Antimatter Catalyzed Nuclear Pulse Propulsion

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. With conventional technologies nuclear explosives can scale down to about 1/100 kiloton (10 tons, 42 GJ; W54), but making them smaller seems difficult. Large nuclear explosive charges require a heavy structure for the spacecraft, and a very large (and heavy) pusher-plate assembly. Small nuclear explosives are believed to stop shrinking in overall size and required fissile nuclear materials at around 25 kilograms weight, so smaller pulse units are much more expensive per delivered unit energy, and much less mass efficient than larger ones. 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, but the concept appears to be feasible using technology and infrastructure likely to be made available during the second half of the 21st century, 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 technically sweet intrinsic advantages.

Tuning of the performance to the mission 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 lower thrust, such as outer-planet probes, a combination of microfission and fusion might be preferred because it reduces 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).

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. The current (2011) record for antimatter storage is just over 1000 seconds performed in the CERN facility, a monumental leap from the millisecond timescales that previously were achievable.