The National Ignition Facility — the USA's flagship laser fusion experiment — is getting close to achieving the goal of laser fusion, with its director Mike Dunne predicting that ignition will be reported before the end of 2012. For fusion to take place, however, it is crucial to achieve symmetric compression of the deuterium–tritium fusion fuel, which is a considerable challenge given the demands it places on the timing and spatial control of the 192 incident laser beams.

Credit: © 2012 NPG

Now, writing in Nature Physics (Nature Phys. 8, 344–349; 2012) a collaboration of scientists from Lawrence Livermore National Laboratory, Los Alamos National Laboratory and the firm General Atomics report a scheme that may be able to help. Their idea is to harness efficient power transfer between multiple laser beams to provide a means of controlling the compression symmetry. Simulations and experiments at the National Ignition Facility show that four-wave mixing in a plasma can change the flow of power between multiple beams, resulting in a much brighter, uniform central beam that propagates deeper into the target (see image).

Inter-beam power transfer occurs because overlapping laser beams generate an interference pattern that creates an electron density modulation in the plasma. This causes a subsequent modulation in the refractive index grating that redirects the flow of incoming laser light from one beam to another. The amount of power transfer between the beams can be controlled simply by changing their respective wavelengths. The team use three tunable wavelengths to create two successive stages of power transfer. In the first stage, power transfer from the outer to the inner beams provides axial symmetry control. In the second stage, power transfer within the inner beams provides azimuthal symmetry control. According to the researchers, the technique could also be used to explore high-intensity physics by combining several beams at the NIF into a single 240 TW pump beam for driving Raman amplification and yielding laser intensities of 1022–1023 W cm−2.