Plasma fluctuations near the shear layer in the ATF torsatron

Abstract

Electrostatic turbulence has been investigated in the edge region of the Advanced Toroidal Facility (ATF). A reversal in the poloidal phase velocity of the fluctuations has been observed (velocity shear) which determines a characteristic plasma radius. The location of this shear layer depends on the magnetic configuration, the limiter radius and the plasma conditions. Using the shear position as a reference point, the density fluctuation levels in ATF (currentless stellarator) are very similar to those previously reported in TEXT (ohmically heated tokamak), suggesting that the plasma current is not an important drive for the edge turbulence. The drives for the turbulence appear to be different inside and outside the shear location (ashear), with T̃e/Te < ñ/n in the scrape-off layer (r/ashear > 1) and possibly larger T̃e/Te in the plasma edge edge (r/ashear < 1). There is a spatial decorrelation in the fluctuations at the shear location; this suggests that the poloidal shear flow has an important influence on the edge turbulence. The poloidal correlation length depends on local plasma parameters (e.g. velocity and temperature). When neutral beam injection is added, the high frequency components of ̃n increase.

Fluctuation-induced flux at the plasma edge in toroidal devices

“…The turbulent flux is measured by means of Langmuir probes and by using the experimental technique described in Ref. 6. The Ohmically heated TJ-I plasmas are characterized by a major radius Rϭ30 cm, minor radius aϭ10 cm, plasma current I p Ϸ30 kA, toroidal field B T Ϸ(1.0-1.4) T, and density n e Ϸ(1 -3)ϫ10 13 cm Ϫ3 .…”

“…1-3 A reversal in the phase velocity of the fluctuations ͑shear layer͒ has been seen in the plasma boundary region in tokamaks and stellarators. 1,4,5 The shear layer location determines a characteristic plasma radius 6 and provides a convenient way to compare the structure of the fluctuations in different magnetic confinement devices.…”

In Search of the Elusive Zonal Flow Using Cross-Bicoherence Analysis

“…Regarding the experimental evidence for zonal flows, a reversal in the poloidal phase velocity of fluctuations has been observed in the edge of plasma tokamaks, stellarators, and reversed field pinch confinement devices [11][12][13]. In this region, the plasma potential has a maximum (i.e., the radial electric field reverses direction) and the phase velocity is generally controlled by the E 3 B velocity.…”

“…Indeed, nonlinear properties of plasma turbulence have been investigated in the boundary of fusion plasmas, providing a link between experimental measurements and turbulence models [4][5][6][7][8]. Bispectral analysis tools have been used to study the strength of nonlinear mechanisms in the proximity of the velocity shear layer, using data from the Advanced Toroidal Facility (ATF) [12] and during L-H transition in the W7-AS stellarator [6,7]. During L-H transition scenarios in the W7-AS stellarator, there is a relatively small enhancement of the stored energy with a concomitant reduction (but without disappearance) in the level of fluctuations [6].…”

Experimental evidence of three-wave coupling on plasma turbulence

“…Edge turbulence was studied using Langmuir probes and the experimental techniques described elsewhere [9,10]. Edge fluctuations have been characterized by the Langmuir probe ion satu-ration current, I s ^ n e (T e ) 1/2 , and by the floating potential, <t>f = <P P -aT e , where (/> p is the plasma potential, T e is the electron temperature, and a« 3 for hydrogen plasmas [11].…”