Electron thermal conductivity from heat wave propagation in Wendelstein 7-AS

Abstract

Heat wave propagation experiments have been carried out on the Wendelstein 7-AS stellarator. The deposition of electron cyclotron resonance heating power is highly localized in the plasma centre, so that power modulation produces heat waves which propagate away from the deposition volume. Radiometry of the electron cyclotron emission is used to measure the generated temperature perturbation. The propagation time delay of the temperature perturbation as a function of distance to the power deposition region is used to determine the electron thermal conductivity χe. This value is then compared with the value determined by global power balance. In contrast to sawtooth propagation experiments in tokamaks, it is found that the value of χe from heat wave propagation is comparable to that calculated by power balance. In addition, inward propagating waves were produced by choosing a power deposition region away from the plasma centre. Experiments were carried out at 70 GHz in the ordinary mode and at 140 GHz in the extraordinary mode. Variations of the modulation power amplitude have demonstrated that the inferred value of χe is independent of the amplitude of the induced temperature perturbations

Electron Pressure Profiles in High-Density Neutral Beam Heated Plasmas in the Large Helical Device

“…Anyhow, it is much weaker than the temperature dependences of α = 1.5 in existing anomalous transport models such as the gyro- Bohm, ETG, and TEM, and even weaker than the Bohm model (α = 1). Therefore, our result does not contradict the small stiffness factor (χ HP ~ χ PB ), which rules out the strong temperature gradient dependence of χ, found in former studies in stellarators [19,21].…”

“…The ratio χ HP /χ PB is called the "stiffness parameter", since such a nonlinear dependence of χ can cause the stiffness. As for the stellarators, on the other hand, studies in Wendelstein 7-AS [18][19][20][21] have shown that no systematic discrepancy between χ HP and χ PB is found [19,21] and therefore strong temperature gradient dependence can be ruled out. The transient transport analysis has been also applied to LHD plasmas with an electron internal transport barrier (e-ITB) [22][23][24].…”

New evidence and impact of electron transport non-linearities based on new perturbative inter-modulation analysis

“…Hence, the Volterra series is only an approximation over time and not over space. The proper choice to define U (k) is to use the power dependent part of the perturbation u (t) = P (t) as has been used in the Taylor expansion in (5). The deposition profile is part of G (1) and G (2) as the input is purely the time dependent part of the perturbation.…”

“…These quantities are generally analyzed in 1D as a function of the dimensionless normalized minor radius ρ, which is permissible due to the device geometry, magnetic field configuration, and relevant time scales of transport [3]. This radial transport is then analyzed in steady-state [4] or in the transient phase, often using a perturbation [2,5,3,6].…”

Determination of electron thermal diffusivity at the WEGA stellarator

“…There are different diagnostics capable of measuring this temperature response, e.g. the electron cyclotron emission (ECE) diagnostic [1,3] and the soft X-ray array [2]. In this paper, however, we employ a fast bolometer system at WEGA for this purpose.…”