Development of beam emission spectroscopy in the helically symmetric experiment stellarator.

This study shows the feasibility of a beam emission spectroscopy (BES) diagnostic in the Helically Symmetric eXperiment (HSX) stellarator for obtaining the spatiotemporal structure of density fluctuation. A beam emission simulation was applied to HSX plasmas to design and optimize viewing chords and to estimate the beam emission spectrum. A Doppler-shifted beam emission spectrum was measured from a 30 kV, 4 A diagnostic neutral beam injected into HSX plasmas. The beam emission was measured with a high-time-resolution avalanche photodiode (APD) assembly to determine the feasibility of BES in HSX. For HSX plasmas heated by 28 GHz electron cyclotron heating, a mode around f = 15 kHz was observed in the BES signal. The coherence between the BES signal and the density fluctuation measured by an interferometer system was significant. A plan for improving the BES system to enable the measurement of higher frequency related to turbulent transport is presented. The array of sightlines proposed in this study can be used to measure beam emission with a Doppler shift larger than 3 nm (blue shift), which enables the use of a wide passband interference filter to obtain higher throughput. The adoption of a large objective optics and a chilled APD assembly will improve the signal-to-noise ratio.

Energetic-electron-driven instability in the helically symmetric experiment.

Energetic electrons generated by electron cyclotron resonance heating are observed to drive instabilities in the quasihelically symmetric stellarator device. The coherent, global fluctuations peak in the plasma core and are measured in the frequency range of 20-120 kHz. Mode propagation is in the diamagnetic drift direction of the driving species. When quasihelical symmetry is broken, the mode is no longer observed. Experimental observations indicate that the unstable mode is acoustic rather than Alfvénic.

Effect of quasihelical symmetry on trapped-electron mode transport in the HSX stellarator.

This Letter presents theory-based predictions of anomalous electron thermal transport in the Helically Symmetric eXperiment stellarator, using an axisymmetric trapped-electron mode drift wave model. The model relies on modifications to a tokamak geometry that approximate the quasihelical symmetry in the Helically Symmetric eXperiment (particle trapping and local curvature) and is supported by linear 3D gyrokinetic calculations. Transport simulations predict temperature profiles that agree with experimental profiles outside a normalized minor radius of rho>0.3 and energy confinement times that agree within 10% of measurements. The simulations can reproduce the large measured electron temperatures inside rho<0.3 if an approximation for turbulent transport suppression due to shear in the radial electric field is included.

Experimental demonstration of improved neoclassical transport with quasihelical symmetry.

Differences in the electron particle and thermal transport are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The thermal diffusivity is reduced in the QHS configuration, resulting in higher electron temperatures than in the nonsymmetric configuration for a fixed power input. The density profile in QHS plasmas is centrally peaked, and in the nonsymmetric configuration the core density profile is hollow. The hollow profile is due to neoclassical thermodiffusion, which is reduced in the QHS configuration.

Experimental evidence of reduced plasma flow damping with quasisymmetry.

Measurements of plasma flow damping have been made in the helically symmetric experiment using a biased electrode to impulsively spin the plasma. There are two time scales in the evolution of the plasma flow, for both the spin-up and relaxation. Compared to a configuration with the quasisymmetry broken, the flow in the quasisymmetric configuration rises more slowly and to a higher value at bias turn-on, and decays more slowly at bias turn-off. The decays of the flows are significantly faster than the neoclassical prediction.