Julien Vialle

Arnaud Badel

Kohki Takahashi

Satoshi Awaji

Pascal Tixador

A high performance insulated REBCO pancake with conductive cooling capability

Keywords: Superconducting magnet, Insulated REBCO magnet, High Magnetic field, Impregnation, Co-winding, Conduction cool capability

REBCO Coated Conductors have high tensile strength along their length. They are however sensitive to delamination when traction force is applied on the tape surface, which in a pancake winding corresponds to radial tensile stress. For this reason, most REBCO magnets are dry-wound (not impregnated). One of the drawbacks of dry winding is that conduction cooling to low temperature (below 20 K) would be inefficient. Edge impregnation structures were developed to mitigate this problem but retaining a soft non-stick insulating layer in between the turn. In this paper, we propose an improved pancake structure with the benefits of edgeimpregnation but using Hastelloy tape with thin insulation coating to reinforce the winding. The mechanical behavior of such structure is first estimated then validated with high current tests under large background magnetic field. The insulation performance is then tested by rapid ramping and fast discharge.

I. INTRODUCTION

HE effort to reach very high fields (30 T and above), requires to operate REBCO inserts with large current densities and high mechanical stresses. In other magnet applications such as energy storage, the trend is the same, to gain performance and/or to reduce the conductor cost.

The recent REBCO tape performances make it possible to consider operation up to intermediate temperatures (in the 10 to 30 K range) even for high field and high current density applications [START_REF] Fujita | Flux-Pinning Properties of BaHfO3-Doped EuBCO-Coated Conductors Fabricated by Hot-Wall PLD[END_REF]. Combined with the growing price of liquid helium, this makes a conduction-cooled magnet (cryogen-free) more and more attractive.

To stably operate a REBCO coil in a cryogen-free system and enable efficient cooling of the winding, it is beneficial to impregnate the coil. However, the variations of thermal contractions between the conductor, the insulator and the bonding (epoxy) during cooling down can lead to dangerous radial tensile stress and critical current degradation due to the delamination. This is especially true for the windings with large thickness-over-diameter ratios, necessary for high field inserts among others. During operation, Lorentz forces distribution in such thick windings will also lead to large radial tensile stress.

A solution was proposed using a non-stick material to separate the turns in several blocks in radial direction, or even separate all turns from each other [START_REF] Miyazaki | Delamination Strengths of Different Types of REBCO-Coated Conductors and Method for Reducing Radial Thermal Stresses of Impregnated REBCO Pancake Coils[END_REF]. It was used successfully in recent magnets wound by Toshiba for HLFSM (High Field Laboratory for Superconducting Materials), where the insulation between turns is a co-wound thin polyimide tape with fluorine non-stick coating [START_REF] Miyazaki | Testing of Stacked Pancake Coils for a Cryogen-Free 25-T Superconducting Magnet[END_REF]- [START_REF] Takahashi | Electromagnetic Characteristics Study of Two-Ply REBCO Tapes Pancake Coils[END_REF]. Using this technique leads to an edge-only impregnation, where the REBCO tape edges are glued to the thin FRP flanges on each side but not to each other. The resulting structure has an intermediate mechanical behavior between solid-block impregnated winding and dry winding, while maintaining a good thermal conductivity required for conduction-cooling [START_REF] Awaji | Robust REBCO Insert Coil for Upgrade of 25 T Cryogen-Free Superconducting Magnet[END_REF], [START_REF] Takahashi | Electromagnetic Characteristics Study of Two-Ply REBCO Tapes Pancake Coils[END_REF]. However, the mechanical performances of these winding can still be improved, as part of the winding cross section is occupied by a mechanically weak material (the polyimide co-wound insulation). Here we present an improved edge-impregnated insulated REBCO pancake concept suitable in particular to magnetic field inserts, with enhanced mechanical performance while retaining conductive-cooling capability. The design, fabrication and tests of a prototype are presented in the next sections, focusing on the mechanical and dielectric performances. Thermal conduction tests have been successfully performed (with the setup already described [START_REF] Abe | REBCO Coil With Robust Behavior Against Local Defects Wound Using Two-Tape Bundle[END_REF]) but are not the topic of this paper, for which the tests were conducted in LHe bath to reach high currents.

II. PANCAKE STRUCTURE AND CHARACTERISTICS

A. Pancake Concept

The novelty of this pancake concept is to use edgeimpregnation concept [START_REF] Awaji | Robust REBCO Insert Coil for Upgrade of 25 T Cryogen-Free Superconducting Magnet[END_REF], [START_REF] Takahashi | Electromagnetic Characteristics Study of Two-Ply REBCO Tapes Pancake Coils[END_REF], but replacing the usual Polyimide insulation co-winding by high strength material. The idea of using high strength reinforcement material with thin insulation coating as insulation co-winding was already proposed and notably used for the REBCO insert of the 32 T magnet at NHMFL (National High Magnetic Field Laboratory) [START_REF] Mutlu | High temperature insulation coating and their electrical properties for HTS/LTS conductors[END_REF].

In our case, we use a Hastelloy tape on which a layer of MgO is deposited to ensure the electrical insulation.

One key difference with the previously made edgeimpregnated pancakes is that there is no non-stick material on the insulation co-winding. In order to guarantee that the turns T Fig. 1. The stacking arrangement of the tapes in the winding.

are not glued together, the thin FRP flanges are glued to the surfaces of the pancake after the winding process.

A two-tape bundle conductor in face-to-back configuration is used [START_REF] Miyazaki | Testing of Stacked Pancake Coils for a Cryogen-Free 25-T Superconducting Magnet[END_REF]. This ensures current redistribution between the tapes in case of local degradation of the critical current of one tape and reduces the risk of thermal runaway [START_REF] Abe | REBCO Coil With Robust Behavior Against Local Defects Wound Using Two-Tape Bundle[END_REF]. Fig. 1 shows the configuration of the tapes in the winding. The pancake was wound with REBCO Fujikura FESC tapes with artificial pinning center [START_REF] Fujita | Flux-Pinning Properties of BaHfO3-Doped EuBCO-Coated Conductors Fabricated by Hot-Wall PLD[END_REF], [START_REF] Fujikura | Recent progress of 2G HTS wires and coils at Fujikura[END_REF]. The insulated Hastelloy reinforcement co-wind is actually a THEVA ISD-MgO buffered substrate [START_REF] Theva Proline Hts Wire | [END_REF] (only the substrate with buffer without the REBCO and silver layers was used). Dimensions of the conductor and pancake can be found in Table I.

B. Realization

Initially the coil is dry-wound with a tension of 40 N (Fig. 2). The two conductors are soldered to the copper connections at the inner and outer diameters of the pancake.

Once this is completed, two 0.2 mm thick FRP flanges are glued by means of araldite epoxy resin mixed with quartz powder (Araldite DBF + Aradur HY 951 + Quarzgutmehl SILBOND® FW 12 EST (mixing ratio Araldite + Quartz = 1:1)) [START_REF] Barth | Degradation free epoxy impregnation of REBCO coils and cables[END_REF] on each side of the pancake to make the thermal link between the turns, improve the rigidity of the coil and reduce the radial stress on the tapes. In this way, the coil turns are only glued to the flanges and not between them.

One goal for this test pancake was also to reproduce as best as possible the space constraints in the high magnetic field inserts. The inner and outer copper contact are thus made very thin: 2 mm at inner and outer radius. Connections with the current leads terminals, representing the connections between pancakes, are made afterward with a lower temperature solder compound (InSn 118 °C).

III. PANCAKE MECHANICAL BEHAVIOR: MODELING AND EXPERIMENTAL TESTS

A. Stress Evaluation

The hoop stress in insert coils is commonly estimated with the following assumptions: for dry winding, with tapes fully separated, the stress follows the BJR equation. For impregnated winding, the stress distribution follows the Wilson formula. Both formulas are only valid for roughly isotropic structures.

The composite structure of an edge-impregnated pancake requires more complex modeling, accounting for the different contact interfaces turn-to-turns and turn-to-flanges. We developed a simple FEM model to represent this structure, in which the contacts are all considered as thin elastic interfaces: A layer with very low Young's Modulus (1 MPa) represent the turn to turn interface, which is not glued, while the epoxy between the conductor and the FRP flanges is represented realistically as a thin layer with a Young Modulus of 10 GPa [11]. The winding tension is neglected in the model. These interfaces can be seen in Fig. 3 in white and blue (white between turns and blue for the epoxy). Each turn is represented independently in this modeling. The equivalent Young's modulus of the conductor itself is evaluated at 143 GPa taking into account both REBCO tapes and the co-wound Hastelloy reinforcement tape with its 5 μm of MgO insulation (135 GPa for Fujikura tapes and 163 GPa for THEVA Hastelloy/MgO reinforcement tape if hastelloy young modulus is 180 GPa). The equivalent radial Young's Modulus of the 0.2 mm FRP flange is 50 GPa.

B. Experimental Test and Performance Limits

A first test up to 80 A in liquid nitrogen is conducted to check for degradation due to winding and/or cooling-down delamination. The winding is then equipped with 7 Kyowa strain gauges [12], [START_REF] Takahashi | Mechanical Properties of Four-Stacked Two Tape Bundled REBCO Pancake Coils[END_REF]. Four on the outer diameter of the pancake to measure the strain caused by the hoop stress, two of which were functional during the tests (SG_HS1, SG_HS2). Three on the FRP flange to measure the strain caused by radial stress. One was functional during the tests (SG_RS1).

To evaluate the mechanical performance of the pancake, the tests are performed under a background magnetic field of 8 T. The goal is to ramp the coil until its mechanical limits without concerns for current margin, reason why the tests are conducted in LHe. A current of 800 A is reached before degradation, which occurred at the inner contact. A second ramp was carried out with the aim to go higher in current, but the inner contact degraded just above 800 A, to reach 1.1 µΩ. Therefore, we stopped the ramp at this level. A third ramp up to 750 A was carried out in 8 T background field and a fourth in self-field up to 800 A without any further deterioration of the inner contact. Fig. 4 presents the mechanical modelling results for the highest current reached experimentally. It illustrates the benefit of using insulated reinforcement tape instead of Polyimide, with a 20 % reduction of stress on the REBCO tape (Orange vs Blue, and Yellow vs. Purple).

For this test pancake, the comparison between edgeimpregnation behavior and BJR is not very favorable. The peak hoop stress is transferred to the inner diameter of the winding compared to a dry winding. It is slightly lower, but not by much. This is because of the pancake dimensions. The reduction of peak stress using edge-impregnation is much larger in a real configuration (a full size REBCO insert made of several pancakes [START_REF] Miyazaki | Testing of Stacked Pancake Coils for a Cryogen-Free 25-T Superconducting Magnet[END_REF], [START_REF] Takahashi | Mechanical Properties of Four-Stacked Two Tape Bundled REBCO Pancake Coils[END_REF]).

According to the modelling results, the maximum hoop stress safely reached was about 505 MPa (purple curve, inner radius) corresponding to 0.38 % elongation. To produce the same field, a dry wound coil with a standard insulation material would have needed to reach more than 620 MPa tensile stress on the tape, resulting in more than 0.45 % elongation, value for which the risks of damage become overwhelming.

C. Measured Strain and Discussion

Fig. 5 presents the strain measurement on the outer diameter for the first ramp to 800 A under 8 T. The measured longitudinal strain went up to 0.29 and 0.31% on the outer diameter for SG_HS1 and SG_HS2 respectively, while the radial strain reached 0.25% for SG_RS1 (Fig. 6).

Fig. 5 and6 show good agreement between experiments and expected values from modelling results, although modelling was slightly optimistic (0.27 % strain on the outer radius of the coil compared to 0.29 and 0.31 % in the experiment). A slight residual strain can be observed after a first cycle of charge and discharge remains in the coil strain Fig. 5, which cannot be represented using our simplified linear mechanical model.

Still, the model agreement with experiment at the outer turn validates the estimation of stress and strain at the inner radius of the winding presented in previous section (Fig. 4) at about 0.38 % for 505 MPa. Moreover, it is to our knowledge the first time that radial strain in such composite structure is measured, at about 0.25% , corresponding to a large 170 MPa radial stress in the flanges (Fig. 7). This high radial stress and strain values illustrate the fact that the turns are not glued together, as such stress would surely cause delamination [START_REF] Muto | Evaluation of Actual Delamination Strength of REBCO-Coated Conductors Based on the Weibull Analysis Considering Size Effect[END_REF], and that radial stress is solely applied to the flanges.

IV. ELECTRIC TESTS AND DIELECTRIC STRENGTH

A. Measurement Setup

For the experimental tests, 4 voltage taps were placed on the coil. Two on each side of the inner and outer contacts in order to measure their resistance and check the superconducting state of the coil during the tests. The first tests in liquid helium were carried in self-field configuration up to 850 A (600 A/mm²) with no dissipation. The contact resistances at the current terminals are 120 nΩ (60 nΩ/cm²) and 200 nΩ (100 nΩ/cm²) for the inner and outer contact respectively.

To protect the pancake from a possible hot spot, we use a pick-up coil to compensate for most of the inductive signal of the coil when it is charged [START_REF] Ciceron | Test in Strong Background Field of a Modular Element of a REBCO 1 MJ High Energy Density SMES[END_REF]. In this way, we can more accurately observe the onset of a thermal runaway. The compensated voltage during the first test up to 800 A under 8 T is shown Fig. 8. As expected, a drift upwards can be observed on both as the current in the coil increases. This is due to redistribution of the current density within the conductor [START_REF] Vialle | Transient voltages and energy balance in REBCO insulated magnet: experimental and numerical studies[END_REF]. Many spikes due to wire movement are visible during this first test, they were much fewer in the following tests.

Validation of Insulation

In order to test the electrical insulation of the winding thanks to the Hastelloy reinforcement on which a 5 µm layer of MgO was deposited, several tests of charge and discharge of the coil at different speeds were carried out to see the inductive behavior of the magnet and its energy efficiency. The energy efficiency of the coil is the ratio between the energy discharged from the coil (either back to the source or to a dump resistance) and the energy supplied to the coil by the power supply during the charge. Considering the conductor structure, the losses should be mostly hysteretic: the energy efficiency should thus be very high and independent from discharge speed.

Using the power supply, we achieved current ramps from 5 to 100 A/s. To go to higher discharge speeds (up to about 300 A/s) we used a dump resistance.

In Fig. 9 we visualize the fastest discharge that we have realized (about 300 A/s). If we make a fit of the discharge current, we find a perfect exponential curve (dotted curve) which overlaps perfectly with the experimental curve. This highlights the good insulation of the coil. The peak voltage during this discharge is 4.7 V which corresponds to 30 mV of voltage between turns. At last, in order to check more precisely the good insulation of the pancake we also look at the energy efficiency of the coil for each cycle and charge/discharge speed. Experiment confirms that the energy efficiency of the coil (inset of Fig. 9) is very good and stays roughly constant, between 97 and 99 % even with very fast discharges. This excludes the possibility of current transfers between turns and confirms that the MgO insulation fulfils its function up to at least 30 mV per turn.

V. CONCLUSION

This work presents the first experimental validation of edgeimpregnated REBCO pancake without soft non-stick material in between turns. The insulation-coated Hastelloy co-wound improved the winding stiffness while retaining the benefits of edge-impregnation (hoop stress redistribution, conduction cooling capability…). The test pancake reached an estimated hoop stress above 500 MPa at 800 A under 8 T background before degradation, operation conditions for which the strain would have exceeded 0.45 % if conventional Polyimide insulation and winding had been used. For the first time, radial strain in such structure was measured and was in good agreement with the expected model values.

Finally, rapid charge and discharge tests of the pancake from 5 to 300 A/s show that the thin MgO coating is an effective electric insulation for turn-to-turn voltages up to at least 30 mV per turn. This innovative winding structure can be used for high field REBCO magnet with fast transient.

Fig. 2 .

 2 Fig. 2. Pancake before impregnation.

Fig. 3 .

 3 Fig. 3. 2D axisymmetric schematic of the edge-impregnated pancake.

Fig. 5 .

 5 Fig. 5. Strain gauge measurements in circumferential direction under 8 T. In dotted black, expected value from modelling.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Strain gauge measurement in radial direction on FRP plate under 8 T. In dotted black, expected value from modelling.

Fig. 4 .

 4 Fig. 4. Hoop stress and longitudinal strain expected in the pancake thickness for 800 A under 8 T background field.


Patrick Bruneteaux

Esra Bayhantopçu

CHILD MARRIAGE IN TÜRKİYE: ANALYSIS OF EXPERIENCED WOMEN'S NARRATIVES

1

As a sociologist working on poverty issues in France, I admire Esra's work. For it wasn't just a question of meeting, for the most part (despite a few cases of households with cultural capital), poor women steeped in the traditional culture of a part of Turkey where this type of practice is still commonplace. We had to be able to discuss and "gather" informations from people who, for the most part, had been traumatized by the choices imposed by the family group and the patriarchal system.

Between the thesis and this book, a long road has been travelled, with a refinement of language and theoretical frameworks, with a renewed distance that brings the final touches to this work so important in feminist or gender.

Among your indisputable contributions, we must mention the breadth, vision and generalization that have enabled you to move from media analysis to a socio-historical analysis of the problem. By devoting yourself to analyzing the discourse on women in Turkey, and by delving into the country's various historical periods, you now offer us a comprehensive look at the issue. The survey was conducted between 2014 and 2015, offering recent first-hand data on the problem, as well as the most up-to-date statistics. This makes your work a reference work.

You underline the relationship between women's own representations of what happened in their lives as forced wives and the social constructions of the problem in dominant ideological discourses, now and in the past. And, of course, at the heart of social practices of domination based on religion, moral codes and reputations, so many "obligations" to be fulfilled, monitored, imposed and, if need be, punished.

Of course, the problem of forced marriages is not unique to Turkey, and the author is right to point out that it is a global problem affecting gender relations. But while "reports" abound, and social medical centers receive women calling for help, there are few researchers in the field who have been able to access the populations concerned in order to grasp their trajectory in the complexity of family relations on both sides of the scale.

2

While legal and political boundaries indicate that child marriage begins under the age of 18, social science looks more broadly at cultures, acceptance phenomena, resilience and resistance, and the tensions between macro-social norms and micro-social interpretations.

Indeed, the dominant discourse, so quick to disqualify the woman ready to leap into the arms of another man, wants from the outset, "at the start of life", to put forward the risk of early or unwanted pregnancy, the honor of the family or all the attributes of the prostitute or dangerous woman. The sooner the woman is married, the better the order will be preserved. As you can see, Esra clearly shows how the social system violates women who don't want these forced marriages, except for a very small part of her sample.

But perhaps the worst thing for families is to love their children and intrinsically want to keep them, while being unable to do so out of necessity. The cost of children is such that, as in many other countries around the world, including Nepal and Bangladesh, parents sell their daughters or, at the very least, arrange to place them on the marriage market as quickly as possible.

What's even more incredible is the circularity of male violence, with the threat of rape and sexual harassment of young girls triggering a spiral of control that ultimately serves the dominant males who are at the root of the problem! Instead of educating men to respect the integrity of women's bodies, patriarchal systems externalize responsibility onto women, in other words, onto fathers who assume the burden of transferring the burden.

The book takes a detailed look at all the "weights" weighing on the mental structures of these trapped women (like the young boys, incidentally) who are going to have to live with a man they didn't choose. Nothing beats the discourse of the actors, the feelings of the people, the representations bearing the values, like this father who petrifies social obligation by evoking his shoulder still used to carry his runaway daughter back into the fold. When routines break down, the crisis reveals "rectification" in the postures of sanction and reparation, a second layer that delivers all the intensity with which symbolic capital is exerted on women's bodies, through those muscular arms that bend the dominated, bent under the weight of muscular repression. The role of the repressive mother-in-law is often overlooked. Almost the owner of her son, she becomes jealous of her wife and torments her as much as possible. If the husband is difficult to bear, he's not the only one. It's the whole family of the "other" that imposes itself through violence and suffering.

And nothing is more dramatically a validation of symbolic capital than to see these young girls validate the power of the other through their own consent. It's not a common occurrence, but then again, parents can have qualms about dictating their choices. The question is: to what extent can the daughter, with all the weight of paternal authority asserting itself in the father's precedence with regard to the plot already defined, the husband found, the contacts established, refuse, oppose herself to the choice of the necessary and the honor of all?

3

The wealth extracted from these lives is that of the 34 women we met, from whom social science can infer representations, experiences and objective analyses of living conditions. This passage from the subjective to the objective presupposed a great deal of know-how, which is clearly accomplished in this research, which readers will be able to verify for themselves. Like all scientific work, it was necessary to define the terms of the survey and the most global concepts (gender, identity, sexual violence, economic necessity, forced marriage, etc.). But it goes without saying that the heart of the demonstration rests on the reconstruction of "pieces" of the lives of these placed women, who showed common reactions but also diverse tactics of resistance.

The theoretical framework clearly indicates that what's at stake is the deconstruction of patriarchalism in this specific dimension of the social construction of women as objects to be controlled, as a kind of cultural product whose value is proportional to the hold that power can exert in order to normalize, smooth and conform as closely as possible to men's expectations. The important thing is to go from father to husband.

In working with the women, we were always concerned to give them the opportunity to feel at ease, to confide in us in complete confidence, and thus to be able to relate their experiences. The methodological approach was not just profoundly ethical: while the interviews conducted are underpinned by respect for the ethical standards laid down by Macnamara, there are many other scientific qualities to report in your work; including critical discourse analysis, the snowball tool, the empathic approach to gaining trust, tree interviews -all epistemological framings that enabled you to report your data.

It should be added that these interviews were mostly carried out in the homes of the interviewees, offering a quasi-ethnographic depth of view on their lifestyles and the organization of their days.

You can't help but admire the methodological order proposed, with all its tables, curves and data classifications. Research ethics is all about respecting people by respecting their information, in other words, by respecting formal transcription. It's one thing to quote what people say, it's quite another to link them into logical chains, categorizations that are knotted together and articulated.

4

And yet, in spite of this "infernal" mesh, since domination is accompanied by orders, vexations and humiliations ("you can't do anything", "you're good for nothing"), wives and daughters-inlaw still manage to create their own spaces, large or small. The incorporation of traditional symbolic frameworks is not complete, since some of these women run away from the marital home, either in the first few weeks of marriage, or later, after several years and having had children.

The sociologist describes the battles fought to gain freedom, to be able to go out alone, to have her own money, to go shopping. The limits of clinical work are undoubtedly reached here, as the women do not expose their resistance "in bed", the tactics used to escape marital rape, the obligation to do one's duty.

While the book shows how difficult it is to make the decision to escape, both materially and psychologically, it also reveals how symbolic escape, through children or even by trying to tame the partner, makes the imposed marital and family situation livable. And yet, in the end, what this remarkable demonstration clearly shows is the majority's rejection of this condition, and the new frustrations and suffering experienced by women on the run, most often to the capital.

The proportion of divorced women reveals these capacities for dissidence and escape.

La proportion de femmes divorcées révèle ces capacités de dissidence, de fuite Perhaps it's a pity that the "traps" of conservatism couldn't be put into perspective with the modernity present in big cities, even if codes of honor aren't totally erased in middle-and upper-class families. But it was important to grasp in minute detail what makes up the cultural world of this patriacalism on the women's side. And it is this view of the vanquished, though resistant in many aspects of their lives, that is delivered to us thanks to the perseverance and quality of Esra Bayhantopçu's analyses.


David A Cordova Morales

Thi-Mai-Trang Nguyen
email: thi-mai-trang.nguyen@lip6.fr

Guy Pujolle
email: guy.pujolle@lip6.fr

Towards a Blockgraph-based Trustless Authentication Scheme for Future 6G Technology

Keywords: 6G architecture, Zero-trust model, Blockgraph, Verifiable Credentials

One of the most important paradigm shifts nowadays regarding future 6G communication is related, from one side to the desire of bringing services and data as close as possible to the end users, and from another side, to empower the user with control over their data and personal information. In this vision, private 6G networks will shape trusted zones where data center services are placed at the edge of the network. The services will follow a Web3 approach, where decentralization and zerotrust mechanisms are predominant. In this environment, a decentralized authentication mechanism is needed. In this paper, we propose a 6G architecture and a blockchain-like authentication scheme based on Verifiable Credentials. Our model uses zerotrust technology for a better and more trusted Internet.

I. INTRODUCTION

As 5G technology is being deployed around the world and new trends are emerging in the Web model, noble ideas are surging for the next generation of cellular networks. Web3 is a new Web paradigm that pushes towards a trustless and decentralized Internet, where users would rather interact with decentralized Applications (dApps), belong to collectivelyowned Decentralized Autonomous Organizations (DAOs), and have control over their personal information than use the current Web model (i.e., Web2), where a few large corporations control most of the data traffic and users have no control over their personal information. These new features are all based on blockchain technology, which requires a distributed architecture [START_REF] Guan | Web3.0: A review and research agenda[END_REF]. In this regard, some work in the literature suggests that the future 6G technology should include a distributed network architecture composed of mobile trusted zones with nodes capable of maintaining a blockchain-like technology [START_REF] Al Agha | Edge Networking: Internet des Edges[END_REF]- [START_REF] Gui | 6g: Opening new horizons for integration of comfort, security, and intelligence[END_REF]. In our vision of a future 6G architecture, multiple private 6G networks (the trusted zones) might coexist and interact using blockchain, thus, removing the need for a trusted third party for authentication.

A Blockchain is a form of Distributed Ledger Technology (DLT) that can make all sorts of data immutable due to its properties. It is at the origin of Bitcoin [START_REF] Nakamoto | Bitcoin: A peer-to-peer electronic cash system[END_REF] and is the underlayer technology for multiple services and applications [START_REF] Di | Blockchain 3.0 applications survey[END_REF]. Blockchain technology could easily be compatible with future 6G architecture [START_REF] Hewa | The role of blockchain in 6g: Challenges, opportunities and research directions[END_REF], [START_REF] Nguyen | Privacy-aware blockchain innovation for 6g: Challenges and opportunities[END_REF], not only for securing future 6G communication [START_REF] Cordova | A preliminary assessment of blockgraph-a mobility-aware solution to secure 6g mesh networks[END_REF] but also to enable Web3 features and zero-knowledge inter-6G authentication. However, since our 6G nodes are mobile, a traditional blockchain cannot cope with network partitions due to the way it handles forks. Blockgraph, on the other hand, is a partition-tolerant blockchain-like data structure that inherits from blockchain properties, which was specially designed to function in dynamic networks with multiple network partitions and network aggregations.

Verifiable Credentials (VCs) are digital representations of information about an individual, an organization, or an entity that can easily be shared in the form of a Verifiable Presentation (VP) and easily verified by a verifier entity. They are usually used to prove claims of a subject issued by trusted entities (e.g., government, universities, and organizations). They use cryptographic primitives to ensure that the claim within the credential has not been tampered with and that it came from a trustworthy source. Decentralized Identifiers (DIDs) are a new type of globally unique identifier. They are designed to enable individuals and organizations to issue, hold, and control their own identifiers. DIDs enable entities to prove control over them by authenticating using cryptographic proofs such as digital signatures. DIDs are an integral part of the VC model and allow for a trustless identification of an entity.

The main objective of this paper is to propose a decentralized authentication scheme to enable the trusted zones to be connected to each other through Virtual Private Network (VPN) associated with zero-trust network access. This works without the need of a trusted third party. Our proposition uses blockgraph as a verifiable data registry where DIDs are stored in-chain. This makes DIDs unique, resolvable with high availability, and cryptographically verifiable. VCs are exchanged off-chain and contain the claims needed for authentication, where the verifier party only needs to cross-check the issuer's DID on the blockgraph to validate its identity.

The remainder of our paper is organized as follows. In section II, we describe the 6G architecture under development. Then, section III presents our distributed authentication scheme for inter-Edge communication between the trusted Fig. 1. The 6G architecture under study zones. Section IV details the blockchain-like technology used for the authentication scheme and its resiliency to nodes' mobility. Finally, section V concludes this body of work.

II. THE 6G NETWORK UNDER STUDY

In Figure 1, we describe the proposed 6G architecture, which is a service architecture incorporating a network that can be cut into two parts: the Internet of the Center and the Internet of Edges.

The Internet of the Center brings together large data centers and telecommunications operators' data centers placed at the edge, that is to say, mainly Multi-Access Edge Computing (MEC) data centers. These are the data centers going from relatively large to infinitely large size and are far from the end-users.

The Internet of Edges is formed from data centers that range from medium to infinitely small. Ideally, they should be tiny data centers embedded into the terminal equipment or very close to it. Fog data centers are also at a short distance from the terminal equipment. This network's architecture allows for vertical and horizontal traffic circulation. The vertical circulation climbs to the Internet of the Center or descends to the periphery, at the Internet of Edges, and the horizontal circulation moves at the same level. The horizontal displacement in the Internet of Edges provides short circuits, low energy consumption, and very strong security by cutting access to the Internet and working independently. On the contrary, vertical displacement gives long circuits, consumes more energy, and allows attacks from the Internet, but obviously, it has much higher computing power.

We recommend an architecture that integrates both horizontal and vertical networks, where the used services can move as close as possible to where they are requested, thus improving the energy consumption, quality of service, and data security of the requested service. The Edges are interconnected between them by secure channels generally crossing the Internet of the Center and are considered as horizontal links between Edges. Vertical networks are often linked through fixed antennas, while horizontal networks have the particularity of incorporating mobile antennas in addition to fixed antennas, especially when using embedded Edges.

As a result, and in contrast with current 5G architectures, our 6G architecture will be composed of multiple 6G private networks. Those networks should be able to remain hermetic to undesired connections and allow access when authorized. In this regard, we believe that the convergence between telecommunication networks and computer networks will be more anchored, and thus, multiple 6G private networks are likely to be deployed. To manage authorization access in this new architecture, a distributed authentication scheme among all trusted zones is needed. Today, blockchain technology is more mature and has proven to be resilient. Thus, we propose an authentication scheme based on blockchain-like technology to manage authorization access among trusted zones.

III. AUTHENTIFICATION SCHEME PROPOSAL

The overall security of our 6G network architecture comes from trusted Edges, which are Edge networks including trusted zones endorsed by zero-trust policies. The confidentiality of an Edge network is ensured by the use of Decentralized Identities (DIDs), Verifiable Credentials (VCs), and Identity and Access Management (IAM) rules in a zero-trust authentication scheme. Moreover, it would be possible to use transparently, several techniques aiming for the protection of the network such as behavioral AI monitoring Edge software and securing code development by design. The security of inter-Edge communication will be enforced by the use of a partition-tolerant DLT, such as the blockgraph, where zero-trust identifiers (i.e., DIDs) are stored in-chain and accessible to the whole network for verifiability.

A verification process between two entities will typically include a VP exchange between the VC holder and the VC verifier. The VP will include VCs containing claims of the VC holder to authenticate to a verifier entity. Claims are issued by well-known trusted issuers and contain the issuer's DID. Once the VP exchange is performed off-chain, the verifying entity will only have to cross-check the information relative to the issuer's DID in the VC with the information in the blockgraph.

Currently, several standards specifications are under revision to represent and request VCs claims [START_REF]Securing verifiable credentials using json web tokens[END_REF], [START_REF]Verifiable credential data integrity 1.0[END_REF], to exchange them [START_REF] Chadwick | Openid for verifiable credentials[END_REF], [START_REF] Curren | Didcomm messaging v2.1 editor's draft[END_REF], and to verify them [START_REF] Jones | Json web signature (jws)[END_REF]. This process creates a zero-trust environment that facilitates the secure authentication, authorization, and access control mechanisms that allow for granular control over the sharing and verification of identity attributes without relying on centralized identity providers. By following this process, certain data centers in the vertical network may be trusted areas by providing evidence for each request. Thus, it would not be possible of sending a message to a zone that cannot demonstrate the authenticity of its identity.

IV. BLOCKGRAPH

Blockgraph is a partition-tolerant DLT design for Mobile Ad Hoc Networks (MANETs). It brings blockchain properties and security while coping with network mobility and topologies changes. In the context of our proposed 6G architecture, MEC data centers in the Internet of Edge may be mobile and interact with each other through a mesh and ad hoc network. In this regard, the effect of mobility may cause links between nodes to break, causing potential network partitions. To cope with this issue, blockgraph can adapt the shape of its only-growing distributed ledger, accordingly to the partitions of the network. In that sense, the blockgraph will not form a traditional linear chain of blocks but a graph of blocks in the form of a Direct Acyclic Graph (DAG).

Cordova et. al. [START_REF] Cordova | Blockgraph: A blockchain for mobile ad hoc networks[END_REF] introduced the concept of blockgraph where detailed characteristics of its architecture are provided. The blockgraph is composed of three primary modules that manage three different aspects of its functioning, naming, the blockgraph protocol, the consensus module, and the group management module. The blockgraph protocol takes care of the management of the DAG data structure and orchestrates the splits and merge functions. The consensus module hosts a compatible consensus algorithm such as C4M [START_REF] Morales | A performance evaluation of c4m consensus algorithm[END_REF] capable of agreeing on the same state of the DLT while managing multiple node associations and dissociations. The group management module runs a topology discovery algorithm that gathers network topology information to alert on changes in the network topology. All modules use dedicated interfaces to exchange relevant information for the proper functioning of the blockgraph system. The role of the blockgraph in our 6G architecture could be two folds: (i) as a means to securing future 6G communication as exposed in [START_REF] Cordova | A preliminary assessment of blockgraph-a mobility-aware solution to secure 6g mesh networks[END_REF] and (ii) as a verifiable data registry for entity identity authentication. In the latter, the blockgraph will be distributed among multiple trusted zones as shown in figure 2, and each trusted zone will have multiple instances of the blockgraph, where identity identifier information (i.e., DIDs) from all trusted zones are stored. The blockgraph will guarantee the immutability and availability of all DIDs, while the distributivity property of the blockgraph will guarantee decentralization, which enhances the security and trust in the information that is being protected by the blockgraph. Moreover, DIDs are natively secure cryptographically and allow binding an identifier with an entity or subject. In our use case, the DIDs are identity information of well-known trusted VC issuers. Thus, a VC can be trusted by verifying that the issuer's DID is included in the VC, which is also cryptographically secured. The exchange of credentials is held off-chain through a standardized protocol that guarantees the authorized entities are allowed to interact. This blockgraph deployment aligns with the principle of zero-trust security since the distributivity of the blockgraph among the trusted zones will ensure decentralization and transparency of the DLT.

V. CONCLUSION

In this paper, we proposed a 6G architecture and an authentication scheme based on blockchain-like technology, called blockgraph, and Verifiable Credentials. We believe that our proposed architecture is an important stone to better achieve security for future 6G technology than its previous generations. However, there is still a lot of work to be done to achieve this goal, especially when moving from one trusted zone to another trusted zone. Using blockchain-like technology and Verifiable Credentials is a promising solution to enforce a decentralized and distributed zero-trust scheme for granular identity authentication and control access. Of course, attackers will always try to deceive any authentication scheme to become trusted before attacking. Thus, we must make sure that the entire system lies in a zero-trust model where resources are accurately verified, and requests are examined in detail to effectively limit the consequences of such attacks. The proposal made in this paper goes in that direction and will be tested in depth by the authors within the framework of research projects. The main challenges of our proposal are on one hand to scale the blockgraph performance for massive use and on the other hand to prove that a zero-trust model for 6G technology is indeed feasible.

Fig. 2 .

 2 Fig. 2. Blockgraph deployment under study

ACKNOWLEDGMENT

This work has been partially supported by the French AMI-5G ENE5AI project and the French DIM RFSI 5G-REISEP project


Épilepsie : « il n'existe pas de sport interdit, mais des sports adaptés » https://sciencespourtous.univ-lyon1.fr/epilepsie-sport-adapte/ Pendant longtemps, l'activité physique était proscrite pour les personnes souffrant d'épilepsie. Axel Lion, doctorant en sociologie au Laboratoire L-VIS, et ancien enseignant en APA, met en lumière le changement de paradigme actuellement en cours, passant de la proscription à la prescription.

Touchant près de 600 000 personnes en France chaque année, l'épilepsie est une maladie chronique neurologique qui se caractérise par un fonctionnement électrique anormal de l'activité cérébrale. Cette maladie se manifeste par des crises répétées, généralisées ou focales qui peuvent prendre différentes formes et varier en intensité selon la zone du cerveau impliquée.

A ce jour, on recense plus de cinquante syndromes épileptiques différents. Par ailleurs, les crises persistent malgré un traitement médicamenteux bien conduit chez 30% des malades, catégorisés dès-lors comme pharmaco-résistants.

Depuis l'apport des sciences sociales, l'épilepsie renvoie aujourd'hui principalement à deux notions : celle de l'incertitude, omniprésente dans le quotidien des malades et de leurs proches du fait des crises qui peuvent se déclencher à n'importe quel moment ; et celle de la stigmatisation, les porteurs d'épilepsie vivant dans la crainte permanente de la révélation fortuite de leur maladie via le déclenchement inopiné d'une crise.

Les personnes souffrant d'épilepsie exclues des pratiques sportives

Considérée en occident jusqu'au XVIII ème siècle comme « l'expression du diable », l'épilepsie a traditionnellement été associée à des croyances et à des représentations sociales négatives. Aujourd'hui encore, les personnes souffrant d'épilepsie subissent un discrédit du fait notamment de la brutalité et de la soudaineté de certaines crises. Les effets de la stigmatisation liée à l'épilepsie incitent les individus à réduire leur participation sociale et à s'isoler, un processus également observé au niveau de leurs pratiques physiques. La peur de voir se dévoiler leur maladie ou celle de se blesser au décours d'une crise freinent les personnes épileptiques à s'engager dans des activités sportives. Le manque d'informations et de recommandations médicales agit également comme une barrière à leur mise en mouvement. De plus, les manifestations physiologiques liées à l'effort telles que l'essoufflement ou la sudation sont parfois identifiéesà tortcomme des facteurs déclencheurs de crise, renforçant leur « autoexclusion » de ces pratiques physiques. Plus dommageable encore, ces idées reçues seraient encore tenaces chez certains professionnels de santé et du sport. Ces éléments peuvent expliquer que l'épilepsie, à l'instar de nombreuses pathologies chroniques, a longtemps été considéréeet l'est parfois encorecomme une contre-indication à la pratique sportive en elle-même.

Activité physique et épilepsie, de la proscription à la prescription

La littérature scientifique permet d'affirmer aujourd'hui la nette supériorité des bénéfices de l'activité physique sur les risques potentiellement encourus par ses pratiquants, à la fois sur les plans physiologique, psychologique et social. Non seulement des travaux ont montré qu'il n'y avait pas davantage d'accident de sport chez les personnes épileptiques que dans la population générale, -il y en aurait d'ailleurs moins -; mais de plus, différentes études ont révélé que la pratique d'une activité physique régulière permettrait de réduire la fréquence et l'intensité des crises tout en améliorant la qualité de vie et l'estime de soi des personnes souffrant d'épilepsie. Ainsi en 2016, un consensus a été publié par la Ligue internationale contre l'épilepsie encourageant la pratique sportive des personnes épileptiques, tout en précisant certaines restrictions (activités en haute altitude, plongée sousmarine, etc.).

Cette reconnaissance grandissante des bienfaits de l'activité physique sur l'épilepsie comme sur d'autres maladies chroniques se traduit cette même année en France par la publication d'un décret [n° 2016-1990 du 30 décembre 2016] permettant aux médecins traitants de prescrire du « sport sur ordonnance » à leurs patients atteints d'affection de longue durée dont font partie les épilepsies graves. Une prérogative qui marque un véritable changement de paradigme sur ce sujet, faisant passer l'activité physique de la « proscription » à la « prescription » auprès de ces publics vulnérables. Pour être plus précis, le décret ne fait pas référence au sport mais à l'activité physique adaptée (APA) et il s'agit là d'un point important. La pratique physique des personnes vivant avec une épilepsie peut effectivement être conditionnée à un certain nombre d'ajustements et doit bénéficier d'un accompagnement spécifique visant à garantir leur sécurité. Ces modalités doivent permettre d'appréhender les singularités de l'épilepsie de chacun : facteurs déclencheurs, fréquence et type de crises, etc. Tout l'enjeu porte donc sur cette notion d'adaptation et sur sa mise en oeuvre.

Avec l'APA, pas de sport interdit mais des sports adaptés

L'APA est le domaine scientifique et professionnel de l'activité physique s'adressant à toute personne ne pouvant pas pratiquer une activité physique ou sportive dans des conditions ordinaires et qui présente des besoins spécifiques de santé (telles que les personnes souffrant d'épilepsie), justifiant la mise en place d'adaptations. Organisées conjointement par le pratiquant lui-même, l'enseignant en APA et le médecin, ces adaptations doivent permettre de donner accès à tout type d'activité, notamment celles générant le plus d'appréhension, parfois en lien avec de mauvaises expériences vécues par le passé. L'idée n'est plus de disqualifier une activité pour les risques supposés ou réels qu'elle ferait encourir au pratiquant épileptique, mais plutôt de chercher à l'adapter pour la rendre accessible, sécurisée et sécurisante.