Experimental Observation of a Magnetic-Turbulence Threshold for Runaway-Electron Generation in the TEXTOR Tokamak

Zeng, Long; Koslowski, H. R.; Liang, Y.; Lvovskiy, A.; Lehnen, M.; Nicolai, D.; Pearson, James L.; Rack, M.; Jaegers, H.; Kh, Finken; Wongrach, K.; Xu, Yuanyuan; Team, Textor

doi:10.1103/physrevlett.110.235003

Cited by 65 publications

(92 citation statements)

References 17 publications

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“…One of the main dependencies for runaway electron generation is thus the ratio between the accelerating electric field and the critical electric field, the latter being proportional to the electron density. Other indirect but known dependencies are the toroidal field B t [7,27] and magnetic fluctuations [19]. For the cases presented in this article, the accelerating electric field is calculated using a 0D coupled circuits calculation which takes into account plasma, vacuum vessel and poloidal field coils.…”

Section: Physics Dependenciesmentioning

confidence: 99%

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Reux¹,

Plyusnin²,

Alper³

et al. 2015

Nucl. Fusion

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110

130

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Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories.

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Section: Physics Dependenciesmentioning

confidence: 99%

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Reux¹,

Plyusnin²,

Alper³

et al. 2015

Nucl. Fusion

Self Cite

110

130

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“…In other words, the time scale of the local growth rate could be longer than suggested by collisional theory [18,3]. Potential loss mechanisms, such as transport of fast electrons due to magnetic field perturbations [20] could therefore act more efficiently on the runaway electrons formed off the magnetic axis than the ones formed on axis which could lead to well confined runaway electrons at the center of the plasma.…”

Section: Effect Of Toroidicitymentioning

confidence: 98%

Kinetic modelling of runaway electron avalanches in tokamak plasmas

Nilsson

Decker

Peysson

et al. 2015

Plasma Phys. Control. Fusion

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Runaway electrons can be generated in tokamak plasmas if the accelerating force from the toroidal electric field exceeds the collisional drag force owing to Coulomb collisions with the background plasma. In ITER, disruptions are expected to generate runaway electrons mainly through knock-on collisions [1], where enough momentum can be transferred from existing runaways to slow electrons to transport the latter beyond a critical momentum, setting off an avalanche of runaway electrons. Since knock-on runaways are usually scattered off with a significant perpendicular component of the momentum with respect to the local magnetic field direction, these particles are highly magnetized. Consequently, the momentum dynamics require a full 3-D kinetic description, since these electrons are highly sensitive to the magnetic non-uniformity of a toroidal configuration. For this purpose, a bounce-averaged knockon source term is derived. The generation of runaway electrons from the combined effect of Dreicer mechanism and knock-on collision process is studied with the code LUKE, a solver of the 3-D linearized bounce-averaged relativistic electron Fokker-Planck equation [2], through the calculation of the response of the electron distribution function to a constant parallel electric field. The model, which has been successfully benchmarked against the standard Dreicer runaway theory now describes the runaway generation by knock-on collisions as proposed by Rosenbluth [3]. This paper shows that the avalanche effect can be important even in non-disruptive scenarios. Runaway formation through knock-on collisions is found to be strongly reduced when taking place off the magnetic axis, since trapped electrons can not contribute to the runaway electron population. Finally, the relative importance of the avalanche mechanism is investigated as a function of the key parameters for runaway electron formation, namely the plasma temperature and the electric field strength. In agreement with theoretical predictions, the LUKE simulations show that in low temperature and electric field the knock-on collisions becomes the dominant source of runaway electrons and can play a significant role for runaway electron generation, including in non-disruptive tokamak scenarios.

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“…There were numerous dedicated experiments to study the problem of runaway current generation during plasma disruptions triggered by massive gas injections (MGI) in the TEXTOR tokamak (see, e.g., (Forster et al 2012;Zeng et al 2013;Wongrach et al 2014)), in KSTAR tokamak (Chen et al 2013), the JET tokamak (Plyusnin et al 2006;Lehnen et al 2011), in DIII-D (Hollmann et al 2010;Commaux et al 2011;Hollmann et al 2013), Alcator C-Mod (Olynyk et al 2013), and others. In these works the dependencies of RE generation on the toroidal magnetic field, on the magnetic field fluctuations, on the species of injection gases have been investigated.…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of plasma disruption and runaway electron losses in the TEXTOR tokamak

et al. 2015

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Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron beams and their losses are proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydro-dynamics (MHD) modes. It is hypothesized that the runaway electron beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor q(0) < 1 the most stable runaway electron beams are formed by the intact magnetic surface located between the magnetic surface q = 1 and the closest loworder rational surface q = m/n > 1 ( q = 5/4, q = 4/3, . . . ). The thermal quench time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench stage is due to the particle transport in a stochastic magnetic field. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant m/n = 1/1 component. The runaway electrons are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch wall and (ii) by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high-energy runaway electrons is formed in the presence of the m/n = 1/1 MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the m/n = 1/1 MHD mode.

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Experimental Observation of a Magnetic-Turbulence Threshold for Runaway-Electron Generation in the TEXTOR Tokamak

Cited by 65 publications

References 17 publications

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Kinetic modelling of runaway electron avalanches in tokamak plasmas

Mechanisms of plasma disruption and runaway electron losses in the TEXTOR tokamak

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