(EX/4-3) Gas Jet Disruption Mitigation Studies on Alcator C-Mod and DIII-D

R.S. Granetz1), E.M. Hollmann2), D.G. Whyte3), V.A. Izzo1), G.Y. Antar2), A. Bader1), M. Bakhtiari3), T. Biewer1), J.A. Boedo2), T.E. Evans4), T.C. Jernigan5), D.S. Gray2), M. Groth6), D.A. Humphreys4), C.J. Lasnier6), R.A. Moyer2), P.B. Parks4), M.L. Reinke1), D.L. Rudakov2), E.J. Strait4), J.L. Terry1), J. Wesley4), W.P. West4), G. Wurden7), J. Yu2)
1) MIT Plasma Science and Fusion Center, Cambridge, MA, USA
2) University of California-San Diego, La Jolla, CA 92093, USA
3) University of Wisconsin-Madison, Dept of Engineering Physics, Madison, WI 53706, USA
4) General Atomics, San Diego, CA 92186, USA
5) Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
6) Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
7) Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Abstract.  Noble gas jet injection on Alcator C-Mod and DIII-D provides good mitigation of deleterious disruption effects, even though the jet does not penetrate deeply into the plasma as neutral gas. Disruption damage can come from overheating of divertor surfaces, electromagnetic loads on conducting structures, and localised impact of relativistic electrons. High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the requirements of fast response time and reliability, without degrading subsequent discharges. Previous experiments on DIII-D showed good success at reducing all of the deleterious effects. More recently, gas jet experiments on Alcator C-Mod have tested the effectiveness of this approach on the higher pressure, higher energy density plasmas representative of C-Mod and ITER. Initial results confirm the ability of this technique to radiate away very high energy densities on timescales consistent with C-Mod's fast current quench. Higher-Z gas jets (Ar, Kr) result in a 50% reduction in halo current and also reduce the heating of divertor surfaces, similar to the DIII-D results, with no deleterious effects on subsequent discharges. High-speed imaging on both DIII-D and C-Mod shows only shallow penetration of the gas jets. However, the plasma core is affected on a timescale which is much faster than normal transport processes. An understanding of this paradox is obtained by modeling with the NIMROD MHD code, which shows that the initial cooling of the plasma periphery triggers a very rapid growth of low-order tearing modes, resulting in a stochastic region over much of the plasma. This allows rapid transport across the entire plasma, and could explain the effectiveness of gas jet mitigation in C-Mod and DIII-D, and presumably in ITER, in spite of the shallow penetration of the neutral gas jet. In DIII-D the onset time of the thermal quench is observed to increase monotonically with increased q=2 surface depth, strongly suggesting that a 2/1 instability is involved, in agreement with the NIMROD modeling. Although runaway electrons are not observed in the gas jet experiments on C-Mod or DIII-D, the quantity of gas atoms injected to date is insufficient to avoid a runaway avalanche on ITER. A new, larger valve ( ∼25× more throughput) will be used soon on DIII-D to test collisional avalanche suppression.

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