First results from the multichannel interferometer system on HSX

Abstract

Measuring the equilibrium electron density distribution and its response to perturbations provides important information to magnetically confined plasma research. A multichannel interferometer system is now routinely operating on the new quasihelically symmetric stellator ͑HSX͒ to measure the equilibrium profile and electron density dynamics. The interferometer system has nine viewing chords with 1.5 cm spacing. The source is a bias-tuned Gunn diode at 96 GHz with passive solid-state tripler providing output at 288 GHz ͑8 mW͒. The HSX plasma is produced by 28 GHz electron cyclotron resonance heating and first results of the interferometer measurement are reported. The density spatial distribution is reconstructed from the measured line-integrated density. At high density ͓n e Ͼ2ϫ10 12 cm Ϫ3 ͔, an mϭ1 density oscillation with frequency of 1-2 kHz is observed. Plans to determine the radial particle flux and transport coefficients will be discussed.

The Wisconsin Plasma Astrophysics Laboratory

“…This system is very similar to interferometers used on the helically symmetric experiment (HSX) and the Madison symmetric torus (MST) (Deng et al. 2003, 2006). Time resolution of density measurements is set by the intermediate frequency (IF) of the variable-frequency source.…”

“…WiPAL is equipped with a heterodyne millimetre-wave (320 GHz) interferometer system designed to measure chord-integrated absolute electron density via wave phase shifts. This system is very similar to interferometers used on the helically symmetric experiment (HSX) and the Madison symmetric torus (MST) (Deng et al 2003(Deng et al , 2006. Time resolution of density measurements is set by the intermediate frequency (IF) of the variable-frequency source.…”

Far-forward collective scattering measurements of density fluctuations in the helically symmetric experiment stellarator

“…1. 3,4 While interferometer systems have been successfully employed to measure density fluctuations ͑typically coherent modes͒ as well as the equilibrium density profile, in an effort to improve sensitivity we have recently operated the diagnostic in a mode whereby far-forward collective scattering from the electromagnetic probe wave is measured. It can be frequency-modulated so that heterodyne detection at the modulation frequency of ϳ1 MHz is possible.…”

H α detector system for the Helically Symmetric Experiment

“…In the modeling a localized neutral gas source at the location of the puff is used in addition to modeled recycling; the electron density profile in the modeling is provided by inverted multi-chord interferometry. 6 The DE-GAS output has been scaled so that the prediction for chord 5 matches the measurement. The agreement between measured and predicted chordal data for the toroidal array is comparable to the poloidal array data presented here.…”

“…Studies of plasma flow damping are in progress, 4 and it is necessary to know the atomic hydrogen density to estimate the damping of the flows due to ion-neutral collisions. Particle transport studies have been initiated using both steady state 5 and perturbative 6 methods. These studies require knowledge of the particle source rate, which is directly coupled to the neutral density.…”