Lockheed Martin’s Skunk Works plans to develop a compact fusion reactor (CFR):
Until now, the majority of fusion reactor systems have used a plasma control device called a tokamak, invented in the 1950s by physicists in the Soviet Union. The tokamak uses a magnetic field to hold the plasma in the shape of a torus, or ring, and maintains the reaction by inducing a current inside the plasma itself with a second set of electromagnets. The challenge with this approach is that the resulting energy generated is almost the same as the amount required to maintain the self-sustaining fusion reaction.
The problem with tokamaks is that “they can only hold so much plasma, and we call that the beta limit,” McGuire says. Measured as the ratio of plasma pressure to the magnetic pressure, the beta limit of the average tokamak is low, or about “5% or so of the confining pressure,” he says. Comparing the torus to a bicycle tire, McGuire adds, “if they put too much in, eventually their confining tire will fail and burst—so to operate safely, they don’t go too close to that.” Aside from this inefficiency, the physics of the tokamak dictate huge dimensions and massive cost. The ITER, for example, will cost an estimated $50 billion and when complete will measure around 100 ft. high and weigh 23,000 tons.
The CFR will avoid these issues by tackling plasma confinement in a radically different way. Instead of constraining the plasma within tubular rings, a series of superconducting coils will generate a new magnetic-field geometry in which the plasma is held within the broader confines of the entire reaction chamber. Superconducting magnets within the coils will generate a magnetic field around the outer border of the chamber. “So for us, instead of a bike tire expanding into air, we have something more like a tube that expands into an ever-stronger wall,” McGuire says. The system is therefore regulated by a self-tuning feedback mechanism, whereby the farther out the plasma goes, the stronger the magnetic field pushes back to contain it. The CFR is expected to have a beta limit ratio of one. “We should be able to go to 100% or beyond,” he adds.
This crucial difference means that for the same size, the CFR generates more power than a tokamak by a factor of 10. This in turn means, for the same power output, the CFR can be 10 times smaller. The change in scale is a game-changer in terms of producibility and cost, explains McGuire. “It’s one of the reasons we think it is feasible for development and future economics,” he says. “Ten times smaller is the key. But on the physics side, it still has to work, and one of the reasons we think our physics will work is that we’ve been able to make an inherently stable configuration.” One of the main reasons for this stability is the positioning of the superconductor coils and shape of the magnetic field lines. “In our case, it is always in balance. So if you have less pressure, the plasma will be smaller and will always sit in this magnetic well,” he notes.
This set of announcements is very premature. When you boil it down they’re announcing a fairly minor advance in terms of the overall effect on our ability to actually produce usable fusion power. As has been pointed out elsewhere, there are a number of problems that come with smaller reactors and in-reactor magnets that have caused enormous headaches in the past – and they don’t propose any way to deal with those headaches. There’s also the issue that a smaller reactor will be less efficient at breeding tritium for its continued operation.
The size claims themselves are also a little misleading. They’ve shrunk the core itself, but all the associated secondary equipment hasn’t changed size, and it amounts to hundred of tons at a minimum.
This sounds fishy to me. Even putting aside the technical challenges of putting magnets inside the plasma volume, such ‘magnetic mirror’ configurations were investigated quite thoroughly early in the history of fusion research, in the 50s and 60s, and found to have a number of undesirable properties which motivated the change to toroidal configurations. Anyway, the capital costs of practicable fusion power plants are so high (much higher than for fission power plants) that they will likely be prohibitive.
The popular accounts point to the compact fusion reactor’s compactness, with an emphasis on how small and portable such a reactor might be, when the designer’s own emphasis is on how he can iterate his smaller design much more quickly — in true Skunk Works fashion.