Verifying raytracing/Fokker-Planck lower-hybrid current drive simulations with a new self-consistent full-wave/Fokker-Planck model

The raytracing/Fokker-Planck (FP) simulations used to model lower-hybrid current drive (LHCD) often fail to reproduce experimental results, especially in experiments where the LHCD is weakly damped. Here, we determined whether or not "full-wave" effects, such as diffraction and interference, and breakdown of the raytracing approximation could explain the discrepancies seen between experiments and simulations. To do this, we performed some of the largest RF simulations ever (~2 million CPU hours), in which we directly solved the plasma Helmholtz equation for non-Maxwellian distribution functions in tokamak geometry using the TORLH full-wave field-solver coupled to the CQL3D Fokker-Planck solver. These simulations determined that raytracing accurately captures the features of the exact Helmholtz equation solution. Diffraction was quantified, and its effect was found to be smaller than other broadening mechanisms such as finite launcher spectral width. Breakdown of raytracing near cut-offs and caustics was also found to not result in important deviations from full-wave results. These results provide the first definitive verification of raytracing/FP simulations of LHCD used for tokamak reactor design against full-wave solutions.