A REVIEW OF MODELS FOR ELMS

The improved confinement regime known as the H-mode is often perturbed by the onset of a quasi-periodic series of relaxation oscillations in volving bursts of MHD activity and D-alpha emission, known as edge loc alized modes (ELMs). These result in rapid losses of particles and ene rgy from the region near the plasma boundary, reducing the average glo bal energy confinement by 10-20%. Furthermore, these transient bursts of energy and particles into the scrape-off layer produce high peak he at loads on the divertor plates which must be accommodated by the dive rtor design. However, the ELMs are efficient, and beneficial, in remov ing density and impurities. Thus they are deemed necessary for the sta tionary H-mode operation of ITER, preventing the build-up of density, impurities and helium ash. It is, therefore, desirable to be able to c ontrol the level and nature of the ELM activity in order to meet these various conflicting conditions; this would be aided by understanding their cause. After briefly describing the phenomenology of ELMs, vario us theoretical models that have been proposed to explain them are disc ussed. These fall into three broad classes. Since ELMs are accompanied by bursts of magnetic activity, the first class of models involves th e excitation of various MHD instabilities: ideal and resistive balloon ing modes, external kink modes and so-called 'peeling modes'. Such mod els envisage the application of auxiliary heating driving the equilibr ium to a state which triggers some such instability, resulting in the loss of plasma, followed by a recovery stage until the cycle is repeat ed. The second class of models involves limit cycle solutions of the t ransport equations governing the plasma edge region, exploiting the bi furcations inherent in theories of the GH transition, for example thos e involving sheared rotation stabilization. In the third class, elemen ts of both types of theory have been combined, with MHD or pressure-dr iven fluctuation transport playing a role.