Cold Electron Effect from Neutral Beam Injection on DIII-D

Electron Cyclotron Emission (ECE) measurements show clear evidence of cooling of T_e by cold electrons from neutral beam injection (NBI) and thus provide a direct experimental indicator of beam deposition, which is normally only available through Monte Carlo (MC) methods. Due to the short thermalization time (~0.1 ms) and low temperature (~10 eV) of beam electrons, cooling effect can be observed at the start of NBI pulses. Its magnitude is proportional to the beam electron source rate and becomes evident around rho=0.5 in high T_e plasma, where T_e drops ~1-10 keV/s for ~10-100 ms. The beam source rate calculated from the cooling rate is compared with predictions from NUBEAM. Deposition profiles from cooling and MC are both Gaussian-like curves peaked at the magnetic axis. However, the deposition from cooling rate is about 2-3 times the MC calculation at rho=0 but drops off about twice as fast, and the total electron source rate is about 20% lower. Lower total source is likely due to missing cooling measurements near the edge, and possible physics behind the discrepancy in the core region is presented. In conclusion, this method has potential to provide a direct experimental measurement for NBI core deposition.