Vincent Mussot

Armelle Bonenfant

Pascal Sotin

Denis Claraz

Philippe Cuenot

From Relevant High-level Properties to WCET Computation Improvement

Keywords: WCET computation, timing analysis, infeasible paths, annotation language

The scheduling of real-time systems requires knowing the Worst Case Execution Time (WCET) of their tasks. WCET analysers compute timings by analysis of the low level behaviour of the target task. This document presents improvements in the WCET computation allowed by taking into account high level behaviours of the tasks. We first classify high level knowledge according to the relevance with respect to WCET estimation. We then propose a systematic method to bring this information back to the low level on which operate most WCET analysers. This approach separates the concerns of stating properties, integrating properties and computing a WCET aware of these properties.

1 Imprecision in Timing Analysis of a Task Critical hard real-time systems are composed of tasks which must imperatively finish before their deadline. In order to guarantee the safety of the system, a task scheduling analysis is performed and requires an a priori Worst-Case Execution Time (WCET) analysis of each task. Moreover for early evaluation of software application resource, it is a great benefit to be able to compute early Software Execution Time.

WCET analysers need to consider architectural details in order to be able to provide timing information. They reason on a Control Flow Graph (CFG) retrieved from the binary. Reasoning at the machine code level makes that information obvious at the design level or at the source code level have been lost. By ignoring such information, the analysers introduce pessimism in the computed WCET. For example, Figure 1 presents a CFG of a loop. Block 2 is the loop head and the worst case execution of the body is assumed to be all blocks from 4 to 8. In practice, this path never occurs because the CFG originates from a loop in the source code shown on Figure 2.

The two main reasons for imprecision in timing analysis of a task are:

1. Pessimism at the architecture level (eg. cache accesses), 2. Pessimism in the considered control flow. Numerous improvements have been suggested for 1. This article tries to tackle 2 with the following this philosophy:

• WCET analyses are by essence low-level analyses,

• Many flow information are known at design level or are easily retrieved at code source level,

• Imprecision in timing analysis can be reduced by transferring the relevant high level knowledge to the low level. Such transfer already exists between the tools Otawa and oRange developed in the IRIT Research Laboratory in Toulouse.

f o r ( i = 0 ; i < 1 0 0 ; i ++) { i f ( i <s ) { . . . } i f ( i >s ) { . . . } } Figure 2: Loop C source
Otawa. Otawa is a toolbox for building WCET analyses [START_REF] Ballabriga | OTAWA: an Open Toolbox for Adaptive WCET Analysis[END_REF]. It handles low level features like caches and pipelines for several architectures (including ARM, PowerPC and Tricore). Following the Implicit Path Enumeration Technique (IPET) the tool eventually converts the program CFG in an Integer Linear Program (ILP) which solution is an over-approximation of the WCET. Loops bounds are required from the user or from an external analyser.

oRange. oRange is a static analyser for C mainly dedicated to infer context-sensitive bounds for the program loops. This tool generates precise source code level information. Its output format is called FFX( [START_REF] Cassé | FFX (Flow Fact in XML) format[END_REF]), an XML-based language for conveying flow information (FFX stands for Flow Fact in XML).

Contributions of the article. The main contributions of this article are:

• A classification of high level information capable of enhancing the WCET analysis,

• A generic method to transfer high level information encoded in FFX format to the Control Flow Graph representation of a WCET analyser, • A proposal to early compute execution time of software application based on software context bounded by data flow definition.

Outline. Section 2 focuses on the user side of the WCET analyses. We list several categories of knowledge held by the system designer/developer. We also discuss how the knowledge harvesting can take place in an development process. Section 3 surveys relevant elements of the FFX format (Flow Facts in XML). This format has been designed to encode facts that hold for a given program or facts that should/can be assumed to hold. A full presentation can be found in [START_REF] Cassé | FFX (Flow Fact in XML) format[END_REF]. Section 4 discuss the integration of high-level knowledge encoded in FFX into a WCET analysis by mean of CFG refinement through product automata. Section 5 presents preliminary results of the process on industrial code. We conclude in Section 6.

Useful Knowledge for Tightening WCET

Handling semantic aspects in timing analysis has been less studied compared to hardware aspects. The semantic analyses used by the WCET computation process are limited to determining loop bounds and rather simple infeasible paths. Once this information is gathered a bound can be found for the WCET even if it may be imprecise or unrealistic.

In order to increase the number of useful properties fed into the WCET computation, we have based our work on the following methodology: identification of relevant properties, extraction, expression, and eventually usage. In 2.1 we present a classification of properties and provide examples. In 2.2 we discuss how useful properties can be retrieved from the designer / developer in an industrial context.

Identification of the Useful Properties

Generic software properties. They are due to (known) design tools/methods (e.g. model-based design, synchronous languages) that may guarantee, by construction, important properties (e.g. bounded memory, bounded loops, memory privacy); these properties must be identified once for a given design method, and can then automatically be taken into account for a particular application.

Hypotheses on inputs/environment/context. Application-specific information may be provided by the software control engineer. It includes basic static information, such as numerical ranges (e.g. possible values from a sensor or on intermediate system data), commands exclusions (e.g. infeasible paths due to the environment, such as for instance different working modes); it may also include more dynamic information, such as known variations of loop indices, causality relations among time (e.g. initialization step, sensor behaviour determined by its environment and achieving a new state in the control). In the context of modular analysis, information about the execution context may also be provided by the software engineer (e.g. being in a certain mode is the consequence of a given execution sequence). When the execution context is unknown, the software engineer may define some execution modes (e.g. automatic or manual gearbox for a vehicle mode) leading to a partition of the parameters values.

We rely on experts to provide this kind of properties. The information they provide can be either asserted system knowledge and thus help to make the estimation more precise, or an execution mode or scenario for which the WCET needs to be computed. In the latter case the computed WCET should not be blindly trusted but can be a useful WCET estimator on the early stages of the system development.

Semantic properties. They derive from the program semantics: exclusion between test branches (e.g. the property of variable value inducing a certain path, excluding other paths), partition of loop executions (e.g. properties on frequencies of branches a loop), loop bounds, etc. Automatically extracting such properties requires to define a semantic model, and to use software static analysis techniques (inspired by/coming from model-checking or abstract interpretation).

In the community of timing analysis, most tools ([MBCS08, Abs07, Tid05, PSK08]) cover extraction and expression of semantic properties (most loop bounds and some infeasible paths). The automatic extraction of richer semantic properties is one of the goals of the project W-SEPT1 but will not be addressed in this article.

Retrieving Properties from the Expert

Generic software properties have to be collected once. Semantic properties have to be kept up to date when the code evolves but this process can be automated. However, hypotheses on inputs/environment/context are numerous, evolving and cannot be systematically collected since they involve the system designers. In this section, we present some guidelines on where to spend the expert time according to our personal experiences.

The collaboration between Continental Automotive and IRIT aims to define the context for improving the WCET accuracy in order to be able to specify requirement for future industrial evaluation of system performance. The overall evaluation of the current work is as follows: Continental Automotive provides a scenario for a piece of code (and the corresponding code). This portion is known by the function developer to be time consuming (typically by measurement). However, due to the huge number of parameters and code branches linked to the system configuration it is laborious to locate the reason of this high consumption and to know whether some improvement can be done. IRIT takes the scenario and outputs the WCET and the corresponding path. The expert studies the path and excludes some part of it, for example incompatible values or exclusion of conditions. Then, the expert provides an improved scenario with these exclusions. Eventually, the developer may consider how to modify the code in order to improve the system performance. This is one possible and promising application of WCET estimation. The evaluation above is summarized in Figure 3. In general, the nature of the feedbacks will be:

Expert

• Ranges of values leading to the WCET. The expert may find an incompatibility in the values leading to WCET. In that case, he can express formally the reason for that incompatibility (e.g. "if mode = critical then sensor greater than 1,000").

• Basic blocks involved in the worst-case path. Due to over-approximation, a worst-case path may not be feasible2 (code structure or system behavior). If the expert realises that, he might the analyser by stating some exclusivity among blocks or feeding the data constraints leading to this exclusivity.

• Estimation of the potential improvements3

3 A Language for Flow Facts: FFX FFX is a description format and an annotation language used to represent flow facts of a program.

The main concepts and ideas that drive the design and the development of FFX include -freedom: tools using FFX do not need to support all features. In the worst-case, unsupported features induce a loss of precision but never invalidate information; expandability: the use of XML allows to easily insert new elements or new attributes without breaking the compatibility with other tools; soundness: all provided information must be ruled by a precision order, ensuring that, at least, information in most generic cases must not be more precise than in particular cases; flexibily: possibility to define context dependent analysis i.e. reuse of component or context. This format presents information in XML, structured as a tree, where tags represent structural elements of the program. Attributes can be attached corresponding to properties of these elements. FFX files are generated by a static analysis tool (in our case oRange [START_REF] Michiel | Static Loop Bound Analysis of C Programs Based on Flow Analysis and Abstract Interpretation[END_REF]) on the program (in C) or provided/enriched by an expert. We provide in Listing 1 some details of tags and attributes used in the paper.

A STATEMENT can be either:

• BLOCK: A block element identifies a piece of code, possibly composed of several execution paths, but with a single entry point,

• CALL: A call element identifies the call to a function. Its location represents the caller and it must contain a function element representing the callee. Multiple call elements that embed functions allow to represent call-chain locations,

• CONDITION: A condition element represents a condition with several alternatives. In the C language, it applies to both if statements and switch statements,

• LOOP: A loop element matches a loop construct in the code. It may contain iteration elements in order to represent properties that are valid only during certain iterations of the loop.

The attributes of the LOOP tag are LOCATION-ATTRS and INFORMATION-ATTRS. LOCATION-ATTRS provides localisation information such as:

• IDENTIFICATION (id="TEXT") allows to make a reference to a part of code represented by another location attribute,

• ADDRESS-LOCATION (address="INT") gives the address but may be invalidated each time the application is compiled,

• SOURCE-LOCATION refers to line number and if necessary file name.

INFORMATION-ATTRS, for loops provides details on the loop bounds:

• maxcount the maximum number of time the loop iterates after each entry

• totalcount the maximum number of time the loop iterates for the whole program run

• exact true if the totalcount is exact or false if it is overestimated

• executed false if the loop is never executed, true if the loop executes in some cases CONDITIONAL, CONDITION and CASE are used to describe a condition with several alternatives. They have attributes similar to LOOP.

The Listing 2 is an example of the simplified output of oRange on the initial example. In addition to the classical maximum of iteration of the loop 1 (maxcount="100"), the mutual exclusion of the two blocks is expressed in the tags <control-constraint>. Here, to simplify, block-IF1T (resp. block-IF2T), is the name of the block of the true condition 1 (resp. 2). The property in the example is that block-IF1T + block-IF2T ≤ 1 for each iteration of the loop.

Properties described in the file are to be exploited by any code analyser, in our case, by OTAWA.

In the current state, OTAWA is fetching the information on loop bounds (and seldom some variable domains), whereas FFX is able to describe a larger amount of properties. We propose a treatment of FFX files in order to fill the gap between expression and usage of properties in our tool chain.

From Knowledge Gain to Precision Gain

Once the high level information relevant for the WCET analysis has been identified, collected and expressed in FFX, it still needs to be integrated in the analysis to yield a precision gain.

The transfer technique sketched in this section relies on viewing the Control Flow Graph handled by the WCET analysers as an automaton accepting sequences of transition between blocks. For example, if we consider the CFG pictured on Figure 1, we can see that after accepting 2 → 4 the CFG can accept Listing 2: Simple example <f l o w f a c t s> <f u n c t i o n name="main" e x e c u t e d=" t r u e " e x t e r n=" f a l s e "> <l o o p i d=" 1 " l i n e=" 4 " s o u r c e=" s r c c / s i m p l e l o o p . c " e x a c t=" t r u e " maxcount=" 100 "> < i t e r a t i o n number=" * "> <c o n t r o l -c o n s t r a i n t> < l e> <add><count i d=" b l o c k -IF1T" /><count i d=" b l o c k -IF2T" /></add> <c o n s t i n t=" 1 " /> </ l e> </ c o n t r o l -c o n s t r a i n t> </ i t e r a t i o n> </ l o o p </ f u n c t i o n> </ f l o w f a c t s> 4 → 5 or 4 → 6. The information contained in the FFX file can be seen as a restriction of what can really be accepted by the CFG. For example, the FFX shown on Listing 2 states that in every iteration of the loop, 4 → 5 and 6 → 7 cannot be accepted simultaneously.

The application of the restriction will be provided by computing a new CFG, more restrictive, by making an automata product of the CFG with an automaton encoding the FFX. Such operation can be unambiguously described and implemented according to a formal description.

Encoding exclusive paths using automata. The automaton Figure 4a encodes the sequences of edges containing no more than one edge 4 → 5 or 6 → 7. If we construct the product of this automata with the loop body CFG we obtain a new CFG where the spurious trace 2 → 4 → 5 → 6 → 7 → 8 no longer exists. This CFG can be seen on Figure 6a. Encoding exclusive paths using automata and constraints. The issue of performing a product with automaton like the one of Figure 4a is that the size of the resulting CFG will grow with each information added by the user, therefore slowing the subsequent analysis.

start / ∈ {4 → 5, 6 → 7} {4 → 5, 6 → 7} / ∈ {4 → 5, 6 → 7} (a) Usual automaton start x + y ≤ 1 {4 → 5}, x {6 → 7}, y / ∈ {4 → 5, 6 → 7} (b) Automaton with constraints
Since we assume that the WCET analyser rely on IPET to compute its prediction we also consider automata enriched with counters and numerical constraints. The automaton of Figure 4b encodes the sequences of edges in which the count of edges 4 → 5 and 6 → 7 is less than one. This automaton directly derives from the <control-constraint> FFX element.

If we perform a product between the loop body CFG and this automaton, we obtain the same CFG, carrying the counters x and y (see Figure 6b). The spurious trace 2 → 4

x → 5 → 6 y → 7 → 8 contains one occurence of counter x and counter y and will thus be rejected by the constraint x + y ≤ 1. Encoding the contexts. The FFX language offers the notion of context which allows to state that a given property is true in a given situation. The automata shown on Figure 4 states an exclusivity property but it remains to take into account that this property holds at each iteration of the loop.

out start q start {4 → 5}, x {6 → 7}, y / ∈ {4 → 5, 6 → 7} in x + y ≤ 1 / ∈ {2 → 4} {2 → 4} {8 → 2}, z z = 100
To do so we rely on hierarchical automata which can recursively carry automata in their nodes. The subautomata are "activated" each time the state containing them is reached.

Figure 5 shows a hierarchical automaton which encodes the FFX information stated by Listing 2. Since Otawa CFG do not have a hierarchical nature, the product of the hierarchical automaton and the CFG have to be flattened. The new CFG is depicted on Figure 6c. Due to the flattening, the spurious trace 2 → 4

x → 5 → 6 y → 7 → 8 is allowed but if that trace is considered taken in 50 iterations then the 50 other iterations must execute 2 → 5 → 8.

Once the generic product and flattening are defined and implemented by a WCET analyser like Otawa, all the burden of expressing the FFX construct in term of automaton is left to the FFX language designers. In [START_REF] Engblom | Modeling complex flows for worst-case execution time analysis[END_REF], Engblom and Ermedahl present a similar integration of flow facts in a CFG.

Experimentation

To demonstrate the effectiveness of our path suppression technique, we use a sample of Continental Automotive code dedicated to the control of the engine speed regulation of an engine management. This code contains mostly conditions and no loops so we are able to obtain a WCET directly. However we used a scenario file where most variables were initialized, in order to correspond to a state of the engine, mostly depending on the engine rotation speed. According to this scenario, oRange is able to specify which of the then and the else branch is taken. It generates an associated FFX file containing several properties for each case (then or else) including a property executed which can be true or false. In order to obtain a reduced WCET, we use the flow information present in the FFX file to perform a path suppression when it does not correspond to the scenario file. The scenario file is a conjonction of facts which can be :

• values for code variables

• ranges for code variables

• conditions between code variables

When oRange is able to set the executed property of a case to false, the corresponding block in the CFG should never be executed. That is why we remove the associated branch in the CFG and we only let the path which goes through the other case of the condition. This way, if the most costly case of the condition cannot be executed, the only path remaining will be taken for the calculation of the WCET and this new estimation will hopefully be lower than the previous one. However this new estimation shall only be used in association with its specific scenario.

Table 1 shows the reduction of the WCET using the specific scenario file. The reduction of the WCET obtained may be different for any other scenario given, depending on which of the two cases of each condition is specified as false.

Conclusion

Over-approximation in WCET computation introduces pessimism and needs to be reduced. Better knowledge of the program, i.e. application domain (in term of variables), execution mode, environment/architecture support, design process, even compilation are valuable sources of information and can be found and exploited all along the development process.

In this work, we presented a classification, aiming to simplify identification and extraction (manual or automatic) of the program properties useful for WCET computation. We express these properties in FFX, which offers an easy way for experts to state their knowledge on the program.

We described an improvement for OTAWA, which was restricted by its ability of reading FFX, by adding a translation step: FFX into CFG thanks to an automaton. Although, this restriction was specific to our tool, the proposal not only levies this restriction, but also helps to enlarge the usage of FFX by other timing analysis tools. The use of FFX as pivot language has been initiated in [ZBdM + 12]. Thus, any FFX file, provided by analysis or expertise, with numerous kind of properties would be readable by OTAWA.

The next step is to establish and validate clear requirement for timing analysis and optimization to be able to optimize but ensure predictive behaviour and soundness of real time application.

  Figure 1: Loop CFG

Figure 3 :

 3 Figure 3: Overall process

Listing 1 :

 1 Extract of FFX grammar <f u n c t i o n LOCATION-ATTRS INFORMATION-ATTRS> STATEMENT * </ f u n c t i o n> STATEMENT : : = BLOCK | LOOP | CALL | CONDITION LOOP : : = . . . | <l o o p LOCATION-ATTRS INFORMATION-ATTRS> < i t e r a t i o n number="INT"> STATEMENT * </ i t e r a t i o n> </ l o o p> CONDITIONAL : : = <c o n d i t i o n a l CONDITIONAL-ATTRS LOCATION-ATTRS> CONDITION? CASE+ </ c o n d i t i o n a l> CONDITION : : = <c o n d i t i o n CONDITION-ATTRS LOCATION-ATTRS> CONDITIONAL | STATEMENT STATEMENT * </ c o n d i t i o n> CASE : : = <c a s e CASE-ATTRS LOCATION-ATTRS> CONDITIONAL | STATEMENT </ c a s e> CONTROL-CONSTRAINT : : = <c o n t r o l -c o n s t r a i n t> CONTROL-PREDICATE </ c o n t r o l -c o n s t r a i n t>

Figure 4 :

 4 Figure 4: Two automata encoding: either transition 4 → 5 or transition 6 → 7

Figure

  Figure 5: Hierarchical automaton

<c o n d

  Figure 7: Code condition

Funded by ANR, http://wsept.inria.fr

In[START_REF] Knoop | Wcet squeezing: on-demand feasibility refinement for proven precise wcet-bounds[END_REF] Zwirchmayer et al. try to automate this process: the worst-case path is checked for feasibility.

So far, a tool like OTAWA only delivers the WCET without giving information on possible improvements. Such information would be a great help for the industrial in order to know where to focus.

Acknowledgements. The authors wish to acknowledge Jakob Zwirchmayer and the anonymous referees for their thorough reviews. This work is supported by the French ANR project W-SEPT.


Emilie Narni-Mancinelli

Eric Vivier

published or not. The documents may come

Clues that natural killer cells help to control COVID

Clues that natural killer cells help tocontrol COVID Emilie Narni-Mancinelli & Eric Vivier

COVID-19 were less able to produce cytokines and chemokines than were such cells from healthy donors or from infected people who have no or minimal COVID-19 symptoms.

To dissect the mechanisms underlying the dysfunction of NK cells, Witkowski et al. assessed gene expression by carrying out single-cell RNA sequencing of these cells from Natural killer cells can destroy cells infected by SARS-CoV-2, but this immune-system defence malfunctions in people with severe COVID-19. Will this finding drive a search for ways to reinvigorate natural killer cells in such cases? people with COVID-19 of differing degrees of severity. The authors found, consistently with another study 7 reporting a similar transcriptional analysis of NK cells from people with COVID-19, that the cells had a gene-expression pattern characteristic of that driven by type I interferons. This gene-expression Most cases of SARS-CoV-2 infection are mild, but a proportion of people progress to severe COVID-19 for reasons that are not entirely clear. Some factors associated with the severe illness are known. For example, people with severe COVID-19 often have other preexisting conditions, such as coronary artery disease, hypertension and diabetes 1 . Unsurprisingly, compromised antiviral defences can also lead to life-threatening illness, as is the case for around 15% of people with severe COVID-19 who have a deficiency in what is called the type I interferon pathway 2 . Indeed, it has been known since the discovery of this pathway in 1957 that it directly interferes with viral repli-cation in an infected cell 3 .

There might also be other mechanisms at play in antiviral defences, such as a role for the natural killer (NK) cells of the immune system. Witkowski et al. 4 report on page 295 that people with severe COVID-19 have higher than normal levels of an anti-inflammatory molecule, transforming growth factor-β1 (TGF-β1), in their bloodstream, and that this is associated with impaired antiviral defence by NK cells and defective immune-system control of SARS-CoV-2.

Natural killer cells are white blood cells that are part of the innate branch of the immune system. They make key contributions to tackling viral infections, and their rapid activation is linked to the efficient control of several types of virus 5 . NK cells can eliminate virus-infected cells in various ways -by releasing cytotoxic granules (termed degranulation) that directly kill the virus-infected cells, or by shaping both the innate and adaptive branches decline in the level of virus (the viral load) and high levels of NK cells in the bloodstream at the beginning of the infection. This correlation was not observed for the T cells or B cells of the immune system. The drastic decrease in NK cells in the bloodstream over the course of infection might be due to their death by a process called apoptosis 7 , or their recruitment from the bloodstream to the lungs.

Crucially, Witkowski et al. report that NK cells from healthy individuals can directly kill SARS-CoV-2-infected cells in vitro, but that NK cells from people with moderate or severe COVID-19 had impaired cell-killing activ-ity (Fig. 1). These defective NK cells express higher-than-normal levels of the cytotoxic molecules that kill cells, but are impaired in signature was more prominent in NK cells from people with severe rather than milder forms of COVID-19. Moreover, a pattern of gene expression characteristic of that driven by the protein TNF-α in NK cells has been described 7 for mild cases of COVID-19. Witkowski et al. also reveal an increase in the expression of genes controlled by TGF-β1 in NK cells purified from either the blood or the lungs of people infected with SARS-CoV-2. This increase seems to corre-late with the patients' symptoms, with a higher prevalence of TGF-β1-driven gene expression corresponding to more severe illness.

TGF-β1 is a cytokine with a central role in tissue remodelling, but it also suppresses the functioning of NK cells 11 Several questions remain. The first is whether NK cells are necessary to fight SARS-CoV-2 infection. Several studies of mouse models of respiratory viral infections, such as influenza A virus, respiratory syncytial virus and Sendai virus, have shown that NK cells contribute to virus control and aid host survival 5 . Consistent with this, the only characteristic shared by people with various types of NK-cell deficiency is a predisposition to viral infections, especially of herpesvirus 12 . The fact that type I interferons are strong activators of NK cells also suggests a link between deficiencies in type I interferons and impaired NK-cellmediated antiviral immunity. All these findings, together with the inverse correlation between the number of functional NK cells and the severity of COVID-19, support a role for these cells in controlling SARS-CoV-2 infection, but formal proof of this hypothesis remains to be obtained.

A second question to be addressed is, what are the mechanisms underlying the high production of TGF-β1 during COVID-19? SARS-CoV-2 contains a spike protein that interacts with the ACE2 receptor on human cells 1 . This receptor is an enzyme that converts the protein angiotensin II (AngII) to a peptide called Ang1-7. The enzymatic activity of ACE2 is disturbed during SARS-CoV-2 infection, resulting in a rise in AngII levels that promotes TGF-β1 expression 1 . Thus, excessive damage to lung tissue caused by a high viral load might induce a high level of TGF-β1, which could affect NK cells. More-over, TGF-β1 is known to promote a type of tissue damage called fibrosis, which is a hallmark of severe COVID-19. A rise in the level of AngII is a common feature of coronary artery disease, hypertension and diabetes 13 , and high levels of TGF-β1 are observed in obesity 14 . It will therefore be interesting to investigate whether the association of such conditions with severe COVID-19 involves defective functioning of NK cells mediated by TGF-β1.

Ways of manipulating NK cells are of increasing interest in anticancer therapy, using approaches such as immunecheckpoint inhibitors, NK-cell 'engagers' and off-the-shelf infusions of NK cells 15 . The importance of NK cells in the control of viruses should similarly prompt the design of innovative treatments based on harnessing these cells to fight viral infections. In the case of severe COVID-19, the high levels of TGF-β1 present might compro-mise strategies to boost the activity of NK cells. Instead, the use of TGF-β1 blockers 16 might be a way to promote NKcell-mediated antiviral defence and prevent lung fibrosis, although the safety of such treatment in this context is a concern that should be addressed. In addition, monitoring TGF-β1 levels in plasma, along with TNFα-mediated gene expression in NK cells from the bloodstream 7 , might help to pre-dict the outcome of infection and to guide a patient's care appropriately.

  . TGF-β1 limits the ability of NK cells to control SARS-CoV-2 of immune defence by producing molecules called cytokines and chemokines, which can modulate the behaviour of other immune cells. Previous studies 6-10 monitored the responses of NK cells in individuals with COVID-19, and reported lower than normal levels of NK cells in people with the disease, but the precise role of these cells in severe COVID-19 was unclear. Witkowski et al. analysed clinical samples, and found a correlation between a rapid

Figure 1 |

 1 Figure 1 | Characterizing immune responses associated with COVID-19. Immune cells called natural killer (NK) cells can help to tackle viral infections by releasing cytotoxic granules that can kill infected cells. a, Previous work 7 indicates that, in cases of mild COVID-19, NK cells express genes that are typically induced through the protein TNF-α pathway. This type of gene-expression profile arises through the action of transcription factors such as the protein NF-κB, and it aids the recovery of function of NK cells 7 . b, Witkowski et al.4 analysed samples of NK cells from individuals with severe COVID-19, and report that these cells had a gene-expression pattern characteristic of that driven by the protein TGF-β1. Gene expression mediated by this pathway depends on transcription factors that include Smad proteins. The authors report that NK cells from people with severe COVID-19 are impaired in their ability to bind to infected cells and to release cytotoxic granules. This finding provides insights into the factors driving severe COVID-19.


Gregory James Smith

James J Ellis

Roba Moussaoui

Cédric Pardanaud

Céline Martin

Jocelyn Achard

Riadh Issaoui

Timo Gans

James Peter Dedrick

Gilles Cartry

Celine Martin

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

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 development of negative ion sources is of significant interest due to their applications in particle acceleration [START_REF] Ueno | Interesting experimental results in Japan proton accelerator research complex H-ion-source development (invited)[END_REF][START_REF] Peters | Negative ion sources for high energy accelerators (invited)[END_REF][START_REF] Moehs | Negative hydrogen ion sources for accelerators[END_REF][START_REF] Lettry | Status and operation of the Linac4 ion source prototypes[END_REF][START_REF] Faircloth | An overview of negative hydrogen ion sources for accelerators[END_REF] , neutron generation [START_REF] Welton | The status of the SNS external antenna ion source and spare RFQ test facility[END_REF][START_REF] Antolak | Negative ion-driven associated particle neutron generator[END_REF] , mass spectrometry [START_REF] Alton | High-intensity, heavy negative ion sources based on the sputter principle (invited)[END_REF][START_REF] Middleton | A survey of negative ion sources for tandem accelerators[END_REF][START_REF] Calcagnile | High-resolution accelerator-based mass spectrometry: Precision, accuracy and background[END_REF][START_REF] Yoneda | AMS14C measurement and preparative techniques at NIES-TERRA[END_REF] , spacecraft propulsion [START_REF] Rafalskyi | Brief review on plasma propulsion with neutralizer-free systems[END_REF][START_REF] Lafleur | Alternate extraction and acceleration of positive and negative ions from a gridded plasma source[END_REF][START_REF] Aanesland | The PEGASES Gridded Ion -Ion Thruster Performance and Predictions[END_REF] , nanoelectronics manufacturing [START_REF] Vozniy | High-energy negative ion beam obtained from pulsed inductively coupled plasma for charge-free etching process[END_REF] , and neutral beam heating for magnetic confinement fusion (MCF) [START_REF] Hemsworth | Overview of the design of the ITER heating neutral beam injectors[END_REF][START_REF] Hemsworth | Status of the ITER heating neutral beam system[END_REF][START_REF] Hemsworth | Positive and negative ion sources for magnetic fusion[END_REF][START_REF] Fantz | Development of negative hydrogen ion sources for fusion: Experiments and modelling[END_REF] .

One application of particular interest is the creation of negative-ion beams suitable for MCF neutral beam injection, which has a proposed requirement of accelerating a 40 A current of deuterium negative ions to 1 MeV [START_REF] Hemsworth | Overview of the design of the ITER heating neutral beam injectors[END_REF] . This primarily utilises negative ion surface production, as distinct from volume production, to increase the density of negative ions close to the extraction grid [START_REF] Belchenko | A powerful injector of neutrals with a surface-plasma source of negative ions[END_REF][START_REF] Vadim | Surface-plasma method for the production of negative ion beams[END_REF][START_REF] Dudnikov | Development and applications of Negative Ion Sources[END_REF] .

Negative ion production from plasma facing surfaces can be enhanced through the application of a low work function alkali metal [START_REF] Dudnikov | Development and applications of Negative Ion Sources[END_REF] . Current methods apply a thin layer of caesium to the extraction region of the ion source [START_REF] Heinemann | Design of the "half-size" ITER neutral beam source for the test facility ELISE[END_REF] . This is achieved by injecting caesium vapour into the plasma and allowing it to condense onto the inside of the ion source [START_REF] Tsumori | Negative ion production and beam extraction processes in a large ion source (invited)[END_REF] . There exist some limitations with this approach, such as controlling the application of the caesium so that it condenses in the right locations and at a rate that is sufficient to maintain an optimum thickness at the extraction grid [START_REF] Fröschle | Recent developments at IPP on evaporation and control of caesium in negative ion sources[END_REF] . Additionally, this method introduces complex engineering challenges, eg. equipment maintenance and potential for caesium pollution [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Kurutz | Investigations on Cs-free alternatives for negative ion formation in a low pressure hydrogen discharge at ion source relevant parameters[END_REF] . Alternative materials to caesium are therefore of interest.

Several studies have been carried out to investigate negative ion production using alternative materials to caesium via their exposure to low pressure electronegative plasmas. Such materials include non-dielectric and dielectric materials including: diamond-like-carbon (DLC) [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF] , novel electrides [START_REF] Sasao | Negativehydrogen-ion production from a nanoporous 12CaO.7Al2O3 electride surface[END_REF] , highly orientated pyrolitic graphite (HOPG) [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Schiesko | Negative ion surface production through sputtering in hydrogen plasma[END_REF][START_REF] Schiesko | A comparative study of H-and D-production on graphite surfaces in H2 and D2 plasmas[END_REF] , diamond [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Schiesko | Negative ion surface production through sputtering in hydrogen plasma[END_REF][START_REF] Schiesko | A comparative study of H-and D-production on graphite surfaces in H2 and D2 plasmas[END_REF][START_REF] Kumar | Enhanced negative ion yields on diamond surfaces at elevated temperatures[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF][START_REF] Achkasov | Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas[END_REF][START_REF] Kogut | Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry[END_REF][START_REF] Dubois | Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry[END_REF] , and low work function materials other than caesium (LaB 6 , MoLa) [START_REF] Kurutz | Investigations on Cs-free alternatives for negative ion formation in a low pressure hydrogen discharge at ion source relevant parameters[END_REF] .

Dielectric materials are of particular interest as an alternative to low work function metals [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF] .

Generally, for atoms approaching a surface, the affinity level of the atom is gradually downshifted until it overlaps with the surface material's valence band. Electrons can then tunnel from the valence band of the surface to the approaching atom and form a negative ion, this is the so-called resonant charge transfer (RCT) process, as summarised in Ref. 37. For a metal, the conduction band is situated on top of the valence band. When a newly created ion begins to leave the surface, the probability of electron loss through tunnelling back to the conduction band of the surface is high due to the resonance between the affinity level of the negative ion and the empty states of the conduction band. This means that most metals produce negligible negative ions through surface ionisation processes [START_REF] Brako | Theory of electronic processes in atom scattering from surfaces[END_REF] .

Unlike most metals, caesium can be used to enhance negative ion production because it has a low work function. This increases the distance at which the resonance between the affinity level of the new ion and the empty conduction states occurs, reducing the probability that the electron tunnels back to the surface [START_REF] Borisov | Negative ion formation in the scattering of atoms and ions from dielectric surfaces[END_REF][START_REF] Brako | Theory of electronic processes in atom scattering from surfaces[END_REF] .

In contrast to metal surfaces, where the conduction band lies on top of the valence band, the band gap of dielectrics suppresses the tunnelling of electrons from a newly created negative ion back to the material's surface. This means that a new negative ion can travel a larger distance away from the surface before reaching a point where its affinity level is in resonance with the empty states of the conduction band. The increased distance of the ion from the surface reduces the probability that the electron associated with the new negative ion will tunnel into the empty states of the conduction band, thereby increasing the negative ion yield [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Los | Charge exchange in atom-surface collisions[END_REF] .

One potential drawback to the use of dielectric surfaces is that to generate a negative ion the atom-surface distance for a dielectric must be much smaller than for a metal. This is due to the larger energy gap that the occupied valence band states lie beneath the vacuum level [START_REF] Borisov | Negative ion formation in the scattering of atoms and ions from dielectric surfaces[END_REF] . Fortunately, the atom-surface interaction process is amplified by the Coulomb interaction between a negative ion and a localised hole in the surface material [START_REF] Borisov | Diabatic Energy Level Confluence: The Mechanism of Negative Ion Conversion of Neutral Atoms in Grazing Scattering from Insulator Surfaces[END_REF] . This can result in a high ionisation efficiency as demonstrated in beam experiments [START_REF] Winter | Collisions of atoms and ions with surfaces under grazing incidence[END_REF][START_REF] Roncin | Evidence for F-Formation by Simultaneous Double-Electron Capture during Scattering of F+ from a LiF(001) Surface[END_REF][START_REF] Auth | Formation of negative ions in grazing scattering from insulator surfaces[END_REF] . For these reasons, dielectrics are of interest as an alternative to low work function metals for the surface production of negative ions.

Carbon surfaces are one prospective category of materials that are of interest for replacing low work function metals where negative ions are to be produced. For instance, DLC has been used to produce negative ions from incoming neutral particles for a spacecraft particle detector, when low work function metals would not have been appropriate [START_REF] Mccomas | IBEX-interstellar boundary explorer[END_REF] . Of the forms of carbon, diamond has particularly beneficial properties:

• It is a dielectric with a large band gap (5.5 eV) [START_REF] Diederich | Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy[END_REF] that suppresses the destruction of negative ions as they leave the material's surface

• It can be grown to have 'designer' properties such as the preferential growth of a particular crystal face to alter the electronic structure of its surface [START_REF] Diederich | Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy[END_REF] • When it is being grown, dopants can be introduced to change its effective work function and electron affinity [START_REF] Liu | Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron-and Nitrogen-Co-doped Nanodiamond[END_REF][START_REF] Tachibana | Diamond films grown by a 60-kW microwave plasma chemical vapor deposition system[END_REF][START_REF] Scholze | Structure of the diamond (111) surface: Single-dangling-bond versus triple-dangling-bond face[END_REF][START_REF] Baranauskas | Nitrogen-doped diamond films[END_REF] • It can have a negative electron affinity when the surface is hydrogen terminated [START_REF] Diederich | Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy[END_REF] , which reduces its effective work function by reducing the energy gap between the valence band and the vacuum level. This is thought to have a positive influence on negative ion production [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF] • When heated to 450 • C, diamond has previously been shown to produce five times more negative ions compared to other forms of carbon e.g. graphite [START_REF] Kumar | Enhanced negative ion yields on diamond surfaces at elevated temperatures[END_REF] As a means of increasing the production of negative ions, previous work with diamond has investigated using single, nano-and micro-crystalline diamond and also p-type doping of micro-crystalline diamond (MCD) using boron [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF] . The n-type doping of diamond using nitrogen has not previously been studied in this context and it is thought that it could lead to favourable properties for negative ion production for two reasons. Firstly, previous studies of the electronic properties of nitrogen doped diamond have demonstrated that nitrogen doping creates a deep donor level in the band gap of the diamond at 1.7 eV [START_REF] Yiming | Effect of CVD diamond growth by doping with nitrogen[END_REF] . This lowers the effective work function to approximately 3.1 eV 51 , which is lower than boron doped diamond (3.9 eV) [START_REF] Diederich | Electron emission and NEA from differently terminated, doped and oriented diamond surfaces[END_REF] and un-doped diamond (∼4.5 eV, with hydrogenated surface and negative electron affinity) [START_REF] Abbott | Effect of average grain size on the work function of diamond films[END_REF] . Secondly, it is thought that having the aforementioned deep donor level of electrons close to the vacuum level could increase the negative ion production from diamond by creating a source of electrons close to the vacuum level [START_REF] Bacal | Negative hydrogen ion production mechanisms[END_REF] .

In this study, we investigate the production of negative ions from nitrogen doped diamond films in a low pressure deuterium plasma. Comparing micro-crystalline nitrogen doped diamond (MCNDD) with un-doped micro crystalline doped diamond (MCD) and previously investigated micro-crystalline boron doped diamond (MCBDD) [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] , we consider 'low energy' (11 eV) and 'high energy' (48 eV) ion bombardment conditions at the surface as a mechanism for increasing the negative ion yield. The experimental methods are described in section 2: plasma source in 2.1, sample holder in 2.2 and the measurement method in 2.3. The micro-crystalline diamond samples are described in section 2.4, with the surface characterisation using confocal microscopy and Raman spectroscopy described in 2.5. The results are presented in section 3.

Method

The experimental setup is shown in figure 1. It consists of a low pressure deuterium plasma source, a temperature controlled sample holder, and a mass spectrometer for the measurement of negative ions produced at the diamond film's surface.

Description of the plasma source

A deuterium plasma, figure 1 (a), is produced via a helicon source operated in capacitive mode (2 Pa, 26 W), which then expands into a diffusion chamber [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF] . The pressure of the diffusion chamber, as measured by a Baratron gauge (MKS), is regulated via a mass flow controller (7.6 sccm, BROOKS 5850TR) in combination with a 150 mm diameter Riber gate valve installed in front of a turbo molecular pump (Alcatel ATP400). To reduce experimental drifts, the experiment source chamber and lower spherical diffusion chamber have a base pressure of 10 -5 Pa, which is lower than the base pressure of a previous setup of 10 -4 Pa 54 .

The relatively low power coupled to the plasma source results in plasma densities of approximately 10 14 m -3 in the spherical diffusion chamber [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF] . The choice of power and pressure was for similarity with previous work [START_REF] Achkasov | Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas[END_REF] . The positive ion composition of the deuterium plasma is measured by the mass spectrometer, described below, to be (84 ± 2) % D 3 + ions, (14 ± 2) % D 2 + ions and (1.1 ± 0.2) % D + ions. The measurement uncertainty represents the day-to-day variation of the measured plasma composition, however the actual error in the plasma composition due to the internal settings of the mass spectrometer may be higher [START_REF] Schiesko | H -production on a graphite surface in a hydrogen plasma[END_REF] .

Temperature controlled sample holder

The sample holder is shown in figure 1 (b). It is attached to a DC voltage source (Equipment Scientific Alimentation de Laboratoire CN7C) that can negatively bias the frame that holds the diamond sample.

The voltage applied to the sample is defined as V DC , which is distinct from the voltage at the sample surface, V S . The sample is positioned 37 mm away from, and perpendicular to, the plane of the mass spectrometer orifice. This is the closest distance that the sample can be placed in front of the mass spectrometer orifice, and is assumed to be sufficiently small to achieve minimal negative ion signal loss. It has previously been demonstrated that this distance has negligible effect on the shape of the negative ion distributions measured by the mass spectrometer [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] .

It is worth noting that the angular dependence of the NIEDFs for carbon materials has previously been shown to be similar [START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF][START_REF] Kogut | Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry[END_REF][START_REF] Dubois | Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry[END_REF] . Therefore, a single measurement can be used to compare between samples. A misalignment of the sample surface normal to the mass spectrometer would produce spurious results, so to prevent this, the alignment is regularly checked by rotating the sample and maximising the negative ion signal.

As shown in figure 1 (b), a tungsten heating element is built into the sample holder, which is used to heat the back of the sample. The heating element is controlled by a PID controller (designed and built by AXESS tech) using a K-type thermocouple inside the the frame of the sample holder. By fixing a second thermocouple to the surface of the sample, its temperature is calibrated against the temperature measured by the PID. The heating element behind the sample increases the temperature of the sample's surface up to (750±20) • C.

2.3

Mass spectrometry for the measurement of negative ions

Mass spectrometer setup

An electrostatic quadrupole plasma mass spectrometer with attached energy analyser (Hiden EQP 300) is positioned in front of the sample surface as shown in figure 1 (c). The mass spectrometer has a 100 µm diameter polarisable orifice separated from the main chamber by a 5 mm hole in a grounded shield. A grounded screen is positioned above the mass spectrometer orifice to reduce radio-freuency (RF) fluctuations from the plasma source (not shown in figure 1) [START_REF] Kogut | Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry[END_REF][START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF] .

The mass spectrometer polariseable orifice potential is calibrated so that a nearly planar plasma sheath is formed in front of the orifice, as determined by a particle-in-cell (PIC) simulation [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF] .

The potential on the surface of the samples accelerates any negative ions created through surface interactions away from the sample and through the plasma to the mass spectrometer. The low pressure of the plasma means there are few collisions between the plasma and the negative ions [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF][START_REF] Schiesko | H -production on a graphite surface in a hydrogen plasma[END_REF] . Any collisions that do occur with the deuterium plasma would predominantly be detachment collisions with deuterium molecules which would neutralise the negative ions, thus preventing measurement of negative ions that have undergone collisions [START_REF] Hemsworth | Positive and negative ion sources for magnetic fusion[END_REF][START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF][START_REF] Schiesko | H -production on a graphite surface in a hydrogen plasma[END_REF] . The plasma potential in front of the mass spectrometer prevents negative ions generated through volume production processes in the plasma from entering the mass spectrometer, therefore the energy of any negative ions that are measured must have been accelerated away from the sample surface [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF][START_REF] Schiesko | H -production on a graphite surface in a hydrogen plasma[END_REF] . The negative ions are detected at an energy corresponding to the energy they possessed when they were created which can then be shifted by the kinetic energy gained between the sample and the mass spectrometer [START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF][START_REF] Schiesko | H -production on a graphite surface in a hydrogen plasma[END_REF] . Presented NIEDFs are shifted to present the kinetic energy the negative ions have at the surface of the samples.

The secondary electrons emitted from the surface of the sample are filtered out within the mass spectrometer.

Positive ions impacting the samples are assumed to dissociate during impact [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Babkina | Energy analysis of hyperthermal hydrogen atoms generated through surface neutralisation of ions[END_REF] , splitting the energy of the ion into its component particles (ie. for D 3 + , 3 deuterium nuclei). This means that because the plasma is predominantly composed of D 3 + ions, the modal energy of the ions striking the samples' surface is E M = e(V S +V p )/3, where V p is the plasma potential, giving approximately 11 eV per particle at V S = -20 V and 48 eV at V S = -130 V. We define these conditions as 'low energy' ion bombardment and 'high energy' ion bombardment, respectively.

The choice of -130 V is made to align with previously published work, whilst -20 V is chosen as this is the lower limit of what can be reasonably used to ensure effective self-extraction of negative ions from the sample surface into the mass spectrometer [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] .

Procedure for measurement of negative ion energy distribution functions (NIEDFs)

Measurements were undertaken using the following method:

• The plasma was brought to steady state as determined by measurements of the positive ion energy distributions using the mass spectrometer • In order to compare the negative ion production yields for distinct material samples, the positive ion current was measured to the sample surface at 30 • C for V DC = -20 V and V DC = -130 V, using a copper electrode in the place of a sample which was insulated from the sample frame deleted:excluding the current to the sample frame [START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] . This method of measurement could not be used at high temperatures due to a temperature sensitive insulator used to isolate the copper electrode from the sample holder frame. Instead, in order to roughly monitor changes in the positive ion flux onto the sample, the positive ion current to the entire sample holder was measured using an ammeter connected to the frame of the sample holder. This showed that there was a thermal drift in the positive ion current to the entire sample holder of approximately 5%

• A bias of either V DC = -20 V or V DC = -130 V
irrespective of sample at both -20 V and -130 V applied biases.

• The negative ion counts for each sample were integrated with respect to energy and then divided by the positive ion current measured neglecting the possible small changes with temperature to the isolated sample to give the relative negative ion yield for the sample [START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] . This is given in arbitrary units as the mass spectrometer is not calibrated to count an absolute number of negative ions.

Recent measurements show that an absolute negative ion flux could be measured using a magnetised retarding field energy analyser via the technique described in Ref. 57. A detailed investigation of this topic remains the subject of future work, but to provide some context for the presented results, preliminary measurements have shown that the yield from HOPG at 30

Micro-crystalline nitrogen doped diamond

The nitrogen doped diamond films were created using a similar PECVD technique to the MCD and MCBDD samples of Ref. 28 so only a brief summary is provided here. The PECVD process utilised a bell jar reactor with a pressure of 200 mbar, microwave power at 3 kW, substrate temperature of 850 • C, background hydrogen gas mixture with a methane concentration of 5%. The ratio of nitrogen in the gas mixture was set as a means to vary the concentration of nitrogen in the MCNDD film. Each film was deposited on to a (100) orientated silicon wafer.

Surface characterisation

The samples were analysed using confocal microscopy and Raman spectroscopy prior to plasma exposure to characterise their properties.

Surface morphology and crystal structure

A laser confocal microscope (S neox, Sensofar) was used to observe the diamond surface morphology as shown in figure 2. From visual inspection, the crystal grains are observed to have grown to exhibit (111) crystal faces, (100) crystal faces, or a mixture of both, dependent on the concentration of nitrogen dopant introduced in the gas phase during sample growth [START_REF] Silva | Structural characteristics of CVD diamond films versus nitrogen impurities coupled to other deposition parameters[END_REF] . As the gas phase nitrogen concentration is increased from 0 ppm to 50 ppm the crystals are observed to exhibit an increased proportion of (111) faces, with predominantly (111) faces observed at 50 ppm. As distinct from these, the crystal grains of the diamond film with 200 ppm gas phase doping displays predominantly (100) faces. The The spectra are presented with the background subtracted (in order to aid clarity and comparison between samples) and normalised to the carbon sp3 peak, observed here at 1333 cm -1 . The laser wavelength used for the measurements is 514 nm.

Raman spectroscopy was undertaken to measure the relative concentration of nitrogen dopant that is introduced into the MCD films with respect to the gas phase nitrogen concentration present during the PECVD process. Raman spectra were generated using a Horiba Jobin Yvon HR800 setup. The measurements were undertaken in air using an excitation wavelength of λ L = 514 nm, ×100 objective (numerical aperture of 0.9, i.e. theoretical spot radius of 0.34 µm), 600 grooves/mm grating (resulting in a resolution of about 1 cm -1 ), and 5 mW laser power. Measurements were taken using 5 acquisitions of 1 second intervals over a range of 0 cm -1 to 3000 cm -1 , of which 750 cm -1 to 2500 cm -1 is presented, with the measurement taken at the centre of a dominant crystal near the centre of the sample. A brief optical microscopic inspection of the samples prior to taking Raman measurements showed relatively uniform crystal distribution across the surface of all the samples and allowed for precise targeting of a dominant crystal near to the centre of the sample to be the subject of the Raman measurement. A quantitative comparison between Raman spectra has not been carried out as this requires the measurement of the polarisation of the Raman emission and a good understanding of the grain orientation [START_REF] Mossbrucker | Determination of local crystal orientation of diamond using polarized Raman spectra[END_REF] .

Figure 3 shows the Raman spectra from samples of nitrogen doped diamond with gas phase nitrogen doping of 0 ppm to 200 ppm. The spectra have been presented with the background fluorescence removed and normalised to the 1333 cm -1 peak, which can be attributed to sp3 bonded carbon (the diamond bond of carbon) [START_REF] Tachibana | Diamond films grown by a 60-kW microwave plasma chemical vapor deposition system[END_REF][START_REF] Prawer | Raman spectroscopy of diamond and doped diamond[END_REF] . The normalisation to this peak is justified due to the transparency of the diamond films such that the measurement is integrated across the sample thickness. The normalised spectra can therefore enable a comparison between samples that accounts for any change in the thickness of the MCNDD film [START_REF] Prawer | Raman spectroscopy of diamond and doped diamond[END_REF] .

The broad peak centred at 2100 cm -1 observed in figure 3 can be attributed to nitrogen vacancy centres (NV 0 ) that have been introduced into the diamond [START_REF] Kumar | Purification method dependent fluorescence from nitrogen-vacancy (NV) centers of nano-diamonds[END_REF] . This broad peak appears, not due to vibrational modes, but due to the electronic signature attributed to nitrogen vacancy centres and in reality lies at an energy level of 2.15 eV. As Stokes Raman spectroscopy is energy loss spectroscopy, this peak appears arbitrarily at 2100 cm -1 when using a 514 nm laser. Using another laser to perform the Raman spectroscopy results in a change in the wavenumber of this peak [START_REF] Smith | Modern Raman Spectroscopy -A Practical Approach[END_REF] .

As the measurement configuration is the same for all samples, a relative comparison of the number of nitrogen centres in the diamond can be made using the broad 2100 cm -1 peak. This can then be used to infer relative nitrogen concentration [START_REF] Achard | Coupled effect of nitrogen addition and surface temperature on the morphology and the kinetics of thick CVD diamond single crystals[END_REF] . As shown in figure 3, the ratio of the NV 0 peak to the peak centred at 1333 cm -1 increases with increasing gas phase dopant concentration, for samples 0 ppm to 50 ppm (200 ppm will be discussed below). This is consistent with previous work, which showed a similar increase in the magnitude of the NV 0 characteristic peak with an increase in the gas phase nitrogen doping [START_REF] Tallaire | Characterisation of high-quality thick single-crystal diamond grown by CVD with a low nitrogen addition[END_REF] .

In figure 3, the Raman spectrum of the 200 ppm nitrogen doped diamond has a peak at 1500 cm -1 that has a much higher intensity than the other samples. This peak is of particular interest as it is associated with the sp2 bond of carbon that has previously been associated to graphite-like bonds [START_REF] Prawer | Raman spectroscopy of diamond and doped diamond[END_REF] .

The ratio of the peaks at 1333 cm -1 and 1500 cm -1 implies that there is a higher ratio of graphite in the 200 ppm diamond film compared to the other samples [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Chu | Characterization of amorphous and nanocrystalline carbon films[END_REF][START_REF] Merlen | A guide to and review of the use of multiwavelength Raman spectroscopy for characterizing defective aromatic carbon solids: From graphene to amorphous carbons[END_REF] . The 200 ppm nitrogen doped diamond sample also exhibits a NV 0 centre peak at 2100 cm -1 , which is slightly lower than the 50 ppm sample, suggesting a reduction in the number of nitrogen vacancies, and therefore, a reduction in the concentration of nitrogen in the diamond.

The observed increase in intensity of the NV 0 peaks, increasing from 0 ppm to 50 ppm, may be attributed to both the increase in nitrogen introduced in the gas phase and by a change in the crystal face from a mix of (100) and (111), figure 2(a) and (b), to a primarily (111) face for which impurity incorporation is higher than that for (100) crystals, figure 2 (c). This is consistent with the results of previous work [START_REF] Tallaire | Characterisation of high-quality thick single-crystal diamond grown by CVD with a low nitrogen addition[END_REF][START_REF] Samlenski | Characterization of homoepitaxial diamond films by nuclear methods[END_REF][START_REF] Satoh | Difference in Nitrogen Concentration & Aggregation among (111) & (100) Growth Sectors of Large Synthetic Diamonds[END_REF] This same process may then account for the slightly lower 2100 cm -1 peak for the 200 ppm sample compared to the 50 ppm samples despite a four fold increase in the nitrogen gas phase content. This decrease could be attributed to the change in the crystal orientation (see figure 2 (c) and (d)), from a (111) dominant crystal surface for the 50 ppm sample to predominantly (100) crystal orientation for the 200 ppm sample.

The surface characterisation of the samples show that the incorporation of nitrogen into the PECVD process has multiple effects on the diamond produced, aside from only substitutional or interstitial incorporation of nitrogen into the diamond lattice. For these samples, separating the difference in negative ion yield due to the influence of the crystal face or the nitrogen content in the diamond is not possible because of the interrelated nature the presence of nitrogen in the gas phase has with the crystal face orientation and the measurable number of nitrogen vacancy centres. This is an active area of research [START_REF] Zhao | The interface and mechanical properties of a CVD single crystal diamond produced by multilayered nitrogen doping epitaxial growth[END_REF] . However for this study, as nitrogen doping is the main influencing factor that generates the differences between the samples, it is reasonable to suggest that it is possible to associate the nitrogen gas phase doping with the negative ion yield and this is how the samples will be defined in the next section.

3 Results In order to understand these trends, it is important to note that the negative ion yield measured using the technique described in this article relies on a conductive sample surface. A non-conductive sample would not allow negative ions to be accelerated to the mass spectrometer at an energy which the mass spectrometer is tuned for. The magnitude of the transitions seen in figure 4 is a feature of this experimental technique which inadvertently highlights the temperature at which samples become conductive, such that

Nitrogen doped diamond: influence of the dopant concentration

V DC = V S .
The difference in trends between MCNDD samples and the un-doped diamond can be attributed to the differences in conductivity between MCNDD and MCD. Previous work with MCD has shown that it has poor conductivity close to room temperature, which explains the increase in yield occurring between 30 • C and 150 • C 26 . Regarding MCNDD, the temperature at which the sharp increase in yield occurs appears to be dependent on the nitrogen doping of the diamond sample. The results of previous work suggest that the level of interstitial nitrogen doping influences the conductivity of diamond [START_REF] Baranauskas | Nitrogen-doped diamond films[END_REF][START_REF] Al-Riyami | Nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition[END_REF][START_REF] Bhattacharyya | Synthesis and characterization of highly-conducting nitrogendoped ultrananocrystalline diamond films[END_REF] and this sharp increase is consistent with increasing nitrogen dopant concentration and its influence on the conductivity of the diamond, supporting the argument that the nitrogen incorporated into the diamond increases as gas phase nitrogen is increased during its production. An exception to this trend are the results for 200 ppm in both figure 4 (a) and (b), which does not exhibit a significant increase in the sample temperature for which the film becomes conductive relative to the 50 ppm MCNDD sample. This can be explained by considering figure 3. As discussed in section 2.5.2, the nitrogen content measured using Raman spectroscopy suggests a nitrogen content that is similar for 200 ppm and 50 ppm MCNDD samples meaning, in the absence of other influences, a similar conductivity for these two samples could reasonably be expected.

The maximum yield from each sample occurs at temperatures between 400 • C and 550 • C, which is highlighted in the insets of figures 4 (a) and (b). For the 0 ppm and 10 ppm samples, the maximum yield occurs at 400 • C, whilst for 20 ppm (and 200 ppm) it occurs at 500 • C and for 50 ppm, at 550 • C.

As mentioned previously, a conductive sample surface is necessary to hold a DC surface bias which is necessary for the acceleration of negative ions into the mass spectrometer. The trend of increasing temperature for maximum yield as dopant increases (excluding 200 ppm) could be related to the maximum yield in these experimental conditions being restricted by the conductivity of the samples.

For example, the maximum yield for MCD and MCNDD (10 ppm) is at the peak of a gradual increase and decrease in yield as temperature is increased from ∼30 • C to ∼400 • C and then from ∼400 • C to ∼750 • C respectively. This is most clearly observed in figure 4 (b) for the 10 ppm sample. This sample is distinct from the other MCNDD samples as the 10 ppm sample exhibits an increase in yield as the temperature is increased (due to a change in conductivity) then a further smaller increase up to a maximum negative ion yield at∼400 • C. The yield then gradually decreases as the temperature is increased further. The other MCNDD samples also undergo an increase in yield due to a change in conductivity, but no further gradual increase in yield is observed as temperature is increased.

It could therefore be reasonable to suggest that the peak yield conditions are not observed due to a lack of conductivity for samples with more than 20 ppm gas phase nitrogen doping. The trends observed for the 0 ppm and 10 ppm samples suggest that a temperature of approximately 400 • C may be the temperature at which these MCNDD with more than 20 ppm gas phase doping produce the highest yield. A technique to measure negative ions that does not require a conductive surface would be necessary to explore this further.

In figure 4 (a), the effect of the nitrogen doping on the maximum yield is not readily observed when a bias voltage V DC = -130 V is applied to the sample. This is unlike figure 4 (b) in which a bias voltage of V DC = -20 V is used. In this data there is an observed difference between the nitrogen doped and non-doped diamond. The yield in figure 4 (a) for the MCNDD samples and MCD samples is also lower than the yields from all of the samples in figure 4 (b). A higher bombardment energy as a result of the high magnitude bias is associated with an increase in sp2 bond formation in diamond [START_REF] Dubois | Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry[END_REF] . It is reasonable to suggest that the reduction in yield for the higher magnitude bias creates more sp2 defects which decreases the yield. Additionally, if the yield is not changing with the addition of nitrogen to the diamond, it is also reasonable to suggest that the nitrogen doped diamond may be more susceptible to defect formation due to high energy bombardment which would result in a surface state that does not enhance the negative ion yield through nitrogen doping. Additional work would be necessary to characterise this process.

The apparent influence of nitrogen doping on the measured negative ion yield is observed in figure 4 (b) where a sample bias voltage V DC = -20 V is applied. When comparing the yield at temperatures above 550 • C, i.e. when all of the MCNDD films are conductive, it is observed that the negative ion yield is higher at similar temperatures, when increasing nitrogen dopant concentration for the 0 ppm to 50 ppm cases. The mechanism for such an increase in yield is not immediately clear and future work will be necessary to identify the specific cause of this increase. For example, it could be solely due to interstitial nitrogen, or a change of crystal orientation or a combination of both. In any case, the increase is correlated to the amount of nitrogen dopant with the exception of the result observed for 200 ppm gas phase doping, which produces a comparatively lower yield compared to the 50 ppm case. Should interstitial nitrogen content be the main cause of an increase in negative ion yield, this result can be explained by the Raman measurements shown in figure 3. As discussed in section 2.5.2, the Raman measurement suggests that the diamond has a similar amount of nitrogen doping when comparing the 2100 cm -1 peak for the 50 ppm and 200 ppm samples. However the Raman measurement also suggests that the 200 ppm MCNDD sample has more carbon sp2 bonds (graphitelike) than the 50 ppm sample. The reduction in yield observed for the MCNDD (200 ppm) sample compared to MCNDD (50 ppm) sample is therefore consistent with previous work, which observed that an increased number of sp2 bonds is less favourable to negative ion production [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] . This work suggests that this is still the case with nitrogen doped diamond samples.

Mechanism for the surface production of negative ions

NIEDFs for MCBDD and MCNDD are presented in figure 5 to compare negative ion production processes between MCBDD, a previously studied material [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF] , and the MCNDD samples. In this figure

the NIEDFs are normalised to the modal negative ion energy at temperatures where MCBDD and MCNDD samples are conductive.

When considering a normalised NIEDF, a reduction in the proportional magnitude of the NIEDF peak at low energies will result in an increase in the apparent proportion of negative ions at high energies. The NIEDFs in figure5 (a), show that for MCBDD the proportion of high energy ions increases as the surface temperature increases. This is because the main contribution to the measured yield is low energy ions, which are predominantly created through the sputtering process, as distinct from backscattering, due to the acceptance angle of the mass spectrometer [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Ahmad | Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments[END_REF] . Previous work has confirmed this interpretation through comparison of experimental results with SRIM simulations [START_REF] Dubois | Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry[END_REF] .

The physical interpretation for the decrease in the sputtering contribution is that this is due to a decrease in the amount of sub-surface deuterium available for sputtering as a result of out-gassing caused by the increase in temperature. [START_REF] Kogut | Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry[END_REF][START_REF] Dubois | Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry[END_REF] .

At a surface bias of V S = -20 V, i.e. the 'low energy' bombardment condition described in section 2.3, the high energy tail observed in the NIEDFs in figure 5 is not produced. Without a high energy tail, the normalised NIEDF shapes are not strongly dependent on the deuterium surface content [START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] , and comparison of the ratio of sputtered to backscattered particles cannot be readily inferred using this approach. For this reason, only results with a surface bias of V S = -130 V are presented.

Comparing figure 5 This implies that MCNDD has similar negative ion production properties to MCBDD.

The trends for MCNDD and MCD observed in figure 4 and discussed in the previous section can be explored in the context of figure 5. Figure 4 shows that the yield for MCD increases up to sample temperatures of ∼400 • C and decreases as its temperature is increased further. This is similar to the trends observed for samples of MCNDD when they are conductive. The increase and then decrease in yield as temperature is increased, from ∼30 • C to ∼400 • C and then from ∼400 • C to ∼750 • C respectively, can be attributed to two processes that combine to generate the observed trend in figure 4. The first process is the removal of defects on the sample surface. The heating of the sample results in an enhancement of the etching of sp2 bonds created by the bombarding positive ions resulting in a surface which results in a higher ratio of sp3 bonds [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF] . The increased proportion of diamond bonds on the surface increases the negative ion yield, as explored in previous work through Raman spectroscopy [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF][START_REF] Kumar | Enhanced negative ion yields on diamond surfaces at elevated temperatures[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] . The second process is the previously discussed decrease in the sputtering contribution to the negative ion yield due to out-gassing of deuterium from the sample surface, as observed in the measurements of figure 5. As temperature is increased, the influence of each of these processes on the measured negative ion yield is observed to vary significantly. At temperatures below ∼400 • C, the reduction in defects increases the yield, whilst the outgassing does not cause a significant decrease in the sputtering contribution. At temperatures above ∼400 • C, the decrease in sputtering contribution reduces the yield by a greater extent than the reduction in defects caused by the elevated temperature, causing a reduction in the the measured negative ion yield [START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF] .

For the samples of nitrogen doped diamond with more than 20 ppm nitrogen added in the gas phase, the MCNDD film is not conductive at temperatures where the previously mentioned reduction in the defects can increase yield, i.e. between 30 • C and 400 • C. A more thorough exploration of the resulting interplay between the reduction of defects and the decreasing sputtering contribution is beyond the scope of this experimental study. However, figure 5 (b) suggests that the decrease in yield due to a decrease in the sputtering contribution is consistent with current understanding of the behaviour of negative ion formation on micro-crystalline doped diamond.

Negative ion yield: comparison between MCNDD, MCBDD, and MCD

The negative ion yield with respect to sample surface temperature of the MCD, MCBDD and MCNDD samples is shown in figure 6, with high energy ion bombardment (V DC = -130 V) shown in figure 6 agrees with MCD and MCBDD. In the high energy bombardment regime, the yield from MCNDD is found to be lower than MCBDD and comparable to MCD. This suggests that the higher positive ion bombardment energy is having a larger influence on MCNDD than MCBDD, though a mechanism for such a difference is beyond the scope of this study.

In figure 6 (b) for V DC = -20 V, the trends for MCD and MCBDD are also observed to be qualitatively similar, showing an increase in yield by a factor of 2 and a factor 1.5 from 150 • C to 400 • C respectively, and a gradual decrease in yield above 400 • C, which has been discussed in section 3.1 [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF][START_REF] Kogut | Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas[END_REF] . is consistent with both MCD and MCBDD, and agrees with current understanding of these diamond films as discussed in the previous section. Of particular interest is that the yield for MCNDD in this low energy ion bombardment condition is observed to be higher than MCD, and also higher than the previously best performing type of diamond, MCBDD [START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF] . At 550 • C, the maximum yield observed, MCNDD has a higher negative ion yield than MCD and MCBDD by a factor of 2 and 1.5, respectively.

This therefore suggests that controlled addition of nitrogen during the growth of diamond using the PECVD process could be an avenue for increasing the negative ion yield from diamond.

Conclusion

In this study, we have investigated the nitrogen doping of diamond films as a means of increasing the negative ion yield during exposure to a low pressure deuterium plasma (2 Pa, helicon source at film with energies of 11 eV and 48 eV, 'low energy' and 'high energy' bombardment, respectively, mass spectrometry measurements are used to determine the negative ion yield as the film temperature is scanned between 30 • C and 750 • C. For 50 ppm nitrogen doping, introduced in the gas phase during diamond growth using the PECVD technique, the application of low energy ion bombardment is observed to increase the negative ion yield by a factor of 2 compared to un-doped diamond and a factor of 1.5 compared to boron doped diamond.

Acknowledgements

The 6 References

Figure 1 :

 1 Figure 1: Schematic of the experimental setup. (a) Representative plasma parameters and potential profile between the biased sample surface at V S and the mass spectrometer, 37 mm away. (b) Sample holder with heating element and thermocouple. (c) Mass spectrometer showing the external grounded shield and internal extractor orifice.

  was applied to the sample • Negative ions produced following positive ion bombardment accelerate through V S , cross the plasma volume and enter the mass spectrometer where the NIEDF was measured for sample temperatures between 30 • C and 750 • C in increments of 50 • C

Figure 2 :

 2 Figure 2: Confocal microscopy images of the diamond films grown with gas phase concentrations of (a) 0 ppm (b) 10 ppm (c) 50 ppm and (d) 200 ppm. The diamond crystals change in shape and size between the 4 gas phase dopant concentrations, from a mixture of (111) and (100) faces at 0 ppm and 10 ppm to predominantly (111) faces at 50 ppm to predominantly (100) crystal faces at 200 ppm.

Figure 3 :

 3 Figure 3: Raman spectra of nitrogen doped diamond samples taken at the centre of a dominant crystal face.The spectra are presented with the background subtracted (in order to aid clarity and comparison between samples) and normalised to the carbon sp3 peak, observed here at 1333 cm -1 . The laser wavelength used for the measurements is 514 nm.

Figure 4 Figure 4 :

 44 Figure 4 presents the negative ion yield from MCNDD for different dopant concentrations, as measured in the gas phase during sample preparation. In both figures 4 (a) and (b) the yield profile for MCNDD has a distinct shape. For example, at 50 ppm, the measured yield is practically zero between a temperature of 30 • C and 400 • C. At 450 • C, the yield rapidly increases by several orders of magnitude to a maximum at 550 • C. This transition is similar for all nitrogen doped samples with the transition

Figure 5 :

 5 Figure 5: NIEDFs for: (a) micro-crystalline boron doped diamond (MCBDD) at 30 • C, 500 • C and 750 • C, (b) micro-crystalline nitrogen doped diamond (MCNDD) at 550 • C and 750 • C. Increases in sample temperature lead to a decrease in the number of low energy negative ions, which results in an increase in the height of the tail of high energy negative ions when the distribution is normalised. Low pressure deuterium plasma operated at 2 Pa and 26 W.

  (a) to figure 5 (b), which presents NIEDFs for MCNDD at 550 • C and 750 • C,i.e. temperatures at which the sample is conductive, it is observed that MCNDD displays a similar increase in the proportion of high energy negative ions as the sample's surface temperature increases.

  (a) and low energy bombardment (V DC = -20 V) shown in figure6 (b). 50 ppm MCNDD is chosen as a comparison to MCD and MCBDD as this produced the highest relative negative ion yield of all the nitrogen doped diamond samples, as shown in figure4.In figure6(a), V DC = -130 V, the trends for MCD and MCBDD are similar, with an increase in yield by a factor of 6 from 150 • C to 450 • C observed for MCD and a factor of 2 observed for MCBDD from 150 • C to 550 • C. The yield then decreases gradually as temperature is increased further. These results are consistent with previous work using MCD and MCBDD[START_REF] Cartry | Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas[END_REF] . The negative ion yield from MCNDD at sample temperatures below 400 • C is effectively zero. After 400 • C there is a rapid increase in yield by several orders of magnitude up to 550 • C, as discussed in section 3.1. After 550 • C, the trend

Figure 6 :

 6 Figure 6: Negative ion yield with respect to film surface temperature for micro-crystalline diamond (MCD), micro-crystalline boron doped diamond (MCBDD) and micro-crystalline nitrogen doped diamond (MCNDD) for (a) V DC = -130 V and (b) V DC = -20 V. Low pressure deuterium plasma operated at 2 Pa and 26 W. Solid lines have been added to guide the eye.

Figure 6 (

 6 Figure 6 (b) has a similar trend as figure 6 (a) where the yield from MCNDD at temperatures below 400 • C is effectively zero. The yield increases by several orders of magnitude between 400 • C to 550 • C, after which it decreases gradually. At temperatures above 550 • C the general trend of decreasing yield

  authors would like to acknowledge the experimental support of Jean Bernard Faure and the PIIM surface group. This work has been carried out within the framework of the French Federation for Magnetic Fusion Studies (FR-FCM) and of the EUROfusion consortium, and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support was received from the French Research Agency (ANR) under grant 13-BS09-0017 H INDEX TRIPLED. The financial support of the EPSRC Centre for Doctoral Training in fusion energy is gratefully acknowledged under financial code EP/L01663X/1. CGI (Commissariat á l'Investissement d'Avenir) is gratefully acknowledged for its financial support through Labex SEAM (Science and Engineering for Advanced Materials and devices) (No. ANR 11 LABX 086, IDEX 05 02).

  • C and a bias of -130 V was approximately 1%[START_REF] Cartry | Production of negative-ions upon positive-ion impact on surfaces in low-temperature low-pressure plasma[END_REF] , compared to caesium which can have a yield of 30%[START_REF]Negative hydrogen ion production in fusion dedicated ion sources[END_REF] .As described in detail in Ref.28, non-doped and boron doped micro-crystlline diamond films, MCD and MCBDD respectively, were prepared in a bell jar reactor using plasma enhanced chemical vapour deposition (PECVD). The boron doped samples used in this study are comparable to the samples used in previous works where the gas phase doping level used is high (1000 ppm) to ensure a fully conductive diamond layer. The method of the creation of MCD and MCBDD samples is described elsewhere[START_REF] Ahmad | Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H-yield[END_REF] .

	2.4 Sample preparation
	2.4.1 Micro-crystalline boron doped diamond and micro-crystalline diamond