Status of research toward the ITER disruption mitigation system

Hollmann, E. M.; Aleynikov, P.; Fülöp, Tünde; Humphreys, D. A.; Izzo, V.A.; Lehnen, M.; Lukash, V.E.; Papp, G.; Pautasso, G.; Saint‐Laurent, F.; Snipes, J. A.

doi:10.1063/1.4901251

Cited by 216 publications

(213 citation statements)

References 76 publications

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“…In order to develop a predictive modelling capability in support of the design and operation of the ITER disruption mitigation system (DMS) [1,2], it is essential to validate models on present devices as well as to understand the physical mech anisms at play. The present paper describes progress made with the JOREK 3D non-linear magnetohydrodynamics (MHD) code in the domain of massive gas injection (MGI), one of the options considered for the ITER DMS.…”

Section: Introductionmentioning

confidence: 99%

Progress in understanding disruptions triggered by massive gas injection via 3D non-linear MHD modelling with JOREK

Nardon

Fil

Hoelzl

et al. 2016

Plasma Phys. Control. Fusion

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3D non-linear MHD simulations of a D 2 massive gas injection (MGI) triggered disruption in JET with the JOREK code provide results which are qualitatively consistent with experimental observations and shed light on the physics at play. In particular, it is observed that the gas destabilizes a large m/n = 2/1 tearing mode, with the island O-point coinciding with the gas deposition region, by enhancing the plasma resistivity via cooling. When the 2/1 island gets so large that its inner side reaches the q = 3/2 surface, a 3/2 tearing mode grows. Simulations suggest that this is due to a steepening of the current profile right inside q = 3/2. Magnetic field stochastization over a large fraction of the minor radius as well as the growth of higher n modes ensue rapidly, leading to the thermal quench (TQ). The role of the 1/1 internal kink mode is discussed. An I p spike at the TQ is obtained in the simulations but with a smaller amplitude than in the experiment. Possible reasons are discussed.

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Section: Introductionmentioning

confidence: 99%

Progress in understanding disruptions triggered by massive gas injection via 3D non-linear MHD modelling with JOREK

Nardon

Fil

Hoelzl

et al. 2016

Plasma Phys. Control. Fusion

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“…The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy. DOI: 10.1103/PhysRevLett.118.255002 Introduction.-Reaching mega-ampere currents and mega-electron volt (MeV) energies during fast shutdown events, runaway electrons (REs) pose perhaps the greatest operational risk to tokamak fusion reactors such as ITER [1][2][3][4]. Because of the severe potential for damage to the reactor walls, opportunities for empirical tuning of RE control actuators will be limited.…”

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confidence: 99%

Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks

et al. 2017

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Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy. DOI: 10.1103/PhysRevLett.118.255002 Introduction.-Reaching mega-ampere currents and mega-electron volt (MeV) energies during fast shutdown events, runaway electrons (REs) pose perhaps the greatest operational risk to tokamak fusion reactors such as ITER [1][2][3][4]. Because of the severe potential for damage to the reactor walls, opportunities for empirical tuning of RE control actuators will be limited. Instead, a first-principles predictive understanding is needed, and present-day experiments fill a crucial need in validating theoretical predictions of RE dissipation.Classical theories for relativistic RE generation in tokamaks based on the effects of Coulomb collisions (small angle [5] and secondary avalanche [6]) determine the critical electric field (E C ) for the growth of RE populations. Further work highlighted the important role of synchrotron damping in elevating the threshold electric field above E C [7,8], and several experiments have since yielded evidence of the elevated threshold [9][10][11][12]. These observations motivated the development of a rigorous analytical theory [13] and computational tools [14][15][16][17][18][19] that clarified the importance of the effects of pitch-angle scattering and synchrotron damping. Alongside quantifying the enhancement of the threshold field, these works predict phase-space circulation around an attractor resulting in a pileup of REs at specific energies potentially resulting in nonmonotonic features in the RE distribution function (f e ). While important to the RE dissipation rate and thus the prospects for control, neither

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“…Without this capability, the locked mode would grow and promptly lead to a disruption. After that, only one last line of defense would remain, namely to mitigate the disruption, for instance by massive gas injection [1,14].Static, rather than rotating fields [11,13], are used here, permitting to align the plasma such that ECCD can be continuously deposited into the location of the locked mode where it has a stabilizing effect on it. By contrast, continuous ECCD on rotating islands is always on, but only stabilizing half of the time [6,13], and modulated ECH/ECCD on rotating islands is on and stabilizing half of the time, if properly phased [7,11].…”

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confidence: 99%

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Avoiding Tokamak Disruptions by Applying Static Magnetic Fields That Align Locked Modes with Stabilizing Wave-Driven Currents

et al. 2015

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Non-rotating ('locked') magnetic islands often lead to complete losses of confinement in tokamak plasmas, called major disruptions. Here locked islands were suppressed for the first time, by a combination of applied three-dimensional magnetic fields and injected millimetre waves. The applied fields were used to control the phase of locking and so align the island O-point with the region where the injected waves generated non-inductive currents. This resulted in stabilization of the locked island, disruption avoidance, recovery of high confinement and high pressure, in accordance with the expected dependencies upon wave power and relative phase between O-point and driven current.The international ITER [1] tokamak has the objective of demonstrating the scientific feasibility of magnetic confinement fusion as a source of energy. A concern towards the achievement of this goal is represented by major disruptions [1]: complete losses of confinement often initiated [2] by a non-rotating ('locked') magnetic island created by magnetic reconnection [3]. During disruptions, energy and particles accumulated in the plasma volume over several confinement times (seconds in ITER, a fraction of a second in present experiments) are lost in a few milliseconds and released on the plasma-facing materials [4]. In addition, multi-MA level currents flowing in the tokamak plasma for its sustainment and confinement are lost, also in milliseconds, thus terminating the plasma discharge and causing electromagnetic stresses that, if unmitigated, could lead to excessive device wear. Here it is shown for the first time that magnetic perturbations can be used to avoid disruptions by "guiding" the magnetic island to lock in a position where it is accessible to millimetre wave beams that fully stabilize it. Stabilization is due to locally wave-driven currents (Electron Cyclotron Current Drive, or ECCD).Magnetic control of island rotation [5] and stabilization of rotating islands by ECCD [6] were separately demonstrated in the past. Currents were either continuously driven [6] or, more efficiently, they were modulated in synch with the spontaneous island rotation [7]. Electron Cyclotron Heating (ECH) was also used for stabilization [8], but is predicted to scale unfavorably to large hot plasmas [9]. Two experiments combined magnetic perturbations -to produce the island-with ECH that stabilized it: in the first one the mode was born locked to a given phase and was stabilized by continuous ECH [10]; the second one controlled island rotation and stabilized the mode by modulated ECH [11].However, if the rotating island (typically a spontaneous, pressure-driven 'Neoclassical Tearing Mode') is not preempted or stabilized (due for example to late intervention, misalignment, or insufficient power being used for this purpose), or if the island does not ever rotate at all, it becomes necessary to suppress the locked mode. This capability was numerically modeled [12], experimentally tested [13], and is fully demonstrated here for the first time. Without this...

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Status of research toward the ITER disruption mitigation system

Cited by 216 publications

References 76 publications

Progress in understanding disruptions triggered by massive gas injection via 3D non-linear MHD modelling with JOREK

Progress in understanding disruptions triggered by massive gas injection via 3D non-linear MHD modelling with JOREK

Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks

Avoiding Tokamak Disruptions by Applying Static Magnetic Fields That Align Locked Modes with Stabilizing Wave-Driven Currents

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