Guillaume Urbanczyk

Ion cyclotron range of frequencies (ICRF) antenna operation and plasma-wall interaction were investigated on the WEST tokamak in the regime where the density at the antenna limiters was sufficiently low for the slow wave (SW) to propagate in front of the antenna. Using a reciprocating emissive probe magnetically connected to the antenna, we measured for the first time the DC plasma potential, V DC , during a radial scan of the lower hybrid (LH) resonance layer across the antenna limiter. V DC peaks when the density at the antenna limiter edge approaches the LH resonance density, but never exceeds typical values of a few hundred volts. From the plasma-wall interaction standpoint, this regime is highly favorable: because particle fluxes are reduced while sheath potentials remain comparable to standard operating conditions at the same antenna voltage, local tungsten sources at the ICRF antenna and other outer-wall components become nearly undetectable. In general, tungsten sputtering from active WEST ICRF antennas is dominated by the particle flux rather than the sputtering yield; that is, it follows variations in the local density rather than the plasma potential. By contrast, sputtering in the divertor is primarily governed by the sputtering yield. Core impurity contamination is likewise significantly reduced when the antennas are positioned far from the separatrix, both with and without ICRF power, and the radiated power fraction decreases accordingly. Despite the modest coupled powers obtained at large antenna-plasma clearance, satisfactory ICRF heating is maintained as the density at the antenna limiter edge falls below the LH resonance, and no deleterious effects are observed in any key core plasma metrics when operating the antenna from this low-density region. Overall, the experimental results point toward minimal coupling to the SW by the fast wave antenna despite it being located in a region where the SW can propagate.

Ion cyclotron resonance heating (ICRH) is one of the common auxiliary heating methods employed in tokamaks. Therefore, optimizing the ICRH scheme for tokamaks is of great importance. In this study, the two-dimensional full-wave code Interative fluid-kinetics (INTFLUK) is employed to systematically investigate ICRH scenarios for the China Fusion Engineering Demonstration Reactor (CFEDR). A comparative analysis is conducted between the D–T– ⁷ Li three-ion heating scheme and the conventional D–T– ³ He minority scheme. Both the D–T– ⁷ Li and D–T– ³ He scenarios exhibit a relatively high target ion absorption fraction. For the D–T– ⁷ Li scheme, increasing the ⁷ Li ion concentration while reducing the T ion concentration effectively minimizes collisional energy transfer to electrons Furthermore, results indicate that the target ion absorption fraction in the D–T– ⁷ Li scenario is superior to that of other three-ion alternatives. Finally, an assessment of alpha particle effects under the optimized scheme reveals that when the alpha particle concentration reaches 3%, its power absorption fraction exceeds 50%. However, assuming an equivalent Maxwellian distribution slightlyoverestimates the alpha particle power absorption fraction compared to a conventional slowing-down distribution.

A systematic experimental study of hydrogen-minority ion cyclotron resonance heating (ICRH) has been performed in deuterium plasmas in WEST to optimize the power deposition. Dedicated scans of the radial position of the ion cyclotron resonant layer and of the minority concentration were carried out while keeping global plasma parameters and antenna coupling nearly constant. The maximum performance is obtained when the resonant layer is shifted towards the high-field side (HFS) by around 7 cm with a hydrogen minority concentration between 5% and 8%. A pronounced degradation occurs for larger inward displacements, whereas the performances are almost halved when the resonance is located in the low-field side. No clear dependence of turbulence characteristics on the resonance position is observed, indicating that the confinement variations are primarily governed by wave absorption and fast-ion confinement modifications. Infrared thermography and calorimetry measurements show that fast-ion ripple-induced losses are minimized for resonance positions beyond a certain value on the HFS, revealing a trade-off between optimal absorption and wall load mitigation. These results demonstrate that coordinated control of the resonant layer position and minority concentration allows simultaneous optimization of ICRH efficiency and fast-ion wall loads in a fully metallic reactor-relevant environment.

The Material Plasma Exposure eXperiment (MPEX) is a linear device under construction at ORNL. MPEX employs a variety of heating systems, including a helicon antenna, which is the subject of this work. Lacking a resonant absorption mechanism, as tokamaks have, modeling the RF power absorption in such antennas requires detailed consideration of cold collisional physics. Previous modeling of the heating in this antenna was done exclusively in 2D, assuming axisymmetry and a single azimuthal mode number m=1. In this work, we introduce a fully 3D model. By including the full 3D physics, with its self-consistent geometry-imposed azimuthal spectrum, we are able to reproduce the experimentally observed transition from edge heating to core heating.

Interaction d’ondes à forte puissance avec la périphérie des tokamaks WEST et EAST: Cette thèse vise à étudier les interactions entre le plasma et les parois de tokamaks liées aux ondes à la fréquence cyclotronique ionique (FCI), les interactions plasma-métaux étant à éviter absolument car elles sont synonymes de dégradations matérielles de l’enceinte et la libération d’impuretés métalliques dans le plasma dont les performances s’en trouvent grandement réduites. Cette problématique affecte concrètement toute machine visant à chauffer les ions via des ondes à la fréquence FCI, ce qui sera notamment le cas d’ITER. Cette thèse s'inscrit dans une collaboration entre le CEA Cadarache (France) et l’Institut de Physique des Plasmas à Hefei (Chine). Divers travaux expérimentaux ont été effectués sur les tokamaks EAST (Chine) et WEST (France) afin d'identifier les paramètres pertinents pour d'une part optimiser l'efficacité par laquelle les ondes FCI utilisées pour chauffer le plasma doivent être excitées afin de maximiser la quantité de puissance couplée au plasma tout en minimisant les interactions du plasma avec les parois dues à ce type d'ondes et souvent attribuées au concept de gaine radiofréquence au cœur de cette thèse.

Ion cyclotron range of frequencies (ICRF) heating is essential for creating plasma in next-generation fusion devices. ICRF antennas often produce localized wall erosion, reducing reliability, material survivability, and overall plasma performance. We show that the wave physics' gyrotropy is among the causes of toroidal and poloidal asymmetries in these erosion spots, and that this source of asymmetry can be compensated for either by controlling the poloidal phasing or by modifying the limiter shapes.

This paper introduces STRIPE (Simulated Transport of RF Impurity Production and Emission), an advanced modeling framework developed to analyze material erosion and the global transport of eroded impurities originating from radio-frequency (RF) antenna structures in magnetic confinement fusion devices. STRIPE integrates multiple physics modules: SolEdge3x for scrape-off-layer plasma profiles, COMSOL for 3D RF rectified sheath potentials, RustBCA for erosion yields and surface interactions, and global impurity transport for 3D ion energy-angle distributions and impurity transport. The framework is applied to an ion cyclotron RF-heated L-mode discharge (#57877) in the WEST tokamak, where it predicts a thirty-fold increase in gross tungsten erosion at antenna limiters during the transition from ohmic to ICRH operation. Additionally, under ICRH conditions, a tenfold enhancement in erosion is observed when comparing RF sheath effects to purely thermal sheath conditions. High-charge-state oxygen ions ( O 6 + and above) are identified as the dominant contributors to tungsten sputtering. To validate the model, a synthetic diagnostic tool based on inverse photon efficiency (S/XB coefficients) from the ColRadPy collisional-radiative model enables direct comparison with spectroscopic measurements. Model predictions using a plasma composition of 1% oxygen and 99% deuterium show good agreement with observed W − I (400.9 nm) emission for discharge #57877, supporting the accuracy of the STRIPE framework. This study focuses specifically on gross erosion calculations to demonstrate STRIPE’s capabilities. Future extensions of this work will incorporate net erosion, re-deposition, self-sputtering effects, and whole-device modeling of sputtered tungsten impurity transport. STRIPE is also being applied to other RF-heated linear and toroidal devices, offering valuable insights for antenna design, impurity control, and performance optimization in next-generation fusion reactors.

For successfully heating plasma with waves in the ion cyclotron range of frequencies (ICRFs), mitigating impurity production is just as crucial as maximizing power coupling, especially in high-Z environments (Urbanczyk et al 2021 Nucl. Mater. Energy 26 100925). ICRF can effectively deposit energy on ions, modify turbulence-driven transport, and enhance fusion reaction efficiency, but only when its power coupling has minimal impact on impurity production. To do so, one must rely on a toroidal array of at least three active elements excited with appropriate phasing and power ratio to reduce the currents induced on the antenna frame below levels critical for physical sputtering. In contrast to classic two-strap antennas, which are optimized for dipole phasing with equal power on both straps, three-strap antennas in ASDEX Upgrade (AUG)—but also four-strap antennas in JET, Alcator C-Mod, SPARC and ITER—offer the possibility to act also on the power ratio between the central and outer straps. With optimal settings, impurity production can be reduced substantially, making the ICRF compatible with the high-Z wall (Bobkov et al 2017 Plasma Phys. Control. Fusion 59 014022). This paper explores the characteristics of the AUG three-strap antennas in terms of impurity production, as well as the key role of plasma composition in this process. Numerical simulations were performed using SSWICH and Petra-M (finite element codes) to quantify impurity production and compare with experimental results. Energies of ions falling on antenna limiters (measured with probes) are well predicted by both codes. These tools are then used to further describe the source of the impurity, namely the gross erosion of tungsten from an ICRF antenna, for different plasma mixtures. Results are also compared to spectroscopy data. Ultimately, we show that deleterious effects of the ICRF on plasma surface interactions will be weaker in plasmas containing larger fractions of highly ionized heavier low-Z impurity, which is typically relevant for experiments relying on impurity seeding.

The objective of this work is to linearly investigate the plasma instabilities that will be observed in the linear SPEKTRE device, currently being assembled at Institut Jean Lamour. Two configurations are considered. In the first configuration, the magnetic field is set to 0.1 T with no ion temperature gradient (ITG), resulting in the observation of only collisional drift waves (DW). In the second configuration, the magnetic field is set to 0.44 T, and ions can be heated using an ion cyclotron radiofrequency heating (ICRH) system to establish an ITG. Under these conditions, two major types of instabilities may be observed: collisional DW and ITG instabilities. ITG instabilities become more unstable than DW when the ratio of the characteristic lengths of the ion temperature to ion density profiles η=ΩT*/Ωn*>2.6. The observation of such a transition between the two types of instabilities will be possible on this machine using the ICRH system. The azimuthal mode number m of the most unstable mode is significantly larger for helium plasma compared to argon plasma. Furthermore, for the plasma parameters considered in both configurations, a fluid model is often sufficient to accurately describe DW, while a kinetic model is required to accurately describe ITG instabilities. There is a 30% difference between the ITG instability growth rates predicted by the fluid model and those predicted by the kinetic model.