Dissipation in the magnetic turbulence of reversed field pinch plasmas

Titus, James; Almagri, A. F.; Terry, P. W.; Sarff, J. S.; Mezonlin, Ephrem; JohnsonIII, J. A.

doi:10.1063/5.0044808

Cited by 3 publications

(1 citation statement)

References 34 publications

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“…Nonlinear interactions involving tearing modes occur not only between global-scale modes but also across an extended range of scales, as directly observed and inferred in a number of experiments [18][19][20]. As tearing modes cascade to small scales, they continue to interact with MHD fluctuations.…”

Section: Introductionmentioning

confidence: 67%

Global linear and nonlinear gyrokinetic simulations of tearing modes

Jitsuk,

Di Siena,

Pueschel

et al. 2024

Nucl. Fusion

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To better understand multi-scale interactions between global tearing modes and microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch (RFP), the global gyrokinetic code GENE is modified to describe global tearing mode instability via a shifted Maxwellian distribution consistent with experimental equilibria. The implementation of the shifted Maxwellian is tested and benchmarked by comparisons with different codes and models. Good agreement is obtained in code-code and code-theory comparisons. Linear stability of tearing modes of a non-reversed MST discharge is studied. A collisionality scan is performed to the lowest order unstable modes (n=5, n=6) and shown to behave consistently with theoretical scaling. The nonlinear evolution is simulated, and saturation is found to arise from mode coupling and transfer of energy from the most unstable tearing mode to small-scale stable modes mediated by the m=2 tearing mode. The work described herein lays the foundation for nonlinear simulation and analysis of the interaction of tearing modes and gyroradius-scale instabilities in RFP plasmas.

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Section: Introductionmentioning

confidence: 67%

Global linear and nonlinear gyrokinetic simulations of tearing modes

Jitsuk,

Di Siena,

Pueschel

et al. 2024

Nucl. Fusion

Self Cite

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Dissipation in the magnetic turbulence of reversed field pinch plasmas

et al. 2021

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Reversed field pinch (RFP) plasmas are subject to tearing instability that creates a broad spectrum of magnetic fluctuations. The dominant fluctuations have poloidal and toroidal mode numbers (m,n)=(1,6−10) and can grow to 2–3% of the mean magnetic field. Through nonlinear coupling, this growth culminates in a strong reconnection event and broadening of the magnetic spectrum extending to the ion gyroradius scale. Multiple developments occur during the reconnection stage: ions and electrons are energized, magnetic fluctuation amplitudes increase, plasma flow is halted, and the toroidal magnetic flux increases in a sawtooth-like fashion as the RFP dynamo becomes stronger. Magnetic fluctuations are measured in the plasma edge at multiple radial locations from r/a = 0.75 to 0.96 to assess and characterize the magnetic turbulence. The measured spectrum perpendicular to the mean field, S(k⊥), can be fit to a model spectrum consisting of power-law and exponential component with one free parameter that characterizes dissipation. The measured dissipation is much larger than estimated from classical viscous or resistive dissipation, but it is consistent with a flow damping measurement of anomalous viscosity. The measurements show an evolution of the spectrum during which fluctuation power builds up in the smallest wavenumbers and cascades to the larger wavenumber due to the nonlinear coupling between the linear (m, n) = (1, > 6) and the nonlinear (m, n) = (0, 1) tearing modes.

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Estimates of the wavenumber wavelet power spectrum of magnetic fluctuations during magnetic reconnection

Titus,

Almagri,

DeHaas

2023

Physics of Plasmas

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Fluctuation analyses of experimental observations generally lack high temporal resolution and are in frequency-space f, contrary to theoretical efforts in wavenumber-space k. This is due to the inherent limits of the Fourier transform, though it is prominent due to the ease of diagnostic implementation. Advances in wavelet-based analysis have provided relief due to its temporal resolution, but in its common use, is still hard to compare to theoretical models. By using the two-point correlation technique in conjunction with large data sets, a wavelet power spectrum in wavenumber-space can be created. Dubbed the wavenumber wavelet power spectrum, this spectrum relates wavenumber to power in time. This analysis technique more closely connects characterizations of experimentally observed fluctuations with other system parameters and theoretical predictions. In this article, we develop the wavenumber wavelet power spectrum using magnetic fluctuations caused by tearing instability driven magnetic reconnection in reproducible, high temperature laboratory plasmas. These dynamic magnetic fluctuations generated in reversed field pinch plasmas are broadband, ranging from the low frequency, 10's of kHz, up to the ion gyroradii frequencies, 100's of kHz. The dominant fluctuations have poloidal and toroidal mode numbers (m,n)=(1,6−10) and can grow to 2%–3% of the mean magnetic field. During these reconnection events, ions, and electrons are energized, magnetic fluctuation amplitudes increase, plasma flow is halted, and the toroidal magnetic flux increases, all on a semi-periodic basis. The newly developed spectrum provides better temporal resolution of spectrum characteristics to correlate with these particle energization phenomena.

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Dissipation in the magnetic turbulence of reversed field pinch plasmas

Cited by 3 publications

References 34 publications

Global linear and nonlinear gyrokinetic simulations of tearing modes

Global linear and nonlinear gyrokinetic simulations of tearing modes

Dissipation in the magnetic turbulence of reversed field pinch plasmas

Estimates of the wavenumber wavelet power spectrum of magnetic fluctuations during magnetic reconnection

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