Bingzhe Zhao, UT-Austin

"Refined Interpretation of Te Response to Neutral Beam Injection at DIII-D"

Neutral beam injection (NBI) is an essential tool in tokamaks as a source of power, particle and torque. Accurate knowledge of the beam deposition profiles is critical for understanding the effect of NBI as well as for transport analysis. Conventionally, the experimental deposition profiles of the beam are calculated based on Monte Carlo (MC) simulations. There were some previous attempts to extract the beam deposition information from the electron temperature (Te) responses through Fourier analysis, but the results fell short when the fast ion slowing-down time becomes significant (~ 100 ms) and comparable to the plasma confinement time.

In this work, the responses of the local Te to NBI perturbation for some DIII-D discharges are reported and discussed quantitatively, and a novel approach to interpret the Te responses was developed by linearizing and modeling the detailed Te evolution trajectory using coherently averaged Te measurements based on the different time scales of the terms in the local power and particle balance equations.

The study is based on DIII-D plasma discharges with modulated NBI, or MNBI, under a variety of operation scenarios, with a focus on the Te measurements made from the 5 kHz electron cyclotron emission radiometer (ECE) system. The results are benchmarked against simulations from ADAS-based MC beam code (NUBEAM) within the framework of plasma transport code (TRANSP or ONETWO). In general, good consistency and reasonable agreements are found, and notably a peculiar anomaly is observed suggesting unexpected interaction between the 3/2 magnetic island chain and the neutral beam. Additionally, an alternative statistical method based on system identification (SI) is also developed for this objective, with limited but promising results.