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 × 1020 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 × 1019 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 plasmas. This finding strongly affects the prospects of future high-performance gas-fueled ECRH scenarios in W7-X and imposes a requirement for turbulence-suppression techniques.
Highlights
The Wendelstein 7-X (W7-X) experiment [1,2,3] is a stellarator with predicted good equilibrium properties and high normalized pressures of up to β ∼ 5%
We first investigate how well the ion temperature clamping as seen in W7-X electron-heated plasmas can be explained by assuming gyroBohm-level turbulent transport without the additional ion temperature gradient (ITG) features described by the theory above
Ion temperature clamping in ECRH heated plasmas in W7-X is likely a result of various factors that lead to a reduction in the local ion temperature gradients and the global plasma performance
Summary
The Wendelstein 7-X (W7-X) experiment [1,2,3] is a stellarator with predicted good equilibrium properties and high normalized pressures of up to β ∼ 5%. This paper discusses a transport regime in which turbulent transport is not suppressed, and the performance is limited with respect to the ISS04 scaling This has been the case for almost all stationary plasmas in W7-X with ECRH heating and standard divertor- or main-chamber gas fueling. Counter to the post-pellet plasmas in [8], in which Ti > 3 keV was transiently achieved, the clamping of Ti appears virtually irrespective of the applied ECRH power and the electron density and electron temperature values obtained All of these plasmas feature flat to slightly peaked density profiles and no density-gradient-aided turbulence suppression.
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