Handling Antimatter

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    In the folklore of science fiction that activity is usually classified as "A Stupendously Bad Idea", unless youíre fond of the prospect of vanishing in a hardradiant flashboom that simultaneously eliminates from existence several hundred square kilometers of neighboring scenery. If half a gram of antimatter meets an equal quantity of matter, the consequent sublimation of mass into photon vapor involves a quantity of energy roughly equal to that released in the explosion of each of the atomic bombs that blew away Hiroshima and Nagasaki in August of 1945. Postage-stamp quantities of this stuff can whiff away cities, never mind what the thousand-tonne lots carried by ships like Star Trekís U.S.S. Enterprise can do. And yet a century from now antimatter may be in common use, not only in space but on Earth and on the surfaces and in the atmospheres of other worlds as well. Our descendants may even use antimatter in their personal appliances, such as cell phones.

    Absurd? Of course! This is antimatter, the ultimate hypergolic fuel, that Iím talking about. How dare I suggest bringing any quantity of it into an environment (e.g. the bottom of Earthís well-pressurized atmosphere) where it might come into contact with matter? Simply by invalidating the tacit assumptions on which that question is based. The key words here are "touch" and "contact". In storage and in use commercial antimatter will touch its material container no more than my typewriter touches the top of my desk.

    Well, does it? In the sense that the structures of the two objects are compressed in order to balance the weight of one, that the distortions are maximal where the two structures come closest together, and that there is no intervening material between the two structures, the typewriter (more precisely its rubber footpads) does, indeed, touch the top of my desk. But in the sense of particles colliding, coming to the same point in space at the same time, the typewriter does not touch the desktop at all: the maximum extent of the footpads is a cloud of electrons and the maximum extent of the desktop is also a cloud of electrons and those two clouds repel each other as they come close together, thereby preventing actual contact. Thanks to the electromagnetic force and the Pauli Exclusion Principle, the typewriter actually floats a minuscule distance above the desktop. What we come down to, then, is that old piece of Gee-Whiz trivia that says that the bulk of "solid" matter is empty space, empty but for the electromagnetic forcefields that shape the arrangement of the pointlike particles that give atomic matter its mass. Itís that fact that gives us hope that antimatter can be incorporated into material bodies without annihilating.

    Atomic antimatter wonít do at all. Consider what would happen if my typewriter were made of antimatter (Yes, I suppose that the world could get along without Los Angeles County, but thatís not what I have in mind). In that case the maximum extent of the footpads would be a cloud of positrons. If there were no annihilation, the slightest contact between the footpads and the desktop (in the first sense I described above) would result in the typewriter sticking to the desk as if it were epoxied there. But there is annihilation and the electrons and the positrons, drawn together by electric attraction, would mutually annihilate into a blast of 0.511-Mev gamma photons. Then the nuclei and antinuclei, left bare by the loss of electrons and positrons, would rush together for an orgy of annihilation and radiation, but where the mutual annihilation of electrons and positrons is a simple one-stage process, the mutual annihilation of nucleons and antinucleons is a more complex affair.

    The simplest case is the co-annihilation of a proton and an antiproton. The average annihilation yields five pions and on average one third of those are neutral pions. Thus, the 1876.56 Mev manifested in the combined masses of the proton and antiproton is remanifested as 465.23 Mev of charged pions (at 139.57 Mev each) carrying 790.9 Mev of kinetic energy and 224.93 Mev of neutral pions (at 134.96 Mev each) carrying 395.5 Mev of kinetic energy, all on average. The mass energy and the kinetic energy that go into the neutral pions is effectively lost unless we can contrive a way to harness high-energy gamma photons (actually 1.17 percent of neutral pion decays yield a gamma photon and an electron-positron pair, but that will have little effect in the commercial use of antimatter). The electric charge on each of the remaining pions offers a handle that can be gripped by an electromagnetic field to harness the particleís kinetic energy. Each charged pion then decays into a muon (at 105.66 Mev of rest mass) carrying 30 Mev of kinetic energy and a neutrino carrying 3.91 Mev. Finally, each muon turns into an electron or positron (in accordance with the sign of its charge) carrying 53 Mev of kinetic energy by blowing off a 52-Mev neutrino-antineutrino pair. On average, then, each proton-antiproton annihilation gives us 1067 Mev of easily harnessable energy and 809 Mev of more problematic energy (of which 189 Mev goes into neutrinos). With suitable technology weíre essentially guaranteed access to 57% of the energy made available in the annihilation, to 90% if we want to push the technology a little harder, and to 100% if we want to work magic (remembering always that a sufficiently advanced technology....), all subject, of course, to the tax laws of thermodynamics.

 

Particle

Mass (Mev)

decays into

mean proper life

proton 938.28 stable
charged pion 139.57 muon+neutrino 2.6x10-8 sec
neutral pion 134.96 2 gamma photons 0.84x10-16 sec
muon 105.66 electron+neutrinos 2.197x10-6 sec
electron 0.511 stable
neutrino 0 stable
photon 0 stable

 

    If we want something more complicated, we will be obliged to modify those figures because annihilations involving neutrons and/or antineutrons produce higher proportions of neutral pions. However, antiprotons are the simplest (and therefore the cheapest) kind of nucleonic antimatter to make and they give back the highest yield of easily usable energy, so we need not concern ourselves with elements beyond antihydrogen (at least, not yet).

    Antiprotons, being negatively charged, repel and are repelled by electrons, so containment of antiprotons would seem to be fairly simple: we need only insert them into the interatomic spaces of a suitable crystal and electric repulsion will keep them away from the nuclei and, thus, out of nihilistic mischief. If only it were so easy! Unfortunately, because theyíre charged like electrons, antiprotons behave in matter more or less like electrons; specifically, they take up orbits around atomic nuclei. But the antiproton is more massive than an electron, so its deBroglie wavelength is much shorter than that of an electron and its array of atomic orbits correspondingly smaller. The Pauli Exclusion Principle, which prevents an atomís outer electrons from falling into lower orbits, is rendered inoperative by the antiprotonís different properties and orbits. With nothing to stop it, an antiproton will skip gaily down the ladder of orbits and zero in on the nucleus.

Antimatter Batteries

    What we want is a material that will contain antiprotons while preventing them from entering the structure of any of the materialís atoms. Ideally the means of achieving that kind of containment will be passive; that is, something frozen into the materialís structure and not continuously renewed from outside the material. Clearly that something must be an electromagnetic field and indeed there is a configuration of electric and magnetic fields that will keep a charged particle away from the fieldsí sources: itís called a Penning trap.

    If we put an antiproton on the line of symmetry extending between two identical electrodes bearing equal negative charges, the electric field between the electrodes will push the antiproton toward the center of that axis and act to keep it there as long as the antiproton remains precisely on that axis. That central point is one of unstable equilibrium: if the antiproton wanders off the axis, as it will inevitably do, the lateral components of the electric field will accelerate it away from the axis. But if we have enough electric current flowing around a coil centered on that equilibrium point, the resulting dipolar magnetic field will deflect the antiproton back toward the axis. Thus laboratory Penning traps comprise a pair of pointed electrodes facing each other across the center of a magnetic coil. But we donít want a Penning trap that creates a point of pseudostable equilibrium: we want a version that creates a line of pseudostable equilibrium. Such a device would comprise two parallel cables carrying electric current in opposite directions and two equally charged wires running parallel to each other above and below the centerline of the plane defined by the cables, that centerline then being where appropriately charged particles would gather.

    We could actually make a crude proof-of-principle prototype of an antiproton storage battery with our relatively crude technology of integrated-circuit manufacture. Each cell of the battery, perhaps the size of a human hair, would be a linear Penning trap comprising an evacuated channel defined by a pair of ferromagnetic strips (instead of current-carrying cables), suitable insulating support, and a pair of thin-film linear electrodes. Laid down with additional electrodes so that they can be operated as charge-coupled devices for moving antiprotons, side by side in layers and layer upon layer, those cells will form an anisotropically porous material into which antiprotons can be pumped for storage and from which they can be retrieved for use. At normal room temperature the average per-particle kinetic energy of antiprotons is 0.026 electron-volts, so moderate field strengths will be sufficient for containment.

    Thatís nowhere near the ideal for two reasons. First, because the electric fields of the Penning traps must be established and maintained by the output from an external voltage source, the battery is not the purely passive "fill-and-forget" type that we want. And second, if weíre very good with our technology, we might be able to make reliably unflawed batteries with container-to-content mass ratios as low (!?) as a billion to one.

    One tonne (1000 kg) of battery to hold one milligram of antiprotons may seem a depressingly unpromising prospect, even when we remind ourselves that one milligram of antiprotons mixed with an equal mass of protons releases 50,000 kilowatt-hours of energy, the equivalent of 43 tonnes of chemical high explosive. (One kilogram contains 5.98x1026 protons, so Ĺ kg protons + Ĺ kg antiprotons yields 8.98755x1016 joules, which is equivalent to 21.5 megatons of TNT or 25x109 kW-hr.) Such a thing will nonetheless be useful, though only to synchrotron laboratories. With its product limited to that one application, the technology of antimatter containment will likely remain frozen at that level of development until substantial improvements are made in the technology of antimatter production.

    The word "inefficient" just does not adequately describe modern techniques of antiproton production. According to one report from CERN, 800,000 protons of 26 Gev apiece slamming into a copper target 11 centimeters thick are needed to create one antiproton, which carries on average 3.5 Gev/c of linear momentum (1.25x10-6 antiproton per incident proton). Similar figures can be had from Fermilab, Serpukhov, and other synchrotron laboratories around the world. However, we are in only the fourth decade (as of 1989) of our ability to create antiprotons (The first antiprotons made in the Bevatron at the Lawrence Berkeley National Laboratory in 1955.). When the breakthrough comes it will bring with it a demand for lighter batteries because the antimatter factories built on that new technology will be located in space where they will draw power from the only source robust enough not to render the whole still-inefficient enterprise absurd, the one that blows four megatonnes of light into the void every second.

    Indeed, future technology may use the light directly to create the antimatter. We know that gamma rays produce electron-positron pairs when they encounter matter, so we infer that sufficiently energetic photons can also realize proton-antiproton pairs from the quantum vacuum. To realize an electron-positron pair the gamma photon must carry at least 1.022 Mev of energy. Such a photon has an electric field that reaches an intensity of 2.01x1018 volts per meter. For ordinary light that field corresponds to an intensity of 1.5x1031 kilowatts per square meter. The corresponding figures for proton-antiproton production are 3.69x1021 volts per meter and 5.057x1037 kilowatts per square meter. Such intensities might be produced by laser beams focused on a diffraction-limited spot. If the intensity doesnít go high enough, then perhaps a relativistic beam of ions through the spot will provide the catalyst for antimatter formation: in the ionsí frame of reference the light will display a higher intensity than it does in the lasersí frame.

    The first improvement in antimatter batteries will likely be in the means of their production. If we are ever to achieve decent container-to-content mass ratios, we will need to have our batteries made by nanoscopic microbots. They will remove all unnecessary bulk from the design and will replace the thin-film electrodes with ferroelectric material, such as barium titanate or lithium niobate. Thus they will create the desired fill-and-forget batteries with mass ratios as low as a million to one or less.

    What could we do with a gram of antiprotons in a one-tonne battery? If we were to mix one gram of antiprotons slowly into 800 tonnes of water in a properly shaped reactor, the resulting stream of hellfury would be capable of boosting a 200-tonne rocketship off Earthís surface and into low Earth orbit (for a delta-vee of 10 km/sec we need 10-9xM of antimatter, in which M represents the mass of the ship and its propellant.). If the cost of making antimatter drops below $200,000,000 per gram, that kind of space transportation system will become more or less competitive with our chemically-fueled Space Shuttle, depending on how much of that 200 tonnes of rocketship is paying cargo. Until the cost drops substantially below that level we can expect that rocket propulsion will remain the only commercial use of antimatter.

    If we want to get more ambitious, we note that for spaceships traveling at speeds less than half the speed of light we have Mantimatter=0.9Mship(v/c)2. At 300 km/sec (0.001c) a ship crosses 1 AU in 139 hrs (5.79 days), which means that the ship would make the trip from Earth to the moons of Jupiter in less than a month. For such a mission a 100-tonne ship needs 100 grams of antimatter, making the ship all battery. We will need something better.

    Can battery mass ratios be dropped further? We could make our rocketships a bit more profitable if we can. But this is about the stage at which we will encounter a phenomenon that is both a problem and an opportunity.

    Up to this point if we had asked for a detailed description of a Penning cell, one of the tacit assumptions in that description would have been that the atoms comprising the cell were motionless at their positions in the crystalline lattices of the materials making up the cell. At the scales weíve been contemplating, thatís a valid assumption. But when we consider the prospect of devoting much less than a million atomic mass units of material to contain each antiproton, then we are obliged to confront the fact that, at any temperature you care to name, atoms move, each atom vibrating about the position it would occupy if it were indeed motionless. Thatís not meant to suggest, though, any imagery of atoms obtruding themselves into a Penning cellís shrunken central channel and snatching up antiprotons that will then blow up their nuclei. The actual motions of the atoms due to thermal vibration are too small for that to be a valid picture. The phenomenon with which we are concerned is more subtle.

    Because the motions of the atoms in a solid are small, solid-state physicists have devised an elegant model of crystalline solids in which the various phenomena can be considered separately or combined like the transparent overlays in an anatomy text. The basic solid is considered to consist of atoms absolutely motionless at their sites in the crystalline lattice. Superimposed upon that static picture we have one of a box, shaped like the crystal in question, filled with a gas of ultrafine particles called phonons. Devised according to the rules of the quantum theory, phonons represent the vibrations of the atoms: they are the acoustic/thermodynamic analogue of those quantized vibrations of the electromagnetic field that we call photons. If the material in question is a metal and we thus add to our picture one of its fog of conduction electrons, we will see that the electrons and the phonons collide with each other, the collisions tending to produce more phonons (heat) and to retard the collective motion of the electrons (electrical resistance). If the metal under consideration is one of a class with relatively high resistivities, indicative of a strong electron-phonon interaction, and if its temperature is sufficiently low, then phonon collisions with the conduction electrons will cause those electrons with equal and oppositely directed momenta to behave as if they were each attracting the other, so bound together in their subatomic pas de deux that neither can interact with any other phenomenon that does not provide the energy necessary to break the pair apart. In their consequent freedom, to float through the metal with no retardation of their collective motion (zero electrical resistance) and to reorient their motions so that the tiny loop of current that they represent can act to negate any magnetic field within the metal (the Meissner effect), those electron pairs produce both of the defining characteristics of superconductivity. That kind of magic, electron-phonon interactions creating electrical resistance and magnetic passivity and also their antitheses, along with the recent discovery of the perovskite superconductors should give us a strong hint that the adventure of solid-state physics is only just beginning.

    In terms of that model our concern that making Penning cells progressively smaller will enable thermal vibration to interfere with their function translates into an equivalent concern that shrinking the channels in which antiprotons are stored will make the phonon-antiproton interaction stronger. That concern then presents us with the prospect that in the spectrum of phononic blows being rained upon the antiprotons some combination will eventually form a knockout punch: imagine an antiproton, being bounced around in its channel, coming close to an atom that has just become the temporary material locus of an unusually large number of phonons and that thus, via its electron-cloud boxing glove, can transfer to the antiproton enough kinetic energy to knock it out of the channel and into the Penning cellís material structure, where it will then work unfortunate mischief. We certainly donít want that to happen, but we do want to continue improving the antimatter capacity of our batteries. Now, with the theory of superconductivity still fresh in our minds, we are pretty much obliged to entertain the thought in which that troublesome phonon-antiproton interaction is turned to our advantage.

    The effects that phonons cause are determined by the spectral distribution of phonons (the relative proportions of phonons with different frequencies), their densities, and their modes of propagation within the material. Those qualities are determined by the masses and electronic structures of the atoms making up the material, the strengths and orientations of the chemical bonds holding those atoms together, the presence of impurities and structural flaws in the material, and so on. We face a virtual infinitude of possibilities and weíre still working on the simple stuff: those new (as of 1987) high-temperature superconductors that were such an astonishment two years ago consist of only four different kinds of atoms (yttrium, barium, copper, and oxygen) organized into unit crystalline cells of less than fifty atoms apiece. To expect similarly simple arrangements of atoms to solve our problem with antimatter batteries thus seems entirely reasonable. We can look forward with some confidence to the creation of a material that will either augment the antiproton-trapping effect of the Penning fields woven into its structure or replace the Penning fields altogether. Some of the more obvious mechanisms that such a material might manifest are:

    1) antiproton superconductivity; in which antiprotons sloshing around in their Penning cells are compelled by the phonon-antiproton interaction to form pseudobound pairs whose members are thereby made immune to any further interactions that might compromise their containment;

    2) phonon-resonance trapping; in which the material contains vacancies into which the phonon spectrum will automatically push anything resembling an electron with 938.28 Mev of rest mass (and thatís a perfectly accurate description of an antiproton).

Whatever mechanism we end up exploiting, it will bring an improved efficiency of containment. If we can achieve efficiencies as high as that represented by only one unit cell of crystal in the material for each antiproton contained, then we can anticipate container-to-content mass ratios as low as a thousand to one.

    With that kind of efficiency of containment a battery pondering one hundred grams and bulking about the volume of a modern D-cell will contain one decigram of antiprotons. Combine that decigram with an equal mass of protons and youíll get the energy released by 43 hundred tonnes of chemical high explosive. If the reactor/transformer into which the antiprotons are fed converts the energy released by annihilation with 90% efficiency, then that battery will provide 4.5 million kilowatt-hours of useable energy, enough to provide one kilowatt (1.36 horsepower) continuously for 513 years.

    A partial list of applications for such a battery looks like this:

    1) personal rocketships; if one gram of antimatter can shove a 200-tonne rocketship into low Earth orbit (which, as we all know, is dynamically halfway to anywhere), then what will a tenth of a gram do for a 10-tonne RV?

    2) android robots; the Asimovian robot may turn out to be more antimaterial than we ever suspected: not only does it have positrons on the brain, but it also has antiprotons in its heart. This may explain why weíve never seen Cee Threepio or Mr. Data recharging.

    3) human metabolism; using nanotechnology, someone will someday create an antimatter-powered organ that, when implanted in the body, will remove all metabolic waste from the blood, transform it, and return it to the bloodstream as oxygen and food. The kind of totally closed-cycle lifestyle that implies would seem to be a useful adaptation for a spacefaring people. Of course, the technology will require some intensely robust power softeners: one would get a helluva case of heartburn if anything goes wrong with the power generator.

    4) weapons; an unpleasant thought, but if weíre going to go boldly startrekking where no human has gone before, I definitely want a hand phaser that will bring the alien equivalent of Tyrannosaurus rex to an immediate smoldering dead stop.

    We can go on listing applications of antimatter, but ultimately the effort is futile. Making such a list is a bit like asking Benjamin Franklin to describe an electric economy two centuries after his work. He simply could not have conceived most of the things that we do with electricity.

    By the time the things on our list are feasible robots of all sizes will be producing virtually all the material goods that people use, including antimatter, so cost will be a minor consideration. Civilization may run almost entirely on antimatter by then purely as a matter of convenience. If that civilization supports a Terran population of ten billion at ten kilowatts of continuous output per person (which represents a truly affluent lifestyle), then it must import to Earth nearly fifty-four tonnes of antimatter every day. Arriving at isolated spaceports in tankers, the antimatter will be distributed to users through a world-spanning network of cables based on battery material, presumably the kind that uses antiproton superconductivity. (The safety measures alone that such a civilization will require would make an interesting study since they will be based on the firm knowledge that any break, however slight, will instantly transform a cable into a giant fuse burning rapidly toward the biggest bomb in history).

Antimatter Tankage

    The batteries weíve been contemplating so far are fine for uses involving gram and subgram amounts of antimatter. But we may want someday to deal in multi-tonne lots, as might be needed for interstellar flight. Itís reasonable to assume that any space-warping hyperdrive, if such a thing is possible, will consume energy at power densities only attainable through matter-antimatter annihilation. For such applications our solid-state batteries wonít do at all: we will need the means to move and to store antimatter in bulk.

    What comes immediately to mind is a more-or-less conventional fuel tank made of solid antimatter. Fusion will be used to convert antihydrogen into heavier elements of the antimatter periodic table and then from those heavier elements we can make a tank that includes superconductors in its structure so that it can be supported inside a material starshipís fuel hold by magnetic fields via the Meissner Effect. Unfortunately, thatís not a purely passive system. If one of the magnetic support fields fails in the wrong way, weíll get a nasty lesson in the meaning of "loose cannon".

    We want a much more passive system, one that more automatically compensates the forces and torques imposed on/by the tank in response to changes in the shipís motion. The ideal system will be one that exploits the internal forcefields of solid matter and their ability automatically to compensate stress, which are summed up in the concepts of Youngís modulus and yield strength used in mechanical engineering. Solid struts and braces will still be the most reliable way to attach a fuel tank to a shipís main load-bearing structure, so what we will want to do is to connect to those struts and braces a material fuel tank that has an antimaterial lining.

    The trick that we need to learn is that of so putting a layer of antimatter against a layer of matter that the layers will adhere to each other without coming into mutually annihilatory contact, even under pressure. At this stage we can only work up the crudest sketch of such an interface, just to convince ourselves that such a thing is possible. The only guarantee we have is that experiments aimed at developing the technology of making such interfaces will be truly spectacular.

    The first phenomenon that we will want suppressed is the electron-positron dance of death. Both substances confronting the interface will thus have to be the best possible electrical insulators, preferably with a deficiency of electrons and positrons at their bare surfaces. Electrons and positrons will still be the first particles to meet if those surfaces come too close together. The specific phenomenon that will be suppressed is the migration of free charges to random concentrations on the surface of either substance, which concentrations will attract their counterparts on the opposite side of the interface in a positive feedback process that results in a cascade of electrons and positrons arcing across the interface with the usual catastrophic consequences.

    In one possible scheme both surfaces are densely covered with pits and bristles so arrayed that when the surfaces are brought together the bristles on one slip neatly into the pits on the other. Each bristle is a single molecule, a kinky polymer chemically bonded to the surface it supports, and each pit contains a deformed Penning field. Initially, only a fraction of each bristleís thermal energy is manifested in the revolution of the moleculeís atomic nuclei about its longitudinal axis. As the bristle slides into a pit the interaction between its thermal motions and the pitís magnetic field will produce two beneficial effects:

    a) because the atomic nuclei are farther from the axis of revolution than are the electrons/positrons that bind them together, there will be a net force tending to squeeze the molecule and shift a higher proportion of the thermal energy into the rotary mode, thereby making the bristle stiffer (interaction with phonons from the pit material will tend to suppress the other rotary mode, the one that would cause the molecule to stretch laterally); and

    b) because the magnetic field is less dense at the pitís center than it is at the pitís perimeter (resulting from superimposing a dipolar field upon an oppositely directed uniform field), any displacement of the bristle toward the perimeter will modify the forces acting on it, adding a net force pushing the bristle back to the center of the pit.

Those effects assist the bristles in resisting buckling under pressure and in giving the composite material the strength to resist shear at the interface. The atomic nucleus at the end of the molecule acts like a bare ion due to the way charge is distributed in the moleculeís structure: it is initially drawn into the pit by the "upper" portion of the Penning trapís electric field, pulled to the equilibrium point, and then prevented from going any further, even under pressure, by the "lower" portion of the Penning trapís electric field.

    The technology to create antimaterial linings that "float" on material substrates, as described above or by some other means, will stimulate the development of other, associated technologies concerned with plumbing. Itís not enough to be able to contain antimatter in bulk (unless we can devise a way to teleport the stuff from place to place): we must also be able to transfer it from one container to another. To see what that means, letís take a look at the flow of antimatter through a production plant, from the interface where the ions and positrons emerge from the creation process to the coupling through which the liquid antimatter is pumped into a tank aboard a starship or interplanetary freighter. By the time this technology is ready the neutral pions produced in annihilations may no longer represent lost energy, so we can make antiwater, with its antineutrons, the output of our plant with a clear conscience.

    The antioxygen and antihydrogen ions emerge from their creation at very high temperature; which means that they have a wide range of kinetic energies. The initial stage of cooling them involves guiding them straight into a magnetic funnel, a magnetic field whose lines of force become progressively denser. That field makes a sharp right-angle turn so that the ions, originally flying parallel to the field lines, are whipped into circular orbits whose centripetal acceleration stimulates the ions into emitting synchrotron radiation. The more energetic ions will radiate more power than will the less energetic ions of the same species as they whirl about their orbits in the magnetic field, so the ions will emerge from the field with a narrower spread of kinetic energies and a lower overall average energy. They then fly through a klystron that converts most of the remaining kinetic energy into microwaves. When they emerge from the klystron the cold ions spray onto a wide membrane. Because the ions donít touch them, the magnetic-field generators and the klystrons can be made of matter, but the membrane, onto which the ions are adsorbed, must be purely antimaterial.

    Microbotic mechanisms draw the ions into the membrane and combine them with positrons to make molecules of antiwater that are then fed into the membraneís tributary system. With the architecture of a leaf, the membrane channels the antiwater into progressively wider veins and then into a pipe that takes it into a storage tank. From the tank a second pipe, made, like the tank, entirely of antimatter, juts into space.

    At a safe distance that pipe is joined to a material pipe with an antimaterial lining. That lining extends around the flange at the end of the pipe, covering both faces so that the two pipes can be put together flange-to-flange and held together with antimaterial clamps. From the join the material pipe conveys the antiwater the rest of the way to a filling station floating rather forlornly in the void.

    When a ship sidles up to the pump and comes to a dead relative stop, the station reaches out robotic arms and hard docks them to couplings in the shipís hull. A hatch opens and a combination of ultraviolet lasers and electromagnetic fields sweeps the fueling bay beyond it free of any particles of dust that may have accumulated. Into the cleansed bay the station extends its transfer tube, aiming it under laser guidance straight at the shipís fill tube. The caps that protect the ends of the fill tube and the transfer tube flip up 180 degrees so that as they come together each protects the otherís antimaterial lining. Now exposed, the ends of the tubes and the plugs that seal them are aligned to within an Śngstrom, brought together, and the tubes clamped together. Solenoidal magnetic fields, generated by coils on the outsides of the tubes, pull on ferromagnetic elements inside the plugs and thus draw the plugs back to open the tubes for transfer.

    Pumping antiwater in weightlessness should be easy, but thereís a proviso: we donít want the pump to have any moving parts that rub against the lining of the pipe and thus risk abrading or eroding it. The pump could be made entirely of antimatter, incorporated into the pipe through joins like the one described above, and driven through vibrating magnetic fields. An alternative is a free-floating centrifugal pump suspended in the center of a bulge in the pipe and spun by a rotating magnetic field. As long as resistance to the flow is weak such a pump will impart to the antiwater the motion we want, even though a gap separates the impeller from the pipeís inner wall.

    Once the shipís tank is full the most delicate part of the transfer procedure must be carried out. Before the transfer tube can be separated from the fill tube, both tubes must be cleared of all residual antiwater. Blowing antihydrogen into the tubes near where they join is the first step of the clearing procedure, the gas splitting the residual antiwater into two portions and pushing them in opposite directions and through check valves a few meters from the ends of their respective tubes. That step often leaves globs of antiwater clinging to the wall of the tube, quivering like Jell-O as antihydrogen blows past them. Those are removed by alternately charging and discharging ring electrodes plated onto the outside of the tube to create the effect of a moving, nonuniform electric field. Because antiwater is a strong dielectric, like its material counterpart, it is pulled toward where the electric field is densest and thus is pulled by the moving field past the check valve, which is then closed. That leaves only droplets of antiwater in the end of the tube. The system removes those by cycling hot, dry antihydrogen through the end of the tube. Finally, the antihydrogen is pumped out of the tube and the plugs pushed back together where the ends of the tubes join, the plugsí slightly convex surfaces meeting at their centers and then so flattening out under pressure that the space between them collapses in a way that pushes any residual molecules of gas back into the tubes.

    With the tubes properly plugged and made safe, the system releases the clamps and withdraws the transfer tube. The protective caps flip back down into their proper places and the shipís hatch closes. When the transfer tube is fully retracted the robot arms undock from the couplers in the shipís hull and the shipís crew blithely fly their vessel to its next mission.

    Blithely?! Will any sane person, can any sane person, ever be so casual about sitting on enough antimatter to fry a continent?

    One of the motifs prominent in the symphony of technologyís growth is the "taming of dangerous powers". From the first domestication of fire the evolution of Humanity from bands of clever animals into nations of civilized godlings has been conditioned by the confrontation of dangerous powers and the consequent transmutation of fear into careful exploitation. A psychoassayist would be compelled to conclude that one of the more important elements of the human spirit is just that urge to play the troubleseeker. Not satisfied to have put copper harnesses on the fire of heaven, we set out to find bigger and hotter matches with which to play and indeed found them. The fear inspired by the actinic fire of fissioning uranium still crackles through our culture, yet to be transformed by reason and invention into the soft glow of calm self-assurance. Itís too early to fear antimatter, but that time will come in its turn and make its own contribution to the ultimate temper of the human soul. And will that end our raceís playing with matches? Well, only until we learn how to play with supernovae.

Appendix: Just for Fun

To be sung to the tune of "Alouette":

Antimatter, potent antimatter

just a small amount will blow your town away.

If you touch some, you will soon regret it.

Annihilation can ruin your whole day.

    Does it make a big, hot boom?

    Yes, it makes a big hot boom!

Big hot boom. Crack oídoom. Oh,

Antimatter, nasty antimatter,

if you see some, you better get away.

efefef

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