Last week in Oxford, an interesting talk from Eric Lerner of Lawrenceville Plasma Physics (LPP) on his proposed fusion device using a dense plasma focus (DPF).
There’s no doubt about the reality of the DPF, discovered back in the 1960’s, or that it can produce temperatures and densities relevant to fusion, which LPP and others have already done. It’s an extraordinary phenomenon: a plasmoid undergoes a self-driven collapse. Part of the trick is that a huge magnetic field is generated, which both helps to confine the plasma, and prevents kinetic energy passing too rapidly from the atomic nuclei to the surrounding electrons. Sandia’s Z-machine, about which I’ll be posting separately, likewise combines the advantages of inertial and magnetic confinement.
However Lerner is determined to aim for the very ambitious proton-boron fusion reaction. This has the advantage of emitting few neutrons compared to bog standard deuterium-tritium fuel (DT). However it requires a temperature and confinement time each about 10 times greater than DT, and yields several times less energy per gram. This means it’s very difficult to get more energy out than you put in. Lerner, talking at Google a few years ago, described a design whose thermal fusion yield is in fact slightly less than the electrical energy input.
That’s not a paradox: the electrical energy doesn’t vanish, so if almost the energy in the machine, from the original input plus the fusion, can be turned back into electricity, a net gain is possible. But for a reasonable output Lerner is hoping that as much as 75% of the fusion fuel heated in the focus will burn, and that the total energy in the machine can generate electricity at 80% or better efficiency. These are extraordinarily high figures: most fusion designs assume more like 30% fuel burn and 50% electricity generating efficiency at best. A further problem is that due while the reaction emits few neutrons, due to the high temperature and the large charge of individual boron ions (5 times that of hydrogen), a significant part of the energy comes out as fiercely energetic X-rays. It’s difficult to see how the business end of the device, the electrodes and surrounding foil layers, will not need refurbishment or replacement at very short intervals.
On paper Lerner reckons he can do it. But at present he is 10,000 times short of the density he requires. While the density is not unreasonable in itself (it has been achieved in similar machines) the combination of density, temperature and magnetic field he wants has never been previously achieved. In the history of fusion, going to new plasma regimes has always yielded new difficulties.
So while it will be a wonderful thing if Lerner’s approach works – small cheap fusion devices in every neighbourhood really would transform the world – the odds against him seem long at present. Personally I wish he would consider using deuterium-tritium fuel. That would mean a bigger machine – more the size of a traditional electricity generating plant than his elegantly compact design – but it would be more likely to succeed, and more likely to convince large investors to back it.
Focus Fusion are currently advertising on Indiegogo for the opposite of large investors: from as little as $10, you can help to upgrade their machine from tungsten to beryllium electrodes. In return you get monthly reports on the progress of their fusion campaign. That’s a lot more interesting than the usual result of buying a lottery ticket, and at that price I will be backing them.