Edge turbulence measurements in electron-heated Helically Symmetric Experiment plasmas

Abstract

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. Fusion 30, 983 (1990)] that has been used to predict the anomalous transport in HSX. While quantitative differences exist (factors of 2-3), there is a general consistency between the turbulence measurements and the TEM drift wave model. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3205884

A comparison of turbulent transport in a quasi-helical and a quasi-axisymmetric stellarator

“…The structure of eigenmodes plays an important role in determining the effectiveness of nonlinear energy transfer from unstable to damped eigenmodes in turbulent saturation physics (Terry et al 2006;Hatch et al 2009). In a configuration like HSX, where eigenmodes are more extended along the field line, both the nonlinear coupling between eigenmodes and therefore the nonlinear transfer of energy to stable modes is more efficient (Hegna et al 2018), allowing for lower saturated heat flux levels.…”

“…HSX has shown that even with the reduction in neoclassical heat transport (Canik et al 2007), there is still an additional component to the experimentally-inferred electron heat transport that is thought to be due to trapped-electron-mode (TEM) turbulence (Guttenfelder et al 2009;Weir 2014;Faber et al 2015). In general, turbulence accesses free energy in thermodynamic forces which subsequently produces turbulent transport fluxes.…”

“…2007), there is still an additional component to the experimentally-inferred electron heat transport that is thought to be due to trapped-electron-mode (TEM) turbulence (Guttenfelder et al. 2009; Weir 2014; Faber et al. 2015).…”

Intrinsic plasma rotation and Reynolds stress at the plasma edge in the HSX stellarator

“…Here ṽ and V f l denote the fluctuating components of local single-fluid × E B velocity and measured floating potential in time, respectively, on the timescale of a few turbulence decorrelation times or less (τ ≈ 10 d μs in these experiments). It is assumed that the fluctuations are purely electrostatic, which should be valid in these low β HSX plasmas [20] (β < − 10 4 where probe measurements are made). This formulation for the fluctuating quantities accounts for any mean T e gradients that may exist between the pins, as the Ṽ f l quantities are mean-free, but it does not account for fluctuating components of T e on the scale length of the probe tip separation.…”

“…It is important to note that the ripple in the edge of HSX, even in the optimized configuration, is an order of magnitude larger than NSTX with RMP coils. Turbulence has also been shown to increase with minor radius [20] and so the turbulence-free, perfectly quasi-symmetric model in neoclassical transport does not apply [5,10]. Previous studies in HSX have shown that for the QHS configuration, thermal transport is dominated by anomalous rather than neoclassical effects throughout the plasma [18,21].…”