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July 07, 2011

Fusion energy without radioactivity: laser ignition of solid hydrogen–boron (11) fuel

Fusion energy without radioactivity: laser ignition of solid hydrogen–boron (11) fuel (8 pages) by Heinrich Hora, George H. Miley, M. Ghoranneviss, B. Malekynia, N. Azizic and Xian-Tu. He.

(H/T Talk Polywell)

The advent of ultra-high power lasers allows laser power levels that are about 1000 times the power of all the power stations in the USA. This opens the way to new approaches for inertial confinement fusions (ICF) that in turn can drastically reduce the laser input energy needed to achieve practical ICF power. The specific approach discussed here involves inducing a fusion burn wave by laser-driven impact of a relatively large block of plasma on the outside of a solid density fusion target. This new method is specifically selected to enable the extremely attractive, but demanding, neutron-free proton–B-11 fusion that potentially can lead to the long sought goal of an ultra ‘‘clean’’ fusion power plant.



New schemes for ICF power have been proposed based on the new type of laser offering more than PW (petawatt) pulses over picoseconds. The basic scheme is to use a slower pulse laser to initially compress a target to reasonably high density and then use this PW laser to heat (ignite) some volume in the target, which will burn into the rest of the high density fuel. Called ‘‘fast ignition’’ (FI), this method significantly reduces input power requirements, hence giving higher energy gain operation. If achieved, this approach promises a higher performance power plant than possible with the conventional direct compression and burn of the ICF operation.

A design by Nuckolls and Wood would use electron beam ignition, initial compression to only about 10 times solid state density is needed. The ignition occurs with very intense electron beams (of 5 MeV energy). These PW laser beams interact with the pre-compressed target through highly non-linear effects. This technique arrives at fusion gains of 10 000. A pre-compression of the target to about 1000 times solid-state density is required to generate the intense electron beam.

This paper now reports on another method that uses PW–ps laser pulses without high pre-compression of the target. It uses side-on ignition of the target at normal solid state or slightly increased density. The technique follows mechanisms which were actually observed in 1972. However, according to these early results, it appeared impossible to use this in a practical system. This pessimism came from recognition of the enormous energy flux densities predicted.

The result of recemt work is to confirm that side-on ignition of uncompressed H–11B fuel is not very much more ‘‘difficult’’ than DT fusion. Further, it is estimated to be possible with laser pulses in the range of ps duration and several dozens of PW power, after 10 PW pulses have been produced. Some slight pre-compression by chemical explosives or the inclusion of other effects, such as thermal flux inhibition may cause a further reduction of these requirements.

A much more detailed analysis is needed but at least the basic characteristics for side-on ignition are clearly visible. Most significant are the very surprising results that uncompressed H–11B can be ignited. This fusion energy generation with laser pulses in the range of few dozens of PW power and ps duration can achieve H–11B power production. The remarkable fuel avoids neutron generation, results in negligible radioactivity, and allows direct energy conversion, which in turn reduces heat pollution. Such a power plant is ideal for stationary electrical generation in a power station or for space propulsion. Modest pre-compression by chemical driving or with high density cluster methods could improve performance even further especially for p–11B. The X-ray radiation produced in the reaction chamber is less than 200 keV which can be screened off and does not lead to nuclear reactions in the power stations. This provides an exciting vision of a very attractive sustainable future power plant for worldwide use. Its achievement will depend on continued advances in laser optics, target physics and power conversion technology. However, the studies reported here show that such a system is rather close at hand—something not realized before, since p-11B ignition had always been viewed as virtually impossible
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