(EX/P8-7) Mitigated Plasma Shut-Down with Fast Impurity Puff on ASDEX Upgrade

G. Pautasso1), T. Pütterich1), C.J. Fuchs1), O. Gruber1), C.F. Maggi1), M. Maraschek1), V. Rohde1), C. Wittmann1), E. Wolfrum1), P. Cierpka1), M. Bech1), ASDEX Upgrade Team1)
1) Max Planck Institut für Plasmaphysik, Garching, Germany

Abstract.  Experiments on disruption mitigation are carried out nowadays on all large tokamaks to investigate the influence of the injected impurities on the development of the disruption. The optimal disruption mitigation requires that all three deleterious effects of disruptions, namely the localized thermal load, the energy carried by runaway electrons and the mechanical forces are minimized. The kind of gas, the rate of injection and the injected quantities differ considerably according to the type of gas injection system available, the volume of the vessel and the specific problems affecting the machine after disruption. In this framework, ASDEX Upgrade has been conducting experiments for years and is routinely employing the fast injection of neon for the plasma shut-down of disrupting plasmas and machine protection. The injection is triggered by the locked mode (LM) signal and leads to the onset of a mitigated disruption within 5 ms. The impurity gas is injected into the plasma with two electromagnetic valves. The valves have an opening and closing time of 2 ms and remain open for 4-5 ms; they have been mostly operating with neon gas at a reservoir pressure of 5 bar and have been typically injecting 180 mbar ( 4.5×1021 atoms) of gas. Dedicated experiments have been carried out with 10 and 15 bar reservoir pressure. This paper will (1) describe the phenomenology of mitigation, (2) the experimental condition in ASDEX Upgrade, (3) the plasma response to the injection of impurities, (4) discuss the understanding of the observed phenomena, and (5) outline the future plans for the development of the mitigation system on ASDEX Upgrade.

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