Vacuum insulation is applied in high voltage apparatus such as power circuit breakers and low loss capacitors. The highest possible electrical breakdown strength should be expected in ideal vacuum, since no charge carriers are present in the inter-electrode gap. Vacuum thus appears as an effective alternative to gas insulators such as sulfur hexafluoride (SF6), perfluorocarbons (CF4, C2F6, or the promising C4F8) which present the drawback of being global warming potential gases. But electrons emitted by the cathode directly cross the gap without any collision phenomena, and pre breakdown is frequently encountered, so limiting the use of vacuum for power circuit breakers. The role played by metallic microprotusions in the field emission mechanism is now well understood but an alternative emission mechanism proposed by Latham et al implies non-conductive or semi-conductive materials such as oxide layers or impurities, including adsorbed gases. Another field of application is in the controlled fusion domain. Fusion reactions in hot plasmas inside future Tokamak reactors (ITER, DEMO) are initiated by injecting high power beams of neutral D° atoms at high energy (1 MeV for ITER) into the plasma. Negative ions are accelerated by an intense electric field between electrodes at high voltage under vacuum and neutralised in a downstream gas target. The connection between the power supplies under SF6 and the electrodes under vacuum is made through an insulating passage called the bushing. Experiments at IRFM on the MV testbed (1MV, 100mA) have shown that the voltage holding is limited by the appearance of breakdowns if two electrodes are too close. The voltage holding with distance follows a square root law for distances larger than 1 cm. This dependence indicates according to the theory by Cranberg an exchange of micro clumps that cause breakdowns when they evaporate on the opposing electrode. Another performance limitation is the appearance of a sizeable electron current (100mA at 400kV) resulting from field emission that appears to follow the Fowler-Nordheim law. This unwanted dark current can be reduced, even eliminated, by the presence of gas in the vacuum vessel. This very beneficial feature is consistent with an increase of the work function of the metal. This would be caused by the adsorption of gas induced by an intense electric field. This physisorption process allows atoms to stick around emitting micro protrusions. Due to this, the emitting surface is reduced and the work function is increased. This research project aims to study the field-induced adsorption process by joining theoretical and modelling work with small-scale and large-scale experiments in several laboratories. The objective is to find physical conditions that favour the increase in surface work function, thus leading to an increased voltage holding in vacuum (by suppression of the dark current and absence of breakdowns) under high electric fields (50-100 kV/cm) between large electrode surfaces. This ANR project proposes specific research on the high-voltage holding under vacuum conditions. Four different laboratories work on 5 different themes: - Fundamental studies and modelling of field induced gas adsorption (LCAR Toulouse). - Simulation of field emission from a realistic surface and micro ionisation around emitting micro tips (LPGP, Paris). - Experimental study of the field emission and field-induced adsorption of gas with adjustable parameters (electrodes surface conditioning, electrodes material, electric field intensity, electrodes gap distance, gas nature and pressure); model validation (Supelec, Paris). - Study of the high voltage holding in vacuum using different materials and surface treatments to eliminate micro particles and micro tips. Large scale application of the previous more fundamental studies (IRFM, CEA Cadarache). - Construction and test of a prototype compact bushing, using all the knowledge gained in this project (IRFM, CEA-Cadarache).

The present project is put into the context of the international projects ITER and DEMO aiming at managing nuclear fusion to produce energy. In tokamaks (nuclear fusion reactors), a hot plasma composed of deuterium and tritium nuclei is magnetically confined to achieve fusion. The heating of the plasma is mainly obtained by the injection of high-energy deuterium neutral beams, coming from the neutralization of high-intensity D- negative-ion beams. D- negative-ions are produced in a low-pressure plasma source and subsequently extracted and accelerated. The standard and most efficient solution to produce high negative-ion current uses cesium (Cs) injection and deposition inside the source to enhance negative-ion surface-production mechanisms. However, ITER and DEMO requirements in terms of extracted current push this technology to its limits. The already identified drawbacks of cesium injection are becoming real technological and scientific bottlenecks, and alternative solutions to produce negative-ions would be highly valuable. The first objective of the present project is to find an alternative solution to produce high yields of H-/D- negative-ions on surfaces in Cs-free H2/D2 plasmas. The proposed study is based on a physical effect discovered at PIIM in collaboration with LSPM, namely the enhancement of negative-ion yield on boron-doped-diamond at high temperature. The yield increase observed places diamond material as the most up to date relevant alternative solution for the generation of negative-ions in Cs-free plasmas. The project aims at fully characterizing and evaluating the relevance and the capabilities of diamond films (intrinsic and doped polycrystalline, single crystal as well as nanodiamond films…) as negative-ion enhancers in a negative-ion source. The second objective is to investigate diamond erosion under hydrogen (deuterium) plasma irradiation, with two main motivations. First, material erosion could be a limitation of the use of diamond as a negative-ion enhancer in a negative-ion source and must be evaluated. Second, the inner-parts of the tokamaks receiving the highest flux of particles and power are supposed to be made of tungsten, but its self-sputtering and its melting under high thermal loads are still major issues limiting its use. It has been shown in the past by one of the partners that diamond is a serious candidate as an efficient alternative-material for fusion reactors. Therefore, diamond erosion in hydrogen plasmas will also be investigated from this perspective. At the moment when all the efforts are put on tungsten, maintaining a scientific watch on backup solutions for tokamak materials is crucial. The project associates partners with complementary expertise in the field of plasma-surface interactions on the one hand, and diamond deposition and characterization on the other hand. Furthermore, in order to span the gap between fundamental science and real-life applications, negative-ion surface-production and diamond erosion will be studied in laboratory plasmas (PIIM in collaboration with LSPM ) as well as in real devices (Cybele negative-ion source at IRFM and Magnum-PSI experiment at DIFFER ). PIIM: Physique des Interactions Ioniques et Moléculaires, Université Aix-Marseille, CNRS LSPM: Laboratoire des Sciences des Procédés et des Matériaux, CNRS, Université de Paris 13 IRFM: Institut de Recherche sur la Fusion Magnétique, Commissariat à l’Energie Atomique, Cadarache DIFFER: Dutch Institute For Fundamental Energy Research, The Netherlands

Energetic particles are ubiquitous in magnetically confined fusion plasmas. They contain a significant fraction of the plasma energy and are thus vital for the performance of fusion devices such as ITER. However, the presence of energetic particles and the fact that fusion plasmas are complex systems heated up to hundred million degrees result in instabilities that reduce the confinement of energetic particles. Understanding, predicting and controlling their transport and losses is of prime importance and constitutes our main goal. This is a high-dimensional multi-scale nonlinear problem, for which a complete description is so far unaffordable. Therefore, we propose a novel and inter-disciplinary approach to develop numerical tools based on Artificial Intelligence techniques applied to two lines of research: (1) derive data-driven reduced models for transport of energetic particles and (2) optimize the information extracted from HPC gyro-kinetic simulations and from experiments.

The project aims at demonstrating the feasibility of a nearly complete neutralization of a beam of H- or D- by photodetachment. This is to be applied to the production of energetic neutral beams, suitable for the heating of fusion plasmas, like the one of ITER and the future fusion reactors (DEMO). This would be an important simplification with respect to the current neutral beam production technique, which relies on collisional neutralization and suffers from important technical drawbacks such as very low overall injector efficiency (~20%) and the necessity to set high-current negative ion sources and all their accessories at very high voltages. Despite its conceptual simplicity, the photodetachment way presents a major difficulty, namely the necessity to illuminate the ion beam with a very high flux of photons, so as to reach the saturation regime. Fortunately, laboratoire ARTEMIS has accumulated an outstanding expertise in high power laser light injection, which makes photodetachment in a high finesse cavity a realistic solution. Using an optical cavity with a finesse of a few thousand would make it possible to reach the multi-megawatt regime suitable for saturation with only one kW of laser input. Recycling of the light is made possible - and necessary - by the very low individual absorption of a single negative ion. The first goal of the project is to make the experiment at a reduced scale, on a negative ion beam of only a few millimetres in diameter, and to increase the cavity finesse, hence the light power progressively. Having several MW of intracavity light power is a complementary objective, for which several technical issues have to be addressed, first of all the thermal effects inside the optical substrates. Association of an industrial partner to the project aims at demonstrating that the cavity mirrors can have the properties necessary for reaching the objective. During the program, the demonstration experiment at LAC will make use of magnetic coils, so as to investigate the amplification of the detachment cross-section at Landau resonances. Though this implies solving additional problems, especially to make the magnetic field homogeneous on large volumes, this could be helpful for future industrial developments by reducing the laser power to be applied.

The International Tokamak Experimental Reactor (ITER) currently underconstruction in South France has been designed as the key step between today's fusion research machines and tomorrow's fusion power plants. Regarding the expected thermonuclear plasma performance, ITER will require an unprecedented effort on the way to controlling plasmas heat and particle fluxes. This will call for the design of optimized plasma scenarios during ITER operation to control the heat flow from the thermonuclear source to the wall. The difficulty to get global experimental measurements in a nuclear environment in ITER, will require complementary numerical simulations based on fluid models to fine tune the magnetic configuration and adjust accordingly the edge plasma conditions. However, the capability of current solvers to perform such simulations, both for magnetic equilibrium and turbulence transport accounting for plasma-wall interactions, is still acknowledged by the international community as being largely insufficient. The SISTEM project aims to successfully achieve the strong scaling-up of plasma simulations in view of the fusion operation in a tokamak of unprecedented size, and with stringent plasma conditions. The effort will be twofold : - enhance numerical performance and capability of solvers resolving fluid models of high-fidelity (3D), in order to tackle a much larger range of spatio-temporal scales than in current machines, and so, the inherent increase in the number of degrees of freedom. - enhance the reliability of low-fidelity models (2D ensemble averaged equations) that will remain the only ones able to perform routine simulations prior to experiment, allowing us to vary engineering plasma parameters (power, pumping, …) as well as geometries of the magnetic equilibrium. On one side, the enhance accuracy and geometrical flexibility of the Hybrid Discontinuous Galerkin (HDG) method has the potential to satisfy a certain number of numerical issues, so as to progress towards predictive ITER simulations. New techniques will be developed to handle the strongly anisotropic equations describing a rapid compressible dynamics in the parallel direction to the magnetic field, and a slower incompressible-turbulence-like dynamics in the transverse direction. Specific nonlinear boundary conditions at the wall for the plasma and the magnetic equilibrium will be also addressed. An original implicit-explicit time-discretization scheme will also be developed in order to exploit HDG capabilities while satisfying HPC requirements for parallelization and memory management to tackle ITER size problems. On the other side, we will explore the development of various data assimilation techniques to improve the reliability of the turbulence modelling, which remain a major challenge nowadays for low-fidelity models. We will use experimental and numerical data from tokamak measurements and high-fidelity simulations, respectively, to reduce uncertainties on the free parameters inherently occurring in the models. The techniques will concern an automative feed-back loop model to a variational approach based on the minimization of a cost function by direct calculations of the derivatives, the number of free parameter being reduced. This way has never been explored in the fusion community. Finally, using the same grids and jointly developed numerical schemes for low and high fidelity models are important assets of the project to prepare future work, either via code-coupling or code-merging. All these challenging issues will be addressed by 3 teams from Ecole Centrale Marseille, CEA Cadarache and University of Nice, which share a multidisciplinary expertise around the same numerical tools. The combined development and use of a chain of codes based on low and high-fidelity models together with the operation of WEST in Cadarache puts our teams in a quasi-unique position in the fusion community and is one of the major assets of the project.