DIRECT FUSION DRIVE BASED ON CENTRIFUGAL MIRROR CONFINEMENT

Abstract

A concept for direct fusion drive based on centrifugal mirror confinement of thermonuclear plasmas (DFD-CM) is described. In centrifugal mirrors, electric and magnetic fields are combined to confine the plasma within a rapidly rotating annulus of burning plasma fixed between two mirror magnets. High-energy fusion products leave the reactor core at a rate determined by the velocity of plasma rotation and the strength of the mirrors. Those departing through the aft jet-side mirror deposit their energy in a “warm plasma” which then expands through a magnetic nozzle to deliver jet power in the 100-1000 kW range. Fusion products departing through the forward, power-side mirror are converted to electricity to power the reactor. Moderate thrusts at attractive specific impulses (15000+ seconds) are possible. Findings are presented on centrifugal mirror reactor dynamics in propulsion applications, to include new insights into the relationship between mirror and centrifugal components of plasma confinement. Additionally, analysis is presented on reactor operability limits and characterization of viable configurations based on power density, technology constraints, and the ability to self-power. Physics of the warm plasma are discussed, to include estimates for fusion energy deposition. Finally, considerations for Alfvén’s “frozen-in” theorem relative to fusion plasmas and magnetic nozzle performance will be outlined.Analysis indicates the DFD-CM system can self-power, and would be relatively compact. For the 200 kW delivered jet power system, the volume of burning plasma in the CM fusion reactor is estimated to be on the order of 1 m3. Self-powering in propulsion applications requires DFD-CM reactor operation at M_θ>9. This in turn requires electric fields ranging from 40-90 MV/m, and mirror strengths up to 15T. The main losses in the propulsion system are due to heating and ionizing the propellant. These losses decrease with increasing specific impulse. This work has resulted in four contributions. To start, it is the first analysis of the end-to-end performance of direct fusion drive based on centrifugal mirror confinement of the burning plasma. It demonstrates that the concept is thermodynamically feasible with nominal cycle efficiencies of 50 percent based on fusion energy entering the propulsion system. The second contribution is characterization of CM fusion reactor performance and operability. A particular finding is that self-powering DFD-CM reactors in propulsion applications may need to operate at centrifugal Mach numbers greater than 9, as previously mentioned. The third contribution is the development and preliminary application of a set of engineering models of the reactor, warm plasma, and plasma acceleration and expansion. These models are considered moderate fidelity in that they account for first order effects, as well as salient second order effects. The fourth contribution is identifying the possibility that the burning plasma in the reactor and the warm plasma may be electrically coupled. The nature and implications of any coupling are uncertain, and the current research proceeds assuming that the coupling does not occur. However, the question indicates the need for further research.