Antimatter The Ultimate Fuel

Perhaps because of television's Star Trek series, everyone has heard of antimatter. Unlike fission and fusion, which convert less than 1% of the reactant mass to energy, all of the mass of a matter/antimatter reaction is converted into energy.

The matter/antimatter reaction violates no physical laws, and the technology of antimatter production and storage is making great strides. The only obstacle to the construction of large antimatter fuelled rockets capable of approaching the speed of light is the enormous cost of this resource.

Forward and Davis (1988) outline the early history of antimatter research. After its prediction by Paul Dirac in 1929, the antielectron (positron) was discovered -using cosmic ray measuring plates - by Carl Anderson in 1932. Discovery of the more massive antiproton required an energetic nuclear accelerator, and was accomplished by a group directed by Emilio Segre in 1955.

A particle and its antiparticle have opposite electrical charge and therefore attract one another. The mass of both interacting particles is converted into gamma rays. Eugen Sanger applied this effect to his annihilation photon rocket concept in 1965. An annihilation photon rocket operates by first combining matter with equal amounts of antimatter, and then reflecting the gamma rays out the ship's rear as exhaust. Unfortunately, the demands of such gamma ray reflection seem somewhat beyond our technology.

Inspired, perhaps, by Robert Forward, a small band of interstellar-flight researchers began to study antimatter in the early 1980s. Massier (1982) reviewed early concepts for antimatter production and long-term storage. Existing antimatter 'factories' are very inefficient, as they operate by projecting a very energetic particle beam against a stationary metal target. A few antiparticles are produced through the beam/target interaction, and a small fraction of these are collected.

An improved and uprated solar powered antimatter production facility was proposed by Chapline (1982). The cost of this facility would be more than US$1012, and about 1 kg per year of antimatter could be produced. Also in 1982, Zito applied cryogenic confinement to design a demonstration matter/antimatter reactor for space propulsion applications.

Forward (1982) examined aspects of the exhaust from a matter/antimatter rocket. The first particles to appear after protons and antiprotons annihilate are pions. From the point of view of an unaccelerated observer, these electrically charged particles travel an average of 21 m before they decay into muons. Because of their high velocity, the muons travel about 2 km before they decay into electrons, positrons and neutrinos. Farther downstream from the spacecraft reaction chamber, the electrons and positrons interact to produce annihilation gamma rays.

Cassenti (1982) investigated the efficiency of an antimatter rocket if it focuses pions or muons by magnetic nozzles. If pions are focused, as much as 67% of the energy released in the proton-antiproton interaction can be transferred to exhaust kinetic energy. If muons are focused, this efficiency is about 40%. Working with these efficiency factors, Morgan (1982) estimated that the exhaust velocity of a pion-relecting matter/antimatter rocket could be in excess of 0.9 c, and the vehicle acceleration could approximate 0.01 g.

Cassenti also investigated the kinematics of antimatter rockets. For velocity increments less than about 0.5 c, the ratio of reaction mass to fuel mass is about 4, since the ratio of antimatter to matter mass can be optimised. Forward (1982) applied this to determine that for velocity increments less than about 0.3 c, the optimum antimatter mass is always less than 1% of the total spacecraft launch mass. He also presented a simple formula relating the antimatter fuel mass (Mf,am) to unfuelled ship mass (M0):

M0 c c where Vin and Vfin are respectively the spacecraft velocities at the beginning and end of antimatter rocket operation.

Exercise 6.6. An antimatter rocket with an unfuelled mass of 107 kg is to be accelerated from rest to 0.1 c. The total fuel mass (from Cassenti, 1982) will be about 4 x 107 kg. Apply equation (6.12) to determine how much antimatter is required; then use an optimistic estimate of antimatter costs from The Star-flight Handbook, US$1010 per gm, to estimate the mission cost.

Magnetic coils

Antiproton input o

Antiproton input o-o-o o

Figure 6.5. The beam-core engine: one type of matter/antimatter annihilation rocket.

Proton input

Figure 6.5. The beam-core engine: one type of matter/antimatter annihilation rocket.

As part of the NASA supported Advanced Space Propulsion effort, recent researchers have attempted to reduce the costs of antimatter propulsion so that serious mission planning can begin. Schmidt et al. (1999) have recently examined the cost savings if tiny amounts of antimatter are used to initiate fission and fusion reactions in much more massive micropellets. Such a strategy could ultimately reduce the antimatter fuel cost of an antimatter propelled interstellar precursor mission to about US$60 million. The specific impulse for such an antimatter interstellar-precursor mission is in the range 13,500-67,000 s.

As discussed by Schmidt et al. (1999), the current world production rate for antimatter is 1-10 nanograms per year. Billions of dollars of investment would be required to obtain milligrams per year and reduce antimatter cost to trillions of US$ per gramme. Because of its extreme volatility, this hazardous substance might be 'mass' produced in orbital or lunar antimatter factories.

Schmidt et al. (1999) also reviewed recent research on antimatter rocket designs, which could be used if the cost of antimatter drops dramatically. One of these - the beam-core engine (Figure 6.5) - could have a specific impulse as high as 107 s, with 60% of the reaction energy transferred to the pion exhaust. The vehicle structure for a beam-core engine would be about 20% of the propellant mass.

As well as engine design and antimatter production, recent researchers have considered approaches to long-term antimatter storage. Gaidos et al. (1999) discussed a portable penning trap (Figure 6.6) that can store up to 330 million ions per cubic centimetre in a 3.5-kiloGauss magnetic field. As discussed by Howe and Smith (1999), higher density antimatter storage for true interstellar

r antiproton plas antiproton plas

Direction of magnetic field -►

Positive electric potential repels antiprotons from walls of trap

Figure 6.6. A penning trap utilises electric and magnetic fields to contain antiprotons.

missions might apply storage-ring techniques being developed for advanced particle accelerators.

Antimatter might remain too expensive a resource for human occupied missions to the stars, but could antimatter ships be miniaturised for application with micro-technology or nanotechnology? This question has recently been investigated in papers by Lewis et al. (1996) and Gaidos et al. (1998). Fermilab, in Batavia, Illinois, produces 5-10 nanogrammes of antimatter per year, and in the near future this production rate could be raised by a factor of about 10, at a cost of about 108 antiprotons per dollar. A scheme involving antiproton induced fission/ fusion could conceiveably boost a small spaceprobe to a velocity of 129 km s-1. In this approach, a small antiproton burst is directed at a small pellet of fissile material. The several-thousand electron volts released in this antimatter induced fission could in turn be used to ignite a larger fusion micropellet.

In a follow-up paper, Gaidos et al. (1999) described how spacecraft mass could be reduced to about 400 kg, and a velocity increment of about 1,000 km scould be achieved with a very small antimatter requirement. Such a craft could traverse 10,000 AU in 50 years. Further efficiencies could be obtained if the fusion fuel cycle were aneutronic.

Halyard (1999) considered antimatter assisted missions to nearby stars that could be accomplished using projections of current technology. An a Centauri flyby mission might require about a century for a 10,000-kg payload. A multistage mission to orbit Barnard's Star (about 6 light years distant) with a 105-kg payload would require an interstellar cruise of almost three centuries. Increased efficiency and lower costs for antimatter production are essential if these travel times are to be reduced.

As noted in Science (284, pp. 1597-1598, (1999)), former NASA administrator Dan Goldin has proposed a research partnership between high-energy physicists and NASA. One of the fruits of this initiative might be greatly increased efficiency and reduced cost in the mass production of antimatter. Such a development will hasten the development of a true interstellar capability.

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