Particle-in-cell simulations of the near-field of a lower hybrid grill

Abstract

Lower hybrid (LH) waves in the frequency range 1 to 10 GHz are used to heat and to drive current in a tokamak. A crucial issue for the future devices is the coupling of the wave power from the launching structure, the grill, to the plasma. A related problem is the formation of hot spots on the grill limiters and other components that are magnetically connected to the grill region. A probable explanation for these asymmetric heat loads is the parasitic absorption of the LH power. In parasitic absorption, the short-wavelength modes are absorbed by electrons within a very short distance in front of the grill. The cold edge electrons (Te ∼ 25 eV) may reach energies up to 2 keV through stochastic acceleration in the electric field in front of the lower hybrid grill.

The particle-in-cell (PIC) technique is one of the most popular methods of kinetic simulations of plasmas. In PIC codes, a spatial grid is used to describe the field quantities, while the particles move in the continuous space. The field and particle motion are solved self-consistently. The advantage of the PIC codes is that they take into account the kinetic effects and the non-linearities of the problem. They also give the deposition profiles unlike pure test particle simulations.

In this thesis, the use of PIC codes has been extended to a whole new branch of applications, the near-field of the LH grill. During the work, the grill model used in the electrostatic PIC code has been developed. The parasitic absorption and the generation of fast electrons in front of the waveguide mouth have been explored. The absorption and the heat load on the grill limiter increases with the edge density, the edge temperature and the launched power density. A weak indication of a power threshold was observed. The simulation results can explain experimental observations in Tore Supra. According to the simulations, the heat loads are not a problem in the next generation launchers used in ITER.