Response of thermal and fast-ion transport to beam ion population, rotation, and Te/Ti in the DIII-D steady state hybrid scenario

Abstract

The thermal and fast-ion transport properties of DIII-D steady-state hybrid discharges with normalized beta beta(N) greater than or similar to 3 are studied at low injected torque and an increased electron to ion temperature ratio T-e/T-i. Linear stability analysis performed with the TGLF turbulent code indicates that a high-k mode is usually dominant at smaller radii, whereas a low-k mode is usually dominant at larger radii in these plasmas. A reduction in the net injected torque from 8.6 to 4.3 N-m leads to reduced E x B shear and hence, an enhanced turbulence that was observed on the Doppler backscattering diagnostic and was also computed with TGLF. As T-e/T-i in the core was increased from 0.57 to 0.66 by adding electron cyclotron current drive (ECCD) to these plasmas, higher levels of transport are observed with increased high-k modes indicated by TGLF. The fast-ion transport level varied over an order of magnitude in these discharges depending on whether Alfven eigenmodes, fishbones, or no instabilities were observed. Hybrid plasmas with fishbones have decreased fast-ion transport, compared to plasmas with Alfven eigenmodes, since they are resonant with a smaller portion of phase space and their resonance is farther from the wall. This reduction in fast-ion transport with ECCD mitigates the increase in turbulent transport, resulting in higher performance than expected during strong electron heating. Similarly, the lowest fast-ion transport was observed in the low torque plasma, which also led to better than expected performance at this torque value. The thermal and fast-ion transport changes observed as the torque/rotation and T-e/T-i are varied indicate possible methods for transferring this scenario to a reactor.