Suppression of Temperature Fluctuations and Energy Barrier Generation by Velocity Shear

Abstract

First measurements of temperature fluctuations in a region of high velocity shear show that absolute and normalized fluctuation levels are reduced across the shear layer, a result that is consistent with weak parallel electron thermal conduction in the electron temperature dynamics. The concomitant reduction of temperature, density, and electric field fluctuations reduces the anomalous conducted and convected heat fluxes.PACS numbers: 52.35. Ra, 52.25.Gj, 52.55.Fa The presence of localized flow shear in fluids, in general, and plasmas, in particular, can reduce turbulence levels and cross-flow transport, creating a transport barrier. Such barriers develop spontaneously in a wide variety of magnetically confined plasmas, producing transitions from a low energy confinement state (L mode) to a higher energy confinement state (H mode) [1]. The L-H transition in magnetically confined plasmas is accompanied by a reduction of turbulence levels [2], confirming the paradigm that microturbulence is responsible for a considerable part of the energy and particle losses [3]. Behavior similar to that of the spontaneous L-H transition is obtained by applying an external radial electric field to the plasma using an electrode [4], indicating that the radial electric field and resulting E 3 B rotation play a crucial role. The shearing of turbulent eddies by differential rotation has been proposed as a universal mechanism to stabilize turbulence in plasmas [5] ( E 3 B generated), but it is also postulated to operate in nonionized fluids, such as two-dimensional Navier-Stokes turbulence and the stratosphere [6]. Electrode bias experiments have clarified the role of the radial electric field and its bifurcation [7] in L-H transitions by correlating the externally applied electric fields with the reduction in turbulence levels [8], formation of a transport barrier, and resulting confinement improvement. The E 3 B velocity shear in plasmas can affect nonlinearly saturated turbulence and reduce transport by acting on both the amplitude of the fluctuations and the phase between the density and potential fluctuations [9]. The shearing rate, v E3 B , must be comparable to Dv D , the nonlinear turbulence decorrelation rate in the absence of shear. Turbulence suppression is achieved in nonlinear simulations when the rate v E3 B ͑v E3 B dV E3 B ͞dr͒ is of the order of the linear growth rate v ins of the dominant mode in the plasma [10]. Turbulent decorrelation theory [9] predicts suppression of arbitrary advected fluctuations, including temperature, under E 3 B shearing. Predictions are consistent with observations of reductions in density and potential fluctuations [10] in particle transport barriers, but no direct measurements exist quantifying temperature fluctuations in a transport barrier. We report those measurements in this paper and examine the associated heat fluxes in light of recent theoretical work [9] that has indicated that if parallel thermal conduction is strong, flow shear can reduce the particle flux, but have little eff...

Transport by intermittent convection in the boundary of the DIII-D tokamak

“…It can therefore be stated that the intermittency, and its response to sheared electric fields that decorrelate turbulent transport resulting in reduced transport and enhanced confinement conditions, has been indirectly characterized extensively in many devices such as CCT, 30,31 PBX-M, 32,33 DIII-D, 34 TEXT, 35,36 and TEXTOR. 37,38 Density profiles from Thomson scattering in the SOL, normalized to the value at the LCFS, are shown for L-and H-mode discharges in Fig. 14, showing that there is a clear difference in the profile decay length near the LCFS ͑0.5 cm H-mode to 1.3 in L-mode͒, but that in the far SOL both profiles are quite flat and the particle density is significant.…”

Transport by intermittency in the boundary of the DIII-D tokamak

“…Since ⌫ r has been characterized extensively in many devices [35][36][37][38] and in particular, DIII-D, 39 the Texas Experimental Tokamak 40 ͑TEXT͒, [41][42] and the Torus Experiment for Technology Oriented Research 43 ͑TEXTOR͒ 44,45 it can therefore be stated that the general properties of the intermittency, such as its response to sheared electric fields that decorrelate turbulent transport resulting in reduced transport and enhanced confinement conditions, have been indirectly characterized.…”

Edge transport barriers in magnetic fusion plasmas

“…Fig. 3 shows Langmuir probe measurements from TEXTOR indicating a clear reduction in the temperature fluctuations with increased shear in E r applied by an electrode inserted at the plasma edge [62]. Further experimental results from TEXTOR [63,64] show that the changes in electron density and improvements in [54].…”

“…However, edge transport barriers have also been produced by directly biasing the plasma edge with an electric field applied through an electrode as in CCT [59] TUMAN-3 [60] and TEXTOR [61][62][63]. Fig.…”