Interpretation of Te Response to Neutral Beam Injection on DIII-D

Electron cyclotron emission (ECE) measurements closely trace electron temperature responses to neutral beam injection (NBI) and thus provide a direct experimental indicator of beam deposition, which is normally only available through Monte Carlo (MC) methods. The power is deposited into the electron population through fast ion collisions with thermal electrons, which has an interaction time scale of 10-100 ms. Meanwhile due to the short thermalization time (~0.1 ms) and low temperature (~10 eV) of the beam electron source, the NBI pulses display a prompt Te cooling effect through dilution. These two opposing effects are separable with ECE data due to difference in effective time scales, and in general the cooling effects dominate at higher Te (Te>2keV). The beam deposition calculated from the Te response is compared with calculations from MC beam code NUBEAM. Both power and source deposition profiles for electrons were overall in reasonable agreement. However, the electron source deposition profile from measurements is more core-biased than the MC calculation: up to 2 times higher on-axis but drops off about twice as fast. This core discrepancy is likely related to beam halo and thermal neutral transport.



Work supported by US DOE under DE-FG02-97ER54415 and DE-FC02-04ER54698.