Microtearing Mode Characterization with Charge eXchange Imaging on the DIII-D* Tokamak

Charge eXchange Imaging (CXI) measurements taken during H-mode hydrogen discharges with high density and current ramps provide strong evidence that microtearing modes become unstable above a critical density/plasma current and is localized near the peak electron pressure gradient despite changing rational surface position with I_p ramps. MTMs () are predicted to cause substantial electron thermal transport, limiting fusion performance on high-power devices. Localized 2D measurements of low-to-intermediate wavenumber inter-ELM density fluctuations in the DIII-D plasma edge are obtained using a new high radial resolution multichannel CXI diagnostic that analyzes the intensity of the CVI n=8-7 emission at 529 nm. Fluctuations are measured in a 12 (radial) x 2 (poloidal) array at 2 MHz with a resolution of ∆R ~ 0.4 cm and ∆Z ~ 1.2 cm located at ρ=0.9-1.05. The mode peaks near with a radial FWHM of 1.0 cm and a fluctuation amplitude of ñ_C/n_C~5% with f~60 kHz in the lab frame. The mode features a broad spectral width [30-180 kHz] and propagates in the electron diamagnetic direction in the plasma frame. Magnetic fluctuation measurements obtained with the RIP System exhibit a similar spectral feature, suggesting the mode is electromagnetic.