Investigation of electron gyro-scale fluctuations in the National Spherical Torus Experiment

Anomalous electron thermal transport in magnetically-confined plasma hinders efforts to achieve feasible magnetic fusion energy, and an adequate understanding of anomalous electron thermal transport remains elusive. Gyrokinetic simulations of plasma turbulence indicate electron temperature gradient (ETG) turbulence may generate experimentally-relevant electron thermal transport. To investigate ETG turbulence, a collective scattering system was installed on the National Spherical Torus Experiment (NSTX). Collective scattering provides fluctuation measurements with spatial and k-space localization, and the NSTX collective scattering system measures fluctuations on the electron gyro-scale. This Dissertation describes the design and operation of the NSTX collective scattering system and presents fluctuation measurements, gyrokinetic calculations, and transport calculations pertaining to ETG turbulence. Measurements and analysis indicate electron gyro-scale fluctuations are present in a variety of NSTX plasma regimes, and the observed fluctuations are consistent with ETG turbulence. Additional results pertain to the ETG critical gradient, flow shear suppression of ETG turbulence, ETG-driven electron thermal transport, fluctuation magnitudes, and k-spectra. The measurements can be a challenging validation test for nonlinear gyrokinetic simulations.