Sciences Naturelles et de l'Ingénieur

Numerical simulation of the fast ion dynamics in thermonuclear plasmas

Auxiliary plasma heating methods are required to achieve the necessary conditions for thermonuclear fusion in magnetically confined plasmas. Ion Cyclotron Resonance Heating (ICRH) and Neutral Beam Injection (NBI) constitute the most practical approaches to increase the temperatures of hydrogen and helium isotopes in a fusion reactor environment. The application of ICRH in the JET tokamak significantly distorts the energetic particle distribution function leading to anisotropy in the plasma pressure. The heat deposition at the resonance layer has the double effect of altering the guiding centre particle trajectories and modifying the underlying equilibrium state. Integrated modelling that incorporates the physical eects of heating on the particle distribution function and on the magnetohydrodynamic (MHD) equilibrium state is thus imperative to obtain accurate solutions. Previously, ICRH heating simulations were only coupled to distribution function solvers. The SCENIC package has been developed that uses an anisotropic pressure version of the VMEC MHD equilibrium code, the LEMan code to calculate the heat deposition due to ICRH and the full-f version of the VENUS code (with Monte Carlo radio frequency collision operators) to follow the guiding centre particle orbits. The fast particle distribution function that is obtained is tted with a bi-Maxwellian model for subsequent updates of VMEC equilibria and LEMan heat deposition. High power (12MW ICRH) minority heating scenarios in axisymmetric JET geometry has successfully converged a solution in which the equilibrium, the heating and the hot particle distributions are mutually self-consistent. This has demonstrated that the anisotropic pressure model we have adopted is an essential feature for the simulations of the JET experimental conditions.

The formalism in the SCENIC suite of codes allows for a three-dimensional (3D) description of the confinement geometry. We have applied SCENIC to a 3D quasiaxisymmetric stellarator system (scaled to the size of the JET tokamak) that has great potential of steady state oper- ation with particle confinement properties similar to an axisymmetric tokamak. The spatial distribution of the modulus of the magnetic eld strength mod-B obtained from VMEC, the heat deposition calculated with LEMan and the corresponding deposition from full-f VENUS are presented in Fig. 1 at four toroidal cross section that encompass one quarter of the torus. The beneficial effect of energetic particles for heating and current drive (NBCD) can be compromised by the presence of microturbulent fields. While turbulent heat redistribution is expected to be negligible, results concerning NBCD are scarce. Steady state experiments, such as ITER, profoundly rely on neutral beam injection for plasma control, stabilization and safety prole tailoring. This interaction is therefore investigated, for the ITER steady state scenario, by coupling the full-f version of the VENUS code and the nonlinear gyrokinetic code GENE. The GENE code is used to simulate the background turbulent fields characterizing tokamak plasmas. The features of the turbulent elds are then extracted and computed by a set of numerical diagnostics to provide a realistic estimate of the particle diusivity of energetic ions. The redistribution of the current driven is then calculated by simulating the neutral beam deposition, energetic ion motion (unperturbed and collisional) and turbulent transport. The numerical platform responsible for this part of the analysis is based on the VENUS code, coupled with a beam deposition module. We have introduce a Monte-Carlo scattering operator in the code for simulating the stochastic microturbulent transport. A schematic of the neutral beam particles simulated by the VENUS code is represented in Fig. 2.