Antimatter catalyzed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel which normally would not be useful in propulsion. The anti-protons used to start the reaction are consumed, so it is a misnomer to refer to them as a catalyst.
Traditional nuclear pulse propulsion has the downside that the minimum size of the engine is defined by the minimum size of the nuclear bombs used to create thrust. A conventional nuclear H-bomb design consists of two parts, the primary which is almost always based on plutonium, and a secondary using fusion fuel, normally lithium-deuteride. There is a minimum size for the primary, about 25 kilograms, which produces a small nuclear explosion about 1/100 kiloton (10 tons, 42 GJ; W54). More powerful devices scale up in size primarily through the addition of fusion fuel. 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 they eventually touch, 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 explode. 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; as such, unlike either the Project Orion-type propulsion system, which requires large numbers of nuclear explosive charges, or the various anti-matter 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 mm 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.
The number of antiprotons required for triggering one thermonuclear explosion were calculated in 2005 to be , which means microgram amounts of antihydrogen.
Tuning of the performance of a space vehicle is also possible. Rocket efficiency is strongly related to the mass of the working mass used, which in this case is the nuclear fuel. The energy released by a given mass of fusion fuel is several times larger than that released by the same mass of a fission fuel. For missions requiring short periods of high thrust, such as manned interplanetary missions, pure microfission might be preferred because it reduces the number of fuel elements needed. For missions with longer periods of higher efficiency but with lower thrust, such as outer-planet probes, a combination of microfission and fusion might be preferred because it would reduce the total fuel mass.
The concept was invented at Pennsylvania State University before 1992. Since then, several groups have studied antimatter-catalyzed micro fission/fusion engines in the lab (sometimes antiproton as opposed to antimatter or antihydrogen).
Work has been performed at Lawrence Livermore National Laboratory on antiproton-initiated fusion as early as 2004. In contrast to the large mass, complexity and recirculating power of conventional drivers for inertial confinement fusion (ICF), antiproton annihilation offers a specific energy of 90 MJ per µg and thus a unique form of energy packaging and delivery. In principle, antiproton drivers could provide a profound reduction in system mass for advanced space propulsion by ICF.
Antiproton-driven ICF is a speculative concept, and the handling of antiprotons and their required injection precision—temporally and spatially—will present significant technical challenges. The storage and manipulation of low-energy antiprotons, particularly in the form of antihydrogen, is a science in its infancy and a large scale-up of antiproton production over present supply methods would be required to embark on a serious R&D programme for such applications.