Europhysics Conference Abstracts 35th EPS Conference on Plasma Physics

Amplitude bifurcation in the peeling-relaxation ELM modelP. Devoy1, P.K. Browning1 and C. G. Gimblett21School of Physics and Astronomy, University of Manchester, Manchester, U.K. 2EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, U.K. Edge Localised Modes (ELMs) in tokamaks are characterised by quasi-periodic losses of both particles and energy from the edge of the plasma to the surrounding vacuum. These instabilities can deposit large amounts of energy on plasma facing components and it has been predicted that the energy released by large ELMs in ITER could significantly limit the divertor lifetime. ELMs are associated with strong current and pressure gradients near the plasma edge in H mode operation, and are thus believed to be triggered either by ballooning (pressure driven) or peeling (current driven) instabilities; here, we consider the latter scenario.We build on an existing ELM theory [1] by introducing the bootstrap current which is associated with the edge pedestal region. The model is based on the idea that a local Taylor relaxation follows ELM onset, in which the current profile in the ELM region is flattened whilst a negative current sheet develops at the plasma-vacuum boundary. The extent of the ELM is determined by finding the largest relaxed region which is peeling stable. Introducing the bootstrap current splits the predicted ELM widths into two classes; one group with large ELM widths, the other with small ELM widths. The percentage of the total plasma energy lost in each ELM is also similarly bifurcated, possibly indicating the distinction between Type I and Type III ELMs. Only small variations in initial q profile are needed to jump between the two classes. We also investigate the dependence of the ELMs on the location, magnitude and distribution of the bootstrap current. Analysis using a small ELM width approximation and a localised bootstrap profile shows that there will be a bifurcation in ELM widths depending on whether the bootstrap current location is reached by the relaxation process, in agreement with the numerical modelling.[1] C. G. Gimblett, R. J. Hastie, and P. Helander, Phys. Rev. Lett. 96, 035006 (2006).AcknowledgmentThis work was funded jointly by the U.K. E.P.S.R.C. and the European Communities.