFe2O4 thin film, Chemical bath deposition (CBD), Rechtschaffner design, Magnetic properties, Electrochemical properties

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The inverse Spinel ferrites with Fd3 ̅ m space group are a class of materials with general structure of AFe2O4 (A corresponds to Mn +2 , Fe 2+ , Co 2+ , Ni 2+ , etc…) where the divalent ions switch with half of trivalent ions between tetrahedral and octahedral sites [START_REF] Polshettiwar | Magnetically Recoverable Nanocatalysts[END_REF]. The ferrites are most prepared in bulk for this many researchers developed ferrites in nanoparticles or thin films [START_REF] Hosni | Semi-hard magnetic properties of nanoparticles of cobalt ferrite synthesized by the co-precipitation process[END_REF][START_REF] Solís | Particular transport properties of NiFe2O4 thin films at high temperatures[END_REF]. However, thin film materials, plays critical role in the development of ferrite materials with great properties [START_REF] Kumar | Effect of non-magnetic substitution on the structural and magnetic properties of spinel cobalt ferrite ceramics[END_REF] and performance [START_REF] Hao | Enhanced resistive switching and magnetic properties of Gd-doped NiFe2O4 thin films prepared by chemical solution deposition method[END_REF]. In fact, the synthesis conditions affect mostly the materials characteristics such as crystallinity, homogeneity, porosity, roughness [START_REF] Bandgar | Metal Precursor Dependent Synthesis of NiFe2O4 Thin Films for High-Performance Flexible Symmetric Supercapacitor[END_REF].

The ferrite thin films are attractive due to their remarkable ferromagnetic properties as unique magnetic structure, high saturation magnetization (Ms), high magnetic anisotropy, low magnetic coercivity, and remarkable electrochemical and thermal stability, which render them suitable for many applications, such as electronics devices [START_REF] Li | Topotactic fluorination induced stable structure and tunable electronic transport in perovskite barium ferrite thin films[END_REF], magnetic information storage [START_REF] Kumar | Effect of non-magnetic substitution on the structural and magnetic properties of spinel cobalt ferrite ceramics[END_REF], microwave absorption [START_REF] Ismail | Electromagnetic interference shielding and microwave absorption properties of cobalt ferrite CoFe2O4/polyaniline composite[END_REF], photocatalysis [START_REF] Kalam | Modified solvothermal synthesis of cobalt ferrite (CoFe2O4) magnetic nanoparticles photocatalysts for degradation of methylene blue with H2O2/visible light[END_REF], supercapacitors [START_REF] Soam | Development of paperbased flexible supercapacitor: Bismuth ferrite/graphene nanocomposite as an active electrode material[END_REF], lithium ion batteries [START_REF] Dongale | Bifunctional nanoparticulated nickel ferrite thin films: Resistive memory and aqueous battery applications[END_REF], electrochemical sensor [START_REF] Ganie | Ferrite Nanocomposite Based Electrochemical Sensor: Characterization, Voltammetric and Amperometric Studies for Electrocatalytic Detection of Formaldehyde in Aqueous Media[END_REF], gas sensors [START_REF] Rao | Nanocrystalline Pd:NiFe2O4 thin films: A selective ethanol gas sensor[END_REF], fuel cells [START_REF] Rethinasabapathy | Cobalt ferrite microspheres as a biocompatible anode for higher power generation in microbial fuel cells[END_REF], etc. Thus, ferrite thin films were successfully synthesis by several techniques which can be categorized into physical and chemical methods. The physical methods which include pulsed laser deposition (PLD) [START_REF] Md | Lead-free (K, Na, Li) NbO3/NiFe2O4 thin films by pulsed laser deposition: Structure, dielectric, magnetic and magnetodielectric behavior[END_REF], molecular beam epitaxy (MBE) [START_REF] Huang | Strain induced magnetic anisotropy in highly epitaxial CoFe2O4 thin films[END_REF] and RF magnetron sputtering [START_REF] Shan | Enhanced magnon spin transport in NiFe2O4 thin films on a lattice-matched substrate[END_REF], however the chemical methods consist electrodeposition [START_REF] Sartale | Novel electrochemical process for the deposition of nanocrystalline NiFe2O4 thin films[END_REF], sol-gel [START_REF] Seifikar | Structural and magnetic properties of sol-gel derived NiFe2O4 thin films on silicon substrates[END_REF], spin-spraying [START_REF] Ran | Effects of aqueous ethanol solutions on the structural and magnetic properties of NiZn ferrite thin films prepared by spin-spray deposition[END_REF], spin coating [START_REF] Ben Khalifa | The effect of the cobalt ferrites nanoparticles (CoFe2O4) on the porous silicon deposited by spin coating[END_REF], dip-coating [START_REF] Byung-Geon | Bismuth ferrite thin film coated on polycarbonate surface and its photocatalytic properties in visible light[END_REF], successive ionic layer adsorption and reaction (SILAR) [START_REF] Kumar | Observation of morphology resembling Hydrangea macrophylla flower in SILAR-deposited MFe2O4 (M=Co 2+ , Ni 2+ , Mn 2+ ) nanocrystallites: synergetic effect on electrochemical performance[END_REF], reflux [START_REF] Manohar | Solvothermal reflux synthesis of NiFe2O4 nanocrystals dielectric and magnetic hyperthermia properties[END_REF], chemical vapor deposition (CVD) [START_REF] Datta | Structural features of epitaxial NiFe2O4 thin films grown on different substrates by direct liquid injection chemical vapor deposition[END_REF] and chemical bath deposition (CBD) [START_REF] Gunjakar | Synthesis of nanocrystalline nickel ferrite (NiFe2O4) thin films using low temperature modified chemical method[END_REF]. Otherwise, the chemical bath deposition (CBD) has many advantageous, the lowcost, the simply, the rapidity and the ecology of this technique. Also, the CBD method can offer the control of the preparative parameters such as precursor concentration, deposition time, bath temperature, pH, etc. However, the possibility to control thus parameters allowed us to use experimental design methodology.

The experimental design [START_REF] Kesraoui Abdessalem | Experimental design methodology applied to electro-Fenton treatment for degradation of herbicide chlortoluron[END_REF] is become very useful to perform the optimization of the variables and preliminary studies of experimental parameters such as the temperature of deposition (Td), the time of deposition (td), the concentration, the pH, the annealing temperature (TR) etc. Traditional univariate approaches are performed by varying each experimental parameter independently of the others, while Rechtschaffner designs represents the best solution to perform the optimization of many variables. However, the motivation behind the use of Rechtschaffner and Doehlert designs [START_REF] Hannachi | Preparation of manganese sulfide (MnS) thin films by chemical bath deposition: Application of the experimental design methodology[END_REF] in the current study is mainly due to its minimal number of experimental runs that does not compromise with the quality of the achieved results and a clear identification between different factor interactions.

In this study, we use the experimental design methodology to investigate the influence of the principal experimental parameters such as temperature deposition (Td), time deposition (td), R' = [Fe]/[OH -] parameter, pH, annealing temperature (TR) and annealing time (tR) on the saturation magnetization (Ms) of NiFe2O4 thin film. The optimal conditions for the deposition of NiFe2O4 thin film has been studied using Doehlert matrix. The obtained thin film into the optimal conditions elaborate by chemical bath deposition has been examined by X-ray diffraction technique, Raman , FTIR, scanning electron microscopy, vibrating sample magnetometer, cyclovoltammetry and Electrochemical Impedance Spectroscopy to study the structural, spectroscopic, morphological, magnetic and electrochemical properties respectively.

Experimental details

All the chemical precursors used in this work were purchased from Aldrich and used without further purification.

Preparation of optimal NiFe2O4 thin film

The optimal NiFe2O4 thin film has been deposited by low-cost chemical bath deposition (CBD) in stainless steel (SS) substrates. The stainless steel (SS) substrate has been polished by polish paper and cleaned with deionized water to remove unwanted oxides formed on the surface. The cleaned stainless steel (SS) substrate was immersed in the basic solution of iron chloride and nickel chloride under constant stirring for 2 h. Then, the film has been taken out from the bath and washed with de-ionized water to remove the excess of particles and finally, the film has been annealed in oven.

Characterization of optimal NiFe2O4 thin film

The NEMROD Software program [START_REF] Mathieu | Software NEMROD[END_REF] has been used to determine the optimal condition of the NiFe2O4 thin film. The structure of the NiFe2O4 thin film has been confirmed by X-ray diffraction technique at room temperature with a scanning range of 2𝜃 = 20 -90• using a D8 advanced X-ray diffractometer (λ(Cu-Kα) = 1.5406 Å). Fourier transform infrared (FTIR) and Raman were utilized to identify chemical bonds of NiFe2O4 thin film in the range of 200-4000 cm -1 . The surface morphology of the film has been studied by Merlin scanning electron microscopy (SEM) equipped with Silicon Drift Detector (SDD)-X-Max 50 from Oxford Instrument. The magnetic behaver of the prepared thin film has been recorded by Vibrating Sample Magnetometer (VSM) at room temperature with maximum applied field of ±15 kA/m. The cyclovoltammetry (CV), the electrochemical impedance spectroscopy (EIS) and the tafel polarization have been performed at room temperature using PGZ-402 potentiostat with a three-electrode cell. The CV, the measurements were recorded in a solution of 1 M NaOH in the potential range of -0.5 to 0.5 V at the scan rate of 100 mVs -1 and the EIS measurements were carried out in open circuit potential in the frequency range of 1 Hz -100 KHz.

Results and discussion

Influence of the operating parameters on saturation magnetization (Ms) of NiFe2O4 thin film using Rechtschaffner design

In order to evaluate the influence of operating parameters on saturation magnetization (Ms) of the NiFe2O4 thin films six main parameters (factors) were chosen: deposition temperature (U1), deposition time (U2), concentration ratio R' = [Fe]/[OH -] (U3), pH (U4), annealing temperature (U5) and annealing time (U6). To determine the influence of these six factors and their interaction on saturation magnetization (Ms) of thin films a two-level full factorial design 2 k-p was used. In these types of designs, variables (k) are set at two levels (minimum) and (maximum) normalized as (-1) and (+1). The linear polynomial model of the experimental response associated to (2 6-1 ) factorial design (for 6 variables) was represented by the following equation:

Y = b0 + b1X1 + b2X2 + b3X3 + b4X4 + b5X5 + b6X6 + b12X1X2 + b13X1X3 + b23X2X3 +b14 X1X4 + b24X2X4 + b34 X3X4 + b15X1X5 + b25X2X5 + b35X3X5 + b45X4X5 + b16X1X6 + b26X2X6 + b36X3X6 + b46X4X6 + b56X5X6 (1)
Were:

Y: experimental response (saturation magnetization (Ms))

Xi: coded variable (-1 or +1) bi: estimation of the principal effect of the factor i for the response Y bij: estimation of interaction effect between factor i and j for the response Y. The variables for the model was chosen referring to previous work [START_REF] Hosni | Synthesis and physicochemical investigations of AgSbS2 thin films using Doehlert design and under DFT framework[END_REF]. The coefficients of the equation model were calculated in the experimental field are listed in As can be seen in Fig. 1a the saturation magnetization is very influenced by the annealing temperature (b5 = 0.038) as it has a positive response on the studied. In fact. the increasing of annealing temperature leads to increase the saturation magnetization. The second most significant factor is the temperature of deposition (b1 = 0.024) which have positive effect on the studied response. There is a significant X1X5 interaction between the deposition and the annealing temperature (b15 = 0.016). Thus, the effect of the deposition temperature (X1) depends on the annealing temperature (X5) in the chosen range and vice versa which has a positive effect on the response. Note also that the effects of the other factors are very negligible.

The Pareto analysis gave more significant information to interpret the results of Rechtschaffner design. In fact, the advantage of this analysis is the calculation of the percentage effect of each factor on the response. The numerical values of this analysis are given by the following relation: As can be seen in Pareto graphic analysis (Fig. 1b), the annealing temperature is the most influential factors on the saturation magnetization with 46.1% contribution response. And the second parameters which influence on the saturation magnetization is the temperature of deposition with total contribution of 18.7%. Furthermore, those factors have an important and meaningful positive interaction with a contribution of 64.8%. We can note also, that more than 72.7% of the response are bringing by the two factors and their interaction. However, the pH, R′, deposition and annealing time and their interactions have a negligible effect with only 27.3% of the response.

𝑃 𝑖 = ( 𝑏 𝑖 2 ∑ 𝑏 𝑖 2 ) × 100 (3)

Determination of the optimal condition of NiFe2O4 thin film using Doehlert design

In order to determine the optimal condition of NiFe2O4 thin film, the approach of Doehlert was applied after determination of the most influential parameters on saturation magnetization. The annealing and deposition temperature are the two factors determined in Rechtschaffner approach which will be investigated in the Doehlert study. The advantages of this approach are the ability to explore the whole of the experimental domain through a minimum number of experiments and the possibility to extend the domain by adding another factor. The Doehlert approach is formed by uniformly distributed of the experimental points within the space filling of the coded variables (Xi). To compare the effects of those factors Ui cited previously in the experimental domain (Table .3), concerned coded variables were used.

For calculation, the experimental value of the factors U1 and U2 were transformed into coded X1 and X2 using the following relation:

𝑋 𝑖 = 𝑈 𝑖 -𝑈 ̅ 𝑖 ∆𝑈 𝑖 (4)
where: Xi is the value taken by the coded variable i, Ui is the value taken by the factor i, Ūi is the value taken by the factor i in the center of the experimental field and ΔUi is the range of variation of the factor i.

𝑈 𝐼 ̅̅̅ =

(𝑢𝑝𝑝𝑒𝑟 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓(𝑈 𝑖 )+𝑙𝑜𝑤𝑒𝑟 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓 (𝑈 𝑖 ))

2

(5)

∆𝑈 𝑖 = (𝑢𝑝𝑝𝑒𝑟 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓(𝑈 𝑖 )+𝑙𝑜𝑤𝑒𝑟 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓 (𝑈 𝑖 )) 2𝛾 𝑖 (6)
The experimental response (saturation magnetization (Ms)) associated to the Doehlert approach can be described as following by a second-degree quadratic polynomial equation:

Y = b0 + b1X1 + b2X2 + b11X1 2 + b22X2 2 + b12X1X2 (7)
Where:

Y: the experimental response (saturation magnetization (Ms)); b0: an independent term; bi: the coefficients of the linear terms;

bii: the coefficients of the quadratic terms; bij: the coefficients corresponding to the interaction term between the factor i and the factor j.

In addition, the calculation of the coefficients of this model was carried through the least squares method:

𝐵 = (𝑋 𝑇 𝑋) -1 × 𝑋 𝑇 × 𝑌 (8)
where:

B: is the estimates coefficients vector; X T : is the transposed model matrix;

X: is the model matrix;

Y: is the measured response vector.

Using Doehlert approach, the number of experiments (N) required is given by the

The effects of the various factors: annealing temperature and deposition temperature were quantified respectively by b1 and b2 as well as their interactions b12. To confirm this result, we plotted the response surface curves in 3D as well as the isoresponse curves. The effects of interaction between deposition and annealing temperature on saturation magnetization were represented in Fig. 3. As can be seen, the maximums of the saturation magnetization (Ms) of NiFe2O4 are obtained for annealing temperatures tend around 750 °C. In addition, to improve the response studied, it would be useful to increase the annealing temperature for deposition temperatures below 60 °C. Consequently, the optimal working condition that allows the formation of NiFe2O4 thin film with a maximum Ms (0.48 T) was found at: TR = 739 °C and Td = 47 °C.

Characterization of the optimal condition of NiFe2O4 thin film

The structural properties of NiFe2O4 thin film prepared under optimal conditions determined from Doehlert matrix, is investigated by X-ray diffraction. planes respectively which correspond to the spinel ferrite NiFe2O4 with a cubic symmetry, and Fd-3m space group. Whereas, peaks located at 43.59°, 50.86° and 74.68° were attributed to stainless steel (SS) substrate. The obtained pattern found to be in good agreement with the JCPDS files (Card No.10-325) which confirmed the formation of cubic NiFe2O4 phase. The average crystallite size (D), microstrain (ε) and dislocation density (δ) of the optimal condition of NiFe2O4 thin film were determined using the following relations:

* * *

Intensity (a.u) 2 q (°) *Substrat * (5 3 3) Intensity (a.u) 2q (°) 𝛿 = 1 𝐷 2 (12)
Where: k is the Scherrer constant, λ is the Cu(Kα) radiation wavelength, β is the full width at halfmaximum (FWHM) and θ is the diffraction Bragg angle.

The crystallite size (D), microstrain (ε) and dislocation density (δ) determined from the above formulas were found to be in the order of 38 nm, 0.40 and 6.92 10 14 m -2 respectively. It is well known that the magnetic parameters for the complex oxides strongly depend on the average and distribution of crystallite size.

Raman and FTIR spectra of the thin film is presented in Fig. 4b. All these peaks and bands can be attributed to NiFe2O4. The peaks at 202 and 516 cm -1 are assigned to T2g modes, and those at 359 and 673 cm -1 are assigned to Eg and A1g modes respectively. The bands located at 470 and 568 cm -1 in FTIR spectra are assigned to Nickel ferrite (NiFe2O4) sample. The band at 359 cm -1 exhibits the stretching vibrations due to Ni-O and the band at 568 cm -1 correspond to stretching vibrations of Fe-O. The spectroscopy analysis confirms the formation of spinel structure of NiFe2O4 found in structural study.

The energy dispersive X-ray spectrometry (EDX) was used to determine the elemental composition of the obtained film. The EDX spectra shows (Fig. 5) that the Fe/Ni atomic percentage was detected by a ratio of RFe/Ni =2 which confirms the formation of stoichiometric ferrite nickel thin film with absence of any impurity. As can be seen, the Ni, Fe and O signals are higher than the C signal. This large amount of C into the film is derived from the doublesided carbon tape of SEM measurements confirmed by element mapping images. SEM images (Fig. 5) demonstrate that the elaborated film has a good coverage with flower-like structure, which is formed by the assembly of flower petal layers. The average crystallite sizes deduced from image treatments for the film are in the range of 52 and 118 nm. However, the homogeneous distribution of Ni, Fe and O was observed also in the element mapping images. In order to correlate the result found in Doehlert methodology to magnetic properties, hysteresis loops (M-H curve) for NiFe2O4 thin film prepared under optimal conditions was investigated. The Magnetic hysteresis loops measured in and out-of-plane in the field range of ±15 kA/m at room temperature are shown in Fig. 6.

H (A/m) H ^ Film H // Film M(T) H(A/m) H ^ Film H // Film
The in-plane measurement of the NiFe2O4 thin film has shown a saturation magnetization (Ms) equal to 0.49 T with a coercivity (Hc) of 12.02 kA/m. The magnetocrystalline anisotropy energy [START_REF] Singh | Investigation of structural and magnetic properties of Ni, NiFe and NiFe2O4 thin films[END_REF] was calculated by the following equation:

𝐾 𝑢 = 𝐻 𝑐 𝑀 𝑠 2
The value of the magnetocrystalline anisotropy energy is equal to 2.95 J/m 3 . The sample was rotated perpendicular from the initial position and the hysteresis loop resulting from the out-plane measurement was recorded. The Ms and Hc values significantly increased to 0.34 T and 12.04 kA/m, respectively. However, this significant difference between in-plane and outof-plane saturation magnetization is due is due to effect of magnetic anisotropy. The NiFe2O4 thin film exhibits a soft ferrimagnetic behavior at room temperature with a large saturation magnetization.

The electrochemical properties of the NiFe2O4 were evaluated by CV, EIS, and Tafel polarization measurements within a three-electrode cell in 1 M NaOH aqueous electrolyte.

The CV curves measured at a scan rate of 100 mVs -1 are represented in Fig. 7a.

Apparently, the CV curve shows a semi-rectangular shape in the potential window of -0.

Conclusion

Rechtschaffner design has been adopted to determine the most influential parameters on saturation magnetization (Ms). In fact, Rechtschaffner design demonstrated that the deposition and annealing temperature are the most influential parameters on Ms. To elaborate the NiFe2O4 thin film, the optimal condition has been predicted by Doehlert matrix. The spinel structure of the obtained film has been investigated by DRX, Raman and FTIR. The nanoflower morphology of the NiFe2O4 film has been confirmed by SEM. To correlate the result found in Doehlert methodology to magnetic properties, hysteresis loops (M-H curve) for NiFe2O4 thin film prepared under optimal conditions has been examined. Electrochemical properties of the NiFe2O4 film has been tested with three electrodes system in 1 M NaOH electrolyte at room temperature. We believe that the elaborate film can be an excellent electrode for EDLC devices.

Fig. 1 .

 1 Fig. 1. (a) Graphical analysis of the effect of : deposition temperature, deposition time, R′, pH, annealing temperature and time, (b) Graphical Pareto analysis.

  following relation: N = k + k 2 + 1, where k is the number of factors. As mentioned previously two factors influence on the saturation magnetization consequently, 7 different experiments were required. However, in order to validate this model, three replicated experiments at the center level were carried out. Doehlert matrix, experimental plan and results for the two selected factors were listed in Table.4.

Figure. 2

 2 represents the effects of the various factors as well as their interactions. The annealing temperature has a positive effect (b1 = 140.43) on Ms of the material, so any increase in the annealing temperature increases the value of Ms. The deposition temperature has a negative effect (b2 = -42.45) on the value of Ms. It can be noted that the annealing temperature is the most influential factor on the response of Ms.

Fig. 2 .

 2 Fig. 2. Graphical analysis of the effect of deposition temperature and annealing temperature.

Figure. 3

 3 shows the isoreponse curve of Ms as a function of annealing temperature and deposition temperature and the 3D surface of Ms in the plane (TR, Td).

Fig. 3 .

 3 Fig. 3. (a) Contour plots of saturation magnetization (Ms) versus deposition temperature (°C) and annealing temperature (°C), (b) The corresponding three-dimensional plot.

  Figure. 4a shown the X-ray diffractogramme of NiFe2O4 thin film. The observed reflection peaks located at 30.13°, 35.50°, 44.47°, 57.19°, 74.93° were assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (5 3 3)

Fig. 4 .

 4 Fig. 4. (a) X-ray diffraction pattern and (b) Raman, FTIR spectra of NiFe2O4 thin film deposited at TR = 739 °C and Td = 47 °C.

Fig. 5 .

 5 Fig. 5. EDX spectra, SEM image with corresponding elemental mapping images of NiFe2O4 thin film deposited at TR = 739 °C and Td = 47 °C.

Fig. 6 .

 6 Fig. 6. M-H hysteresis loops of NiFe2O4 thin film deposited at TR = 739 °C and Td = 47 °C.

5 to 0. 5 V

 5 without any oxidation/reduction peak. The semi-rectangular shape observed in CV curves in the negative and positive potential, indicate the double-layer capacitance (EDLC) of NiFe2O4 electrode. To confirm the double layer capacitance of the NiFe2O4 electrode the EIS measurement has been study. The Nyquist plot and the equivalent circuit are shown in Fig.7b. Nyquist plot consist two parts, a semicircle in the high-frequency region and a straight line in the lowfrequency region. The large radius of the semicircle is assigned to the charge transfer resistance of the electrode and the vertical line is attributed to the dominant capacitive behavior of the electric double layer formed at the electrode/ electrolyte interface. The values of Rs, Rct, C and W were estimated qualitatively from the fittings of experimental impedance spectra which are equal to 29.4 Ω, 43.5 Ω, 0.43μF and 0.27 Ω.s -5 respectively.

Fig. 7 .

 7 Fig.7. (a) The cyclic voltammograms, (b) Nyquist plot with equivalent circuit, (c) Bode plots and (d) Tafel plot of NiFe2O4 thin film deposited at TR = 739 °C and Td = 47 °C.

Table 1 .

 1 Experimental domain investigated for NiFe2O4 thin films

	Coded variables	Factors	Unit	Experimental field
	(Xi)	(Ui)		Lower limit	Upper limit
	X1	U1: deposition temperature (Td)	°C	40	70
	X2	U2: deposition time (td)	min	60	180
	X3	U3: R' = [Fe]/[OH -]	-	1.11	1.75
	X4	U4: pH	-	9	10
	X5	U5: annealing temperature (TR)	°C	550	750
	X6	U6: annealing time (tR)	h	4.30	6

Table 1

 1 and the results are represented in the Table2. The coefficients of the linear polynomial model are calculated using only extremes by NEMROD Software program[START_REF] Mathieu | Software NEMROD[END_REF] which is represented in

	the following equation:
	Y = 141.665 + 19.072 X1 + 2.643 X2 -8.514 X3 -6.508 X4 + 29.910 X5 -4.980 X6 ̶ 0.845
	X1X2 -5.155 X1X3 + 7.022 X2X3 -9.656 X1X4 -4.097 X2X4 -0.934 X3X4 + 12.426 X1X5 +
	3.482 X2X5 ̶ 4.410 X3X5 -7.203 X4X5 ̶ 1.549 X1X6 ̶ 3.442 X2X6 -3.610 X3X6 -7.673 X4X6
	+ 5.183 X5X6	(2)

Table 2 .

 2 Experimental domain investigated for NiFe2O4 thin films

	Experimental	Experimental design			Experimental plan			Results Y (T)
	number	X1 X2 X3 X4 X5 X6	U1	U2	U3	U4	U5	U6
	1	-1 -1 -1 -1 -1 -1	40	60	1.11	9		4.30	0.112
	2	-1 1 1 1 1 1	40 180 1.75	10		6	0.156
	3	1 -1 1 1 1 1	70	60	1.75	10		6	0.18
	4	1 1 -1 1 1 1	70 180 1.11	10		6	0.231
	5	1 1 1 -1 1 1	70 180 1.75	9		6	0.282
	6	1 1 1 1 -1 1	70 180 1.75	10		6	0.093
	7	1 1 1 1 1 -1	70 180 1.75	10 4.30	0.232
	8	1 1 -1 -1 -1 -1	70 180 1.11	9		4.30	0.169
	9	1 -1 1 -1 -1 -1	70	60	1.75	9		4.30	0.143
	10	1 -1 -1 1 -1 -1	70	60	1.11	10 4.30	0.181
	11	1 -1 -1 -1 1 -1	70	60	1.11	9		4.30	0.286
	12	1 -1 -1 -1 -1 1	70	60	1.11	9		6	0.180
	13	-1 1 1 -1 -1 -1	40 180 1.75	9		4.30	0.145
	14	-1 1 -1 1 -1 -1	40 180 1.11	10 4.30	0.151
	15	-1 1 -1 -1 1 -1	40 180 1.11	9		4.30	0.182
	16	-1 1 -1 -1 -1 1	40 180 1.11	9		6	0.111
	17	-1 -1 1 1 -1 -1	40	60	1.75	10 4.30	0.162
	18	-1 -1 1 -1 1 -1	40	60	1.75	9		4.30	0.138
	19	-1 -1 1 -1 -1 1	40	60	1.75	9		6	0.106
	20	-1 -1 -1 1 1 -1	40	60	1.11	10 4.30	0.185
	21	-1 -1 -1 1 -1 1	40	60	1.11	10		6	0.112
	22	-1 -1 -1 -1 1 1	40	60	1.11	9		6	0.156

Table 3 .

 3 Experimental domain investigated for NiFe2O4 thin films

	Coded	Factors	Unit	Experimental domain
	variables (Xi)	(Ui)
				Lower limit	Upper limit
	X1	U1: annealing temperature	°C	550	750
	X2	U2: deposition temperature	°C	40	70

Table 4 .

 4 Experimental domain investigated for NiFe2O4 thin films

	Experimental	Experimental design	Experimental plan	Results Y(T)
	number	X1	X2	U1	U2
	1	1.0000	0.0000	750	55	0.427
	2	-1.0000	0.0000	550	55	0.115
	3	0.5000	0.8660	700	70	0.277
	4	-0.5000	-0.8660	600	40	0.154
	5	0.5000	-0.8660	700	40	0.453
	6	-0.5000	0.8660	600	70	0.142
	7	0.0000	0.0000	650	55	0.366
	8	0.0000	0.0000	650	55	0.365
	9	0.0000	0.0000	650	55	0.366

Acknowledgements

This work is mainly supported by Ministry of Higher Education and Scientific Research Tunisia and the French National Center for Scientific Research (CNRS).


domain: info scco

text:
Errare humanum est, perseverare autem diabolicum: A Follow-Up Study on the Human-Likeness of an AI Othello Player

Keywords: Applied computing → Computer games;, Human-centered computing → Human computer interaction (HCI) AI agent, board game, Othello, human-likeness

Othello, also known as Reversi, is a popular 2-players board game. Olivaw is an intelligent agent playing Othello. Compared to the most famous ones (such as Saio), it exploits limited resources by autonomously learning how to improve its gameplay by playing against itself. In previous occasions, Othello players reported the impression of a sort of human-likeness in how Olivaw plays. We designed and ran an experimental study to better investigate these impressions in a controlled setting. Participants were asked to watch the moves of pre-recorded Othello games played by a human expert player against either another agent (i.e., Olivaw, Saio) or another human. The identity of the opponent, the outcome of the game (i.e., whether the human expert or the opponent player won), and the color of the players (i.e., black or white, black always playing first) were manipulated. We then asked participants to evaluate the human-likeness of the opponent player. Results confirm that the outcome of the match affects the perception of human-likeness of the players.

INTRODUCTION

Othello, also known as Reversi, is a popular 2-players board game.

Olivaw is an Artificial Intelligent (AI) Othello playing agent adopting the design principles of the AlphaGo programs [START_REF] Silver | Mastering the game of go without human knowledge[END_REF]. Instead of using a traditional approach based on handcrafted rules leveraging human players-knowledge, databases of moves, and brute-force move enumeration, Olivaw can play against worldclass human players (and sometimes beat them) by playing against itself and autonomously learning how to improve its gameplay. Olivaw has defeated other Othello programs, such as Zebra or Saio, at superhuman levels. On average, these programs explore hundreds of millions of positions of the game tree per move, a clear indication of brute force at play. Also, by using a database of openings, they leverage accumulated human knowledge crucially. In contrast, Olivaw explores only a few thousand positions and uses no prior human knowledge.

Previous work [START_REF] Norelli | OLIVAW: Mastering Othello without Human Knowledge, nor a Penny[END_REF] has shown that top-notch human players playing against Olivaw tended to attribute a sort of human-likeness to the way Olivaw plays.

These previous insights align with the fact that human-likeness in virtual agents is elicited not only by verbal and non-verbal behavior but also by playing strategies, especially in computer games context [START_REF] Umarov | Creating believable and effective AI agents for games and simulations: Reviews and case study[END_REF]. The fact that Olivaw is perceived as more human-like than other AI agents may be related to the way it learned. Indeed, reinforcement learning is similar to how humans learn [START_REF] Wan | Reasoning like human: Hierarchical reinforcement learning for knowledge graph reasoning[END_REF], i.e., it involves trial and error instead of relying on handcrafted rules based on human experts.

In this paper, we investigate the perception of Olivaw's humanlikeness in a controlled way. We ran an experimental study where participants were asked to evaluate the human-likeness of an Othello player after watching the moves of a match against a human player. This study extends the work from [START_REF] Lauletta | Errare humanum est? a pilot study to evaluate the human-likeness of a AI othello playing agent[END_REF] by addressing its limitations through a more complex experimental design. In particular, we included the outcome of the match as an independent variable and we compared Olivaw with another AI agent.

RESEARCH QUESTIONS AND HYPOTHESES

This work is part of a larger project whose aim is to answer the following research questions: [RQ1] "How is Olivaw's behavior perceived in terms of humanlikeness?";

[RQ2] "What characteristics of Olivaw's playing strategy contribute to its human-likeness?"

In this work, we focus on RQ1, in particular, so we are interested in how people would place Olivaw in terms of human-likeness compared to a human player and compared to another artificial player using more traditional (and computationally expensive) algorithms. We selected Saio 1 as the second AI player in our study, as it is one of the programs mostly used by Othello players to train against and to analyze the matches of other players. Our first hypothesis is: H1: the participants' impressions of the human-likeness of an Othello player vary according to its identity (i.e., human vs artificial). In particular, we expect that Olivaw would be perceived as more human-like than Saio.

In addition, previous results [START_REF] Lauletta | Errare humanum est? a pilot study to evaluate the human-likeness of a AI othello playing agent[END_REF] showed that, in general, participants tend to assume that a computer player is, in general, a "strong" player, so when a player loses a game, they tend to identify it as a human. Thus, we formulate our second hypothesis: H2: the outcome of a match influences the perception of humanlikeness of an Othello player. In particular, according to previous results, we expect that winners would be generally perceived as computers, and losers would be generally considered as humans.

METHODOLOGY

The Othello Game Evaluation App and the Matches. To investigate the perception of Olivaw's human-likeness, we used the Othello Game Evaluation App. It was designed and implemented following a user-centered iterative design approach involving an expert Othello player. The App allows a human participant to watch and analyze the moves of pre-recorded Othello matches [START_REF] Lauletta | Errare humanum est? a pilot study to evaluate the human-likeness of a AI othello playing agent[END_REF]. Independent Variables. In our experiment, all the pre-recorded matches evaluated via the App were played by the same expert Othello player, whom we call the human player, playing against different types of opponents, as reported below. In particular, we manipulated the following characteristics of the opponent player:

• identity: the opponent player could be another expert human player (H), Olivaw (O) or Saio (S); • outcome: the opponent player could either win (+) or lose (-) the match; • color: the opponent player could be either black (B) or white (W).

We manipulated this variable since the player's color determines who starts the game (i.e., the black player), thus, we need to check whether the order of playing influences human-likeness.

The competence level of the players (i.e., Master level), the number of errors (i.e., similar between the two players), and the openings (i.e., XOT openings, that is, pre-determined standard openings of 8 moves, guaranteeing an even position on the board for both players) of the matches were controlled. The latter choice was made as AI players tend to use non-standard openings compared to humans, which was indicated by expert players as a possible cue of the identity of the player. Dependent Variables. After each match, participants were asked to rate their perception of the identity of the opponent player: "Knowing that the [color] player is human, how would you define the identity of the [other color] player?". The answer was given by selecting a percentage pair through a sliding bar ranging from "100% human -0% computer", to "0% human -100% computer". Procedure. We recruited volunteers from several national Othello associations. The participants were asked to evaluate 12 matches (all the combinations of the independent variables, in a counterbalanced order). For each match, the human player was always indicated to be a human. The participants were asked to evaluate the identity of the opponent, in terms of human-likeness. The participants could complete the task across several sessions, but we encouraged them not to interrupt the evaluation of the same match. The mean duration of the evaluation of one match was of 3min 36s ± 3min 28s and was not influenced by the condition (𝐹 (11, 99) = 1.46, 𝑝 = 0.16).

RESULTS

Among the 26 recruited volunteers, 10 (all males, 48.8 ± 13.2 years old) evaluated the 12 matches. Six of them were European, among which 3 were Italian. 56% of participants owned at least a Master's degree. They all self-assessed their own expertise from advanced to master level, except for one participant who was a beginner.

Table 1 and Figure 1 show descriptive statistics of the humanlikeness scores for each match. We run a 3x2x2 repeated-measures ANOVA with identity, outcome, and color as factors and humanlikeness as a measure. A significant effect of outcome was found (𝐹 (1, 9) = 6.06, 𝑝 = 0.036). In particular, the opponent player was perceived to be more human-like when losing (M= 73.17%) than when winning the match (M= 57.67%, 𝑝 𝑎𝑑 𝑗 = 0.04). No interaction effects were found. More details about data are available on OSF 2 .

These results validate H2 and replicate what was found in [START_REF] Lauletta | Errare humanum est? a pilot study to evaluate the human-likeness of a AI othello playing agent[END_REF], that is, the tendency to attribute human-likeness to the players losing the match. H1 is rejected as no effect of identity on humanlikeness was found.

The participants' comments seem to indicate that the use of predefined openings influenced the perception of the opponent player towards human-likeness. In fact, most of the participants stated that those kinds of openings are not typical of an AI agent.

CONCLUSION AND FUTURE WORK

We presented an extended and improved version of a previous study [START_REF] Lauletta | Errare humanum est? a pilot study to evaluate the human-likeness of a AI othello playing agent[END_REF] investigating the perception of human-likeness of Olivaw Othello player. Results confirm the influence of the outcome of the match on participants' ratings. The potential impact of pre-defined openings on participants' ratings encourages us to continue our investigations, for example by only showing the middle part of the matches to avoid influencing their evaluation of the opponent player's human-likeness. Also, from the comments we received in our experiment we realized that our question to evaluate humanlikeness could be interpreted ambiguously by participants: some of them commented, for example, that "the opponent made so many mistakes that it looked like it was either a computer playing randomly or a human being a complete beginner"; which means that the human-likeness rating they provided for the same match could be either very low or very high, depending on their personal interpretation of our question. So, in future investigations we could add extra questions to evaluate human-likeness-related characteristics of the opponent's playing style: for example, by asking participants to evaluate how much "defensive" or "attacking" the opponent played like, or if its playing style looked "random" or "planned", and so on.

Figure 1 :

 1 Figure 1: Distribution of human-likeness scores for each match, in decreasing order (by mean).

Table 1 :

 1 Mean and standard deviation (SD) of human-likeness for each match, in decreasing order.

	Match Mean SD	Match Mean SD
	BH-	90%	19.4 BO-	65%	39.5
	BS-	84%	23.7 WO-	62%	39.7
	WH-	70%	38.9 BO+	55%	37.2
	WS-	68%	28.2 WH+ 55%	40.6
	BH+	67%	30.9 WS+	52%	35.5
	BS+	67%	33.0 WO+	50%	39.4

https://osf.io/23dyn/?view_only=95a509b6951249e19f8e7a9f23061b8a

ACKNOWLEDGMENTS

This work was partially supported by a Google Focused Award and by BICI (Bertinoro International Center for Informatics). The authors would like to thank Paolo Scognamiglio for playing the matches used in the study and his contribution in the iterative design of the Othello Game Evaluation App.