Fluid-model analysis on discharge structuring in the RF-driven prototype ion-source for ITER NBI

Abstract

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.

Negative Hydrogen Ion Sources for Fusion: From Plasma Generation to Beam Properties

“…Modeling efforts, mainly by fluid codes, deepened the insights into the physics behind the vertical plasma drift. It is demonstrated that the drifts are driven by the interplay of the magnetic filter field with gradients of the electron temperature (diamagnetic drift, close to the exit of the RF drivers) and with electric fields (E × B drift, close to the PG) [35]. Simple transport codes as well as particle-in-cell (PIC) code modeling predict that also nonuniformities within the single extracted beamlets can occur [36,37] (see also Beam Extraction and Formation.…”

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…The steady-state plasma column is radially unbounded and typically 1.5 m long. It can be conveniently measured in the main vacuum chamber, axially and radially, by using the B-dot probe [134] as shown in Fig. 25.…”

“…A significant effort has been devoted in the past 10 years to model the plasma behavior of fusion type NIS with fluid [134][135][136] and Particle-In-Cell models with Monte-Carlo-Collisions (PIC-MCC) [66,[137][138][139][140]. The latter have been optimized to run on supercomputers via a parallelization of the algorithms using MPI and OpenMP libraries.…”

A regularized high-order moment model to capture non-Maxwellian electron energy distribution function effects in partially ionized plasmas

“…Standalone fluid models for the electrons are used to study high plasma density (10 16 − 10 19 m −3 ) discharges at low pressure (< 50 mTorr) [18][19][20] . Altough fluid models under these conditions are compromised by the fact that the electron-electron mean-free path is comparable to the size of the device, the dimensions of the system are often too large for full kinetic simulations.…”

“…Instead, most of these multi-fluid models, e.g. [18][19][20] , obtain the closure by taking moments of a kinetic equation with a Bhatnagar-Gross-Krook (BGK) operator for the electronneutral collisions where the collisional frequency is computed as an average rate computed with Maxwellian electrons. As it will be shown in this paper, the resulting system of fluid equations is incomplete (missing effects such as thermal friction or the effect of non-Maxwellian electron energy distribution function (EEDF)) and overestimates the transport coefficients and collisional rates, in particular the ionization and excitation rates.…”