A 2D self-consistent fluid plasma model is employed for studying the plasma in the radiofrequency (RF)-driven negative hydrogen ion source prototype developed to equip the neutral beam injector systems for ITER. The source is considered in its usual configuration with a cylindrical driver and magnetic filter field (MF) produced by permanent magnets arranged in a magnet frame movable along the expansion chamber. The model accounts for the RF injection, the bias potential applied at the plasma grid (PG) and the losses of particles and electron energy in the third dimension, i.e. along the magnetic field lines, are evaluated. The study is focused on the role of the processes which govern the spatial structuring of the plasma parameters caused by the topology of the MF, the PG bias and the losses along the MF. The obtained numerical results are analyzed based on the contribution of the local and nonlocal processes in the electron-and negative hydrogen ion-balance and electron energy balance equations. It is shown how the fluxes associated with the diamagnetic drift caused by the gradient of the electron temperature, and the E×B-drift as well as the electron heating and the negative ion drift in the dc electric field are involved in the formation of the pattern of the plasma parameters. Effects due to the partial penetration of the MF in the driver are also investigated. A comparison with the experimental data shows that, accounting for the losses along the MF, the present model is a reliable tool for study and optimization the ITER relevant sources.

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Radio frequency (RF) power coupling in inductively coupled plasmas (ICPs) is investigated numerically using a self-consistent fluid model. Hydrogen discharges are simulated at pressures from 0.3 -10 Pa and at RF powers of around 1 kW. At the low excitation frequency of 1 MHz a high magnetic RF field of around 30 G is generated by the RF coil, meaning that discharges at low pressures are in the nonlinear skin effect regime. Therefore, a description of the RF power coupling by simple collisional Joule heating is not appropriate. Moreover, models that account for collisionless heating by means of a stochastic collision frequency or as diffusion of the RF current density (as is state-of-the-art for discharges operated in the anomalous skin effect regime at higher frequencies of e.g. 13.56 MHz) are incapable of describing the RF power coupling in the nonlinear skin effect regime properly. This is due to their total neglect or simplified treatment of the RF Lorentz force. Instead, this work demonstrates that the RF power coupling mechanism for discharges operating at low radio frequency in the nonlinear skin effect regime can be described by an electron momentum balance retaining the nonlinear RF Lorentz force as well as electron inertia and advection. The crucial role of the RF Lorentz force in generating the RF plasma current density and thus in shaping the plasma parameter profiles is validated successfully with experimentally obtained electrical and spatially resolved plasma parameters for pressures as low as 0.5 Pa. Below this pressure the results obtained from the model and the ones from the experiment diverge. Most likely this is caused by a sudden change in the electron distribution function at the lowest pressures.

RF-driven negative hydrogen ion sources are typically operated at low frequencies around 1 MHz, gas pressures around or below 1 Pa and large power densities up to 10 Wcm-3. Owing to these conditions as well as the current discharge geometries and antenna layouts, the RF power coupling is far from optimized, i.e. only a fraction η of the power delivered by the generator is absorbed by the plasma. This considerably limits the performance and reliability of RF-driven ion sources. To study the bidirectional RF power coupling a self-consistent fluid model is introduced. Taking into account the interplay between the nonlinear RF Lorentz force and the electron viscosity (usually neglected in state-of-the-art fluid models) a steady state solution is obtained, where the trends reflect the experimental data. Solutions calculated in hydrogen but with increased ion masses indicate that the latter are responsible for the systematically increased η, which is observed experimentally when deuterium instead of hydrogen is used as feed gas.

Abstract.The study provides results of the influence of the filter field topology on the plasma parameters in the RF prototype negative ion source for ITER. A previously developed 2D fluid plasma model has been extended to take into account for the particles and energy losses along the magnetic field lines and the presence of a magnetic field in the driver as it is the case for the BATMAN and ELISE test-beds. Since the main role of the magnetic filters in these two sources is the same (to reduce the co-extracted electron current) they are studied and compared for the geometry of the prototype source. For the magnetic field configuration used at BATMAN, the axial position of the filter has been varied according to experiments with an external magnet frame. For the magnetic field configuration of ELISE the magnitude of the field has been varied, as can be done in the experiment by varying the current flowing through the first grid of the extraction system. The obtained results show the same main features in the patterns of the plasma parameters for the two configurations of the magnetic filter. The presence of a magnetic filter in the driver has a local impact on the plasma parameters, showing no significant influence on their spatial distributions in the expansion chamber.