2013 Paterson medal and prize

Dr Ian Chapman, CCFE Fusion Association. For his experimental investigations of the stability of fusion experiments that have provided critical insights into enhanced fusion performance.

Sawtooth oscillations and edge localised instabilities (ELMs) in tokamaks can reduce plasma confinement and are thus a threat to high fusion performance in ITER (the international fusion experiment being built in France). In a series of experiments Ian Chapman has elucidated the physics of these instabilities and shown how they may be avoided, stabilised or, their impact minimised. His results have a profound effect on the current fusion programme and on ITER’s future performance.

The sawtooth instability produces oscillations in the central temperature of tokamaks. When they are small oscillations they are relatively benign; when they are large they can trigger other instabilities that disrupt the plasma. Such disruptions will be intolerable in ITER. In a series of experiments on many machines worldwide Ian showed how to stabilize and trigger the instability and thereby control their impact. He showed how control of the energetic particle distribution function – particularly the distribution of alpha particles born in fusion reactions – can provide the needed stability. It is now believed to be a solved problem. Edge localized modes (ELMs) are periodic eruptions from the edge of tokamak plasmas that resemble solar flares. In ITER and future fusion reactors such eruptions could rapidly erode the wall surrounding the plasma. Experiments on the MAST experiment at Culham have shown that randomizing fields at the edge of the plasma can reduce the size of the eruptions and in certain conditions completely remove the instability. Using both theory and experiment Ian has shown how the randomizing fields modify the stability. ITER is now considering employing this method to stabilize ELMs.