Nonthermal particle and full-wave diffraction effects on heating and current drive in the ICRF and LHRF regimes

Abstract

Fast waves (FW) are a primary technique for heating and current drive (CD) on the proposed burning plasma device, International Tokamak Experimental Reactor (ITER) and lower hybrid (LH) waves are a candidate for edge current profile control. The models used to simulate these two waves rely on assumptions of Maxwellian populations that allow efficient analytic implementations of the plasma response, and in the case of the LH wave, the ray tracing models used are able to follow the very small wavelengths in a continuum manner without requiring a fine computational grid. Recent advances in algorithms and parallel computational methods have allowed these assumptions to be tested, permitting more accurate estimates of heating deposition and CD efficiencies in a burning plasma. Absorption by energetic particles for both waves can be significant, reducing electron heating and associated CD. Wave propagation and absorption are dependent on the velocity space distribution of particles in the plasma and geometric effects of focusing and diffraction. Fusion-born alpha particles and neutral beam ions may interact with these waves in a manner that cannot be accurately modelled by Maxwellian distributions. The AORSA2D code has been modified to use a generalized non-Maxwellian conductivity and applied to ITER reference scenarios. The effects of diffraction on LH waves in toroidal geometry are not well understood because computational limits have prohibited full-wave simulations at those small wavelengths. Simulations of LH waves have been restricted to WKB ray tracing techniques and one-dimensional full-wave in the past, but the availability of massively parallel architectures has made full-wave calculations using an electromagnetic field solver tractable. The TORIC code has been adapted to run on parallel architectures making it possible to resolve the slow electrostatic LH wave. We present full-wave simulations of LH slow and FW in toroidal geometry using a Maxwellian distribution with non-relativistic electron damping in Alcator C-Mod at values of (ωpe/ωce)2 comparable to those expected in the ITER device.