Hybrid Antimatter-Fusion/Fission Propulsion for Interstellar Exploration

Aaron Palke
March 17, 2011

Submitted as coursework for Physics 241, Stanford University, Winter 2011

Nuclear-Powered Transportation

Since the development of the atomic bomb and the more benign development of domestic nuclear energy production mankind has been thinking up ways to harness the immense amount of energy stored in the nucleus for our transportation needs. To date the most successful foray into nuclear-powered transportation has been in seafaring vessels such as submarines, aircraft carriers, and civilian ice breakers. Nuclear energy is useful for such endeavors since the large size of the vessels allows for adequate shielding from the nuclear reactor. However, this apparent size constraint didn't stop the scaling down of the concept for use in personal automobiles (see for instance the Ford Nucleon, a concept car developed in the late 1950's). In addition to land- and sea-based transportation the other obvious choice is nuclear-powered space exploration.

The first attempt at a nuclear-propelled rocket for space exploration was project ORION. In this design the rocket carries a load of nuclear explosives which are released behind the ship and detonated. The nuclear explosives are designed to direct the energy from the blast toward a pusher plate on the back of the rocket which absorbs the energy and propels the spacecraft forward. [1] This design is similar in many ways to a concept currently being studied called Antiproton-Catalyzed Mircofission/Fusion (ACMF) or alternatively Antimatter Initiated Microfusion (AIM).

Spacecraft Design

The design is a hybrid of various concepts including antimatter, fusion, and fission spacecraft propulsion. Each of these concepts alone has serious problems. Antimatter propulsion would require an unthinkable amount of antimatter given current methods of antimatter production. Fusion propulsion would require huge laser or particle beams or a large magnetic torus to drive the fusion reaction. Most commonly proposed fission propulsion systems are nuclear thermal rockets in which a fission reactor heats liquid hydrogen fuel and expels it creating the required thrust. Nuclear thermal rockets, however, suffer from a low specific impulse, a measure of rocket efficiency defined as the change in momentum of the rocket per unit weight of propellant on earth.

The basic design for AIM and ACMF involves the injection into a cloud of antiprotons confined within a Penning trap of a fuel pellet composed of either deuterium (D) and tritium (T) or D and He-3 in a 9:1 molar ratio with an actinide metal such as U-238. [2] The fissioning of U-238 upon interaction with antiprotons has been documented previously. [3] The energy from the fission reaction provides the driving power for heating and ionizing of D+, He++, and/or T+ and subsequent production of a plasma. The superheated plasma would be confined by a strong nested well potential supplied by the Penning trap. A 600 kV potential supplied by the Penning trap could produce a 100 keV plasma with a density of 6 × 1017 ions/cm3 which is sufficient to sustain a full Fusion burn. [2] Specifically, a cloud of 1011 antiprotons would be needed and a 42 ng pellet of D-3He or D-T could be used which would consume only 5 × 108 antiprotons. Each pulse would last 20 ms and could be repeated 50 times without reinjection of antiprotons. [2] This pulsed propulsion technique is similar to project ORION in that the spacecraft would be driven by a series of nuclear explosions except that in the case of AIM/ACMF the "explosion" is more like the detonation of a small thermonuclear device. Most of the energy from the pulse would be in the form of radiation thus a means of utilizing this energy is needed. In the ICAN-II concept this comes in the form a spherical mass of SiC which would absorb the radiation, heat up to keV temperatures, and form a plasma of the inner surface which would expand and be expelled producing the necessary thrust. [4] The advantage of this type of propulsion system is that, like ORION, it is capable of producing a high specific power (defined as power provided divided by mass of the rocket) while also having a high specific impulse. [4] The ICAN-II concept has been suggested as the rocket of choice for interplanetary missions and it has been calculated that with a maximum Δv = 100 km/s and specific impulse = 13,500 s the total mass of the ship would be 707 tonnes. [4]

Unresolved Issues

The main problems with this concept, as of now, involve the storage and transportation of antiprotons. It is estimated that ICAN-II would need 30 ng of antiprotons (~1017 antiprotons) [4]. Currently, antiproton traps can hold only a much smaller number of antiprotons for only a few weeks at a time. In addition, a Penning trap has not been developed which could confine the plasma of D+, He++, and/or T+ to the requisite density. However, work is ongoing for both of these problems and it is the belief of many researchers that the problems are inherently engineering problems and not limitations of a physical nature.



[1] B. Stetler, "Project Orion", Physics 240, Stanford University, Fall 2010.

[2] G. A. Smith et al., "Aimstar: Antimatter Initiated Microfusion for Precursor Interstellar Missions", Acta Astronautica 44, 183 (1999).

[3] B. Chen et al., "Neutron Yields and Angular Districutions Produced in Antiproton Annihilation at Rest in Uranium", Phys. Rev. C 45, 2332 (1992).

[4] G. Gaidos et al., "Antiproton-Catalyzed Microfission/Fusion Propulsion Systems for Exploration of the Outer Solar System and Beyond", AIP Conference Proceedings 387, 1499 (1997).