1 - Nuclear Sciences and Applications - IAEA

SAWTOOTH ACTIVITY AND CURRENT
DENSITY PROFILES IN JET
D.J. CAMPBELL, J.G. CORDEY, A.W. EDWARDS,
R.D. GILL, E. LAZZARO, G. MAGYAR, A.L. McCARTHY 1 ,
J. O'ROURKE, F. PEGORARO, F. PORCELLI, P. SMEULDERS,
D.F.H. START, P. STUBBERFIELD, J.A. WESSON,
E. WESTERHOF 2 , D. ZASCHE 3
JET Joint Undertaking,
Abingdon, Oxfordshire,
United Kingdom
Abstract
IAEA-CN-50/A-VH-2
SAWTOOTH ACTIVITY AND CURRENT DENSITY PROFILES IN JET.
Extensive investigations of sawtooth activity in JET have led to a major revision of the understanding
of sawteeth and to further theoretical developments. In addition, experiments with powerful
auxiliary heating have revealed a new plasma regime in which sawteeth are stabilized and quiescent
plasmas are achieved for periods of over 3 s. Measurements of current density profiles using a far
infrared multichannel polarimeter show that the central safety factor q0 may be below unity in normal
sawtoothing plasmas and may reach values as low as 0.6-0.8 during the stable periods. Analysis of pellet
ablation and of density perturbations at rational q surfaces ('snakes') yields further information on the
evolution of the q-profile during sawteeth. These results have significant implications for theories of
the sawtooth instability and for explanations of the sawtooth stabilization.
1. Introduction
In many tokamak plasmas, the sawtooth instability plays a primary role in
determining heating and confinement in the plasma centre. In the
near—ignition regime, now being explored by large tokamak experiments,
sawteeth may cause a substantial reduction of the fusion reactivity and a
spatial redistribution of fusion products. Considerable attention is,
therefore, focussed on the understanding of the instability mechanism and
the investigation of possible stabilization techniques.
Previous studies on JET [1,2] have shown that there are considerable
discrepancies between conventional models of the instability and
experimental observations. In addition, measurements of the central value
of the safety factor q0 in TEXTOR [3] and observations of the 'snake' in
JET [4] suggest that the instability does not involve complete reconnection
1 Flinders University, Bedford Park, 5042 South Australia, Australia.
2 FOM Instituut voor Plasmafysica 'Rijnhuizen', Nieuwegein, Netherlands.
3 Max-Planck-Institut fur Plasmaphysik, D-8046 Garching, Federal Republic of Germany.
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