Experimental study of plasma transport using X-ray imaging spectroscopy in Alcator C-Mod

Abstract

The high resolution X-ray spectrometer with spatial resolution (HiReX-Sr) is intensively used in the studies of impurity, momentum and energy transport in Alcator C-Mod. This thesis investigates three phenomena using HiReX-Sr: (1) Non-local heat transport (the breakdown of a local expression for the heat flux) is observed in linear Ohmic confinement (LOC) regime plasmas in cold pulse injection experiments. This effect disappears in saturated Ohmic confinement (SOC) regime plasmas. The plasma rotation reverses direction across this transition. Transport analysis show that a transient internal transport barrier (ITB) forms in the core plasma. Cold pulse modulation experiments suggest a non-diffusive heat transport behavior. The change of dominant turbulence mode from electron mode dominance in LOC plasmas to ion mode dominance in SOC plasmas is proposed to be the underlying mechanism for the correlations among the disappearance of the non-local effect, the rotation reversals and the LOC/SOC transitions. This work extends the understanding of non-local phenomena in the tokamak plasmas. (2) Evidence of inward momentum pinch in ICRF modulation experiments. The momentum transport coefficients are calculated based on a simplified model. It is found that within 0.1 < r/a < 0.5, the momentum diffusivity is lower than the ion thermal diffusivity and the momentum pinch is inward. (3) ICRF induced argon pumpout in hydrogen-deuterium plasma is observed in Alcator C-Mod when the H-to-D level is relatively high (nH/nD ~ 35% - 50%). The pumpout effect is maximal when nH/nD ~ 40% ± 5%, at which level the hydrogen-deuterium hybrid layer is close to the Ar16+ 2nd harmonic resonant layer. This suggests that the pumpout of argon could be due to the energy absorbed from the enhanced left-hand polarized electric field through 2nd harmonic resonance. Further experiments are proposed to verify this impurity-wave interaction mechanism, which is potentially useful for active impurity control in fusion devices. Thesis Supervisor: Dr. John E. Rice