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Alternative approaches: concept improvements in magnetic fusion research

While the conventional tokamak, as embodied in JET, is the front–runner in the magnetic–confinement approach to fusion, other concepts are being developed that might, in the long term, prove more attractive for power generation (‘concept improvement’ research). Also, in the shorter term, these concepts might offer a more–rapid and cheaper way of studying ignited or near–ignited plasmas. Their advantages, and disadvantages, are described. The stellarator offers steady–state operation at the cost of coil complexity, new advanced stellarators with overall size comparable to JET are either under construction or being commissioned in Japan and Germany. ‘Spherical–torus’ configurations, of which the most promising is currently the spherical tokamak, offer high–pressure containment in a compact device, but the high power density may mean power–plant technology will be more challenging (e.g. suitable materials). New spherical tokamaks will soon come into operation in the UK, USA and Russia and will test properties in larger higher–current longer–pulse plasmas. Meanwhile, other concepts such as the ‘reversed–field pinch’, magnetic–mirror systems and the dense Z–pinch, have their own advantages, though they are less well developed. The status of the various concepts are summarized as are their potential fusion applications that include electricity generation, acting as a fusion neutron source, and providing a driver for inertial fusion.