Plasma confinement with external helical fields: what's up and what's coming

Full text: The various toroidal confinement fusion devices that use rotational transform produced by external helical magnetic fields are called stellarators, advanced stellarators, heliotrons, torsatrons, heliacs, etc. but all share the property of using non-axisymmetric fields to produce closed flux surfaces that do not (necessarily) surround a net toroidal plasma current. This feature has important effects on transport and plasma stability which will be put to the test in the new generation of experiments now beginning, which range from LHD (Japan) through Wendelstein VIIAS (Germany) to CHS (Japan), H-1 (Australia) and HSX (Wisconsin-USA). These devices were all designed using modern techniques of 3-D computation and configuration optimisation. The theoretical designs of these different devices place varying weights on specific features of the transport and stability optimisation of the vacuum and finite-pressure-plasma field configurations. But many aspects of how these design features work together can only be tested experimentally. The key issues to be tackled in the experiments are the role of the helical field in determining the net particle and energy transport and limiting plasma pressure through particle drifts and instabilities localised in the helical ripple wells. I will review what we know from the existing experimental database, and what we might learn in this new generation of experiments. (author)