STELLARATOR THEORY AND NON-AXISYMMETRIC SHAPING

Stellarator Theory and Non-Axisymmetric Shaping

Allen H. Boozer

Department of Applied Physics and Applied Mathematics

Columbia University, New York, NY 10027

The magnetic fusion program throughout the world is focused on the tokamak in which the fusion takes place is an axisymmetric, toroidal plasma. Axisymmetry simplifies many things but also excludes much. With the advance of physics understanding and computational design and manufacturing, what is excluded is becoming ever more important and what is being simplified is become ever more irrelevant. Perfect axisymmetry is not possible in practice, even one part in ten-thousand breaking of symmetry can be important, so effects of non-axisymmetric shaping must be understood even for the tokamak, which is ideally axisymmetric. The two most important benefits of strong non-axisymmetry, as in a stellarator, are: (1) The state of the fusion plasma is externally controllable to an extent unparalleled by any other fusion concept, whether magnetically or inertially confined. (2) The external control is through the magnetic fields produced by coils, which have approximately ten times more degrees of freedom than are available for practical control as a tokamak. In particular, the strong plasma current needed in a tokamak requires an invention to ensure disruptions and beams of relativistic electrons will not do unacceptable damage at the fusion-scale. The stellarator concept becomes particularly important when the demonstration of fusion energy is needed on the fastest possible time scale. The seriousness with which the issue of the increase in the atmospheric carbon dioxide level is being taken implies the cost of each year's delay in developing attractive carbon-free energy sources, such as fusion, is trillions of dollars. This is discussed in a paper supported by this grant, Nuclear Fusion 60, 065001 (2020). A critical element for speed is a reduction in the number of generations of experiments required before a reactor-scale device can be built. Computational design of stellarators can be used to reliably skip generations of experiments. The reliability is made possible by the external plasma control of stellarators and spectacularly demonstrated by the initial operations of the W7-X stellarator in Germany. The principle investigator on this grant is responsible for developing mathematical and other concepts that have made stellarators so successful and his continued innovations are represented by the Nuclear Fusion article.