Experiments at ASDEX Upgrade indicate that with eight newly fitted magnetic control coils, the strength magnitude of plasma edge instabilities, so called ELMs, can be significantly reduced. Magnetic ELM suppression was first observed at the DIII-D tokamak in San Diego, USA. The ASDEX Upgrade experiments are enhance the understanding of the physics behind the process and will help to extrapolate the results to ITER. The project was partly funded by EFDA under priority support.

Full set of magnetic perturbation coils at ASDEX Upgrade. The eight red coils are already in operation. (Image: IPP)

Edge Localised Modes, or ELMs, are one of the big headaches posed by fusion physics: They cause sudden outbursts of the plasma thus expelling particles and depositing large heat flux onto the vessel wall. The plasma loses severe amounts of energy. In high-power fusion devices such as ITER, ELMs pose a serious risk to the wall and will probably need to be mitigated or suppressed. On the other hand, the less violent ELMs also serve to purge the plasma of impurities. These are relevant to ASDEX Upgrade with its tungsten-lined wall since tungsten atoms cause considerable energy losses if they penetrate the plasma centre. The vessel has been equipped with eight magnetic coils along the inside wall, four at the top and four at the bottom. They perturb the magnetic field which confines the plasma. In recent experiments, started in December 2010, the operation of the coils successfully mitigated ELMs in certain plasma regimes by converting large ELMs into harmless, but frequent, bursts. At the same time, the impurity content remained low and good plasma confinement was also maintained.

In 1998 and 2004, respectively, COMPASS-D (Culham, U.K.) and DIII-D (San Diego, U.S.A.) were the first experiments to report ELM mitigation using magnetic perturbations. The exact stabilisation mechanism, however, is not yet understood: The plasma resists the magnetic perturbations induced by the coils and thus far, no theory has been developed which is able to accurately predict the effects of the net magnetic field that penetrates the confined plasma. The current experiments being carried out at ASDEX Upgrade also show that ELMs can only be mitigated in certain plasma parameter ranges. For example, the effect could not be produced below a certain plasma density. Comparing previous observations with the new measurements from ASDEX Upgrade should now enhance the understanding of the physics and will help to establish a theory about this phenomenon. Around August 2011, another eight coils will be installed at ASDEX Upgrade, resulting in a total of eight coils at the top and eight at the bottom of the inside wall. This extension will allow the field strength of the induced magnetic perturbations to be increased and ensure even more accurate definition of their spatial structure.