Progress Toward a Technique for Measuring Electric Field Fluctuations in Tokamak Core Plasmas

Measurements of electric field fluctuations in magnetic confinement experiments are desired for validating turbulence and transport models. A new diagnostic to measure $E_{z}(r$,$t)$ fluctuations is in development on the Pegasus Toroidal Experiment. The approach is based on neutral beam emission spectroscopy using a high-throughput, high-resolution spectrometer to resolve fluctuations in wavelength separation between components of the motional Stark effect spectrum. Fluctuations at mid-minor-radius, normalized to an estimated MSE field, are estimated to be $\delta E$/$E_{MSE}\approx $ 10$^{-3}$. In order to resolve fluctuations at turbulent time scales ($f_{Ny}\approx $ 500 kHz), beam and spectrometer designs minimize broadening and maximize signal-to-noise ratio. The diagnostic employs a Fabry-P\'{e}rot spectrometer with \'{e}tendue-matched collection optics and low noise detectors. The interferometer spacing is varied across the face of the etalon to mitigate geometric Doppler broadening. An 80 keV $H^{0}$ beam from PBX-M with a divergence $\Omega $\textless 0.5 degrees is being refurbished for this project. The beam includes a new ion source to maximize full energy species fraction and is designed to provide $\approx $ 2 cm spatial resolution and 50 ms of 6 mA/cm$^{2\, }$current density at the focal plane. Successful development and demonstration on Pegasus will guide future deployment on larger fusion facilities.