CNR ExploRA

Articolo in rivista, 2015, ENG, 10.1088/0741-3335/57/4/045001

Effect of tungsten off-axis accumulation on neutral beam deposition in JET rotating plasmas

Koskela T.; Romanelli M.; Belo P.; Asunta O.; Sipilae S.; O'Mullane M.; Giacomelli L.; Conroy S.; Mantica P.; Valisa M.; Angioni C.; Kurki-Suonio T.

JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK; 1 Department of Applied Physics, Aalto University, FI- 00076 AALTO, Finland; 2 Culham Center for fusion energy, Abingdon, OX143XU, UK; 3 IPFN Laboratorio Associado, IST, P-1049-001 Lisboa, Portugal; 4 Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, Scotland; 5 Department of Physics, Università degli Studi di Milano-Bicocca, Milano, Italy; 6 Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden; 7 Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany; 8 Istituto di Fisica del Plasma, CNR/ENEA, Milano, Italy; 9 Consorzio RFX-CNR/ENEA, I-35127 Padova, Italy; 10 See the appendix A of Romanelli et al 2012 (Proc. of the 24th IAEA Fusion Energy Conf. (San Diego, USA, 8-13 October 2012). (T Koskela 1, M Romanelli 2, P Belo 3, O Asunta 1, S Sipilä 1, M O'Mullane 4, L Giacomelli 2, 5, S Conroy 6, P Mantica 8, M Valisa 9, C Angioni 7, T Kurki-Suonio 1 and JET-EFDA contributors 10).

Evidence for low field side accumulation of tungsten is often observed in bolometry and soft x-ray emissivities of highly rotating JET ITER-like wall (ILW) plasmas. Poloidal variation of the density of high-Z impurities, such as tungsten, in the core of NBI heated plasmas is expected from neoclassical theory due to charge displacement and parallel electric field generated by the centrifugal force. We calculate the poloidally asymmetric distribution of tungsten using fluid equations and a 1D transport simulation with the JETTO/SANCO code. Peaking of tungsten on the outboard side of the plasma is found and verified with soft x-ray and bolometry measurements. We then study the effect of a poloidally asymmetric tungsten distribution on the distribution of the NBI heat source by simulations with the Monte Carlo code ASCOT. The simulations show that the poloidally asymmetric tungsten profile redistributes the fast NBI ions radially through shifting their ionization profile and poloidally through enhanced pitch-angle scattering at high energy. The amplitude of the redistribution is in the order of 10% for the highest n(W)/n(e) ratios of similar to 10(-4) measured in recent JET H-mode plasmas. As a result of the scattering of the beam particles, the core heat deposition is changed less than 10%, which does not have a significant impact to the performance of JET plasmas. The modelling is in qualitative agreement with measurements by the vertical neutron camera that sees a broadening in the 2.5 MeV neutron profile when tungsten peaks on the outboard side of the plasma.

Plasma physics and controlled fusion (Print) 57 (4)