Benchmarking of Lower Hybrid Current Drive Codes with Application to

This paper discusses the results of a benchmark exercise in which the predictions of several simulation models for lower hybrid current drive (LHCD) were compared using parameters typical of the steady state operating scenario in the ITER device (the so-called Scenario 4). The most complete LHCD simulation models that were used combined 2D velocity space Fokker Planck solvers with toroidal ray tracing packages. These models also predicted the highest LHCD efficiencies with 2.0 – 2.6 MA of driven current for 30 MW of coupled LHRF power. Codes that solved the Fokker Planck equation using a Green’s function approach and then computed the driven LH current using a wave induced RF flux based on 1D parallel velocity damping along ray trajectories were found to predict LH currents 35-40% lower than the 2D Fokker Planck models. This discrepancy is understood in terms of the approximate nature of the 1D wave induced flux that fails to properly capture 2D velocity space effects that occur in LHCD owing to the significant distortion of the electron distribution function. We also used an orbit-following Monte Carlo code to study the possible parasitic damping of LH waves on fusion generated alpha particles. The effect of magnetic field ripple and fast ion anomalous transport on the alpha population was considered. It was found that for a large anomalous diffusion coefficient (1m/s), the absorption on fusion alphas can be as high as 7.7% using a LHRF source frequency of 3.7 GHz. This result gives some confidence in the source frequency choice of 5.0 GHz in order to minimize the possibility of this parasitic interaction.