Effect of pressure-driven MHD instabilities on confinement in reactor-relevant high-beta helical plasmas

Abstract

Through the experiment data analysis in the large helical device (LHD), the influence of the global MHD instability and the relatively short wave length MHD instabilities driven turbulence on the confinement performance in reactor-relevant high-beta helical plasmas is studied. The comparison of the energy confinement time between just before global MHD instability disappears and after that, and the estimation of the saturated mode structure by the multi-channel soft x-ray measurement enable us to quantitatively estimate the influence of the global interchange type MHD instability with different saturated mode structures on the confinement performance. According to the comparison between thermal conductivities in experiments and those predicted by theoretical transport models, the transport properties in the peripheral region of high beta LHD plasmas are quite similar with anomalous transport model based on an interchange type MHD instability driven turbulence, and that result is supported by the dependence of the density fluctuation with relatively short wave length on beta value.

Characteristics of magnetic island formation due to resistive interchange instability in helical plasma

“…In the LHD experiments, we observed 5%-40% radial displacement normalized by the minor radius for the interchange instability. 1,13 Also, the MHD activities are observed around 10 5 < S < 10 7 . 1,2 It can be seen from Fig.…”

“…1 In this section, we investigate these effects for the first, the second, and the third eigenfunctions. Figure 5 shows the linear growth rates of each eigenfunction as a function of the S number for various b plasmas with 2% < b c < 5%.…”

“…Moreover, due to the presence of the fluctuations in the marginally unstable plasma, an apparent degradation in the electron temperature gradients and in the confinement performance is observed. 1 On the other hand, the characteristics of the resistive interchange instability have been studied from the viewpoint of the dependence of the linear growth rate and the width of the radial perturbed profile on the b and the S number, and from the aspect of their nonlinear saturation behavior as well. An early theoretical work on the resistive interchange instability was made by Furth et al 5 They clarified that the linear growth rate and the width of the radial profile are proportional to S À1=3 .…”

“…It is known that the resistive interchange instability causes a degradation of plasma confinement properties. 1 Therefore, it is one of the crucial issues for the realization of the fusion reactor to study the characteristics of the resistive interchange instability.…”

Identification of MHD Mode Structure using ECE and SX Measurements

“…In the Large Helical Device (LHD) [1], some magnetohydrodynamic (MHD) modes, including the interchange-type instability, excited in the edge region of a torus plasma have been observed in the high-beta regime [2], which does not lead to disruptive discharge termination. However, the instability is considered to be related to the decision mechanism of the achieved achieved beta values [3].…”