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(ITERP1/11) Neoclassical Islands, $\beta$-Limits, Error Fields, and ELMs in Reactor-Scale Tokamaks

   
F. Perkins ¹, A. Bondeson ², R. Buttery ³, J. Callen ⁴, J. Connor ³, A. Garofalo ⁵, D. Gates ⁶, R. Harvey ⁷, C. Hegna ⁴, T. Hender ³, G. Huysmans ⁸, Y. Kamada ⁹, R. J. La Haye ¹⁰, A. Pletzer ⁶, M. Rosenbluth ¹, T. Taylor ¹⁰, J. Wesley ¹, H. R. Wilson ³, H. Zohm ^11
 
1 ITER Joint Central Team, 11025 N. Torrey Pines Rd., La Jolla, CA 92037, USA
² Chalmers University of Technology, S-412 96 Göteborg, Sweden
³ UKAEA Fusion, Culham, Abingdon, Oxfordshire OX14 3DB, U. K.
⁴ University of Wisconsin, Madison, WI 53706, USA
⁵ Dept. of Applied Physics, Columbia University, New York, NY 10027, USA
⁶ Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA
⁷ CompX, 12839 Via Grimaldi, Del Mar, CA 92014, USA
⁸ JET Joint Undertaking, Abingdon, Oxfordshire, OX14 3EA, U. K.
⁹ JAERI, Naka Fusion Research Establishment, Naka-gun, Naka-machi, Ibaraki-ken, Japan
¹⁰ General Atomics, PO Box 85608, San Diego, CA 92186, USA
¹¹ Institut für Plasmaforschung, Pfaffenwaldring 31, D-70569 Stuttgart, Germany

Abstract
An assessment is presented of the impact of recent magnetohydrodynamic research results on performance projections for reactor-scale tokamaks as exemplified by the ITER  Final Design Report facility. For nominal ELMy H-mode  operation, the presence and amplitude of neoclassical tearing modes governs the achievable $\beta$-value. Recent work finds that the scaling of $\beta$ at which such modes onset agrees well with a polarization drift model, with the consequence that, with reasonable assumptions regarding seed-island width, the mode onset $\beta$ will be lower in reactor-scale tokamaks than in contemporary devices. Confinement degradation by such modes, on the other hand, depends on relative saturated island size which is governed by principally by $\beta$ and secondarily by $\nu^*$-effects on bootstrap current  density. Relative saturated island size should be comparable in present and reactor devices. DT ITER Demonstration Discharges in JET  exhibited no confinement degradation at the planned ITER operating value of $\rm\beta_N = 2.2$. Theory indicates that Electron Cyclotron Current Drive can either stabilize these modes or appreciably reduce saturated island size. Turning to operation in candidate steady-state, reverse-shear, high-bootstrap-fraction configurations, wall stabilization of external kink modes is effective while the plasma is rotating but (so far) rotation has not been maintained. Recent error field observations in JET imply an error-field size scaling that leads to a projection that ITER/FDR will be somewhat more tolerant to error fields than thought previously. ICRF experiments on JET and Alcator C-Mod  indicate that plasmas heated by central energetic particles have benign ELMs  compared to the usual type 1 ELM of NBI-heated discharges.

                

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