Internal measurements of magnetic and density fluctuations from magneto-hydro-dynamic (MHD) to sub-ion cyclotron frequency range using radial interferometer-polarimeter in DIII-D

Tokamak transport is believed to be dominated by fluctuations such as those arising from Magneto-Hydro-Dynamic (MHD) instabilities, turbulence and Energetic-Particle-driven instabilities. Internal measurements of radial magnetic (δb) and density (δn) fluctuations from MHD to sub-ion cyclotron frequency range have been achieved with high resolution (δb/b~O(10^-5), δn/n~O(10^-6)) using a radial interferometer-polarimeter in the DIII-D tokamak. Coherent magnetic and density fluctuations with multiple harmonics in the 10-100 kHz range are usually observed before and after a sawtooth crash. Broadband magnetic and density fluctuations from 10 to 600 kHz exhibit a direct link to global plasma confinement: suppression of 10-100 kHz magnetic and density fluctuations closely correlates with low- (L-) to high-confinement (H-) mode transitions, whereas a ~50% reduction of 200-600 kHz magnetic fluctuation amplitudes is measured during transition from L- to improved-confinement (I-) mode. In sub-ion cyclotron frequency (3-4 MHz at ~1 T magnetic field), density fluctuations with a mixture of continuous and bursting behavior are measured to correlate with neutral beam injection. These measurements will help develop a better physics understanding to optimize transport in a fusion reactor.