Spheromak Physics Development

Abstract

The spheromak is a Magnetic Fusion Energy (MFE) configuration, which is a leading alternative to the tokamak. It has a simple geometry which offers an opportunity to achieve the promise of fusion energy if the physics of confinement, current drive, and pressure holding capability extrapolate favorably to a reactor. Recent changes in the US MFE program, taken in response to budget constraints and programmatic directions from Congress, include a revitalization of an experimental alternative concept effort. Detailed studies of the spheromak were consequently undertaken to examine the major physics issues which need to be resolved to advance it as a fusion plasma, the optimum configuration for an advanced experiment, and its potential as a reactor. As a result of this study, we conclude that it is important to evaluate several physics issues experimentally. Such an experiment might be appropriately be named the Sustained Spheromak Physics Experiment (SSPX). It would address several critical issues, the solution to which will provide the physics basis to enable an advanced experiment. The specific scientific goals of SSPX would be to: * Demonstrate that electron and ion temperatures of a few hundred electron volts can be achieved in a steady-state spheromak plasma sustained by a magnetic dynamo (``helicity injection``). * Relate energy confinement quantitatively to the magnetic turbulence accompanying the dynamo and use this knowledge to optimize performance. * Measure the magnetic field profiles and magnetic turbulence in the plasma and relate these to the science of the magnetic dynamo which drives the current in the plasma. * Examine experimentally the pressure holding capability (``beta limit``) of the spheromak. * Understand the initial phases of the transition of the plasma from an equilibrium supported by a magnetic-flux conserving wall to one supported by external coils. These goals could be achieved in an experiment with duration of a few milliseconds, and can consequently be addressed at a relatively low cost. There are additional goals which would be addressed in a larger, follow-up experiment, the Advanced Spheromak Physics Experiment. These include the achievement of temperatures in the multi-kev range, the control of low mode-number instabilities (perhaps with a feedback system), and the technology of long-pulse current drive. This document reviews past work in the field and describes a number of new results. Recent publications which complement this report are also referenced. These publications also describe the characteristics of an experiment to examine the important spheromak physics issues.