Nonlinear δf particle in cell gyrokinetic simulations of the microtearing mode

Chowdhury, Jugal; Chen, Yang; Parker, Scott

doi:10.1063/1.5141353

Cited by 5 publications

(3 citation statements)

References 83 publications

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“…The growth rates of the KBM modes increase with β; these growth rates peak at long wavelength, k y ρ s ∼ 0.15 but the growth rate in the long wavelength limit is not much lower than the peak growth rate, as in the density-gradient driven case. For all but the lowest β case, the vector potential of the modes in the electron diamagnetic direction exhibit even parity (consistent with tearing modes [52][53][54]) and are therefore microtearing modes [52,[55][56][57][58]. A comparison for A ∥ in the ballooning space is shown in figure 6 for β = 8.1% and for k y ρ s = 0.414 and k y ρ s = 0.77 which exhibit real frequency respectively in the ion and electron direction.…”

Section: Linear Simulations Two-species Casesmentioning

confidence: 76%

Low n electromagnetic modes in spherical tokamaks

Chowdhury

McMillan

2021

Plasma Phys. Control. Fusion

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The performance of spherical tokamak reactors depends on plasma β, and an upper limit is set by long-wavelength kinetic ballooning modes (KBMs). We examine how these modes become unstable in spherical-tokamak reactor relevant plasmas, which may contain significant fast-ion pressure. In a series of numerically generated equilibria of increasing β, the KBM becomes unstable at sufficiently high plasma β, and for such cases, it is also significantly unstable even in the long-wavelength limit. The β threshold for the KBMs is similar to the ideal Magnetohydrodynamics (MHD) threshold, and in cases without fast ions, their frequencies are as predicted by diamagnetic-drift stabilised MHD. To isolate and explore the KBMs, simulations are performed where the pressure gradient is entirely due to the density profile, or entirely due to the temperature profile; the resulting KBMs have similar properties in the long-wavelength regime. The introduction of energetic ions restricts the KBMs to longer wavelengths, and reduces the β threshold somewhat; for parameter regimes of current-day devices, this is such long wavelength that a global analysis would become necessary. Mode frequencies in plasmas with a significant fast particle population are seen to be controlled by fast particle precession frequencies.

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Section: Linear Simulations Two-species Casesmentioning

confidence: 76%

Low n electromagnetic modes in spherical tokamaks

Chowdhury

McMillan

2021

Plasma Phys. Control. Fusion

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“…Possible explanations include (1) a MTM localized at the pedestal top where both the Doppler shift and ω e * are much smaller (note that pedestal-top MTMs have been described in Ref. [22,31,32,21]), and (2) a KBM or ITG mode with ion frequency in the plasma frame.…”

Section: Magnetic Fluctuation Datamentioning

confidence: 99%

“…Subsequent work [11,12] elucidated the role of the parallel wavenumber, k || , the separate influence of density and temperature gradients, and the possibility of collisionless variations on the theme [13,14,15,16]. Gyrokinetic simulations have further explored the nonlinear saturation physics [17,18,19], and its role in spherical tokamaks (both core [20,19,21], and edge [22,23]). In an interesting recent development, Ref.…”

Section: Introductionmentioning

confidence: 99%

Microtearing modes as the source of magnetic fluctuations in the JET pedestal

Hatch,

Kotschenreuther,

Mahajan

et al. 2020

Preprint

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We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data, to establish the physical basis for their origin, role, and distinctive characteristics.We demonstrate quantitative agreement between gyrokinetic simulations of microtearing modes (MTMs) and two magnetic frequency bands with corresponding toroidal mode numbers n=4 and 8. Such disparate fluctuation scales, with substantial gaps between toroidal mode numbers, are commonly observed in pedestal fluctuations. Here we provide a clear explanation, namely the alignment of the relevant rational surfaces (and not others) with the peak in the ω * profile, which is localized in the steep gradient region of the pedestal. We demonstrate that a global treatment is required to capture this effect. Nonlinear simulations suggest that the MTM fluctuations produce experimentally-relevant transport levels and saturate by relaxing the background electron temperature gradient, slightly downshifting the fluctuation frequencies from the linear predictions. Scans in collisionality are compared with simple MTM dispersion relations. At the experimental points considered, MTM growth rates can either increase or decrease with collision frequency depending on the parameters thus defying any simple characterization of collisionality dependence.

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Finite β effects on short wavelength ion temperature gradient modes

et al. 2020

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The electromagnetic effect is studied on the short wavelength branch of the ion temperature gradient mode in the linear regime for the first time using a global gyrokinetic model. The short wavelength ion temperature gradient mode growth rate is found to be reduced in the presence of electromagnetic perturbations at finite plasma β. The effect on real frequency is found to be weak. The threshold value of ηi is found to increase for the mode as the magnitude of β is increased. The global mode structure of the short wavelength branch of the ion temperature gradient mode is compared with the conventional branch. The magnetic character of the mode, measured as the ratio of mode average square values of electromagnetic potential to electrostatic potential, is found to increase with increasing values of the plasma β. The mixing length estimate for flux shows that the maximum contribution still comes from the long wavelengths modes. The magnitude of the flux decreases with increasing β.

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Nonlinear δf particle in cell gyrokinetic simulations of the microtearing mode

Cited by 5 publications

References 83 publications

Low n electromagnetic modes in spherical tokamaks

Low n electromagnetic modes in spherical tokamaks

Microtearing modes as the source of magnetic fluctuations in the JET pedestal

Finite β effects on short wavelength ion temperature gradient modes

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