Compressible magnetohydrodynamic sawtooth crash

Sugiyama, L.

doi:10.1063/1.4865571

Cited by 9 publications

(25 citation statements)

References 62 publications

(80 reference statements)

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“…Influenced by the Kadomtsev model, the fast temperature drops observed in many experiments have almost universally been assumed to be caused by fast magnetic reconnection, and a number of numerical studies have been published reporting to observe this fast reconnection in the simulation of a sawtooth event caused either by anomalous electron viscosity [15], two-fluid effects [16][17][18], high-n ballooning modes [19], plasmoids [20], or plasma compressibility [21]. However, a common feature of these studies is that they only simulate a single sawtooth event, and the initial conditions are such that the central safety factor is much less than unity so that the configuration is strongly unstable from the beginning of the simulation.…”

Section: The Kadomtsev Modelmentioning

confidence: 99%

A new explanation of the sawtooth phenomena in tokamaks

2020

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The ubiquitous sawtooth phenomena in tokamaks are so-named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974, it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new theory for the sawtooth phenomena in auxiliary heated tokamaks that is motivated by our recent understanding of "magnetic flux pumping". In this theory, the role of the (m, n) = (1, 1) mode is to generate a dynamo voltage which keeps the central safety factor, q0, just above 1.0 with low central magnetic shear. When central heating is present, the temperature on axis will increase until at some point, the configuration abruptly becomes unstable to ideal MHD interchange modes with equal poloidal and toroidal mode numbers, m = n > 1. It is these higher order modes and the localized magnetic stochasticity they produce that cause the sudden crash of the temperature profile, not magnetic reconnection. Long time 3D MHD simulations demonstrate this phenomena, which appears to be consistent with many experimental observations.

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Section: The Kadomtsev Modelmentioning

confidence: 99%

A new explanation of the sawtooth phenomena in tokamaks

2020

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“…New types of 1/1 instabilities with n = 1 helical density concentrations inside and around the q = 1 surface have been observed in recent experiments [3][4][5]. Nonlinear MHD numerical simulations [3,4,6,7] carried out with the extended MHD initial value code M3D [8,9] demonstrate that these 'snake' density quasi-steady states are inherently nonlinear and dynamic. They are strongly influenced by the toroidal mode coupling of poloidal harmonics m due to R = R o + r cos θ and by several types of nonlinear mode coupling, which are enhanced by compressible ∇ • v = 0 effects.…”

Section: Introductionmentioning

confidence: 86%

“…Large scale numerical simulations show that compressible MHD at low resistivity, applied to realistic plasma configurations (e.g. [7][8][9]), explains many experimentally observed properties of macroscopic nonlinear instabilities in a torus surprisingly well, considering the simplicity of the physics. Nonlinearity plays a major role.…”

Section: Introductionmentioning

confidence: 98%

Mode coupling and aspect ratio effects on low and high-nplasma instabilities

Sugiyama

2015

Nucl. Fusion

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In magnetically confined toroidal plasmas such as tokamaks, magnetohydrodynamic (MHD) instabilities experience strong toroidal and nonlinear mode coupling effects. Resistive MHD simulations with the M3D code show the importance of mode coupling and compressible MHD effects, which contribute to stronger mode coupling. For the m/n = 1/1 internal kink mode and sawtooth crash and for the edge localized mode (ELM) at higher n, MHD reproduces many features of the experimental observations, including the fast sawtooth crash and the moderate n ∼ 10 toroidal harmonics of the ELM. A general property of the perpendicular momentum equation in toroidal fusion plasmas is that the unbalanced radial forces remain relatively small, so that the terms that are lowest order in small inverse aspect ratio mostly cancel. The higher order terms then have significant effects, even at small r/Ro and small amplitude. Effects are strongest for the lowest toroidal harmonics n ≃ 1 and the most strongly driven ones with highest amplitude. Unlike the n = 1 internal kink mode, the small amplitude ELM ballooning/peeling-type mode, and thus ELM MHD marginal stability, may be reasonably described by the lowest order in aspect ratio, for moderate and large n ≳ 10. The ELM crash, however, depends on higher order.

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“…An extended magnetohydrodynamics (MHD) code M3D [Park, et al (1999)] includes separate density and temperature, over the entire region toroidal simulation [Sugiyama and Park (2000); Sugiyama (2008); Sugiyama (2013); Sugiyama (2014)]. The simulations show that Edge Localized Modes (ELMs) in high temperature, toroidal fusion plasmas a rise from a nonlinear plasma instability.…”

Section: Resistive Wall Modesmentioning

confidence: 99%

Alfvén Cavity Modes, Fast Ions, Alpha Particles and Diagnostic Neutral Beams

Horton¹,

Benkadda²

2015

ITER Physics

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ITER will have a high-energy population of the fusion product alpha particles with a velocity distribution function f α (v, r) called the slowing-down distribution. At each radius r, or more precisely, on each magnetic flux surface Ψ, the distribution in energy E = 1 2 m α v 2 of the particles is derived from their birthrate of the 3.5 MeV alpha particles produced by nuclear reaction rate σv DT times the product of the local deuterium and tritium densities [Bosch and Hale (1992)]. After their birth, the alpha particles slow down and scatter in their direction from the Coulomb collisions with the thermal plasma. In contrast, the 14.1 MeV neutrons born with the alpha particles travel in straight lines through the chamber wall into the outer boron-lithium blanket designed for breeding new tritium atoms for maintaining the tritium fuel supply. The blanket also converts the neutron kinetic energy into thermal power to drive the electric generators for fusion electric power. ITER will test tritium breeding blanket designs, but the conversion of the thermal energy into electric power awaits the next machine called DEMO.The phase-space distribution of the magnetically-trapped alpha particles is of primary interest since this distribution function is the source of a high-frequency ioncyclotron wave and the low-frequency Alfvén eigenmodes. The tritium experiments in JET [Keilhacker, et al. (1999)] showed the presence of both these instabilities. The high-frequency, ion-cyclotron waves were easily observed in the first DT experiments on TFTR (1996) and then more clearly on JET [Jet Team (1998)]. The power at the fundamental cyclotron frequency of the alpha particle, which is close to the deuteron cyclotron frequency, showed peaks in the wave frequency power spectrum with several cyclotron harmonics excited by the alpha particle distribution function. The amplitude of the fundamental wave at approximately 18 MHz increased linearly with the alpha-particle power as reported in the IAEA publication for TFTR discharges with the progression of 1.8, 4.7 and 8.9 MW of neutral beam heating power.The alpha particle density was inferred from measurements of the spectrum of the knock-on deuterons produced in the elastic nuclear collisions of the Z = 2 alpha particles with the plasma ions D + and T + . The measurements of the spectrum used the neutral particle analyzer that collected the D and T atoms leaving the 47 ITER Physics Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 10/03/15. For personal use only. ITER Physics Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 10/03/15. For personal use only.

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Compressible magnetohydrodynamic sawtooth crash

Cited by 9 publications

References 62 publications

A new explanation of the sawtooth phenomena in tokamaks

A new explanation of the sawtooth phenomena in tokamaks

Mode coupling and aspect ratio effects on low and high-nplasma instabilities

Alfvén Cavity Modes, Fast Ions, Alpha Particles and Diagnostic Neutral Beams

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