The kinetic tandem concept: theory and computer simulations of the potential barriers

The Kinetic Tandem fusion plasma confinement concept is a member of the class of open magnetic confinement systems whose magnetic topology is that of a tube of magnetic flux open at both ends. In open-ended systems the central problem is that of limiting the rate of plasma losses out the ends. In a conventional tandem mirror system end-plugging is accomplished by the generation of positive potential barriers within special short mirror cells located at each end of a long central confinement cell. The kinetic tandem concept accomplishes the same end result by employing dynamic effects, but without the necessity of special end cells. The field employed in the kinetic tandem is a simple axially symmetric solenoidal field whose intensity tapers to low values at the ends. Since the field line curvature is everywhere positive such a field is stabilizing for MHD interchange modes. Into each end are injected ion beams that are aimed nearly parallel to the field line direction. The ions from these beams then are radially compressed, stopped, and reflected back by magnetic mirror action in climbing up the magnetic gradient. In this way ion density peaks are formed between which the plasma is to be confined. As in the original tandem mirror concept, a localized ambipolar potential arises to maintain quasi-neutrality between the ions and the electrons. provided the plasma density in the plugs is higher than that of the plasma contained between them the ions of the central plasma will be confined between the plugs by the positive potential barriers represented by the plugs. The plasma electrons will at the same time be confined by the overall positive potential of the plasma with respect to the ends. In this report some analytical calculations of the formation of the plugs will be given. These calculations were then confirmed and extended by computer simulations, using the LLNL code ICEPIC. Within the assumptions made in the theoretical calculations and in the code representation, fusion-relevant plug plasma parameters were achieved, and no evidence of unstable behavior was detected in the simulations