Novel rapid shutdown strategies for runaway electron suppression in DIII-D

Abstract

New rapid shutdown strategies have been recently tested in the DIII-D tokamak to mitigate runaway electrons (REs). Disruptions in ITER are predicted to generate multi-MeV REs that could damage the machine. The RE population in large tokamaks is expected to be dominated by avalanche amplification which can be mitigated at high density levels by collisional drag. Particle injection schemes for collisional suppression of RE have been developed and tested in ITER-relevant scenarios: massive gas injection, shattered pellet injection (SPI) and shell pellet injection. The results show an improved penetration of particles injected with the SPI. Another strategy has been developed to harmlessly deconfine REs by applying a non-axisymmetric magnetic perturbation to worsen their confinement. This technique appeared to deconfine seed RE before the avalanche process could amplify the RE beam. The last method tested was to use the plasma position control system on the RE beam to prevent it from contacting the wall. This proved effective in preventing high current RE beam from touching the wall and providing more time to mitigate an existing RE beam but a successful ‘soft landing’ (without fast final losses) of the RE has not been observed yet.

Runaway electron mitigation by 3D fields in the ASDEX-Upgrade experiment

“…This might also affect the primary generation of runaway electrons, thus reducing the initial seed available, and/or the efficacy in the avalanche mechanisms. Also in past experiments in DIII-D a similar phenomenology was observed[46] when n = 3 perturbations were applied before the disruption; there, in particular, discharges with RMPs application were characterized by current quench decay rates among the highest ones. In DIII-D too, these facts have been interpreted as a general worsening of the confinement properties due to the applied perturbations.V.…”

Physics of runaway electrons in tokamaks

“…In TEXTOR, an applied n = 3 field had a clear affect on final RE levels [35]. In DIII-D, less effect was seen: both n = 1 and n = 3 were attempted and some effect may have been observed, but limited statistics made clear conclusions difficult to achieve [254]. Similar experiments on RMP effects on RE prompt loss were tried in JET, but no clear effect was seen.…”

Disruptions in ITER and strategies for their control and mitigation

“…Present experiments show that the assimilation with MGI is below 20%. SPI can achieve about a factor of 2 higher assimilation of D 2 compared to MGI [55], but the spatial distribution and the potential for further improvement of the assimilation efficiency for SPI (and MGI) remain open questions. For the ideal case, the necessary quantities of He and D 2 exceed the injection limits, whereas argon or neon injection is likely to be compatible with these limits, but because of the high cooling rate of these gases, the CQ time is expected to fall below the eddy current limit.…”