R. L. Forward
A major new form of propulsion has just graduated from thenever-never land of science fiction and has now become a serioustopic for scientific and engineering investigation. In the pastit was called antimatter propulsion, but to emphasize thedifference between past fiction and present reality, I prefer tocall it antiproton annihilation propulsion,1for the use of antiprotons as the form of the antimatter is crucial tothe use of antimatter for propulsion.
For every particle known to exist, there is a mirror imagetwin particle that has its charge, spin, and quantum statesreversed from that of normal particles. As shown in Figure 1,the stable particles that make up atoms--electrons, protons, andneutrons--have mirror twins called positrons, antiprotons, andantineutrons. Conceptually, these could be combined to formantiatoms, such as antihydrogen.
When a particle comes near its antiparticle, they attracteach other and annihilate each other totally converting all oftheir rest mass into energy. When positrons and electronsannihilate they produce gamma rays, which are difficult toconvert to thrust. On the contrary, when antiprotons annihilatewith protons, the annihilation process does NOT produce gammarays immediately. Instead, the products of the annihilation arefrom three to seven particles called pions. On the average thereare three charged pions and two neutral pions. The neutral pionshave a very short lifetime and almost immediately convert intotwo high energy gamma rays. The charged pions have a normalhalf-life or 28 ns. Because they are moving at 94% of the speedof light, however, their lives are lengthened to 70 ns. Thus,they travel an average of 21 m before they decay. These chargedpions contain 60% of the annihilation energy.
Because of the long lifetime and interaction length of thecharged pions that result from the annihilation of antiprotonswith protons, it is relatively easy to collect the charged pionsin a thrust chamber constructed of magnetic fields and to obtainpropulsion from them. As is shown in Figure 2, the energy in the pionscan then either be used to heat a working fluid, such ashydrogen, to produce thrust, or the high speed pions themselvescan be directed by a magnetic nozzle to produce thrust. Evenafter the charged pions decay, they decay into energetic chargedmuons, which have even longer lifetimes and interaction lengthsfor further conversion into thrust. Thus, if sufficientquantities of antiprotons could be made, captured, and stored,then presently known physical principles show that they can beused as a highly efficient propulsion fuel 1.
Since antimatter does not exist naturally, it must be made,one particle at a time. It is a synthetic fuel. It will alwaysrequire much (~l04 times) more energy to produce antimatter thancan be extracted from the annihilation process. Its majoradvantage is that it is a highly concentrated form of energystorage. A tenth of a milligram, about the size of a singlegrain of salt, contains the energy of 2 tonnes of the best rocketfuel known, liquid oxygen/liquid hydrogen. A study that comparedantihydrogen propulsion systems with chemical propulsion systems2 found that antiproton propulsion could possibly be cost effectivefor space propulsion. More importantly, it was mission enabling,in that it would allow missions to be performed that areessentially impossible to perform with chemical fuels.
Antimatter in the form of antiprotons is being made today,albeit in small quantities. As is shown in Figure 3, theantiprotons are generated by sending a high-energy beam ofprotons into a metal target. When the relativistic protonsstrike the dense metal nuclei, their kinetic energy, which ismany times their rest-mass energy, is converted into a spray ofparticles, some of which are antiprotons. A magnetic fieldfocuser and selector separates the antiprotons from the resultingdebris and directs the antiprotons into a storage ring. Thesecollecting rings have stored as many as 1012 antiprotons for daysat a time. To give some scale as to what has already beenaccomplished at these research facilities, 1012 antiprotons havea mass of 1.7 pg. When this amount of antimatter is annihilatedwith an equivalent amount of normal matter, it will release300 J, an engineeringly significant amount of energy.
In a recent experiment,3 a team of scientists took the lowenergy antiprotons in one of these rings, slowed them down toalmost zero velocity, and captured a few hundred antiprotons in asmall electromagnetic ion trap. Other experiments planned forlate 1987 will attempt to capture many millions of antiprotons ina trap no bigger than a thermos bottle. The electromagnetic trapwill be made portable so the antiprotons can be transported toother laboratories for experiments.
In order to use antiprotons as a propulsion fuel, it will benecessary to find a more compact method of storage than an iontrap, which is limited to relatively low ion densities. AnotherAir Force sponsored research program is looking into addingpositrons to the antiprotons in the ion traps and slowly buildingup "cluster ions" of antihydrogen. These cluster ions are largeagglomerations of neutral antihydrogen atoms clustered around asingle antiproton ion. The net negative electric charge of thecluster ion allows it to be kept in the ion trap, yet the mass ofeach ion can be increased until we have an ice crystal withenough charge that it can be electrostatically levitated withouttouching the walls of the cryogenically cooled trap
The use of antihydrogen to power antimatter engines is fairlystraightforward. The small antihydrogen microcrystals, eachweighing about a microgram and having the energy content of 20 kgof LOX/hydrogen, would be extracted electromagnetically from thestorage trap, directed by electric fields down a vacuum line withshutters (to maintain the trap vacuum) , then electrostaticallyejected with a carefully selected velocity into the rocketchamber, where the antiprotons would annihilate with the reactionfluid, heating it up to provide high thrust at high specificimpulse. The annihilation cross section increases dramaticallyat low relative velocity, so the annihilation process occursmostly at the center of the chamber.
Designs of rocket engines to use antimatter are well underwayat a number of engineering laboratories. One simple design4 is based on the NERVA nuclear rocket, with the nuclear reactorreplaced with a tungsten heat exchanger core. The reactionproducts (both gammas and pions) would be stopped in the tungstenand the energy used to heat hydrogen gas passing though the heatexchanger. This engine would use 13 µg/s of antiproton fuel toproduce a specific impulse of 1100 s at a thrust level of4.4x105 N (100,000 lb) for a power level of 2.7 GW. Such anengine could take 100 tonnes of payload to Mars and back in sixmonths (only three months each way) with a mass ratio of 4. Bycomparison, a LOX/hydrogen system would require a mass ratio of18 and would take 12 months to get there and 9 months to getback.
Studies have also started on magnetic bottle reaction chambers5 that have the potential of attaining higher specificimpulse than engines limited by the thermal properties of matter.Analysis of plasma transport coefficients has identified twoparameter regimes of practically lossless operation of a magneticnozzle with a pure hydrogen plasma. The one of interest for anantimatter-heated hydrogen plasma thruster is optically thick,with a density of 3xl019 ions/cm3, a temperature of 2 eV(23,000K), a magnetic field of 5 T, a throat dimension of 1 m,and a pressure of 1000 psi (67 atm).
Because antiproton propulsion promises a major advance inspace propulsion capability, the recently completed Air ForceSystems Command Project Forecast II study recommended that theAir Force start a new program in antimatter propulsion. As adirect result of the Project Forecast II recommendations, the AirForce Astronautics Laboratory at Edwards AFB in California hasreorganized its advanced propulsion activities and formed a newproject called ARIES (Applied Research In Energy Storage). Theproject has two major thrusts - chemically bound excited statesand antimatter. The Air Force Office of Scientific Research hasinitiated a new program on antimatter research in the Physicaland Geophysical Sciences Branch under Col. Hugo Weichel. TheProgram Manager for Antimatter is Maj. John Prince, who evaluatesunsolicited proposals for research on antimatter sciences. InEurope, an Antimatter Research Team (ART) has been formed atTelespazio, SpA per Ie Comumicazioni Spaziali in Italy. Theirresearch work6 will cover antiproton and positron production andstorage, and engine simulations, leading ultimately to technologydemonstrations
The number of workshops concerned with the science andtechnology of antiprotons is growing with each passing year. Ihave been involved in one way or another with most of thefollowing workshops. The Workshop on the Design of a Low EnergyAntimatter Facility in the USA was held at the University ofWisconsin-Madison from 3-5 October 1985. The Antimatter Physicsat Low Energy Workshop was held at Fermi National AcceleratorLaboratory, Batavia, Illinois from 10-12 April 1986. The AGSTime-Separated Antiproton Beam Workshop was held at BrookhavenNational Laboratory, Upton, New York from 18-22 August 1986. TheCooling, Condensation, and Storage of Hydrogen Cluster IonsWorkshop was held at SRI International, Menlo Park, Californiafrom 8-9 January 1987. The Antiproton Science and TechnologyWorkshop was held at RAND Corporation from 2l-22 April l987. TheWorkshop on Intense Positron Beams was held at Idaho NationalEngineering Laboratory, Idaho Falls, Idaho from 18-19 June 1987.Additional planned workshops will be the IV LEAR (Low EnergyAntiproton Ring) Workshop to be held in Villars, Switzerland from6-13 September 1987, and the 2nd Antiproton Science andTechnology Workshop to be held at RAND Corporation, Santa MonicaCalifornia from 6-8 October 1987.
If the next decade of experimental research on cooling andtrapping of antiprotons, the growth and storage of antihydrogen,and the design studies of antimatter rockets and antimatter-powered missions shows promise, then engineering studies will commence on the design and ultimate fabrication of an antiprotonfactory capable of producing about a microgram a year (comparedto the present nanogram per year) . A microgram of antiprotonswith usable energy of 100 MJ could power a test stand run of a1 MW feasibility demonstration rocket engine for 100 s. At thatpoint a lot more would be known about the engineeringfeasibility, cost effectiveness, and desirability of antiprotonannihilation propulsion. Then a decision could be made whetherto proceed with the construction of an antiproton factory thatcould produce the hundreds of milligrams a year needed to run aspace program. Such a factory could be designed to be self-powering, but would require a capital investment comparable tobuilding a 10 GW power plant.
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