February 10, 2012

Magnetized Inertial Fusion with energy gains of energy gains between 200 and 1,000 shown in simulations

Physical Review Letters - High-Gain Magnetized Inertial Fusion

Magnetized inertial fusion (MIF) could substantially ease the difficulty of reaching plasma conditions required for significant fusion yields, but it has been widely accepted that the gain is not sufficient for fusion energy. Numerical simulations are presented showing that high-gain MIF is possible in cylindrical liner implosions based on the MagLIF concept with the addition of a cryogenic layer of deuterium-tritium (DT). These simulations show that a burn wave propagates radially from the magnetized hot spot into the surrounding much denser cold DT given sufficient hot-spot areal density. For a drive current of 60 MA the simulated gain exceeds 100, which is more than adequate for fusion energy applications. The simulated gain exceeds 1000 for a drive current of 70 MA.

Ars Technica has coverage

For many years, fusion scientists had thought that if a magnetic field were used to compress the target, then a more complete fusion burn might be possible. The basic idea is that the role of the lasers changes. Instead of being responsible for compressing the pellet, it is only required to pre-heat the deuterium and tritium. Then, before the pellet explodes, the magnetic field is turned on, compressing it and initiating fusion.

The magnetic field acts on all charged particles, so it confines both electrons and nuclei, keeping the energy within the pellet. Furthermore, because everything is confined, the speed at which the nuclei need to be moving is reduced to just 1 million meters per second. If you think that isn't significant, consider that energy is proportional to the square of speed, so we are talking about requiring a thousand times less energy to initiate fusion.

But the magnetic field itself uses energy, and early calculations showed that it might slow down the expansion of the burn shell, which would also result in an incomplete burn. It would help—the total gain in energy production from magnetically confined inertial fusion was predicted to be a factor of ten. But we need gains on the order of a factor of 50 to make fusion break even. So the entire idea seemed destined for the scrap heap.

This is where this latest bit of research comes in. Slutz and Vesey from Sandia National Laboratories have shown that, if you modify the structure of the pellet, then energy gains between 200 and 1,000 are possible. The major finding is that the pellet and initial heating stage need to be modified. Slutz and Vesy start with a fairly standard pellet: a cylindrical piece of cryogenically cooled deuterium/tritium, surrounded by either aluminum or beryllium (this is the conductor that the magnetic field acts on).

The pellet is fabricated so that the density of the ice is very high just inside the metal shell. And, it seems (though the authors never explicitly say) that the whole cylinder is large enough in diameter so that the only the center of the pellet is heated by the incoming laser beams. The laser beams themselves don't hit it from every direction, but only along the axis of the cylinder.

The laser pulse heats the material at the very center of the pellet, creating a gas in that location. Before the outside of the pellet can heat up, the magnetic field is turned on, crushing the metal liner and compressing the gas. Fusion initiates, and the expanding shell of fusing material runs right into the layer of dense ice, slamming it into the shell before it can escape outwards. The result is a nearly complete burn.

The researchers calculated the amount of current and the duration of the current pulse required to produce the magnetic fields, and the numbers they came up with are not unreasonable (50-70MA for ~100ns). They also looked into the fabrication of the pellet. One critical issue is the smoothness of the inner shell of the surrounding metal layer. They show that they require the surface to be perfect to within about 20nm, while current technology routinely manages 30nm. The additional precision should be feasible with current technology.

Where I think the authors may have missed the mark is earlier in their calculations. It seems that they require the laser beam to create a gas with a relatively sharp boundary so that the shell of dense ice is left untouched, even as the interior is vaporized. It is unclear from the paper if they calculate the heating stage explicitly or not. I believe they do, but that leaves unanswered questions about how it's done.

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