Turbulence Measurements by Interferometry and Far-forward Scattering on the HSX Stellarator

After neo-classical transport was reduced by restoring symmetry along the helical axis, a primary physics goal for HSX is to study how 3-D shaping can reduce turbulence thereby requiring measurement of turbulence with $k_{y}\rho_{s} $up to 1. For characteristic HSX parameters (Te \textasciitilde 200 eV at r/a \textasciitilde 0.5 where the density gradient peaks), this condition corresponds to $k_{y} $up to 7 cm$^{\mathrm{-1}}$. To accommodate this goal, a new 9-chord HSX interferometer/far-forward scattering system has been designed to measure density turbulence at higher k. The new system employing two high-power (30 mW each, 320 GHz), solid-state sources with frequency offset up to 6 MHz. This will permit true heterodyne detection, thereby realizing faster measurement time response, increased bandwidth and reduced noise. High power sources and high sensitivity planar-diode mixers will allow us to reduce the aperture of the receiver optics to a few mm thereby increasing the maximum wavenumber to k\textasciitilde 15 cm$^{\mathrm{-1}}$. Reconfiguring the interferometer system into a finite-angle collective scattering arrangement is also planned as it will increase the measured k-spectrum up to 18 cm$^{\mathrm{-1}}$ with some spatial resolution (core or edge).