Plasma stability made to measure

After barely a year of modification work the first experiments conducted have already proved successful. Eight magnetic control coils on the wall of the plasma vessel of the ASDEX Upgrade fusion device have now succeeded in reducing perturbing instabilities of the plasma, so-called ELMs, to the level required. If these outbursts of the edge plasma become too severe, they can cause major damage to the plasma vessel in devices of the ITER class. The results now achieved go a long way towards solving this important problem for ITER.

The research objective of Max Planck Institute for Plasma Physics (IPP) at Garching is to develop a power plant that, like the sun, derives energy from fusion of atomic nuclei. Whether this is feasible is to be demonstrated with a fusion power of 500 megawatts by the ITER (Latin for "the way") experimental fusion reactor, now being built at Cadarache, France, as an international cooperation. This requires that the fuel, an ionized low-density hydrogen gas -- a plasma -- be confined in a magnetic field cage without touching the wall of the plasma vessel and heated to ignition temperatures of over 100 million degrees.

The complex interaction between the charged plasma particles and the confining magnetic field can cause all kinds of perturbations of the plasma confinement. Edge Localized Modes (ELMS) are very much under discussion at present in relation to ITER. These cause the edge plasma to briefly lose its confinement and periodically hurl bundled plasma particles and energies outwards to the vessel wall. Up to one-tenth of the total energy content is thus ejected. Whereas the present generation of medium-sized fusion devices can easily cope with this, it might cause overloading in large-scale devices such as ITER of, in particular, the divertor -- specially equipped collector plates at the bottom of the vessel, to which the plasma edge layer is magnetically diverted. This would make continuous operation inconceivable.

This ELM instability is, however, not altogether unwelcome, because it expels undesirable impurities from the plasma. Instead of the usual hefty impacts the aim is therefore to achieve weaker but more frequent ELMs. The 300-million-euro decision, originally scheduled for last year, on how to achieve this tailor-made solution for ITER was postponed by the ITER team, pending incorporation of the control coils in ASDEX Upgrade. This was because other fusion devices using similar coils -- DIII-D at San Diego being the first -- came up with conflicting results.

The experiments on ASDEX Upgrade now pave the way to clarification: Shortly after the power in the new control coils is switched on, the ELM impacts decline to a harmless level. But they occur often enough to prevent the accumulation of impurities in the plasma. The good confinement of the main plasma is also maintained. The ELMs do not regain their original intensity till the coil field is switched of. This experimental result goes a long way to answering the question how the energy produced in the ITER plasma can be properly extracted.

But the goal has not quite been attained: This is because the plasma edge of ITER cannot be completely simulated in smaller devices such as ASDEX Upgrade. It is therefore all the more important to understand exactly the processes underlying the suppression of ELMs; this calls for sophisticated measuring facilities for observation and a powerful theory group for clarification. The physical theory hitherto acquired at IPP does fit the present results, but has yet to be checked and expanded. Till the decision on ITER scheduled for 2012 there is time for solving the problem for the test reactor -- and for a future power plant.

The possibilities afforded by control coils on ASDEX Upgrade are then still far from being exhausted: Another eight coils as of 2012 are to make lots of new investigations possible.