Progress Toward a New Technique for Measuring Local Electric Field Fluctuations in High Temperature Plasmas

A new diagnostic measuring local E$_{\mathrm{z}}$(r,t) fluctuations is being developed at the Pegasus Toroidal Experiment. A novel multiple volume phase holographic grating spectrometer, designed to have high resolution (0.25{\AA}) and high \'{e}tendue (U $=$ 0.01cm$^{2}$-ster), measures the line separation of the $\pi $ components of the H$_{\mathrm{\alpha}}$ motional Stark spectrum of emitted beam light. The spectra are recorded at high frequency ($f_{\mathrm{Ny}} \approx $ 500kHz) by a high speed CMOS imaging detector. The groove density of the objective grating is varied linearly along its surface to counter geometric Doppler broadening. A low divergence ($\Omega \approx $ 0.5$^{\mathrm{o}})$, 80kV, 2.5A H$^{0}$ diagnostic neutral beam is being deployed on Pegasus. The beam uses a washer-stack arc ion source to maximize full energy species fraction in the injected neutral beam. Laboratory tests of the ion source demonstrate stable, repeatable plasmas with T$_{\mathrm{e}} \le $ 20eV and n$_{\mathrm{e}} \approx $ 5x10$^{17}$m$^{-3}$, sufficient to sustain a 6mA/cm$^{2}$ current density at the focal plane for up to 20ms. A three phase resonant converter power supply, with low amplitude ($\delta $V/80kV $\approx $ 0.05{\%}), high frequency ($f_{\mathrm{rip}}\approx $ 280kHz) ripple, is in development to provide the 80kV accelerator power.