17.3.2 Controlled Fusion Rocket
Another highly promising starship propulsion system is the technique of using a controlled, continuous thermonuclear fusion reaction as the main power source. Because of the tremendous temperatures involved (upwards of 10,000,000 K), no known material can physically contain a fusion reaction. Magnetic fields therefore must be used to contain, compress, and heat the plasma fusion fuel (hydrogen, deuterium, tritium, etc.). Scientists are currently engaged in designing and testing various "magnetic bottle" configurations strong enough to hold such an energetic plasma, in connection with electrical power generation at major fusion research facilities around the world.
The biggest problem is to make the magnetic bottle leakproof enough so that fusion reactions occur in sufficient abundance for the process to become self-sustaining. In terms of propulsion, however, a leaky bottle is exactly what is required. Hot plasma, energized by fusion energy, streams rapidly from the site of the "leak" and produces the desired rocket thrust.
According to Dr. Robert L. Forward, Senior Scientist at Hughes Research Laboratories, a deuterium fusion rocket capable of a steady 1-gee acceleration and consisting 90% of fuel (by mass) could reach a final velocity of 10%c.718 This would mean a travel time to Proxima Centauri of 45 years. Theoretically, a fusion-powered starship could be fueled by hydrogen isotopes drained from the atmosphere of Jupiter or from the icy rings of Saturn, and writer Alan Bond has estimated that a 10 light-year mission could be completed in 60 years.1159
One major problem to be overcome from a practical standpoint is to learn how to deal with the various forms of energy released by a fusion engine. Only about 20% of the energy liberated by nuclear reactions appears as kinetic energy -- direct propulsive thrust -- in the leaking plasma stream. Ten percent is thrown off as heat and ultraviolet radiation, but the lion's share (70%) is released as X-rays. G.L. Matloff and H.H. Chiu have suggested that this energy may be reclaimed by using an auxiliary laser thruster surrounding the fusion reaction chamber.2754 Waste X-rays, absorbed by, say, xenon-doped gas in the laser, are converted into a collimated light beam which serves as a photon thruster.*
* A wide variety of related but "futuristic" propulsive energy systems may be possible. These may include condensed cold neutron reactions, pion fusion (which has been demonstrated experimentally), muon catalysis fusion,2737 Hawking black hole induced fusion,1947 and compact monopole fusor and energy storage devices or magnetic monopole metamatter.1224 Fission rockets are not ruled out,2758 and high-energy superpropellants such as monatomic hydrogen, metallic hydrogen,2684 cryogenic metastable triplet helium (stores 0.5 megajoule/gram) may be available.2736