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 involving bursts of MHD activity and emission, known as edge localized modes (ELMs). These result in rapid losses of particles and energy from the region near the plasma boundary, reducing the average global energy confinement by 10-20%. Furthermore, these transient bursts of energy and particles into the scrape-off layer produce high peak heat loads on the divertor plates which must be accommodated by the divertor design. However, the ELMs are efficient, and beneficial, in removing density and impurities. Thus they are deemed necessary for the stationary H-mode operation of ITER, preventing the build-up of density, impurities and helium ash. It is, therefore, desirable to be able to control 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, various theoretical models that have been proposed to explain them are discussed. These fall into three broad classes. Since ELMs are accompanied by bursts of magnetic activity, the first class of models involves the excitation of various MHD instabilities: ideal and resistive ballooning modes, external kink modes and so-called `peeling modes'. Such models envisage the application of auxiliary heating driving the equilibrium to a state which triggers some such instability, resulting in the loss of plasma, followed by a recovery stage until the cycle is repeated. The second class of models involves limit cycle solutions of the transport equations governing the plasma edge region, exploiting the bifurcations inherent in theories of the L-H transition, for example those involving sheared rotation stabilization. In the third class, elements of both types of theory have been combined, with MHD or pressure-driven fluctuation transport playing a role.