Suppression of runaway electron generation by massive helium injection after induced disruptions on TEXTOR

Disruptions with runaway electron generation have been deliberately induced by injection of argon using a disruption mitigation valve. A second disruption mitigation valve has been utilised to inject varying amounts of helium after a short time delay. No generation of runaway electrons has been observed when more than a critical amount of helium has been injected no later than 5 ms after the triggering of the first valve. The required amount of helium for suppression of runaway electron generation is up to one order of magnitude lower than the critical density according to Connor & Hastie (1975) and Rosenbluth & Putvinski (1997).

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Electron runaway in ASDEX Upgrade experiments of varying core temperature

Journal of Plasma Physics ◽

10.1017/s0022377821000416 ◽

2021 ◽

Vol 87 (3) ◽

Author(s):

O. Linder ◽

G. Papp ◽

E. Fable ◽

F. Jenko ◽

G. Pautasso ◽

...

Keyword(s):

Runaway Electron ◽

Initial Temperature ◽

Fluid Model ◽

Runaway Electrons ◽

Avalanche Multiplication ◽

Core Electron ◽

Electron Generation ◽

Seed Population ◽

Small Seed ◽

Order Of Magnitude

The formation of a substantial postdisruption runaway electron current in ASDEX Upgrade material injection experiments is determined by avalanche multiplication of a small seed population of runaway electrons. For the investigation of these scenarios, the runaway electron description of the coupled 1.5-D transport solvers ASTRA-STRAHL is amended by a fluid model describing electron runaway caused by the hot-tail mechanism. Applied in simulations of combined background plasma evolution, material injection and runaway electron generation in ASDEX Upgrade discharge #33108, both the Dreicer and hot-tail mechanism for electron runaway produce only ${\sim }$ 3 kA of runaway current. In colder plasmas with core electron temperatures $T_\textrm {e,c}$ below 9 keV, the postdisruption runaway current is predicted to be insensitive to the initial temperature, in agreement with experimental observations. Yet in hotter plasmas with $T_\textrm {e,c}$ above 10 keV, hot-tail runaway can be increased by up to an order of magnitude, contributing considerably to the total postdisruption runaway current. In ASDEX Upgrade high-temperature runaway experiments, however, no runaway current is observed at the end of the disruption, despite favourable conditions for both primary and secondary runaway.

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Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade

Journal of Plasma Physics ◽

10.1017/s0022377820000793 ◽

2020 ◽

Vol 86 (4) ◽

Cited By ~ 1

Author(s):

K. Insulander Björk ◽

G. Papp ◽

O. Embreus ◽

L. Hesslow ◽

T. Fülöp ◽

...

Keyword(s):

Runaway Electron ◽

Kinetic Modelling ◽

Electron Distribution Function ◽

Real Space ◽

Tokamak Plasmas ◽

Runaway Electrons ◽

Plateau Phase ◽

Electron Generation ◽

Current Dissipation ◽

Threshold Amount

Massive material injection has been proposed as a way to mitigate the formation of a beam of relativistic runaway electrons that may result from a disruption in tokamak plasmas. In this paper we analyse runaway generation observed in eleven ASDEX Upgrade discharges where disruption was triggered using massive gas injection. We present numerical simulations in scenarios characteristic of on-axis plasma conditions, constrained by experimental observations, using a description of the runaway dynamics with a self-consistent electric field and temperature evolution in two-dimensional momentum space and zero-dimensional real space. We describe the evolution of the electron distribution function during the disruption, and show that the runaway seed generation is dominated by hot-tail in all of the simulated discharges. We reproduce the observed dependence of the current dissipation rate on the amount of injected argon during the runaway plateau phase. Our simulations also indicate that above a threshold amount of injected argon, the current density after the current quench depends strongly on the argon densities. This trend is not observed in the experiments, which suggests that effects not captured by zero-dimensional kinetic modelling – such as runaway seed transport – are also important.

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