Full wave simulations of lower hybrid wave propagation in tokamaks

Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons at (2.5−−3)× vte, where vte ≡ (2Te/me) is the electron thermal speed. Consequently these waves are well-suited to driving current in the plasma periphery where the electron temperature is lower, making LH current drive (LHCD) a promising technique for off-axis (r/a ≥ 0.60) current profile control in reactor grade plasmas. Established techniques for computing wave propagation and absorption use WKB expansions with non-Maxwellian self-consistent distributions. Higher order WKB expansions have shown some effects from diffraction on the parallel wavenumber evolution and consequently on the damping location. In typical plasma conditions with electron densities of several 1019 m−3 and toroidal magnetic fields strengths of 4 Telsa, the perpendicular wavelength is of the order of 1mm and the parallel wavelength is of the order of 1cm. Even in a relatively small device such as Alcator C-Mod with a minor radius of 22 cm, the number of wavelengths that must be resolved requires large amounts of computational resources for the full wave treatment. These requirements are met with a massively parallel version of the TORIC full wave code that has been adapted specifically for the simulation of LH waves [J. C. Wright, et al., Commun. Comput. Phys., 4, 545 (2008), J. C. Wright, et al., Phys. Plasmas 16 July (2009)]. This model accurately represents the effects of focusing and diffraction that occur in LH propagation. It is also coupled with a Fokker-Planck solver, CQL3D, to provide self-consistent distribution functions for the plasma dielectric as well as a synthetic HXR diagnostic for direct comparisons with experimental measurements of LH waves. The wave solutions from the TORIC zero FLR model will be compared to the results from ray tracing from the GENRAY/CQL3D code via the synthetic HXR diagnostic and power deposition.