Inertial Confinement Fusion


Peter Norreys
Justin Wark

Fusion - the process that powers the sun - in principle holds great promise as a source of almost limitless clean energy on earth. However, the obstacles that oppose its realisation are great. In order to get light nuclei to fuse, and release nuclear energy, the positively charged nuclei, that thus repel each other owing to the Coulomb force, must be pushed close enough together such that the strong nuclear force takes over. This requires the nuclei to be moving towards each other rapidly, which is achieved by heating them to very high temperatures (of order 10s of millions of Kelvin). The problem is then keeping such a hot plasma confined for long enough for the fusion reaction to occur. Two main approaches exist - one is to confine a very low density of the charged particles in a 'magnetic bottle', and keep it there for several seconds for the plasma to react - this is magnetic confinement, where although the plasma is very hot, it is low density such that the overall pressure is only a few times atmospheric pressure. The other approach, which we study, is inertial confinement fusion. In this method, laser beams are used, either directly or via conversion to x-rays, to heat the outside surface of a millimetre-sized spherical fusion capsule, causing it to act a bit like a spherical rocket. The fusion material is accelerated to the centre, compressing and heating, and causing a fusion reaction. The densities are enormous, such that the reaction occurs in a few tens of picoseconds, before the target can disassemble (hence the name, inertial confinement).

The major system world-wide engaged in this effort is the National Ignition facility, based at LLNL in California. The Oxford group has close links with the laboratory, and has a collaborative research project funded from EPSRC with them looking at the main problem that currently besets the experiments. This problem is the Rayleigh-Taylor Instability, that causes the shell of material to break up before peak compression, owing to non-uniformities in the drive, or in the construction of the fusion target.

The above approach is called 'hot-spot' ignition (the fusion gas in the middle of the solid target is at the same pressure as the shell on stagnation, but as it is at lower density, it is very hot, and thus a fusion 'spark' is created that sets the whole reaction going). Other approaches are also under study in the Oxford group, using methods where the compression and the 'spark' are separated. One such method is called 'fast ignition', where the compression phase uses nanosecond lasers, and then the heating is performed with very high power picosecond lasers. For this reason there is also a lot of research performed in the centre in the area of producing energetic (so-called 'hot') electrons that could be used in this heating phase.