CNR ExploRA

Articolo in rivista, 2019, ENG, 10.1088/1361-6587/ab2100

Analysis of metallic impurity content by means of VUV and SXR diagnostics in hybrid discharges with hot-spots on the JET-ITER-like wall poloidal limiter

Czarnecka, A.; Krawczyk, N.; Jacquet, P.; Lerche, E.; Bobkov, V.; Challis, C.; Frigione, D.; Graves, J.; Lawson, K. D.; Mantsinen, M. J.; Meneses, L.; Pawelec, E.; Pütterich, T.; Sertoli, M.; Valisa, M.; Van Eester, D.

a Consorzio RFX (CNR, ENEA, INFN, Università di Padova, Acciaierie Venete SpA), Corso Stati Uniti 4 - 35127 Padova, Italy; b Institute of Plasma Physics and Laser Microfusion, Hery 23 Str., Warsaw, 01-497, Poland; c CCFE, Culham Science Centre, Abingdon, OX14 3DB, United Kingdom; d Laboratory for Plasma Physics, ERM/KMS, EUROfusion Consortium Member, Renaissancelaan 30, Brussels, B-1000, Belgium; e Max-Planck-Institut für Plasmaphysik, Boltzmannstr.2, D-85748, Germany; f ENEA, Fusion and Nuclear Safety Dep., C.R. Frascati, Frascati (Roma), Italy; g Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne, CH-1015, Switzerland; h ICREA, Barcelona, Spain; i Barcelona Supercomputing Center, Barcelona, Spain; j Associação EURATOM/IST, Instituto de Plasmas e Fusão Nuclear-Laboratório Associado, Instituto Superior Técnico, Lisboa, P-1049-001, Portugal; k Institute of Physics, Opole University, Oleska 48 Str., Opole, 45-052, Poland.(Czarnecka, A.b, Krawczyk, N.b, Jacquet, P.c, Lerche, E.d, Bobkov, V.e, Challis, C.c, Frigione, D.f, Graves, J.g, Lawson, K.D.c, Mantsinen, M.J.h,i, Meneses, L.j, Pawelec, E.k, Pütterich, T.e, Sertoli, M.c,e, Valisa, M.a, Van Eester, D.d, JET Contributors)

In preparation for the upcoming JET D-T campaign, great effort has been devoted during the 2015-2016 JET campaigns with the ITER-like wall (ILW) to the extension of the high performance H-mode phase in baseline and hybrid scenarios. Hybrid discharges were the only ones that have been stopped by the real-time vessel protection system due hot-spot formation on the outboard poloidal limiter. Generation of hot-spots was linked to the application of high neutral beams injection and ion cyclotron resonance heating (ICRH) power. In tokamaks with high-Z plasma components, the use of ICRH heating is also accompanied by an increased metallic impurity content. Simultaneous control of hot-spot temperature and the core impurity content was crucial due to the fact that the same plasma-wall interaction mechanism is responsible for both phenomena. Impurity data collected by SXR, EUV and VUV diagnostics were able to provide for the first time comprehensive information concerning tungsten and mid-Z impurities such as nickel, iron, and cooper. To determine absolute mid-Z impurity concentrations a new relative calibration technique, compatible with JET-ILW, has been developed based on cross-calibration with a calibrated spectrometer via the quasicontinuum of W in the 200-400 Å wavelength range. In hybrid discharges, it was found that local D2 gas injection, plasma current, separatrix density, and fast ion losses appeared to impact hot-spot temperature and core impurity levels. Analysis showed a reduced maximum hot-spot temperature and impurity concentration at higher gas rate. Changes in the plasma current had a strong impact on the plasma-wall interaction, both via modifications in the edge density and in the fast ion losses. At constant gas injection rate, both the hot-spot temperature and the core impurity content decreased with the separatrix density. The main mechanism responsible for the formation of the hot-spots was found to be linked to the fast ion losses, but RF sheath effects may also be playing a role in the high limiter temperatures observed in these experiments. © Institute of Plasma Physics and Laser Microfusion.

Plasma physics and controlled fusion (Print) 61 (8), pp. 085004-1–085004-11