Overview of Runaway Electron Control and Mitigation Experiments on Tore Supra and Lessons Learned in View of ITER

Disruptions in ITER and strategies for their control and mitigation

“…It was therefore proposed to inject impurities during the current quench phase to destabilise MHD modes by increasing locally the resistivity. Dedicated experiments at Tore Supra and ASDEX Upgrade showed neither an impact on the current decay nor a correlation of the injection with the observed MHD activity [58]. Magnetic perturbations that are imposed by external coils are able to suppress the formation of a RE beam above a certain amplitude threshold [60,61].…”

“…Such control schemes were successfully applied at ToreSupra [58] and DIII-D [19]. However, even with optimised vertical position of the plasma before the disruption, this control scheme appears unlikely to be feasible at ITER.…”

WEST Physics Basis

“…Disruption mitigation experiments have been carried out previously with this system on Tore Supra to study various injection scenarios and to investigate gas jet penetration and mixing, showing in particular that runaway mitigation is more efficient using light gases [44]. Additional experiments on runaway mitigation have been performed using a high pressure cartridge [45]. This later system could be reinstalled in the WEST configuration if required.…”

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

“…A possible explanation may be that the RE beam, which is localized in the core of the plasma, extending perhaps up to mid-radius, is "shielded" by the cold background plasma surrounding it via a gas braking effect similar to the one described above. However, it is essential to note that on Tore Supra [31], DIII-D [32] or ASDEX-U [33], MGI clearly affects the RE beam, so the gas must reach the beam in these machines. It is therefore interesting to investigate possible reasons for differences in terms of gas penetration between the machines.…”

“…To finish this section, we would like to draw attention to an interesting observation made in the above-mentioned Tore Supra experiment. Figure 7b in [31] shows that the neutral pressure measured with a pressure gauge in the vacuum vessel decreases abruptly from ≃ 2.5 Pa to ≃ 1 Pa in about 20 ms (which is the response time of the gauge) just after the extinction of the plasma, after which it stabilizes slightly below 1 Pa, the latter being the expected pressure for a uniform filling of the vessel at the vessel temperature of 120 • C. The pressure of ≃ 2.5 Pa in the runaway plateau phase suggests that the gas is confined to a smaller volume and/or hotter than 120 • C. This is consistent with the findings presented in this paper that gas-plasma interaction prevents most of the gas from penetrating the plasma volume (even when part of the gas does penetrate the plasma and affects the runaway beam) and also heats up the gas. An interesting direction for future work is to investigate the possible role of this gas surrounding the plasma.…”