How might a realistic antimatter reactor work?

[Darth Wong]

AM power is actually completely worthless as an actual power source for society, because unless you can find a source of naturally existing antimatter, its full-cycle net energy production is negative.

It's really a no-brainer to say that it is easily out-competed by nuclear power, which uses a fuel that you simply dig up out of the ground, rather than one which you have to create yourself (at enormous energy cost, exceeding whatever you would get out of it later).

One could argue that practical and efficient use of AM as a energy carrier is an arguement though, correct?

The words "practical" and "antimatter" do not belong in the same sentence. It is wildly impractical, and incredibly dangerous. The only reason you would use it is some extraordinary need for energy density. It certainly does not constitute a viable power source for society at large. At most, it would be a highly specialized way of packing a lot of power into some kind of exotic device or spacecraft.

[Singular Intellect]

The words "practical" and "antimatter" do not belong in the same sentence. It is wildly impractical, and incredibly dangerous. The only reason you would use it is some extraordinary need for energy density. It certainly does not constitute a viable power source for society at large. At most, it would be a highly specialized way of packing a lot of power into some kind of exotic device or spacecraft.

Like I said, I understand it's a negative net energy process. Thus assuming it were somehow possible, it would merely be a very high density energy carrier (not source) system.

[Turin]

If you have accelerators which can fire the ferrous antimatter atoms in a particular direction, you could, in theory, build up a tank atom by atom, provided the impact velocity is not too great. Another method would be to fabricate a chunk of anti-iron and then use lasers to remove excess material until you get the shape you want. Of course, this would take a very long time and yes, the resulting piece of equipment would be really expensive, but that was part of the premise anyway. The bonus is that it allows you to use many kinds of antimatter fuel which are normally impractical for storage, and to pump them far more easily.

In that case, why build a tank at all?

If we're handwaving the idea that this civilization can make anti-matter of whatever element in the first place, why not just use anti-iron as your fuel? You can contain it using ferromagnetism, and just have it be stored in the form of very small anti-iron pellets (condensed into droplets in some kind of microgravity forge, for example). Remove the pellets from the containment area on as-needed basis. AFAIK, the energy density for M/AM reactions is directly proportional to the mass regardless of its elemental composition, so the density of the fuel is actually an advantage -- you can store it in a smaller volume.

[Darth Wong]

If you have accelerators which can fire the ferrous antimatter atoms in a particular direction, you could, in theory, build up a tank atom by atom, provided the impact velocity is not too great. Another method would be to fabricate a chunk of anti-iron and then use lasers to remove excess material until you get the shape you want. Of course, this would take a very long time and yes, the resulting piece of equipment would be really expensive, but that was part of the premise anyway. The bonus is that it allows you to use many kinds of antimatter fuel which are normally impractical for storage, and to pump them far more easily.

In that case, why build a tank at all?

If we're handwaving the idea that this civilization can make anti-matter of whatever element in the first place, why not just use anti-iron as your fuel? You can contain it using ferromagnetism, and just have it be stored in the form of very small anti-iron pellets (condensed into droplets in some kind of microgravity forge, for example). Remove the pellets from the containment area on as-needed basis. AFAIK, the energy density for M/AM reactions is directly proportional to the mass regardless of its elemental composition, so the density of the fuel is actually an advantage -- you can store it in a smaller volume.

Because anti-iron is bound to be vastly more difficult to make than anti-water. You only have to make a tank once, but you might refill it hundreds or thousands of times with fuel. It makes sense to front-load the costs into the tank, not the fuel. I already pointed this out to MoO.

[Turin]

Because anti-iron is bound to be vastly more difficult to make than anti-water. You only have to make a tank once, but you might refill it hundreds or thousands of times with fuel. It makes sense to front-load the costs into the tank, not the fuel. I already pointed this out to MoO.

I missed that, apologies. That makes sense -- this obviously isn't the sort of situation where, like a nuclear submarine, you're going to fuel it once with the assumption that the fuel will last it the reactor lifetime.

[Junghalli]

Atomic Rocket has a section on antimatter-fueled starships that talks about another possible way of storing antimatter.

http://www.projectrho.com/rocket/rocket ... l#valkyrie

...Since antimatter and matter annihilate each other on contact, releasing enormous bursts of energy from literally microscopic amounts of propellant, you cannot simply fill a shuttle tank with liquid antihydrogen and let it slosh around inside.

The only storage method that has a hope of working is solid antihydrogen, supercooled within one degree of absolute zero (within one Kelvin of -273 degrees C). At this temperature, antihydrogen condenses into "white flake," with an extremely low evaporation rate.

Particles of solid antihydrogen will be suspended and held away from the "pod" walls, probably by electrostatic forces and/or magnetism. According to our latest models, near 0.0005° K, antihydrogen should be sufficiently stable as to allow, in the form of matter-antimatter micropellets or wafers (we are presently working to determine which design, layered pellets or wafers, will provide optimal thrust). With one-fifty thousandth of a degree Kelvin, matter-antimatter storage becomes thinkable because wave functions do not overlap enough to produce an appreciable reaction, at least in principle.

(And in practice?)

We do not know. It has not been practiced yet, and can only be verified by experimentation. Personally, carrying matter-antimatter pellets already assembled, even at 0.0005° K, gives me nightmares. I keep seeing a cosmic ray particle stopping at the matter-antimatter interface, giving off its heat, and triggering a horrible chain reaction... Jim says we can prevent that, but I am still opting for storing our antihydrogen in complete isolation from matter until virtually the moment it is needed. I am reminded of that scene from the movie version of 2010, in which Roy Scheider describes the aerobraking maneuver his ship is about to make through Jupiter's atmosphere. "It's dynamite on paper," he says. "Of course, the people who came up with the numbers on paper aren't here."...

Another idea I've heard is keeping the antihydrogen as ice and using a laser to vaporize away a little bit at a time.

[Covenant]

You need to refine your definition of an antimatter-powered spaceship. Why use it to generate fuel? Why not a matter/antimatter particle beam hookup to generate the thrust? Antimatter is useful in terms of energy density, which is important for a spaceship, but why use your ultradense energy hookup to generate mundane electricity like anything else can? Use it to generate thrust. Ion Engines and stuff are great too, but depending on your theme, may not have the kind of impulse you need to achieve a high acceleration in a reasonable amount of time.

One of the brainbugs is that the engine must always be some powered apparatus fed from an antimatter reactor (which is somewhat inefficent in bulk) instead of just a big antimatter rocket.

Antimatter has it's uses, but it's not the most useful universal energy source, and nobody says you need star trek wierdass field drives. What's wrong with just a nice big rocket?

[Darth Wong]

Atomic Rocket has a section on antimatter-fueled starships that talks about another possible way of storing antimatter.

That's an old one. The problem is that the magnetic interactions with supercooled hydrogen are still extremely weak compared to ferromagnetism. Do you really want to use a piss-weak interaction to keep a chunk of antimatter away from the storage pod's walls, when any contact would lead to the obliteration of your ship?

[Fingolfin_Noldor]

I'm more inclined to think that the best use of antimatter is to use it as a catalyst for fusion than an actual energy source.

Some interesting articles here: http://www.engr.psu.edu/antimatter/documents.html

[Fingolfin_Noldor]

Using antimatter as a catalyst for fission is something I've read about before as a concept to get more efficient nuclear pulse drives. Since the amount of antimatter needed is miniscule, it actually sounds pretty workable.

But, this actually brings up a question about the main power plant subject again: could a side effect of the antimatter reaction include radioactivity? Suppose some stray anti-protons hit a random heavy nucleus, like one in the reactor's wall or one of the pipes. Could this make it fission or at least decay to cause additional damage?

This may be yet another thing to drive up the cost.

I would guess that one has to introduce some measure of a magnetic field/electromagnetic field(which ever is most convenient) to contain the antimatter the same way they do so in a Fusion reactor.

[Cykeisme]

A question regarding the hypotheteical ferromagnetic tank made of anti-iron used to store anti-matter fuel.. it would have to be suspended by electromagnets, right?

Aside from the fact that the strength of the suspensor electromagnets would have to be varied if the entire assembly underwent any variable acceleration (say, a vehicle of any sort).. doesn't that also mean that failure of the electromagnets would result in the anti-iron tank falling (or touching) against the sides of whatever outer container it's in, and thus result in an uncontrolled reaction?

[Darth Wong]

A question regarding the hypotheteical ferromagnetic tank made of anti-iron used to store anti-matter fuel.. it would have to be suspended by electromagnets, right?

Aside from the fact that the strength of the suspensor electromagnets would have to be varied if the entire assembly underwent any variable acceleration (say, a vehicle of any sort).. doesn't that also mean that failure of the electromagnets would result in the anti-iron tank falling (or touching) against the sides of whatever outer container it's in, and thus result in an uncontrolled reaction?

Of course. The difference between this and most antimatter storage schemes is that the tank is ferromagnetic, so it responds much more strongly to magnetic fields, hence it is much easier to manipulate in the storage chamber. But since it is a tank rather than a blob of material, we can select the actual fuel to be something which is much easier to make than anti-iron.

[Cykeisme]

Ah, understood.

So.. the anti-iron tank will be hard to make, but it only needs to be made once, since it can be refilled with other antimater fuel many times.

Meanwhile, the actual consumable fuel can be some other form or element of antimatter that's easier to make and use, even if it can't be reliably manipulated with magnetic fields.

Got it now, I think.


On another note, if a process was made to reliably create and safely store anti-matter, how does it look as a way of transporting energy from the sun to Earth?
I'm only talking about quantities of energy around Earth's current electricity usage level, or maybe just an order of magnitude more.

[Cykeisme]

Mweh, lest I sound like an idiot, I meant if we had a station built somewhere near to the sun to soak up a greater intensity of solar energy per unit area of solar panels.. or some other form of energy collection, maybe thermal.

Since anti-matter is only useful as a method of storing energy, that's the only realistic application I could think of (aside from storing energy for sudden release as a bomb, perhaps).

[Sriad]

A question regarding the hypotheteical ferromagnetic tank made of anti-iron used to store anti-matter fuel.. it would have to be suspended by electromagnets, right?

Aside from the fact that the strength of the suspensor electromagnets would have to be varied if the entire assembly underwent any variable acceleration (say, a vehicle of any sort).. doesn't that also mean that failure of the electromagnets would result in the anti-iron tank falling (or touching) against the sides of whatever outer container it's in, and thus result in an uncontrolled reaction?

Of course. The difference between this and most antimatter storage schemes is that the tank is ferromagnetic, so it responds much more strongly to magnetic fields, hence it is much easier to manipulate in the storage chamber. But since it is a tank rather than a blob of material, we can select the actual fuel to be something which is much easier to make than anti-iron.

On a ship not intended to see combat (and, since realistically if your ship gets hit you're dead anyway, combat ships too) the electromagnetic assembly might be rigged up very near the outside of the ship so that if the magnets failed the anti-tank would fall away from the rest of the ship through some hard-vacuum tunnel.

Of course, failure of something as utterly trivial as electromagnets in your monstrously complex anti-matter fueled interstellar spaceship probably means you're right fucked anyway, but might as well design to allow the possibility of survival.

[Darth Wong]

On a ship not intended to see combat (and, since realistically if your ship gets hit you're dead anyway, combat ships too) the electromagnetic assembly might be rigged up very near the outside of the ship so that if the magnets failed the anti-tank would fall away from the rest of the ship through some hard-vacuum tunnel.

Of course, failure of something as utterly trivial as electromagnets in your monstrously complex anti-matter fueled interstellar spaceship probably means you're right fucked anyway, but might as well design to allow the possibility of survival.

What makes you think trivial components cannot fail on complex systems? Or be taken down for maintenance? If you need to do maintenance work on the storage system, you dump the tank into space, and pick it up again when you've got the system working again.

[Fingolfin_Noldor]

On a ship not intended to see combat (and, since realistically if your ship gets hit you're dead anyway, combat ships too) the electromagnetic assembly might be rigged up very near the outside of the ship so that if the magnets failed the anti-tank would fall away from the rest of the ship through some hard-vacuum tunnel.

Of course, failure of something as utterly trivial as electromagnets in your monstrously complex anti-matter fueled interstellar spaceship probably means you're right fucked anyway, but might as well design to allow the possibility of survival.

You might like to note that US and Russian submarines both have gotten into collisions, and their reactors didn't go critical. I would imagine that if a reactor were that fragile, it wouldn't be used at all on a navy vessel. Safety and survivability comes first.

[Sriad]

On a ship not intended to see combat (and, since realistically if your ship gets hit you're dead anyway, combat ships too) the electromagnetic assembly might be rigged up very near the outside of the ship so that if the magnets failed the anti-tank would fall away from the rest of the ship through some hard-vacuum tunnel.

Of course, failure of something as utterly trivial as electromagnets in your monstrously complex anti-matter fueled interstellar spaceship probably means you're right fucked anyway, but might as well design to allow the possibility of survival.

What makes you think trivial components cannot fail on complex systems? Or be taken down for maintenance? If you need to do maintenance work on the storage system, you dump the tank into space, and pick it up again when you've got the system working again.

Good points. Really, good engineering practice dictates that systems should be designed so that when they fail, their spaceship doesn't explode in microseconds.

[Winston Blake]

An idea I had about this anti-iron storage tank is that since we're building anti-elements from the bottom-up anyway, you might as well stick some anti-blocks of high temperature super conductor on the outside. The tank can be suspended by the Meissner effect. Then there's no control system or power supply to worry about - the whole thing can be a sealed unit.

You also wouldn't strictly need to make the tank's bulk out of ferromagnetic material - elements with much smaller atomic numbers should be easier to make. E.g. carbon fiber reinforced graphite would only need 6 antiprotons per anti-atom, instead of 26 for anti-iron. (Note I don't actually know if that stuff can hold hydrogen).

[Winston Blake]

Right after posting I had another idea - if you were willing to let such tanks be expendable, you could make them really small and use them as granular fuel. They could then flow in pipes and whatnot, or be spilled without hurting anyone. Perhaps a particular reactor design could make these granules reusable.

[His Divine Shadow]

Antimatter sounds like a bitch to get to work properly. Wonder if it wouldn't be more plausible to try and make really, really tiny black holes that collapse instantly into hard radiation? Maybe not.

[Darth Wong]

Antimatter sounds like a bitch to get to work properly. Wonder if it wouldn't be more plausible to try and make really, really tiny black holes that collapse instantly into hard radiation? Maybe not.

People who propose the use of tiny black holes usually invoke some handwavium about using a "field" to make it not act the way a black hole of that size should.

It would be nice if some future Star Trek writer just called it a "physics cancellation field". At least it would be honest.

[His Divine Shadow]

I was thinking of some magic way of concentrating matter (maybe some using kind of particle accelerator from hell) so it turned into a small black hole, from what I understand it would at once evaporate into radiation because small black holes cannot hold together and even big ones evaporate.

[Sikon]

It's been pointed out before in this thread that antimatter production is a net energy loss. But the topic deserves more.

An antimatter reactor wouldn't be just what the average person thinks of vaguely as somewhat inefficient, which gives no understanding of how many orders of magnitude are involved here.

Antimatter is currently literally about a *billion* times more expensive per unit of energy than nuclear fuel. After foreseeable future technological development with dedicated antimatter production facilities, it would become on the order of a *million* times more expensive instead.

---------------------

Approximate order-of-magnitude cost illustrations now and for the foreseeable future, outside of extreme situations like a post-singularity uber civilization filling the solar system with self-replicating factories:

• 1a. Total cost of terrestrial nuclear electricity generation (mostly capital and not fuel expense) =~ $0.01 / megajoule (electric)
• 1b. Methods reducing cost are conceivable, especially for a space civilization, such as =~ $0.001 / megajoule (electric)
• 2. Fuel cost of uranium from seawater, without breeding, for thermal energy from fissioning the U235 fraction =~ $0.0001 / megajoule (thermal)
• 3. Energy release from current thermonuclear bombs relative to total production cost =~ $0.0001 to $0.001+ / megajoule (thermal, uncontrolled)
• 4. Fuel cost of lithium-6 deuteride for fusion (if tech using it available) =~ $0.00001 / megajoule (thermal)

versus

• 5. Cost of antimatter production using current particle accelerators not designed for it (1 per 100000 collisions) =~ $100000 / megajoule-antimatter
• 6a. Estimated cost of antimatter with a dedicated production facility (different GeV, 1 per 20 collisions collected) =~ $100 / megajoule-antimatter
• 6b. Optimistic assumptions for a future antimatter factory =~ $1 to $100 / megajoule-antimatter

---------------------
Basis:

1. Several cents per kilowatt-hour (per 3.6 megajoules) can be the total cost for bulk industrial production of nuclear electricity today. Of course, you probably see a higher figure such as $0.10/kWh or more on your electricity bill, but total prices for residential customers are higher wherever one lives, including distribution cost to all the individual end-users, etc.

2. Thermonuclear device cost can be as low as between a fraction of a million dollars and several million dollars per megaton ... per 4 billion megajoules.

3. #3 is based on estimates in prior posts here and here. The cost of current uranium production from mining is also a comparable order of magnitude.

4. Deuterium is currently on the order of $3000 per kilogram (e.g. $600 to $700 per kilogram of heavy water); Li6D =~ $1000 / kilogram. Energy release for such is 64 kilotons per kilogram or 270 million megajoules per kilogram before inefficiencies.

5. Currently antimatter production in particle accelerators is extremely inefficient, e.g. on the order of 1 antiproton per 100000 proton collisions, with an input energy cost on the order of $60+ million per microgram of antimatter with $0.10/kWh or nominally potentially less with cheaper electricity. A microgram of antimatter reacting with a microgram of matter results in annihilation of the two total micrograms involved; E = mc^2, so that is 180 megajoules. (Some discussions refer only to the energy equivalence of the antimatter itself, 90 MJ/µg).

6a. A dedicated production facility could successfully collect an antiproton per 20 collisions of 200-GeV protons against high-atomic-mass material, increasing efficiency so as to reduce antimatter production expense to $25000 per microgram. That's described here and also corresponds approximately to Robert Forward's estimate for a purpose-built antimatter factory.

6b. Optimistically, one could consider a possibility that the assumed electricity input cost might be reduced by an order of magnitude if future power is eventually cheaper. Better than 1 antiproton per 20 collisions might also be obtained, maybe. But the result is still expensive.
---------------------

Applications where an antimatter reactor could be worthwhile are hard to find with its astronomical cost. Plus, storing a really large quantity of antimatter that will all go up upon the slightest contact with matter is about the most impractical method imaginable for power generation from a safety and engineering perspective ... any air or gas leaking in, a sub-microscopic speck of dust, an unanticipated shock or too much vibration, any penetrating damage...

Stationary general power plants running on antimatter are obviously implausible.

What about using antimatter in starship propulsion?

There's the example of the Valkyrie idea, to have a starship massing 200 tons total accelerating to most of lightspeed. Hypothetically, about 50 tons of antimatter would be stored as solid antihydrogen, apparently thought to have low enough vapor pressure at under 1 Kelvin, to be kept away electrostatically or by magnetic fields from the tank walls. In their idea, some antihydrogen would be ionized and guided out of a magnetic bottle to react with matter, spraying extreme-velocity particle exhaust to propel the ship. The crew compartment would be on a ten-kilometer tether so most of the very penetrating radiation didn't reach it, protected by a small shield.

There are many questionable aspects about that, including whether or not heat transfer versus the limits of materials was analyzed from a quantitative engineering perspective ... not overlooking it but performing calculations. Outside of doubtful assumptions, there's no point in trying for 92% of lightspeed on a journey to a nearby star, not if the time to accelerate without more power than materials could handle would be enough years that a few percent of lightspeed would be more appropriate anyway.

(The only self-propelled starship concepts I know to have been based on proper thermal calculations are various forms of external nuclear-pulse propulsion including inertial confinement fusion, not this one; skipping much longer discussion, just note that this lacks a number of the factors which should make the performance goals of the former workable).

Worst of all, even if antimatter production expense became as efficient as an antiproton per 20 collisions at 200 GeV proton energy as previously discussed, at $25000 per microgram, the ship's 50 tons of antimatter would have an economic and industrial opportunity cost nominally around a *million* trillion dollars.

---------------------

In contrast, for example, consider sending a 1000-ton dry-weight colonization starship at 10% lightspeed to reach a nearby star in several tens of years, sent by a space civilization to set up another on the asteroids and comets of the destination. Neglecting details like whether a magnetic brake could be used for deacceleration, the nominal minimum energy requirement is 500 trillion megajoules before (enormous) inefficiencies.

When energy requirements are so enormous, one likely does not want to use astronomically expensive energy like antimatter but rather cheap energy, more like #3 / #4 than #5 / #6 in the earlier list.

With potential future thermonuclear pulse propulsion, the base fuel cost for Li6D fusion fuel can be $0.00001 / megajoule or less, e.g. the earlier $1000 per 270 TJ being ~ 3.7E-6 $/MJ.

The nominal fuel expense for Li6D would thus be on the order of $2 billion; large inefficiencies would raise that figure greatly, although other factors such as deuterium production expense potentially decreasing below $3000/kg for a space civilization could reduce the equivalent cost. Of course, total starship expense would be more than fuel alone.

However, at least with such nuclear fuel, a colonization starship could reach another star system for a relatively conceivable allocation of resources for a hypothetical future civilization, a plausible number of billions of dollars per thousand tons.

But what if one used antimatter costing a figure like the $100 / megajoule of #6? Then the 500 trillion megajoules becomes a crazy 50000 trillion dollars nominally just for fuel cost even before considering how drive inefficiencies could raise that figure further. Outside of singularity-type assumptions, the equivalent of thousands of years of current world GDP being spent on just that project would be doubtful, to say the least.

*That's* one of the huge disadvantages of antimatter propulsion compared to nuclear pulse propulsion.

(In this post and elsewhere, I often take unpopular positions compared to what's typical in science-fiction, like nuclear pulse propulsion versus antimatter or mass drivers versus space elevators ... but that's because of considering matters quantitatively including real-world factors like plausible economics).

---------------------

If one made the starship instead just a probe returning some scientific data, decreasing mass by orders of magnitude, production expense for the lesser quantity of antimatter fuel would be correspondingly less. However, it'd still be enormous expense, and, for a really small probe, beamed energy is more likely.

(That's even aside from the matter of how much antimatter propulsion could be scaled down without too much penetrating radiation uselessly escaping omnidirectionally when it needs to be directed for the "photon rocket"'s thrust).

Even a cost such as $0.001 to $0.01 per megajoule for electricity from large power stations powering with moderate inefficiency a beam propelling a lightweight probe to fractional lightspeed is likely to be much better than spending $100 per megajoule for antimatter.

For example, nominally, a 100-kilogram probe can be sent to 10% lightspeed with an X inefficiency factor for $X * 0.5 billion of electricity. The exact figure isn't important, but what matters is that it isn't astronomical.

---------------------

A antimatter starship would work better if one could throw economic considerations out the window.

Consider a totally different scenario than almost all sci-fi: an astronomical posthuman post-singularity civilization fully utilizing the solar system's resources with self-replicating factories. Then economic / industrial output millions of times greater than today could allow them to do almost anything they wanted, even build antimatter starships whether small or large. There's still some question of whether such would be preferred to alternatives in that case, though.

---------------------

In the foreseeable future, plausible applications for antimatter involve using exceedingly tiny amounts, as opposed to using it as a bulk source of energy.

For example, as discussed more in a prior post here, the ICAN II concept for a high-performance interplanetary spaceship using antiproton-catalyzed microfission/fusion had pellets of 0.8 kg inert propellant mass around a 0.003-kg nuclear fuel capsule, each one releasing 302 billion joules of nuclear energy but with only 6 joules of antimatter used to help ignite its very center, a tiny fraction of a trillionth of a gram of antiprotons used per pellet.

Like matches igniting forest fires, the ship's total antimatter energy equivalent of a few hand grenades indirectly leads to the nuclear energy equivalent of tens of millions of tons of TNT, not all at once but spread out over half-a-million pellet detonations. For example, the basic technology could do a quick journey to Mars or really anywhere in the solar system with some modification.

Currently, Penning traps are used to store tiny amounts of antimatter. At liquid helium temperature and with a magnetic field, a cloud of antiprotons can be stored, a limited amount with their mutual repulsion.

The ICAN II antiproton-catalyzed microfission/fusion spaceship would be based on having 1000 antiproton traps each 0.3 meters in diameter and a meter long. Each trap would be able to store 10^14 antiprotons without too much loss over a several month period, an improvement over the current HiPAT device. The total amount of antimatter stored would be 140 nanograms. An accident would present the special concern of penetrating radiation, but there is the energy equivalent of merely several-percent as much as a hand grenade per trap (each the size of a small trashcan but 125 kilograms each).

In such a special application with small amounts needed, antimatter is feasible. Likewise, antiprotons have been used in some medical imaging even today. Merely a small fraction of a trillionth of a gram of antiprotons is sufficient.

In principle, greater antimatter density than today's Penning traps could be obtained by storing antimatter as antihydrogen. Electrically neutral antihydrogen atoms in a condensate would not repel each other like the antiprotons, if such could be produced someday in sufficient quantity without too many additional inefficiencies and difficulties. In 2002, for example, CERN produced clouds of tens of thousands of antihydrogen atoms. But a single microgram of antihydrogen would take about 6E17 atoms, trillions of times more.

---------------------

So the most realistic antimatter reactor is no antimatter reactor as such but, rather, only antimatter used in small quantities in specialized applications.

[Turin]

Right after posting I had another idea - if you were willing to let such tanks be expendable, you could make them really small and use them as granular fuel. They could then flow in pipes and whatnot, or be spilled without hurting anyone. Perhaps a particular reactor design could make these granules reusable.

That was pretty much the idea I mentioned above ("why build the tank at all?"), but as DW noted, you don't want to use-up the anti-iron tanks because anti-iron is going to be a lot more difficult to make than, say, anti-hydrogen.