Aneutronic fusion

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Aneutronic fusion is any form of fusion power where neutrons carry no more than 1% of the total released energy.^[1] The most-studied fusion reactions release up to 80% of their energy in neutrons. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as ionizing damage, neutron activation, and requirements for biological shielding, remote handling, and safety.
Some proponents also see a potential for dramatic cost reductions by converting energy directly to electricity. However, the conditions required to harness aneutronic fusion are much more extreme than those required for the conventional deuterium–tritium (DT) fuel cycle.

Candidate aneutronic reactions

There are a few fusion reactions that have no neutrons as products on any of their branches. Those with the largest cross sections are these:

2D	+	3He	→		4He	(3.6 MeV)	+  1p (14.7 MeV)
2D	+	3He	→		4He		+  1p	+ 18.3 MeV
2D	+	6Li	→	2	4He			+  22.4 MeV
1p	+	6Li	→		4He	(1.7 MeV)	+ 3He (2.3 MeV)
1p	+	6Li	→		4He		+   3He	+ 4.0 MeV
3He	+	6Li	→	2	4He		+   1p	+ 16.9 MeV
3He	+	3He	→		4He		+ 2 1p	+ 12.86 MeV
1p	+	7Li	→	2	4He			+ 17.2 MeV
1p	+	11B	→	3	4He			+ 8.7 MeV
1p	+	15N	→		12C		+ 4He	+ 5.0 MeV[2]

The two of these which use deuterium as a fuel produce some neutrons with D–D side reactions.^{[citation needed]} Although these can be minimized by running hot and deuterium-lean, the fraction of energy released as neutrons will probably be several percent, so that these fuel cycles, although neutron-poor, do not qualify as aneutronic according to the 1% threshold.
The next two reactions' rates (involving p, ³He, and ⁶Li) are not particularly high in a thermal plasma. When treated as a chain, however, they offer the possibility of enhanced reactivity due to a non-thermal distribution. The product ³He from the first reaction could participate in the second reaction before thermalizing, and the product p from the second reaction could participate in the first reaction before thermalizing. Unfortunately, detailed analyses do not show sufficient reactivity enhancement to overcome the inherently low cross section.
The pure ³He reaction suffers from a fuel-availability problem. ³He occurs in only minuscule amounts naturally on Earth, so it would either have to be bred from neutron reactions (counteracting the potential advantage of aneutronic fusion), or mined from extraterrestrial sources. The top several meters of the surface of the Moon is relatively rich in ³He, on the order of 0.01 parts per million by weight,^[3] but mining this resource and returning it to Earth would be relatively difficult and expensive. ³He could in principle be recovered from the atmospheres of the gas giant planets, Jupiter, Saturn, Neptune and Uranus, but this would be even more challenging. The amount of fuel needed for large-scale applications can also be put in terms of total consumption: According to the US Energy Information Administration, "Electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kW·h" (1.14×1015 W·h). Again assuming 100% conversion efficiency, 6.7 tonnes per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency.
The p –⁷Li reaction has no advantage over p –¹¹B, given its somewhat lower cross section.^{[citation needed]}
For the above reasons, most studies of aneutronic fusion concentrate on the reaction, p –¹¹B.^{[4] [5]}

Current research

• Lawrenceville Plasma Physics has published initial results and outlined a theory and experimental program for aneutronic fusion with the Dense Plasma Focus (DPF),^[19] building on earlier discussions.^[20] The private effort was initially funded by NASA’s Jet Propulsion Laboratory.^[21] Support for other DPF aneutronic fusion investigations has come from the Air Force Research Laboratory.^[22]

• Polywell fusion was pioneered by the late Robert W. Bussard and funded by the US Navy, uses inertial electrostatic confinement. Research continues at the company he founded, EMC2.^[23][24]

• The Z-machine at Sandia National Laboratory, a z-pinch device, can produce ion energies of interest to hydrogen–boron reactions, up to 300 keV.^[25] Non-equilibrium plasmas usually have an electron temperature higher than their ion temperature, but the plasma in the Z machine has a special, reverted non-equilibrium state, where ion temperature is 100 times higher than electron temperature. These data represent a new research field, and indicate that Bremsstrahlung losses could be in fact lower than previously expected in such a design.

None of these efforts has yet tested its device with hydrogen–boron fuel, so the anticipated performance is based on extrapolating from theory, experimental results with other fuels and from simulations.

• A picosecond laser produced hydrogen–boron aneutronic fusions for a Russian team in 2005.^[26] However, the number of the resulting α particles (around 10³ per laser pulse) was extremely low.