Fusion Propulsion Reactor Concepts

The history of fusion concepts for space propulsion goes back almost to the very beginning of the US fusion program for power generation (the Matterhorn Project). At that time plasma was imagined confined inside a "magnetic bottle'' by means of a specially shaped magnetic field, with hydrogen isotopes fusing while traveling back and forth between the two bottle ends. About half century later, we are still struggling with the many facets of confining plasma [Miyamoto, 2007], but substantial progress has enabled plasma technology to achieve fusion, albeit for the time being by injecting inside the plasma more energy than that due to the fusion process itself: the so-called energy "breakeven" condition must still be reached. Independently, many researchers, quite a few of them belonging to the visionary type, have proposed fusion propulsion concepts. Among them, the more promising appear to be those where plasma is not kept confined to generate electric power, but rather those where the hot plasma products are allowed to escape at their extremely high energy, sometimes after having been mixed with inert propellant. Devices of this class are called open magnetic confinement (OMC) reactors, and are discussed in Section 8.10. Details of their application to propulsion is in [Romanelli and Bruno, 2005] and Appendix B.

In the following discussion of conceptual fusion propulsion systems the level of detail is purposely kept modest, since emphasis is on the effect of fusion power on propulsion, rather than on the specifics of fusion reactors themselves.

By far, the best-known and tested fusion machine is the tokamak, to(roidal) ka (chamber) mak (machine). Fusion reactions are prevented from quenching on the cold reactor walls by magnetic confinement. This word means that the fusing plasma is guided by a magnetic field shaped in such way as to always keep it from touching reactor walls. This class of fusion reactor is called a magnetic confinement reactor, MCR. The conceptual operation of MCR is steady, but the actual mode of operation may depend on the electric transformer needed by the electromagnet supplying the magnetic field imposed. The transformer is a fusion reactor component that links the plasma, viewed as a classic secondary "electric circuit", to the external power supply. If the electromagnet is not superconducting, the unavoidable ohmic heating forces reactor operation to be intermittent, say, stopping once per hour. In any event, the slow degradation of the plasma due to unwanted matter, e.g., detached from the walls by plasma interaction, makes periodic shutdown and cleaning inevitable on MCR conceived for ground power generation. In space operations such regularly scheduled maintenance may be impracticable or impossible because of safety and radiation hazards, and this is a major concern. Space-qualified MCR will probably have to meet much more stringent reliability requirements than are envisaged at the moment for ground fusion powerplants. Note that whatever experience is available for MCR comes from ground fusion tests and experiments: extrapolating to future space propulsion is premature and may be very risky.

Other configurations, embodying different fusion plasma confinement strategies, have been proposed, or tested, or are still at the stage of conceptual suggestions. Among alternatives the second most investigated is inertial confinement fusion reactors (ICR) in which extremely high (gigawatt) laser energy pulses are sent to a very small pellet containing the fuel(s). The energy pulse ablates (i.e., volatilizes) the external layer of the fuel pellet and raises the pellet temperature. The temperature of the volatilized gas is so high that the gas becomes a plasma, radiating very effectively. It is precisely this radiation that compresses ("implodes") the fuel, driving its density and temperature up, and (hopefully) to the point of fusion ignition. Radiative compression obtained in this way may reach 0.1 Mbar (105 atm).

For continuous power generation ICR need to be fed a stream of pellets; each pellet is then "lased", fused and releases power. Thus operation of ICR is necessarily always pulsed, the repetition rate determined by the power demand. This feature may seem awkward to chemical rocket engineers, but is advantageous or convenient when releasing power at destructive energy levels. For instance, the gasoline automotive engine reaches in-cylinder temperatures above 2,500 K, far higher than the melting point of most structural materials; its pulsed operation, however, reduces heat transfer and temperatures to quite acceptable average values. In contrast, gas turbine engines are limited to a much lower 1,800-1,900 K precisely by their steady combustion mode of operation. The Orion concept [Dyson, 2002], in which pulsed nuclear explosions were proposed to push a spaceship, is similar in many ways to an ICR, also because of its ablation physics.

It is far too soon to quantify practical merits of MCR vs. ICR, so both will be summarily described and their issues and shortcoming discussed in what follows.

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