Effect of Quasihelical Symmetry on Trapped-Electron Mode Transport in the HSX Stellarator

Abstract

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.

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

“…15 The regions of bad normal curvature and particle trapping are highly correlated, and previous linear gyrokinetic modeling has indicated that the TEM is highly localized in the bad curvature region of HSX. 16 Combined with temperature or density gradients, there is strong drive for curvature driven modes such as the TEM. 17 Figure 8 shows unstable flux tube samples the region of low magnetic field strength on the inboard side of the device.…”

Edge turbulence measurements in electron-heated Helically Symmetric Experiment plasmas

“…Recently, a modified version of the Weiland ITG/TEM model 6,7 was used to model the anomalous transport in HSX. 28 The results suggest that the anomalous transport in HSX can be explained by TEM transport driven by both electron temperature and density gradients in the helically symmetric geometry. It is natural to question whether the measured turbulence is consistent with this model prediction, or if the transport prediction is simply fortuitous.…”

“…It is important to note that while the local electron tem- perature gradient may be relatively small in the edge with the diagnostic probes inserted, a strong density gradient ͑a / L n Ϸ 2-3͒ remains that is capable of destabilizing the TEM, 28 which can drive the resulting turbulence and transport.…”

“…Linear and nonlinear three-dimensional ͑3D͒ gyrokinetic simulations performed for many stellarator configurations [22][23][24][25][26][27][28] demonstrate that they are susceptible to the same type of drift wave microinstabilities present in tokamaks, in-cluding ion temperature gradient ͑ITG͒, trapped electron mode ͑TEM͒, and electron temperature gradient ͑ETG͒. Recently, a modified version of the Weiland ITG/TEM model 6,7 was used to model the anomalous transport in HSX.…”

Geometry Dependence of Stellarator Turbulence

“…1) aims at mitigating the neoclassical (nc) losses to the point where anomalous transport becomes dominant over most of the plasma column. Attention is now turning to understanding the effects of 3D geometry on microturbulence, [2][3][4][5][6][7] aided by nonlinear gyrokinetic codes valid for 3D. Supported by the scant, albeit promising, existing findings in this area, we attempt to identify key geometrical factors which contribute to the development and, subsequently, control of turbulent transport levels.…”