XXXVI international conference on plasma physics and CF, February 9 – 13, 2009, Zvenigorod.

Beta Limit Studies on JET

M.P. Gryaznevich*, and JET-EFDA Contributors

JET-EFDA, Culham Science Centre, OX14 3DB, Abingdon, UK
*Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK

Advanced tokamak regimes are associated with increased normalised beta, N = tBta/Ip (t = 20<p>/Bt2) and are often limited by MHD instabilities. Although the presence of a conducting wall increases this -limit, it is important to know the no-wall ideal -limit to be able to prevent/avoid the most dangerous pressure-driven instabilities in JET and ITER.

Systematic studies of -limits have been carried out on JET in several advanced regimes: the hybrid low-shear regime with q(0) close to 1, the high- low or slightly negative shear regime without or with weak ITB and 1< qmin < 3.0 and in a reversed shear high- ITB regime. During these studies, the probing of the plasma stability has been performed by measuring the plasma response to an externally applied helical magnetic perturbation. It is found that an externally applied helical magnetic field is strongly enhanced when the plasma exceeds the ideal no-wall stability limit or approaches proximity to other marginally stable (i.e. current-driven) modes. This effect is known as the Resonant Field Amplification (RFA) and was used for the systematic probing of stability in different advanced regimes on JET.

It was shown that increase in the target qmin results in significant reduction in the measured RFA threshold (RFAT) going down from RFAT ~ 3 at q(0), or qmin (in a case of a slightly reversed shear) ~1 to RFAT ~ 1.5 at q(0) or qmin going up above 2. Numerical stability simulations based on JET pulses have been performed and confirm this observed trend. It has been demonstrated experimentally and confirmed by stability simulations that RFA as a diagnostics of the no-wall -limit is sensitive to the edge current, which can reduce the RFA threshold, and n=1 and n=2 RFA has been observed at low-N preceding 1st ELM after prolonged ELM-free period. At high-N, the measured RFA threshold has been exceeded on JET by up to ~ 70%. Values of N ~ 3 at qmin~ 1.5-2.5 (ITER steady-state-relevant domain) have been reached by optimisation of the heating scenario, limited by a rotating n=1 kink or tearing mode, sensitive to q-profile. It has been demonstrated, that the Resonant Field Amplification is an empirical precursor to this observed n = 1-limiting mode, so potentially can be used in scenario optimisation towards high-.

This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was carried out within the framework of the European Fusion Development Agreement.


[1].See annex to M.Watkins et al, Fusion Energy 2006 (Proc. 21st IAEA Conf., Chengdu, 2006) IAEA Vienna.