Submegajoule Liner Implosion of a Closed Field Line Configuration

Abstract

Adiabatic compression of a preformed closed field line configuration by an imploding liner is considered. Three configurations are discussed: the field-reversed configuration, the spheromak, and the Z-pinch. It is shown that by employing a two-dimensional compression, one can reach a breakeven condition with an energy input into the plasma as low as 100 kJ. Typical initial dimensions of the liner are length, 5 to 6 cm; radius, ∼1 cm; and wall thickness, ∼0.01 cm. Liner mass is in the range of a few grams. It is assumed that the initial plasma beta is of the order of unity; in this case, the final beta is much greater than 1, and the plasma is in a wall confinement regime. Typical plasma parameters for the final state (for the linear compression ratio equal to 10) are density, 10²¹ cm⁻³; temperature, 10 keV; and magnetic field, 10⁷ G. A brief discussion of various phenomena affecting the wall confinement is presented (magnetic field diffusion, radiative losses, and impurity penetration); the conclusion is drawn that the heat losses to the walls are modest and are not a factor that limits plasma enhancement Q. It is shown that at least for relatively thin liners, whose compressibility can be neglected, what limits Q is a relatively short liner dwell time near the maximum compression point. The scaling law for the Q versus the input parameters of the system is derived, which shows a relatively weak dependence of Q on the input energy. Possible ways for increasing the dwell time are discussed. Reactor potentialities of the system are briefly described. It is emphasized that the possibility of performing crucial experiments on small- to medium-scale experimental devices may considerably shorten the development path for the system under consideration. Some nonfusion applications of the system described are mentioned. Among them are burning and transmutation of long-lived fusion products, medical isotope production, a pulsed source of hard X rays, and fusion neutrons.