Antimatter-catalyzed nuclear pulse propulsion (also antiproton-catalyzed nuclear pulse propulsion) is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel to initiate a nuclear chain reaction for propulsion when the fuel does not normally have a critical mass.
Technically, the process is not a '"catalyzed'" reaction because anti-protons (antimatter) used to start the reaction are consumed; if they were present as a catalyst the particles would be unchanged by the process and used to initiate further reactions. Although antimatter particles may be produced by the reaction itself, they are not used to initiate or sustain chain reactions.
Description
Typical nuclear pulse propulsion has the downside that the minimal size of the engine is defined by the minimal size of the nuclear bombs used to create thrust, which is a function of the amount of critical mass required to initiate the reaction. A conventional thermonuclear bomb design consists of two parts: the primary, which is almost always based on plutonium, and a secondary using fusion fuel, which is normally deuterium in the form of lithium deuteride, and tritium (which is created during the reaction as lithium is transmuted to tritium). There is a minimal size for the primary (about 10 kilograms for plutonium-239) to achieve critical mass. More powerful devices scale up in size primarily through the addition of fusion fuel for the secondary. Of the two, the fusion fuel is much less expensive and gives off far fewer radioactive products, so from a cost and efficiency standpoint, larger bombs are much more efficient. However, using such large bombs for spacecraft propulsion demands much larger structures able to handle the stress. There is a tradeoff between the two demands.
By injecting a small amount of antimatter into a subcritical mass of fuel (typically plutonium or uranium) fission of the fuel can be forced. An anti-proton has a negative electric charge, just like an electron, and can be captured in a similar way by a positively charged atomic nucleus. The initial configuration, however, is not stable and radiates energy as gamma rays. As a consequence, the anti-proton moves closer and closer to the nucleus until their quarks can interact, at which point the anti-proton and a proton are both annihilated. This reaction releases a tremendous amount of energy, of which some is released as gamma rays and some is transferred as kinetic energy to the nucleus, causing it to split (the fission reaction). The resulting shower of neutrons can cause the surrounding fuel to undergo rapid fission or even nuclear fusion.
The lower limit of the device size is determined by anti-proton handling issues and fission reaction requirements, such as the structure used to contain and direct the blast. As such, unlike either the Project Orion-type propulsion system, which requires large numbers of nuclear explosive charges, or the various antimatter drives, which require impossibly expensive amounts of antimatter, antimatter-catalyzed nuclear pulse propulsion has intrinsic advantages.
A conceptual design of an antimatter-catalyzed thermonuclear explosive physics package is one in which the primary mass of plutonium usually necessary for the ignition in a conventional Teller–Ulam thermonuclear explosion, is replaced by one microgram of antihydrogen. In this theoretical design, the antimatter is helium-cooled and magnetically levitated in the center of the device, in the form of a pellet a tenth of a millimeter in diameter, a position analogous to the primary fission core in the layer cake/Sloika design. As the antimatter must remain away from ordinary matter until the desired moment of the explosion, the central pellet must be isolated from the surrounding hollow sphere of 100 grams of thermonuclear fuel. During and after the implosive compression by the high-explosive lenses, the fusion fuel comes into contact with the antihydrogen. Annihilation reactions, which would start soon after the Penning trap is destroyed, is to provide the energy to begin the nuclear fusion in the thermonuclear fuel. If the chosen degree of compression is high, a device with increased explosive/propulsive effects is obtained, and if it is low, that is, the fuel is not at high density, a considerable number of neutrons will escape the device, and a neutron bomb forms. In both cases the electromagnetic pulse effect and the radioactive fallout are substantially lower than that of a conventional fission or Teller–Ulam device of the same yield, approximately 1 kt.