Multi-field Ion Distribution Fluctuation Measurements of T_e/T_i-dependent Transport in DIII-D

The electron-ion temperature ratio (T_e/T_i) has minimal impact on measured long-wavelength ion temperature fluctuation amplitudes in DIII-D L‑mode plasmas, while parallel velocity fluctuations are found to decrease at higher T_e/T_i, accompanied by large increases in local ion thermal transport and reduced energy confinement. These carbon ion fluctuation measurements are obtained with the novel Ultra‑Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic at two line-averaged densities; neutral beam (NBI) heated and neutral beam plus electron cyclotron (ECH) heated phases are examined to systematically vary collisionality (ν^*) and T_e/T_i. In addition, low and intermediate wavenumber density fluctuation (ñ/n) measurements show virtually no change. Increasing T_e/T_i is expected to excite drift wave instabilities which typically drive most turbulent transport. TGLF modeling using experimental profiles shows decreased ion mode growth rates and increased electron mode growth rates when increasing T_e/T_i at both densities. The changes in growth rates are, in part, consistent with observations of increased electron temperature fluctuations. TGLF results are also consistent with poloidal turbulence velocity measurements, which show mode propagation trending from the ion diamagnetic towards the electron diamagnetic direction in the plasma frame as T_e/T_i is increased. TGLF, however, predicts a significant decrease in relative T_i fluctuations with increasing T_e/T_i at low density, which is not observed. This discrepancy provides an excellent test case for nonlinear gyrokinetic turbulence codes at burning plasma relevant parameters with T_e~T_i and low rotation.