Density Collapse Events Observed in the Large Helical Device

Ohdachi, S.; Sakamoto, R.; Miyazawa, J.; Morisaki, T.; Masuzaki, S.; Yamada, H.; Watanabe, K. Y.; Jacobo, V.R.; Nakajima, N.; Watanabe, F.; Takeuchi, Masaki; Toi, K.; Sakakibara, S.; Suzuki, Y.; Narushima, Y.; Yamada, I.; Mianami, T.; Narihara, K.; Tanaka, Kaoru; Tokuzawa, T.; Kawahata, K.

doi:10.1002/ctpp.200900051

Cited by 19 publications

(17 citation statements)

References 11 publications

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“…29 A candidate of the driving mechanism is the ballooning MHD instability, but it is still unclear. 33 In the high-central-beta scenario, the study of the MHD instability effect on the confinement is out of the scope of the present paper, and one of our future subjects.…”

Section: -8mentioning

confidence: 98%

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

et al. 2011

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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.

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Section: -8mentioning

confidence: 98%

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

et al. 2011

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“…The importance of the interplay between the ITG turbulence and zonal flows in the large helical device (LHD) (Kaneko et al 2013), which is a heliotron device, was reported by the means of electrostatic gyrokinetic simulations with adiabatic electrons (Watanabe et al 2008), and it is reported that a neoclassical optimization of the magnetic configuration leads to the reduction of heat transport through the enhancement of zonal flows. The LHD plasma is normally free from the current driven instability, while the pressure driven instabilities such as ballooning modes can degrade the confinement of plasmas in high-beta discharges (Ohdachi et al 2010), and the threshold of KBM is normally lower than that of the ideal MHD ballooning mode (Tang et al 1980;Hirose 1994).…”

Section: Cyclone Base Case (Cbc) Tokamakmentioning

confidence: 99%

Electromagnetic gyrokinetic simulation of turbulence in torus plasmas

et al. 2015

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Gyrokinetic simulations of electromagnetic turbulence in magnetically confined torus plasmas including tokamak and heliotron/stellarator are reviewed. Numerical simulation of turbulence in finite beta plasmas is an important task for predicting the performance of fusion reactors and a great challenge in computational science due to multiple spatio-temporal scales related to electromagnetic ion and electron dynamics. The simulation becomes further challenging in non-axisymmetric plasmas. In finite beta plasmas, magnetic perturbation appears and influences some key mechanisms of turbulent transport, which include linear instability and zonal flow production. Linear analysis shows that the ion-temperature gradient (ITG) instability, which is essentially an electrostatic instability, is unstable at low beta and its growth rate is reduced by magnetic field line bending at finite beta. On the other hand, the kinetic ballooning mode (KBM), which is an electromagnetic instability, is destabilized at high beta. In addition, trapped electron modes (TEMs), electron temperature gradient (ETG) modes, and micro-tearing modes (MTMs) can be destabilized. These instabilities are classified into two categories: ballooning parity and tearing parity modes. These parities are mixed by nonlinear interactions, so that, for instance, the ITG mode excites tearing parity modes. In the nonlinear evolution, the zonal flow shear acts to regulate the ITG driven turbulence at low beta. On the other hand, at finite beta, interplay between the turbulence and zonal flows becomes complicated because the production of zonal flow is influenced by the finite beta effects. When the zonal flows are too weak, turbulence continues to grow beyond a physically relevant level of saturation in finite-beta tokamaks. Nonlinear mode coupling to stable modes can play a role in the saturation of finite beta ITG mode and KBM. Since there is a quadratic conserved quantity, evaluating nonlinear transfer of the conserved quantity from unstable modes to stable modes is useful for understanding the saturation mechanism of turbulence.

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“…In the super-density core plasmas, which were obtained by repetitive pellet injection in the outward shifted LHD configurations, core density collapse phenomena are observed (Ohyabu et al 2006;Sakamoto et al 2007). From the MHD analysis for such plasmas, it is found that ideal ballooning modes with γ τ a ∼ 0.1 are unstable (Ohdachi et al 2010(Ohdachi et al , 2017. Since the linear growth rate is sufficiently larger than the precession drift frequency of the deeply trapped ions, i.e.…”

Section: Discussionmentioning

confidence: 99%

Ion kinetic effects on linear pressure driven magnetohydrodynamic instabilities in helical plasmas

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2020

J. Plasma Phys.

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The linear MHD (magnetohydrodynamic) stability for high beta plasmas in the inward shifted Large Helical Device (LHD) configurations has been investigated for a wide range of magnetic Reynolds numbers $S$ using numerical simulations based on the kinetic MHD model with kinetic thermal ions where the beta is the ratio of the plasma pressure to the magnetic pressure. It is found that the dependence of the linear growth rate of the resistive ballooning modes on the $S$ number changes from $\unicode[STIX]{x1D6FE}\propto S^{-1/3}$ to $\unicode[STIX]{x1D6FE}\propto S^{-1}$ by the kinetic thermal ion effects so that the resistive ballooning modes are significantly suppressed as the $S$ number increases. For a high $S$ number comparable to experimental values, the most unstable modes are interchange modes. The kinetic thermal ion effects change the most unstable interchange mode from the ideal mode to the resistive mode. This transition of the interchange modes by kinetic thermal ion effects is consistent with the shift of the marginal stability boundary for the ideal interchange modes observed in the LHD experiments.

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Density Collapse Events Observed in the Large Helical Device

Cited by 19 publications

References 11 publications

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

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

Electromagnetic gyrokinetic simulation of turbulence in torus plasmas

Ion kinetic effects on linear pressure driven magnetohydrodynamic instabilities in helical plasmas

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