Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas

Abstract

The neoclassical transport optimization of the Wendelstein 7-X stellarator has not resulted in the predicted high energy confinement of gas fueled electron-cyclotron-resonance-heated (ECRH) plasmas as modelled in (Turkin et al 2011 Phys. Plasmas 18 022505) due to high levels of turbulent heat transport observed in the experiments. The electron-turbulent-heat transport appears non-stiff and is of the electron temperature gradient (ETG)/ion temperature gradient (ITG) type (Weir et al 2021 Nucl. Fusion 61 056001). As a result, the electron temperature T e can be varied freely from 1 keV-10 keV within the range of P ECRH = 1-7 MW, with electron density n e values from 0.1-1.5 × 10 20 m −3 . By contrast, in combination with the broad electron-to-ion energy-exchange heating profile in ECRH plasmas, ion-turbulent-heat transport leads to clamping of the central ion temperature at T i ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of n e = 7 × 10 19 m −3 , an apparent 'negative ion temperature profile stiffness' was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇T i /T i decreases with increasing ion heat flux. The experiment was conducted in helium, which has a higher radiative density limit compared to hydrogen, allowing a broader power scan. This 'negative stiffness' is due to a strong exacerbation of turbulent transport with an increasing ratio of T e /T i in this electron-heated plasma. This finding is consistent with electrostatic microinstabilities, such as ITG-driven turbulence. Theoretical calculations made by both linear and nonlinear gyro-kinetic simulations performed by the GENE code in the W7-X three-dimensional geometry show a strong enhancement of turbulence with an increasing ratio of T e /T i . The exacerbation of turbulence with increasing T e /T i is also found in tokamaks and inherently enhances ion heat transport in electron-heated

Confinement in electron heated plasmas in Wendelstein 7-X and ASDEX Upgrade; the necessity to control turbulent transport

“…As the ECRH power increases in the hydrogen plasmas, the ratio of T e /T i (figure 5(a)) increases strongly but the increase of Q i (figure 5(b)) is only modest for the higher density plasmas and null for the lower density. This is due to the strong T −3/2 e dependence in (1). Moreover, the ratio of Q i /Q gB (figure 5(c)), relevant for micro turbulence, is even stronger suppressed due to the T 5/2 i dependence in equation ( 2).…”

Coarse-grained gyrokinetics for the critical ion temperature gradient in stellarators

“…Introduction.-The excitation of the ion temperature gradient (ITG) mode in magnetically confined fusion devices leads to turbulence that is responsible for energy losses, which deteriorate plasma confinement. For instance, it has been argued that, during operation of the Wendelstein 7-X (W7-X) stellarator in electron heating scenarios, the ITG mode leads to so-called "ion temperature clamping" [1], thus preventing the heating of ions in the plasma core above 2 keV. In order to lessen the negative effects of the ITG mode, a possible strategy to follow involves the manipulation of the density and electron profiles, as implemented in W7-X using pellet injections [2].…”

Seeking turbulence optimized configurations for the Wendelstein 7-X stellarator: ion temperature gradient and electron temperature gradient turbulence

“…Ion temperature gradient turbulence is found in the W7-X experiment (Carralero et al 2021), and is responsible for strong turbulent heat fluxes, especially for scenarios involving ECRH power. First, we consider the linear simulations shown in figure 2(a).…”

“…This feature has been attributed to the strong stiffness of the turbulence transport driven by the ion temperature gradient (ITG) (Beurskens et al. 2021).…”

“…During the last campaign of the W7-X experiment, it has been observed that for electron cyclotron resonance heated (ECRH) plasmas, the confinement time is similar to that predicted by the International Stellarator Scaling 2004 (ISS04) scaling law (Weller et al 2009), and the ion temperature does not rise beyond an upper limit, of around 1.5 keV, independently of the input power. This feature has been attributed to the strong stiffness of the turbulence transport driven by the ion temperature gradient (ITG) (Beurskens et al 2021).…”