Researchers create extremely hot and dense plasma

Oxford researchers have created and analysed an extremely hot, dense plasma from a solid substrate of aluminium foil.

Plasmas such as this one differ markedly from well-characterised dilute ‘gaseous’ plasmas and could simulate conditions within stars with a view to emulating fusion power.

The team carried out the experiments at the Linac Coherent Light Source (LCLS) within the US Department of Energy (DOE), which creates X-rays.

‘There is unfortunately no comparable facility in the UK,’ said project collaborator Dr Sam Vinko of Oxford University.

‘Although Diamond [Light Source, at Harwell] is a high spectral-brightness X-ray source compared to other synchrotrons, that is, other third-generation light sources, the spectral brightness of LCLS is more than a billion times greater. This puts it into a whole new league in terms of the science one can do.’

Scientists have long been able to create plasma from gases and study them with conventional lasers, but until now no tools have been available for doing the same at solid densities.

‘Conventional lasers cannot penetrate such systems but are reflected off their surface, so we cannot see inside them. Also, once heated they tend to blow up very quickly, making them extremely short-lived and necessitating ultra-fast diagnostics,’ said Vinko.

‘This is a problem because many systems in the universe are dense, rather than dilute, and not being able to study them in a controlled way severely limits our understanding of their behaviour and material properties.’

The LCLS, with its ultra-short wavelengths of X-ray laser light, is the first that can penetrate a dense solid and create a uniform patch of plasma — in this case, a cube one-thousandth of a centimetre on a side — and probe it at the same time.

The resulting measurements could feed back into theories and computer simulations of how hot, dense matter behaves. This could help scientists analyse and recreate the nuclear fusion process that powers the sun.

Specifically, it could help understand inertial confinement fusion (ICF) power, where lasers ignite a solid pellet of deuterium fuel. This differs from the circular ‘Tokamak’ fusion reactors, which hold a very dilute plasma in magnetic confinement.

‘Part of the problem is that, in order to have a successful heating and compression mechanism, a detailed knowledge of the equation of state — namely how the pressure varies with density and temperature of the dense and hot plasma — is required,’ said Vinko.