Stellarator pro and con

Abstract

The cancellation of the National Compact Stellarator Experiment ( Physics Today July 2008, page 25 ) leaves a hole in the US and world fusion programs that are focused on ITER. Two physics points define the importance of the hole that NCSX filled. First, the shape of the plasma is the primary design freedom of magnetically confined fusion plasmas. The other determinants of plasma equilibria, which are the pressure and current profiles, are largely self-determined. Second, the excellent confinement of tokamaks, such as ITER, does not require axisymmetry. Only quasi-axisymmetry is required, which greatly increases the freedom of plasma shaping.In quasi-symmetry the magnetic field lines lie on nested toroidal surfaces, and the magnetic field strength on those surfaces has a symmetry—even when the shape of the surfaces does not. Particle trajectories are determined by the magnetic field strength, independent of the shape of the magnetic surfaces, and quasi-symmetry ensures the preservation of the constant of the motion that gives good confinement in axisymmetry. The deviation from axisymmetry can have any magnitude as long as it is constrained by quasi-axisymmetry. Axisymmetric shaping—aspect ratio, ellipticity, triangularity, and squareness—is considered essential to achieving the ITER mission, but most of the shaping freedom of toroidal plasmas requires the breaking of axisymmetry.The NCSX stellarator was the only experiment in the world designed to study quasi-axisymmetric shaping other than in the axisymmetric limit. Although the project is canceled, its costs do establish a required financial scale. The highest cost estimates for NCSX construction and research were about 15% of the annual US non-ITER construction budget for fusion, or about 1% of the envisioned world ITER budget. Expertise on quasi-axisymmetric shaping would give the US unique capabilities in exploiting the information from ITER to make fusion a reality, if that expertise were developed by the time the ITER information becomes available.As the primary design freedom, quasi-axisymmetric shaping is clearly important. It is the only type of non-axisymmetric shaping that can be applied to ITER-like plasmas when the fusion program moves to the design of a demonstration power plant. Non-axisymmetric shaping provides the only known solutions to a number of issues that must be addressed before magnetic fusion energy can be a reality. 1 1. For a discussion of issues facing magnetic fusion, see Priorities, Gaps, and Opportunities: Towards a Long-Range Strategic Plan for Magnetic Fusion Energy, Fusion Energy Sciences Advisory Committee, US Department of Energy, Washington, DC (2007); available at http://www.ofes.fusion.doe.gov/FESAC/Oct-2007/FESAC_Planning_Report.pdf. Management problems led to the cancellation of NCSX. Such problems cannot be allowed to undermine the fundamental strategic objectives of US fusion research: to develop the knowledge base for fusion energy, to have a world-leading fusion program, and to ensure the success of the ITER mission.REFERENCESection:ChooseTop of pageREFERENCE <<1. For a discussion of issues facing magnetic fusion, see Priorities, Gaps, and Opportunities: Towards a Long-Range Strategic Plan for Magnetic Fusion Energy, Fusion Energy Sciences Advisory Committee, US Department of Energy, Washington, DC (2007); available at http://www.ofes.fusion.doe.gov/FESAC/Oct-2007/FESAC_Planning_Report.pdf. Google Scholar© 2008 American Institute of Physics.