Effect of radial transport on the LH current drive efficiency in tokamaks featuring an internal transport barrier

Abstract

In the present paper we study the effects of occurrence of radial transport of particles in a tokamak, and the effects of the presence of an Internal Transport Barrier (ITB), on the current drive efficiency and power deposition profiles in the case of lower hybrid waves generating an extended tail in the electron distribution function. The results are obtained by numerical solution of the Fokker-Planck equation which rules the evolution of the electron distribution function. We assume that the radial transport of particles is due to magnetic or electrostatic fluctuations, and introduce a model to describe the ITB, with adjustable parameters. The presence of an Edge Transport Barrier (ETB) is simulated by Neumann boundary conditions at the plasma edge. The results obtained show very different behavior for current drive whether we have electrostatic or magnetic transport origin. The change in the plasma current due to magnetic transport has been observed to be more significant than the change due to electrostatic transport, basically because the magnetic transport is more effective in diffusing high-velocity particles of the electron tail.