Distinct core turbulence transport regimes in high-confinement HSX plasmas

Record core electron temperatures and on-axis electron beta have recently been achieved in HSX. Improved confinement conditions are observed in hydrogen and helium plasmas with large core electron temperature gradients and flat core density profiles. Hydrogen plasmas show more peaked profiles compared to helium, believed to result from changes in turbulence-driven transport. A re-designed and upgraded Thomson scattering diagnostic enables high quality measurements of the electron pressure profile and gradient scale lengths. Reflectometry and interferometry provide electron density fluctuation measurements and a 16-channel correlation electron cyclotron emission diagnostic measures core electron temperature fluctuations.  These measurements are supported by gyrokinetic flux-tube modelling using the GENE and, for the first time, the global GENE3D code. Simulations show density-gradient-drivenTEMs likely drive transport in the core. Strong on-axis ECRH destabilizes temperature-gradient-driven TEMs that drives heat flux but minimal particle flux, flattening core density profiles.  Simulations of helium plasmas show larger TEM growth rates and electron heat fluxes compared to hydrogen, corroborating the observation of less peaked helium pressure profiles. Measurements of plasma flows, as well as confinement in new HSX coil current configurations with smaller linear growth rates will be presented.

Acknowledgements: Supported by U.S. DoE Grants DE-FG02-93ER54222, DE-SC0021972.