Hotter than the Sun....petawatt lasers heat solid density matter to 10 million degrees (5/17/2008)

Vulcan petawatt target chamber with Mr Dan Hey

A milestone in high energy density science has been reached by an international team of physicists from Japan, the EU and the US. They have heated significant volumes of solid density matter to temperatures of 10 million degrees Kelvin using intense laser pulses from the Vulcan petawatt laser facility at the STFC Rutherford Appleton Laboratory, Oxfordshire, UK. Previously only ultra-thin layers of matter (less than 1-micron in thickness) had been heated to similar temperatures and this milestone takes scientists one step closer to laser fusion.

The Vulcan petawatt laser facility provides staggeringly powerful pulses of energy to target. One petawatt (10¹⁵ Watts) is 100 times the entire world's electricity production and the laser beam is focused to a spot of a few microns across - about one tenth the size of a human hair. It only lasts for less than 1 picosecond (10-12 of a second) but during that time, it is possible to heat materials above their normal melting point - allowing conditions that are found in exotic astrophysical objects such as supernova explosions, white dwarfs and neutron star atmospheres, to be created.

A reasonable volume of matter is needed to initiate the fusion process to enable energy gain (to get more energy out than the energy needed to produce it). Previously only ultra-thin layers of matter (less than 1-micron in thickness) had been heated to similar temperatures. This made them interesting, but of limited value for applications since the expansion of the material inevitably introduces density variations. This new work confirms that the heated material stays at this temperature and at solid density for a least 20 picoseconds - which is more than enough time for high speed instruments, such as X-ray spectrometers, to probe the heated material. "This is an exciting development - we now have a new tool with which to study really hot, dense matter. Careful selection of the target parameters allows access to this new regime" said Professor Peter Norreys from STFC Rutherford Appleton Laboratory and Imperial College London - the Principal Investigator of the experiments.

The measurement was made possible by the deployment of an innovative optical diagnostic, called HISAC (high speed sampling camera), that provided both spatial and temporal resolution needed for the measurements. The scientists measured the black-body radiation from the reverse surface of irradiated and heated foil targets and compared them with sophisticated computer modelling. "HISAC was developed in my laboratory at Osaka University and is a powerful tool for study of ultra-high speed phenomena in extreme conditions", explained Professor Ryosuke Kodama from Osaka University, Japan.

The temperatures reached are only one tenth of those needed for ignition of fusion capsules with only 300 Joules of energy on target. The team found that at least 15% of the laser energy was transferred to the fast electron beam. That transfer fraction informs designs for ignition of fusion targets on the proposed HIPER laser facility. "Efficient coupling of the laser energy to the target is crucial for fast ignition fusion, and is one of the main questions on which the design of the European laser fusion laboratory, HiPER, depends", said Dr Jonathan Davies from Instituto Superior Technico, Lisbon who performed the modelling of the experiment.

The work appears in press in the 'New Journal of Physics' - the new online, free access physics journal published by the Institute of Physics. 'Space and time resolved measurements of the heating of solids to ten million Kelvin by a petawatt laser'.

Note: This story has been adapted from a news release issued by the Science and Technology Facilities Council

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