March 24, 2010

Computational Studies Suggest that Laser Ignition of Aneutronic Fusion is Only Ten Times More Difficult than Deuterium-Tritium Fusion and Not One Hundred Thousand Times Tougher

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A team led by Heinrich Hora at the University of New South Wales in Sydney, Australia has carried out computational studies to demonstrate that new laser technology capable of producing short but high energy pulses could be used to ignite hydrogen/boron-11 fuel using side-on ignition.

The high energy laser pulses can be used to create a plasma block that generates a high density ion beam, which ignites the fuel without it needing to be compressed, explains Hora. Without compression, much lower energy demands than previously thought are needed. 'It was a surprise when we used hydrogen-boron instead of deuterium-tritium. It was not 100 000 times more difficult, it was only ten times,' says Hora.

Fusion reaction   Neutron fraction
DT                   0.80  (80% of the energy is neutrons == radiation)
DD                   0.336
D3 He                0.02
p6Li                 0.05

Journal of Energy and Environmental Science - Fusion energy without radioactivity: laser ignition of solid hydrogen–boron (11) fuel

The full paper is here

The usual laser compression developed for burning deuterium–tritium (DT) fuel cannot be used for H–11B because densities of 100000 times the solid are needed. Instead, the alternative laser fusion scheme of side-on ignition with uncompressed fuel is proposed to enable ignition of the H–11B fuel along with PW laser interactions. This approach employs a recently discovered laser-plasma interaction technique that uses very high contrast ratio laser pulses (i.e. pulses nearly free from pre-pulses). Plasma blocks of modest temperature are generated causing highly directed ion current densities above 10^10 A cm−2. This new ignition process is termed side-on block ignition, and it is described here in some detail.

The NIF laser is expected to produce ignition by delivering pulses of 1.1 MJ on the ICF target over a few nanoseconds. The ignition campaign at NIF is scheduled to achieve ignition in 2010–2011, demonstrating for the first time on Earth a controlled fusion reaction capable of generating more energy than delivered by the input laser pulse. The DT fusion reaction burns (reacts with) isotopes of heavy hydrogen (deuterium, D) with the super-heavy hydrogen isotope (tritium, T), where the laser irradiation compresses the fuel to more than 1000 times the solid density, causing heating to ignition temperatures of several tens of millions of degrees centigrade. Following on this success, LLNL scientists have proposed a prototype power station for 2020, based on use of a very compact, high efficiency, and high repetition rate diode pumped laser which builds on current laser technology. Simultaneous development of a power plant using similar technology is also proposed for use as an actinide burner to resolve the radioactive waste problem from existing light water reactors.

Parallel to these developments, 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. For one of these FI options, a design by Nuckolls and Wood using 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 10000. 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 method8 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.

The use of nonlinear force driven plasma blocks with the ultra-high current densities using a PW–ps laser-plasma interaction permits a come-back of the side-on ignition of uncompressed DT.

Side-on ignition for fusion using plasma blocks driven by nonlinear forces laser interactions is based on preventing self-focusing of the laser beam as previously described. This requires strong suppression of laser pre-pulses, i.e. a contrast ratio higher than 10^8, for times dozens of ps before arrival of the main pulse. The resulting plasma blocks have high momentum and are directed back towards the incoming laser beam. Momentum conservation causes an imploding block of plasma towards the inner portion of the target fuel. This implosion produces an inward moving thermonuclear reaction shock front as elaborated in the work by Chu. If the laser is obliquely hitting the plane target, the direction of the nonlinear force accelerated blocks is mainly perpendicular to the target surface, with minor deviations due to collision absorption, anticipating the later derived TNSA (target normal sheet acceleration) by Wilks from his discovered particle-in-cell computations

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 methods55 could improve performance even further especially for p–11B. The X-ray radiation produced in the reaction chamber is 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

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.

Conventionally, the fusion process occurs with deuterium and tritium as fuel. The fuel is spherically compressed - meaning compression occurs from all directions - with laser irradiation to 1000 times its solid state density. This ignites the fuel, producing helium atoms, energy and neutrons which cause radiation. Fusion is also possible with hydrogen and boron-11, and this could produce cleaner energy as it does not release neutrons, explains Hora.


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