Investigating the impact of Electron Cyclotron Heating on Electron Temperature Gradient turbulence with Phase Contrast Imaging at DIII-D

The Phase Contrast Imaging (PCI) diagnostic system at DIII-D observes short wavelength broadband turbulence () consistent with Electron Temperature Gradient (ETG) modes in the core of plasmas with off-axis Electron Cyclotron Heating (ECH) (1). PCI measures line-integrated electron density fluctuations within a large range of wavenumbers (k_PCI = [1 - 30] cm^-1), frequencies ( f = 10 kHz - 2 MHz), and radii (ρ > 0.4). Measurements are performed in lower single null deuterium H-mode plasmas with ECH applied in either the core (ρ = 0.5) or edge (ρ = 0.7) regions. Linear CGYRO simulations predict unstable ETG modes within the range (2). PCI measurements indicate that high wavenumber (k_⊥ρ_s » 1) and high frequency (f = [500 - 1000] kHz) turbulence is driven unstable when ECH resonates within the plasma core region. Measurement of high-k turbulence is localized to the core through correlation of observations with changes in rotation profile and ECH location. These findings support the conclusion that these measurements are consistent with ETG turbulence.

Work supported in part by the US Department of Energy, Office of Science under awards DE-FC02-04ER54698 and DE-SC0016154.

(1). E. M. Davis et al, Rev. Sci. Instruments, 89, 10B106 (2018)

(2). N.T. Howard et al Nucl. Fusion 61 106002 (2021)