ENERGY-TRANSPORT DURING OFF-AXIS NEUTRAL BEAM HEATING IN THE DIII-D TOKAMAK

Energy transport in discharges with off-axis neutral beam injection (N BI) has been compared with transport in similar discharges with on-axi s NBI. A two fluid power balance analysis assuming purely diffusive tr ansport indicates substantial improvements in the plasma centre in bot h the ion and the electron channels during off-axis NBI. The ion therm al diffusivity, chi(i), and the angular momentum diffusivity, chi(phi) , in the plasma centre improve by over 2 orders of magnitude, and chi( i) decreases below the Chang-Hinton neoclassical level. A detailed err or analysis of the energy transport has been carried out. The electron thermal diffusivity in the plasma centre improves by an order of magn itude during off-axis NBI. The predicted peak ion temperature, assumin g the same ion diffusivity profile for off-axis NBI as with on-axis NB I, is nearly a factor of 2 lower than the measured ion temperature pro file with off-axis NBI. In addition, assuming unchanged diffusivities, significant inward energy flow is required to explain the observed io n temperature profiles during off-axis NBI.