As plasma parameters such as pressure are raised to the high levels needed for an economical high performance magnetic fusion reactor, various helical perturbations from a nominally axisymmetric plasma equilibria can become unstable. Uncontrolled growth of these helical perturbations can then lead to a deterioration of energy confinement, or even to a complete loss of the plasma altogether through a violent plasma disruption event. An engineering approach which has been considered for mitigating this situation is to provide an active feedback system which stabilizes the high performance plasma.

Engineering issues relevant to the design of magnetic based systems for plasma feedback stabilization of internal tearing modes and of resistive wall modes are discussed herein. Proposed design optimization methods are delineated, and are then illustrated in practice by applying them to the example design of a hypothetical experimental plasma feedback stabilization demonstration facility.

In addition to the physical dynamics of the plasma itself and of the eddy current behavior of metallic structures close to the plasma, the plasma feedback system includes magnetic field sensors distributed spatially near the plasma, digital signal processing electronics to implement an optimal state-estimating "observer" and an optimal controller, and also electromagnets and their associated power circuitry. The magnetic field sensors must be sufficiently numerous and spatially well distributed so that each unstable plasma eigenmode can be resolved without spatial aliasing. Digital signal processing algorithms must be sufficiently fast to continually estimate the amplitude, phase, and rotation rate of each unstable eigenmode based on the magnetic field sensors' signal histories. They must be sufficiently sophisticated to discriminate between field amplitude components produced directly by the plasma perturbations and the components produced in response to optimal controller commands, and they should be sufficiently robust for a range of plasma parameters. The electromagnets must be located close enough to the plasma to drive a controlled opposing field with the proper resolution of helical shapes, and their power circuitry must be capable of producing the commanded amplitude, phase, and rotation rates for the driven field.

*Work supported by U.S. DOE Contract No. DE-AC02-76-CHO3073.