Neutral Beam Source and Target Plasma for Development of a Local Electric Field Fluctuation Diagnostic

A new diagnostic measuring local $\vec{{E}}(r,t)$ fluctuations is being developed for plasma turbulence studies in tokamaks. This is accomplished by measuring fluctuations in the separation of the $\pi $ components in the $H_{\alpha } $ motional Stark spectrum. Fluctuations in this separation are expected to be ${\tilde{{E}}} \mathord{\left/ {\vphantom {{\tilde{{E}}} {E_{MSE} \sim 10^{-3}}}} \right. \kern-\nulldelimiterspace} {E_{MSE} \sim 10^{-3}}$. In addition to a high throughput, high speed spectrometer, the project requires a low divergence ($\Omega \approx 0.5^{\circ})$, 80 keV, 2.5 A $H^{0}$ beam and a target plasma test stand. The beam employs a washer-stack arc ion source to achieve a high species fraction at full energy. Laboratory tests of the ion source demonstrate repeatable plasmas with $T_{e} \sim 10$ eV and $n_{e} \approx 1.6\times 10^{17}$ m$^{\mathrm{-3}}$, sufficient for the beam ion optics requirements. $T_{e} $ and $n_{e} $ scalings of the ion source plasma are presented with respect to operational parameters. A novel three-phase resonant converter power supply will provide 6 mA/cm$^{\mathrm{2}}$ of 80 keV $H^{0}$ at the focal plane for pulse lengths up to 15 ms, with low ripple $\delta V/80$ keV $\approx 0.05\% $ at 280 kHz. Diagnostic development and validation tests will be performed on a magnetized plasma test stand with $\sim 0.5$ T field. The test chamber will utilize a washer-stack arc source to produce a target plasma comparable to edge tokamak plasmas. A bias-plate with programmable power supply will be used to impose $\tilde{{E}}$ within the target plasma.