Superthermal electron dynamics in the HSX stellarator

The Helically Symmetric eXpriment (HSX) is a novel machine with quasi-helical symmetry in mod B&ar; which promises improved single particle confinement. Superthermal electrons are generated in HSX during 2nd harmonic X-mode ECRH using a 28 GHz Gyrotron. The normal magnetic configuration is Quasi-Helically Symmetric (QHS), with a dominant n=4, m=1 component in the magnetic field spectrum. With a set of auxiliary coils, the quasihelical symmetry can be broken (Mirror and AntiMirror configurations). In this work the resolved hard x-ray emission in HSX is analyzed using a CdZnTe detector. The hard x-ray spectra were accumulated in a series of similar ECRH discharges for densities in the range of 0.2 to 0.6 x 1012 cm-3. The magnetic configuration has been altered between QHS, Mirror and AntiMirror configurations in order to determine the effect of magnetic field structure on characteristic energies and x-ray fluxes. Pulse height analysis of the hard x-ray emission shows the presence of x-ray photons with energies as high as 1 MeV during the microwave discharge. The hard x-ray spectra show higher x-ray intensities and photon energies in QHS compared to the Mirror configuration, while no signal was detected in the AntiMirror configuration. The time evolution of the hard x-ray signals show that QHS has longer superthermal electron confinement time (on the order of 7 msec) compared to the Mirror configuration (on the order of 1 msec). Calculations of single particle heating in the microwave electric field using a relativistic Lorentz model and single particle orbits using guiding center equations in magnetic coordinates indicate little difference in the heating rate between configurations. The improved confinement in the quasisymmetric configuration is responsible for the higher energies and fluxes observed.