Experimental studies of the radial diffusion of energetic electrons in lower-hybrid wave heated plasmas in the Alcator-C tokamak

Abstract

An upper limit for the cross-field spatial diffusion coefficient of energetic electrons in moderate density tokamak discharges has been measured. The energetic electrons were generated by square wave-modulated lower-hybrid (LH) wave injection. The decay of the fast electron population for electron energies εd≥100 keV following LH radio-frequency (RF) power shutoff was inferred from the time dependence of the radial profiles of the hard x-ray bremsstrahlung emission. The experimental results were fitted to a diffusive model that includes collisional losses and electric field acceleration. The fast electron diffusion coefficient consistent with the data is Df≤0.2 m2/s, at least in the inner half of the plasma. The corresponding global confinement time of the energetic electrons is estimated to be at least a factor of 2 greater than the energy confinement time of the bulk plasma in discharges of density n̄e=8×1019 m −3.