CNR ExploRA

Articolo in rivista, 2015, ENG, 10.1088/0029-5515/55/9/093013

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Reux, C.; Plyusnin, V.; Alper, B.; Alves, D.; Bazylev, B.; Belonohy, E.; Boboc, A.; Brezinsek, S.; Coffey, I.; Decker, J.; Drewelow, P.; Devaux, S.; de Vries, P. C.; Fil, A.; Gerasimov, S.; Giacomelli, L.; Jachmich, S.; Khilkevitch, E. M.; Kiptily, V.; Koslowski, R.; Kruezi, U.; Lehnen, M.; Lupelli, I.; Lomas, P. J.; Manzanares, A.; Martin De Aguilera, A.; Matthews, G. F.; Mlynar, J.; Nardon, E.; Nilsson, E.; von Thun, C. Perez; Riccardo, V.; Saint-Laurent, F.; Shevelev, A. E.; Sips, G.; Sozzi, C.

[ 1 ] CEA, IRFM, F-13108 St Paul Les Durance, France [ 2 ] Univ Lisbon, IST, Inst Plasmas & Fuso Nucl, P-1699 Lisbon, Portugal [ 3 ] Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England [ 4 ] Karlsruhe Inst Technol, IHM, D-76021 Karlsruhe, Germany [ 5 ] Culham Sci Ctr, EFDA CSU, Abingdon OX14 3DB, Oxon, England [ 6 ] Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany [ 7 ] Queens Univ Belfast, Dept Math & Phys, Belfast BT7 1NN, Antrim, North Ireland [ 8 ] Max Planck Inst Plasma Phys, D-17489 Greifswald, Germany [ 9 ] ITER Org, F-13115 St Paul Les Durance, France [ 10 ] Belgian State Inst ERM KMS, EURATOM Assoc, Lab Phys Plasmas, B-1000 Brussels, Belgium [ 11 ] Ioffe Inst, St Petersburg 194021, Russia [ 12 ] CIEMAT, EURATOM Assoc, E-28040 Madrid 3, Spain [ 13 ] AS CR, Inst Plasma Phys, Prague 18200 8, Czech Republic [ 14 ] EURATOM, CNR, Ist Fis Plasma, I-20125 Milan, Italy [ 15 ] Culham Sci Ctr, JET, EUROfus Consortium, Abingdon OX14 3DB, Oxon, England

Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m(3) high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries.

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