Expert: Steve Nelson Date: 7/2/2008 Subject: Questions about aneutronic laser fusion.
Question Hi:
Is it possible to fuse hydrogen and boron using a laser that emit 400 nm wavelength light? If so, what is the minimum amount of photons-per-second-per-square-meter required to induce the H-B fusion?
If 400 nm wavelength is too long to have the energy needed to cause the H-B fusion, then what is the shortest-required wavelength in order to facilitate such fusion? What is the minimum amount of photons-per-second-per-square-meter of that wavelength necessary for that fusion?
Note that I am interested in H-B fusion and not deuterium-tritium fusion. This is because the former is aneutronic fusion and can be directly converted to electricity while that latter requires a steam turbine and causes neutron pollution.
Thanks,
Green
P.S. These may seem like homework questions but they aren't. They are simply questions of my genuine interest.
Answer The National Ignition Facility uses lasers of close to that wavelength (350 nm), 192 of them, with a total of 2 MJ of power. It's estimated that you need approximately 10-45 times that much power (depending on the laser quality and confinement of the plasma) to achieve breakeven H-11B fusion, and bremsstrahlung radiation losses would be more than the power achieved. Plasma focus devices offer far more hope for creating this type of fusion than laser fusion, the confinement quality and laser technology to do that is probably decades away at best (and still inefficient).
As far as aneutronic goes, there will be neutrons and radioactive byproducts. There are side reactions that occur even with isotopically pure fuels which create neutrons, and the 16 MeV gammas produced by proton capture reaction will definitely require massive shielding and cause photonuclear reactions that will create neutrons and radioactive material. Nothing is perfect, it's just far less radioactive than D-T fusion.
Energy extraction is technically possible from an asymmetric plasma, but it's never been done for something like what you're talking about. The technology isn't even developed. Yes, there are charged particles flying about, but they don't have a direction that allows their energy to be extracted except as noise. Best bet would be to use the ions and gammas/x-rays to heat the shielding walls and extract the heat with a cooling gas (nitrogen or something like that) that could be used to drive a turbine. Far more efficient energy extraction. Even with some direct extraction mechanism, there will be massive heating and you wouldn't want to waste that heat energy. High temperature heat sources like that could be used to create synthetic gasoline from CO2 and water in the atmosphere, closing the carbon cycle...so I say go for the extraction of heat. Far more useful. A project involving this technology already exists at Los Alamos National Lab (Project Green Freedom, it's called).
