EXPERIMENTAL STUDIES OF THE RADIAL DIFFUSION OF ENERGETIC ELECTRONS IN LOWER-HYBRID WAVE HEATED PLASMAS IN THE ALCATOR-C TOKAMAK

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 epsilon(d) greater-than-or-equal-to 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 D(f) less-than-or-equal-to 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 n(e)BAR = 8 X 10(19) m-3.