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Impact of electron trapping on RF current drive in tokamaks

Radiofrequency current drive in tokamak plasmas in the presence of trapped electrons is investigated. The appropriate response function, allowing the current drive efficiency, J/P, to be expressed by a simple analytical formula, is evaluated. It is shown that a significant degradation of the current generated away from the plasma axis is to be expected for methods relying on perpendicular diffusion only, even for appreciably high resonant parallel velocities.