K.M. Likin

Internal electron transport barrier due to neoclassical ambipolarity in the helically symmetric experiment

Electron cyclotron heated plasmas in the Helically Symmetric Experiment (HSX) feature strongly peaked electron temperature profiles; central temperatures are 2.5 keV with 100 kW injected power. These measurements, coupled with neoclassical predictions of large “electron root” radial electric fields with strong radial shear, are evidence of a neoclassically driven thermal transport barrier. Neoclassical transport quantities are calculated using the PENTA code [D. A. Spong, Phys. Plasmas12, 056114 (2005)], in which momentum is conserved and parallel flow is included. Unlike a conventional stellarator, which exhibits strong flow damping in all directions on a flux surface, quasisymmetric stellarators are free to rotate in the direction of symmetry, and the effect of momentum conservation in neoclassical calculations may therefore be significant. Momentum conservation is shown to modify the neoclassical ion flux and ambipolar ion root radial electric fields in the quasisymmetric configuration. The effect is much smaller in a HSX configuration where the symmetry is spoiled. In addition to neoclassical transport, a model of trapped electron mode turbulence is used to calculate the turbulent-driven electron thermal diffusivity. Turbulenttransport quenching due to the neoclassically predicted radial electric field profile is needed in predictive transport simulations to reproduce the peaking of the measured electron temperature profile [Guttenfelder et al. , Phys. Rev. Lett.101, 215002 (2008)].

Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment

Measurements of particle and heat transport have been made in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technology 27, 273 (1995)]. Experimental differences in the density and temperature profiles are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The electron temperature is higher in the QHS configuration, due to a reduction in electron thermal diffusivity that is comparable to the neoclassical prediction. The density profile in plasmas with the symmetry broken is measured to be hollow, while in QHS plasmas the profile is centrally peaked. Calculations of the radial particle flux using the DEGAS code [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)] show that the hollow profile observed with the symmetry broken is due to neoclassical thermodiffusion. Thermodiffusion is reduced in the QHS configuration, resulting in a peaked density profile.

Gyrokinetic studies of trapped electron mode turbulence in the Helically Symmetric eXperiment stellarator

Gyrokinetic simulations of plasma microturbulence in the Helically Symmetric eXperiment are presented. Using plasma profiles relevant to experimental operation, four dominant drift wave regimes are observed in the ion wavenumber range, which are identified as different flavors of density-gradient-driven trapped electron modes. For the most part, the heat transport exhibits properties associated with turbulence driven by these types of modes. Additionally, long-wavelength, radially localized, nonlinearly excited coherent structures near the resonant central flux surface, not predicted by linear simulations, can further enhance flux levels. Integrated heat fluxes are compatible with experimental observations in the corresponding density gradient range. Despite low shearing rates, zonal flows are observed to regulate turbulence but can be overwhelmed at higher density gradients by the long-wavelength coherent structures.

OVERVIEW OF RECENT RESULTS FROM HSX

Abstract Recent results are summarized for the Helically Symmetric Experiment (HSX), which has the capability of running as a quasi-helically symmetric stellarator or as a more conventional, nonsymmetric stellarator. From X-ray measurements, we have demonstrated improved confinement of energetic particles. With central electron cyclotron heating, the density profiles in the quasi-symmetric configuration are peaked, in contrast to the hollow or flat profiles when the symmetry is broken. The difference in profiles is attributed to the lowering of the neoclassical thermodiffusive flux when the symmetry is present. The central electron temperature is ~200 eV higher for the quasi-symmetric configuration over the nonsymmetric case. The power deposition profiles are similar for the two cases, implying that the neoclassical electron thermal conductivity is reduced with quasi-symmetry. Related to the good confinement characteristics in the quasi-symmetric mode of operation, fluctuations in the density and magnetic field, consistent with that of a global Alfvén eigenmode (GAE), are observed. While the neoclassical characteristics of the quasi-symmetric and nonsymmetric configurations are very different, we have yet to find, under present operating conditions, any significant difference (other than the possible GAE mode) in turbulence characteristics or blob formation at the plasma edge.

Three-dimensional scrape off layer transport in the helically symmetric experiment HSX

The edge topology of helically symmetric experiment (HSX) in the quasi-helically symmetric configuration is characterized by an 8/7 magnetic island remnant embedded in a short connection length scrape-off layer (SOL) domain. A 2D mapping of edge plasma profiles within this heterogeneous SOL has been constructed using a movable, multi-pin Langmuir probe. Comparisons of these measurements to edge simulations using the EMC3-EIRENE 3D plasma fluid and kinetic neutral gas transport model have been performed. The measurements provide strong evidence that particle transport is diffusive within the island region and dominantly convective in the SOL region. Measurements indicate that phenomenological cross-field diffusion coefficients are low in the SOL region between the last closed flux surface and edge island (i.e. m2 s−1). This level of transport was found to increase by a factor of two when a limiter is inserted almost completely into the magnetic island. A reduction in gradients of the edge electrostatic plasma potential was also measured in this configuration, suggesting that the reduced electric field may be linked to the increased cross-field transport observed.

Profile stiffness measurements in the Helically Symmetric experiment and comparison to nonlinear gyrokinetic calculationsa)

Stiffness measurements are presented in the quasi-helically symmetric experiment (HSX), in which the neoclassical transport is comparable to that in a tokamak and turbulent transport dominates throughout the plasma. Electron cyclotron emission is used to measure the local electron temperature response to modulated electron cyclotron resonant heating. The amplitude and phase of the heat wave through the steep electron temperature gradient (ETG) region of the plasma are used to determine a transient electron thermal diffusivity that is close to the steady-state diffusivity. The low stiffness in the region between 0.2 ≤ r/a ≤ 0.4 agrees with the scaling of the steady-state heat flux with temperature gradient in this region. These experimental results are compared to gyrokinetic calculations in a flux-tube geometry using the gyrokinetic electromagnetic numerical experiment code with two kinetic species. Linear simulations show that the ETG mode may be experimentally relevant within r/a ≤ 0.2, while the Trapped Electron Mode (TEM) is the dominant long-wavelength microturbulence instability across most of the plasma. The TEM is primarily driven by the density gradient. Non-linear calculations of the saturated heat flux driven by the TEM and ETG bracket the experimental heat flux.

Performance of the Thomson scattering diagnostic on Helical Symmetry Experiment

We report the design and the performance of the ten-point Thomson scattering system installed on Helical Symmetry Experiment (HSX). Based upon the design of the GA Thomson scattering system, the HSX Thomson scattering system covers the complete plasma cross section, providing a ten-point plasma parameter profile measurement during a single plasma shot. The system consists of a commercial 1 J–10 ns yttrium–aluminum–garnet laser, ten polychromators from GA, the specially designed collection optics, a CAMAC data acquisition unit, and a controlling computer. The high throughput collection optics enables measurement even at a plasma density of 0.5×1012 cm−3. The system configuration, spectral response calibration, Raman calibration of the system, and operational results are presented.

Comparison of electron cyclotron heating results in the helically symmetric experiment with and without quasi-symmetry

The extraordinary wave at the second harmonic of the electron cyclotron frequency produces and heats the plasma in the helically symmetric experiment. Ray-tracing calculations predict 40% first pass absorption at a plasma density of 1.5 x 10 18 m -3 and an electron temperature of 400 eV To measure the wave absorption, a set of absolutely calibrated microwave detectors is installed along the machine. It was found that the absorption efficiency is very high (about 0.9) in the quasi-helically symmetric (QHS) and mirror configurations, and it drops to 0.6 in the anti-mirror mode. The confinement of particles in the different configurations is studied in the neutral gas breakdown experiments. With the same gas pressure and heating power, the density for the QHS configuration has a larger growth rate (10 4 s -1 ) compared with the mirror (5 x 10 3 s -1 ) and anti-mirror modes (2 x 10 3 s -1 ). A study of the stored energy versus launched power and plasma density shows that it increases linearly (up to 50 J) with power and has a maximum at a low plasma density (at about 0.4 × 10 18 m -3 ). The central electron temperature measured by Thomson scattering also rises linearly with heating power and reaches 600 eV at 100 kW of launched power.

Edge turbulence measurements in electron-heated Helically Symmetric Experiment plasmas

This paper presents edge measurements utilizing Langmuir probes to characterize plasma turbulence in the Helically Symmetric Experiment (HSX) [F. S. B. Anderson et al., Fusion Technol. 27, 273 (1995)]. Normalized density and potential fluctuations exhibit strong intensities but are comparable to mixing length estimates using measured correlation lengths. The correlation lengths are isotropic with respect to radial and poloidal directions and follow local (gyro-Bohm) drift wave expectations. These observations are common to measurements in both the optimized quasihelically symmetric (QHS) configuration, as well as a configuration where the symmetry is degraded intentionally. The resulting turbulent particle flux in higher density QHS discharges is in good quantitative agreement with transport analysis using three-dimensional neutral gas simulations. The measured turbulence characteristics are compared to a quasilinear trapped electron mode (TEM) drift wave model [H. Nordman, J. Weiland, and A. Jarmen, Nucl...

Core density turbulence in the HSX Stellarator

Broadband turbulent density fluctuations are explored in the helically symmetric stellarator experiment (HSX) by investigating changes related to plasma heating power and location. No fluctuation response is observed to occur with large changes in electron temperature and its gradient, thereby eliminating temperature gradient as a driving mechanism. Instead, measurements reveal that density turbulence varies inversely with electron density scale length. This response is consistent with density gradient drive as one might expect for trapped electron mode (TEM) turbulence. In general, the plasma stored energy and particle confinement are higher for discharges with reduced fluctuations in the plasma core. When the density fluctuation amplitude is reduced, increased plasma rotation is also evident suggesting a role is being played by intrinsic plasma flow.