18 October 2011

Fusion Power, Steampunk-style

Harnessing the power of fusing hydrogen isotopes together has long been a staple of science fiction, a means of achieving otherwise unachievable power densities, so as to make so many gee-whiz devices within the realm of the possible.  In reality, fusion design concepts are generally massive, cantankerous, and incredibly expensive (see magnetic confinement fusion and internal confinement fusion).  Fusion has been practical for awhile now, but it generally requires more energy input than it outputs, and the prototypes have been ludicrously expensive. The joke is with fusion, with research that's been ongoing for fifty years, is that it's always and always will be, "twenty years away." 

In order to make hydrogen fuse, the first step is to get it hot enough that it ionizes, such that the electrons are no longer attached to a nucleus, forming the forth-form of matter, a plasma.  Generally, the aim is to bring together a Deuterium (proton+neutron) ion and a Tritium (proton+2 neutrons) ion close enough together that the strong nuclear force affects the fusion of the two ions.  However, the electrostatic charge on the ions is a longer range force, and it tends to mess up collision trajectories, such that only very high energy ions on a direct collision course could ever fuse.  So the temperatures required are quite massive.

The magnetic confinement tokamak design that most people will be familiar with due to its widespread coverage in popular science magazines, tries to achieve more or less steady-state fusion power.  Steady-state fusion tends to be plagued by energy losses, particularly turbulence in the plasma, that bleeds off power. In comparison, pulsed concepts like internal confinement are easier to initiate, but the natural tendency of an extremely hot gas is to expand rapidly, so fusion rapidly slows and stops, limiting the overall efficiency of the process.

Ok, enough background: enter General Fusion, a company based in British Columbia, that is angling to build a fusion power generator that, well, seems like it would fit right into a Steampunk science fiction novel! It's the one fusion concept that I've seen that one could conceivable build using relatively low-technology components: pistons, microwave ovens, that sort of thing.  It's something MacGyver might build. 

Magnetized Target Fusion (MTF) is a hybrid concept that is supposed to be low-cost.  It was first proposed in 1976 as the LINUS concept, and it relies on first forming a small ball of  deuterium and tritium plasma, called a plasmoid (or sometimes a spheromak).  The plasmoid is given some angular momentum, such that it's actually a vortex, so that it has an inherent magnetic field that holds the plasmoid together for a brief moment.  The plasmoid, which is already pretty warm, is then compressed so that a pulse of fusion occurs.

The main advantage of forming a plasmoid first, over the plain inertial design, is efficiency in transferring energy from electricity into the plasma.  Lasers, plain and simple, aren't efficient at converting electricity to coherent light — I don't know what the lasing efficiency is at the National Ignition Facility, but commercial solid-state lasers are usually in the single digits. In comparison, a plasmoid can be formed with basically a high-tech microwave, using radio-frequency radiation, and the conversion efficiency is very very high.

Of course, the next question is, how to compress the plasmoid?  A plasmoid has a lifetime of approximately 100 μs according to General Fusion, so compression has to occur on that timescale. The proposed solution is to use over two-hundred pistons driven by compressed air to smash into the 'pot' holding the plasmoid, inducing a converging acoustical wave. As the wave converges, its strength increases and it collapses the plasmoid to very high pressures, ~1 Megabar and results in a enormously high magnetic field within the collapsed plasmoid, on the order of 1000 Telsa.  Effectively, it's like an artificial implosion nuclear bomb, using a very small amount of material. By using pneumatically-driven pistons instead of say, lasers, to achieve compression General Fusion is again gaining major efficiencies in terms of their energy input to output ratio (aka 'gain' in the fusion world).  Air can be compressed relatively efficiently up to thermodynamic limits, so the whole concept doesn't have massively lossy steps that crush the overall system efficiency.

Since the pistons are basically flat, the shock wave will actually not be perfectly spherical.  Also, it's pratically impossible to get all the pistons to hit the sphere at the exact same time — General Fusion claims they have accurate control of the impact time down to 5 μs which is 'good enough.'  Since there will always be some error in the impact timing, the shock wave will imperfectly compress the plasmoid and one can expect a lot of cavitation and other hypervelocity fluid dynamical effects.  The cavitation is similar to shaped-charge explosives, in that very high-speed jets are formed.  I am not very clear on the physics of these plasma jets, but I would guess that they are basically the source of the ultra-high temperatures that make fusion possible with this concept. So cavitation early in the compression of the plasmoid is bad, because it bleeds off energy and reduces the ultimate compression achieved.  However, a certain amount is probably desirable once the pressure reaches its ultimate limit.
Figure 1: General Fusion's pneumatic fusion reactor concept (http://www.generalfusion.com/generator_design.html). Plasmoids are formed in the plasma injectors (cones on the top and bottom) and then injected into the 'pot' of liquid lead and lithium. The two plasmoids collide in the middle and are metastable for a brief instant.  The pot is surrounded by 220 pneumatically driven pistons which hammer the side of the pot, creating an imploding acoustical wave that compresses the plasmoid, causing a pulse of nuclear fusion.
The pot itself is actually full of a mixture of liquid lead and lithium metals, as there's a need for an 'aether' to transmit the acoustical energy.  The lead acts as a neutron/thermal heat sink, absorbing the energy from neutrons produced by the fusion event both to recover it in the form of heat but also to protect the rest of the machine from high-energy neutron radiation.  The lead-lithium mixture carries the heat produced by the fusion pulse to some working fluid (i.e. water) which can then produce electrical power.

The lithium is a slow neutron absorber, but it also undergoes fission to hydrogen and helium isotopes (via n + 6Li → T + 4He and n + 7Li → T + 4He + n), thus acting as a source of tritium, which is very expensive, radioactive, has a tendancy to leak through solid materials, and dangerous, since it can be used to make hydrogen bombs.  Hence the reactor is designed to have a high breeding ratio (claimed at 1.6:1), so that once a little tritanium is given as a starter, more comes out.

Flowing the lead-liquid mixture in and out of the pot is likely a little tricky because the mixture has to spin in the pot, so as to setup favourable conditions for the plasmoid collision.

For the test-bed unit, which is smaller than an industrial scale reactor would likely be due to efficiencies of scale, about 100 Megajoules of mechanical energy is required as an input and about 600 MJ of thermal energy is produced.  The heat can then be used to make steam, just like any other thermal power plant, and recovered at around a 33 % efficiency, so that 200 MJ of electrical energy is produced per shot.  Hence the net would be 100 MJ per shot, and the target goal is 1 shot per second, thus producing 100 MW of power.

There are of course a variety of problems with the concept.  One of the biggest is getting the two plasmoids to collide and combine in the desired manner to form a little vortex of plasma in the centre of the pot. This is a hard thing to test without two working plasma injectors and a pot of liquid lead-lithium.  Currently they are relying on simulations, and there is plans for an explosive-based compression test to see if their plasma injector is working as desired.  The disadvantage of the explosive-based method is that it's destructive, so they can only get one test per boom-boom.  This makes iterating the design expensive and manpower intensive, but they are planning a shot in the fall of 2012 without Tritium.

Another problem is that material from the pot or the plasma injector nozzles (called spalling in the tokamak field) will be absorbed into the lead-lithium liquid, and that these impurities will radically increase the rate at which the plasmoids dissapate.

Irradiation of the machine itself is also a problem.  The lead-lithium matrix will absorb 99.9999 % of the neutrons but the walls of the vessel will still become too radioactive after about six months of use. Fortunately neutron embrittlement should not be a problem because the neutrons should be moving at relatively low velocities by the time they get to the shell of the pot.

Lifetime of the shell and pistons is also a concern, due to the thermal and shock stress caused by the impacts. This is actually something that improves as the machine gets bigger, because the pistons can move slower in order to achieve the same overall compression ratio. 

When I describe this concept as being steampunk-themed, I am exaggerating a bit.  In fact, this concept requires exquisite timing to control all the pneumatically driven pistons, and to form and inject the plasmoids into the liquid lithium-lead chamber, and that means lots of fibre-optics and other high-speed network devices unavailable 20-30 years ago. It is definitely the hipster of fusion power schemes, however. 

The bottom line: I remain skeptical that nuclear fusion can be more economical than either photovoltaics, which will eventually be the cheapest source of power on the planet, or advanced fission reactors.  Fusion is one of those gee-whiz things that sounds really exciting, until you start getting into the details and wonder how it will be economical, and the radiation waste aspect isn't really any better than fission (there's no worry about products decaying into Radon, which is a radioactive gas, but they do have to worry about Tritium contamination of the reactor, and it's a gas that can flow in-between the molecules of solid metal). The company has raised about $40 million thus far, and they probably need more than double that to finish their prototype in 2013/14, so it will be interesting to see if they find it. On the other hand, this concept is ripe for science-fiction fodder.