Formation and termination of runaway beams in ITER disruptions

Martı́n-Solı́s, J. R.; Loarte, A.; Lehnen, M.

doi:10.1088/1741-4326/aa6939

Cited by 112 publications

(217 citation statements)

References 41 publications

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“…[31]. This estimation is even much smaller than or comparable to seed currents produced by the tritium decay and the Compton scattering of gamma-rays in fusion devices [34].…”

Section: Current Quench (Cq) and Rosenbluthputvinski Modelmentioning

confidence: 82%

Analysis of Avalanche Runaway Generation after Disruptions with Low-<i>Z </i>and Noble Gas Species

Matsuyama

Yagi

2017

Plasma and Fusion Research

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The effects of impurities on runaway electron generation are studied using a zero-dimensional disruption simulation code. For describing collisions between fast electrons and partially stripped ions, a charge-resolved expression of the Coulomb logarithm is employed. Numerical analysis of the avalanche growth rate using the adjoint Fokker-Planck method is compared with two existing semi-analytic models, showing (i) the convergence of the growth rate to strong electric field limit of the Rosenbluth-Putvinski (R-P) model and (ii) the cancellation of the effect of second-order collisional diffusion for intermediate electric fields. Using the developed current quench (CQ) simulations, the parametric study is performed with the aid of the power balance analysis, which characterizes the onset of strong avalanche amplification in the presence of low-Z and noble gas species. Thermal quench (TQ) simulations are also developed for self-consistent evaluation of hot-tail seed electrons. The deposition timescale of impurity neutrals is shown to have significant impacts on hot-tail seeds, depending nonmonotonically on the pre-TQ temperature and the injected impurity density.

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“…[31]. This estimation is even much smaller than or comparable to seed currents produced by the tritium decay and the Compton scattering of gamma-rays in fusion devices [34].…”

Section: Current Quench (Cq) and Rosenbluthputvinski Modelmentioning

confidence: 82%

Analysis of Avalanche Runaway Generation after Disruptions with Low-<i>Z </i>and Noble Gas Species

Matsuyama

Yagi

2017

Plasma and Fusion Research

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“…The flux of gamma rays Γ γ in ITER is estimated to be of order 10 18 m −2 s −1 during strong DT power production but quickly drops to of order 10 15 m −2 s −1 once nuclear reactions are terminated [7] . The critical energy for runaway is proportional to the background electron density while the Compton scattering cross section is inversely proportional to the energy reached by a cold electron as a result of the scattering.…”

Section: Seed From Compton Scatteringmentioning

confidence: 99%

Pivotal issues on relativistic electrons in ITER

Boozer

2018

Nucl. Fusion

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IntroductionThe construction of ITER has placed scientific challenges on the plasma physics community. Perhaps none is greater than that of the transfer of the plasma current from thermal to relativistic electrons, which could result in unacceptable damage to the device. To achieve its mission, less than one in a thousand ITER pulses can result in the transfer of a significant fraction of the plasma current to relativistic electrons that subsequently strike the walls [1]. Consequently, (1) ITER operations must be constrained to avoid rapid quenches in the electron temperature, (2) either natural or mitigation techniques must prevent electrons from running away to relativistic energies, or (3) relativistic electron currents must be benignly terminated.The reliable termination of a plasma current faster than its natural decay without disruption, especially at a safety factor less than ten, has not been demonstrated and may be AbstractThe transfer of the plasma current from thermal to relativistic electrons is a threat to ITER achieving its mission. This danger is significantly greater in the nuclear than in the nonnuclear phase of ITER operations. Two issues are pivotal. The first is the extent and duration of magnetic surface breaking in conjunction with the thermal quenches. The second is the exponential sensitivity of the current transfer to three quantities: (1) the poloidal flux change required to e-fold the number of relativistic electrons, (2) the time τ a after the beginning of the thermal quench before the accelerating electric field exceeds the Connor-Hastie field for runaway, and (3) the duration of the period τ op in which magnetic surfaces remain open. Adequate knowledge does not exist to devise a reliable strategy for the protection of ITER. Uncertainties are sufficiently large that a transfer of neither a negligible nor the full plasma current to relativistic electrons can be ruled out during the non-nuclear phase of ITER. Tritium decay can provide a sufficiently strong seed for a dangerous relativistic-electron current even if τ a and τ op are sufficiently long to avoid relativistic electrons during non-nuclear operations. The breakup of magnetic surfaces that is associated with thermal quenches occurs on a time scale associated with fast magnetic reconnection, which means reconnection at an Alfvénic rather than a resistive rate. Alfvénic reconnection is well beyond the capabilities of existing computational tools for tokamaks, but its effects can be studied using its property of conserving magnetic helicity. Although the dangers to ITER from relativistic electrons have been known for twenty years, the critical issues have not been defined with sufficient precision to formulate an effective research program. Studies are particularly needed on plasma behavior in existing tokamaks during thermal quenches, behavior which could be clarified using methods developed here.

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“…As the presently available fluid approximations to hot-tail generation are limited in applicability ( [15], [16]), we do not consider the hot-tail generation mechanism in our model at present, while a term describing it can be added later on or an ad-hoc seed distribution can be initialized. Also, it must be noted that, while the secondary RE generation occurs at timescales close to the resistive timescale τ res = µ 0 L 2 /η ∼ 0.1 s for a 10 eV plasma, the Dreicer generation is a relatively faster process occuring at a timescale τ Drecier ∼ 10 −5 -10 −6 s in a typical tokamak plasma.…”

Section: Introductionmentioning

confidence: 99%

Simulating the nonlinear interaction of relativistic electrons and tokamak plasma instabilities: Implementation and validation of a fluid model

et al. 2019

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For the simulation of disruptions in tokamak fusion plasmas, a fluid model describing the evolution of relativistic runaway electrons and their interaction with the background plasma is presented. The overall aim of the model is to self-consistently describe the non-linear coupled evolution of runaway electrons (REs) and plasma instabilities during disruptions. In this model, the runaway electrons are considered as a separate fluid species in which the initial seed is generated through the Dreicer source, that eventually grows by the avalanche mechanism (further relevant source mechanisms can easily be added). Advection of the runaway electrons is considered primarily along field lines, but also taking into account the E × B drift. The model is implemented in the non-linear magnetohydrodynamic (MHD) code JOREK based on Bezier finite-elements, with current-coupling to the thermal plasma. Benchmarking of the code with the one-dimensional runaway electron code GO is done using an artificial thermal quench on a circular plasma. As a first demonstration, the code is applied to the problem of an axisymmetric cold vertical displacement event in an ITER plasma, revealing significantly different dynamics between cases computed with and without runaway electrons. Though it is not yet feasible to achieve fully realistic runaway electron velocities close to the speed of light in complete simulations of slowly evolving plasma instabilities, the code is demostrated to be suitable to study various kinds of MHD-RE interaction in MHD-active and disruption relevant plasmas.

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Formation and termination of runaway beams in ITER disruptions

Cited by 112 publications

References 41 publications

Analysis of Avalanche Runaway Generation after Disruptions with Low-<i>Z </i>and Noble Gas Species

Analysis of Avalanche Runaway Generation after Disruptions with Low-<i>Z </i>and Noble Gas Species

Pivotal issues on relativistic electrons in ITER

Simulating the nonlinear interaction of relativistic electrons and tokamak plasma instabilities: Implementation and validation of a fluid model

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