Inward thermodiffusive particle pinch in electron internal transport barriers in TCV

Abstract

Electron internal transport barriers (eITBs) are obtained in TCV with different heating and current drive schemes. They are sustained in steady-state conditions for several energy confinement and current redistribution times. In these scenarios, the density profile displays a different behaviour with respect to normal L-mode plasmas, with or without auxiliary heating. In fully noninductive discharges developing an eITB, the density profile shape is strongly correlated with the electron temperature profile, with the normalized density gradient equal to 0.45 times the normalized temperature gradient, revealing the existence of a significant inward pinch of a thermodiffusive type. The coupling of the two profiles is observed from the 'foot' of the barrier inwards. The effect of small inductive current perturbations on fully non-inductive sustained eITBs shows that ∇n/n is only indirectly coupled to the current profile, through its effect on local confinement, contrary to the standard L-mode.

Non-linear gyrokinetic simulations of microturbulence in TCV electron internal transport barriers

“…The former study, based on mixing length estimates, has shown that the stabilization of trapped electron modes (TEMs) by (s − α) effects provides an explanation for the reduced heat transport in the barriers. The latter study enabled us to show that the stabilization of TEMs can furthermore explain the significant electron thermo-diffusive pinch and resulting density peaking observed, albeit with strong central electron heating [6]. More specifically, the quasi-linear model presented in [11] has shown how the interplay of ion temperature gradient (ITG) and TEMs may lead to a zero electron particle flux for the correct combination (R/L n e , R/L T e , R/L T i ) of normalized electron density, electron temperature and ion temperature gradients, respectively.…”

“…Such electron barriers have been systematically obtained in TCV under conditions of low or negative magnetic shearŝ, leading simultaneously to sharp density and electron temperature gradients [3][4][5][6][7]. Although heating (in particular, electron cyclotron resonance heating, ECRH) is applied to the core of the plasma, no external particle fueling is provided to this central region in these discharges.…”

“…Although heating (in particular, electron cyclotron resonance heating, ECRH) is applied to the core of the plasma, no external particle fueling is provided to this central region in these discharges. The e-ITBs are thus maintained during the discharge over hundreds of diffusion confinement times with a zero radial electron particle flux, which in itself is a remarkable feature of these barriers [6].…”

“…It has been shown experimentally that the degree of magnetic shear reversal is the key to the formation and sustainment of the e-ITBs and furthermore that they can be maintained with zero loop voltage [3][4][5]. One can thus conclude that the improved density gradient in these barriers is not due to a Ware pinch effect [6]. Simulations of TCV e-ITBs using the ASTRA transport code have confirmed that the confinement improvement indeed increases with an increased negative magnetic shear profile [7].…”

Understanding the core density profile in TCV H-mode plasmas

“…The density peaking in the C-Mod ITBs was thought to result from the Ware pinch, with the ITB density gradient during electron heating determined by the balance of Ware pinch and TEM turbulent fluxes, thus acquiring a strong inverse dependence on temperature. In TCV electron ITBs, where transport barriers are seen mainly on the electron temperature profile [5], the density peaking results from a strong thermodiffusive pinch [6] component which can be explained [3] using the quasi-linear model that shall be used in this paper .…”