Measurements and modeling of turbulent transport in the HSX stellarator.

摘要

Measurements of electron cyclotron resonance heated (ECRH) plasmas in HSX demonstrate a reduction in experimental electron thermal diffusivity in the core, consistent with that predicted by neoclassical theory due to quasihelical symmetry. However, the transport across most of the minor radius is anomalous. Because of the low electron collisionality, the trapped electron mode (TEM) instability is predicted to be the dominant turbulent transport mechanism. To assess this possibility, the Weiland model has been used to predict the linear stability and quasi-linear transport of HSX plasmas. This axisymmetric model, evaluated using the local curvature and trapped particle fraction in the outboard, low field, bad curvature region of HSX, is capable of reproducing the experimental transport within an order of magnitude. Localized edge turbulence measurements using Langmuir probes demonstrate the existence of fluctuation amplitudes consistent with mixing length arguments. The scale lengths, density-potential cross phases, and inferred growth rates are also in reasonable quantitative agreement with linear TEM predictions. The TEM growth rates, calculated more accurately for 3D HSX equilibria using the gyrokinetic code GS2, exhibit scaling with collisionality, density and temperature gradients, and trapped particle fraction, similar to that in tokamaks. The Weiland model, with local HSX geometry approximations, is shown to agree within 30% of the GS2 calculations for experimental parameters in HSX. One-dimensional transport simulations have been performed using the Weiland model in addition to neoclassical transport calculations. These simulations predict the energy confinement times within 10% of measured values. The predicted density and electron temperature profiles are in good agreement with measurements outside the inner 30% of the minor radius. The predictions inside this core region are sensitive to the radial electric field through both the neoclassical and anomalous transport models.