Effects of trapped electrons on off-axis lower hybrid current drive in tokamaks

The effects of trapped electrons on off-axis lower hybrid current drive (LHCD) in tokamaks are studied, A computercode for solving the Fokker–Planck equation in a toroidal geometry is developed and employed. The code is suitablefor various auxiliary heating and current drive schemes in tokamak plasmas. The influence of the resonance regimeon the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency areemphasized. It is shown that, as an electrostatic force, the lower hybrid wave causes some of the trapped electrons tobe untrapped and lose their energy, which can cut the LHCD efficiency by about 30%. The ITER scaling law is alsoused to estimate the trapped electron effects.