Edge transport studies in the edge and scrape-off layer of the National Spherical Torus Experiment with Langmuir probes

Abstract

Transport and turbulence profiles were directly evaluated using probes for the first time in the edge and scrape-off layer (SOL) of NSTX [Ono et al., Nucl. Fusion 40, 557 (2000)] in low (L) and high (H) confinement, low power (Pin∼ 1.3 MW), beam-heated, lower single-null discharges. Radial turbulent particle fluxes peak near the last closed flux surface (LCFS) at ≈4×1021 s−1 in L-mode and are suppressed to ≈0.2×1021 s−1 in H mode (80%–90% lower) mostly due to a reduction in density fluctuation amplitude and of the phase between density and radial velocity fluctuations. The radial particle fluxes are consistent with particle inventory based on SOLPS fluid modeling. A strong intermittent component is identified. Hot, dense plasma filaments 4–10 cm in diameter, appear first ∼2 cm inside the LCFS at a rate of ∼1×1021 s−1 and leave that region with radial speeds of ∼3–5 km/s, decaying as they travel through the SOL, while voids travel inward toward the core. Profiles of normalized fluctuations feature levels of 10% inside LCFS to ∼150% at the LCFS and SOL. Once properly normalized, the intermittency in NSTX falls in similar electrostatic instability regimes as seen in other devices. The L-H transition causes a drop in the intermittent filaments velocity, amplitude and number in the SOL, resulting in reduced outward transport away from the edge and a less dense SOL.

Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation

“…From experiments on magnetically confined plasmas, it is commonly found that the average profile is indeed exponential in the far SOL with a scale length which increases with increasing line-averaged density. [17][18][19][20][21][22][23][24][25][26][27][28][29]32,33 It has also been found that this profile broadening is accompanied by larger average radial blob velocities. 29,32,33,65 The stochastic model presented here clearly reveals how such an increase in the pulse velocity leads to broader profiles in the SOL.…”

“…The wave-form for largeamplitude bursts in the time series is well described by an exponential function. [11][12][13][14][15][23][24][25][26][27][28][29][30][31][32] The present study incorporates these features in a stochastic model for intermittent plasma fluctuations in the SOL, described as a super-position of uncorrelated pulses. [34][35][36][37][38][39] This model explains many of the salient experimental findings and empirical scaling relations, including broad plasma profiles and large fluctuation levels, skewed and flattened probability density functions, and a parabolic relation between the skewness and flatness moments.…”

“…[11][12][13][14][15] The turbulence-driven transport results in broad plasma profiles in the SOL and enhanced levels of plasma--wall interactions that may be an issue for the next generation plasma confinement experiments and future fusion power reactors. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The radial motion of blob-like structures leads to largeamplitude bursts in single-point measurements. The statistical properties of these bursts in the SOL of several tokamak plasmas have recently been elucidated by the analysis of long data time series from probe measurements and gas puff imaging.…”

Ion temperature measurements of L-mode filaments in MAST by retarding field energy analyser

“…The main limitations of probes are the perturbation they can cause to the plasma and the recycling of ablated probe material which can affect plasma properties. Electrical probes have been used to study filaments in a number of tokamaks including Alcator C-Mod [7], ASDEX Upgrade (AUG) [8], DIII-D [9], JET [10,11], MAST [12], NSTX [13] and TCV [14].…”

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

“…[3][4][5] Multiple diagnostic systems are in use to study plasma turbulence, including probes and beam emission spectroscopy. [6][7][8][9] One of the most successful diagnostics to study SOL turbulence is gas-puff imaging (GPI), a technique which is based on imaging the line radiation emitted by a locally puffed neutral gas such as deuterium or helium, as its atoms are excited by the edge plasma. 3,10 There are GPI diagnostic systems currently installed on Alcator C-Mod, 11 NSTX, 12 EAST, 13 and TEXTOR.…”