Target practice for synthetic diamond
High-power laser targets that incorporate single-crystal chemical vapour deposition synthetic diamond have been developed by UK scientists. The behaviour of diamond under the extreme pressures of laser beam shocks is being observed to simulate conditions in planetary cores (warm dense matter) for research into new forms of energy generation such as inertial confinement fusion (ICF) energy.
Christopher Spindloe, a member of the Target Fabrication Group at Rutherford Appleton Laboratory (RAL) in Harwell, explains that targets are made from a range of materials, from standard metals such as aluminium, gold and copper, to germanium, gadolinium, niobium and dysprosium. The material used depends on the objective.
Using diamond, he says, ‘is fairly novel’. It is high strength and can tolerate greater energy pulses before vapourising. ‘The most important properties are clean polished edges [and transparency] so that you can see inside, [and] the shock of the laser beams can be tracked through the sample,’ Spindloe notes.
The team has produced diamond targets for experiments in France, as well as for RAL’s most powerful lasers – the Vulcan and Astra Gemini. Researchers are now designing structures for the GEKKO XII high power neodymium-doped glass laser based at Osaka University’s Institute for Laser Engineering in Japan.
Spindloe says, ‘Target design is based on a number of parameters. These include the laser geometry, the energy of the laser beams (the more powerful the laser, the bigger the target), [and] if shielding has to be designed to protect diagnostics from background signals’.
For example, the first diamond sample was coated on one side with a 300nm aluminium film. There was also a gold micromachined collimator and other foils (sub-micrometre) to enable X-rays to collimate through the structure, allowing shock from the laser beam to be probed. Plastic and gold foils on the other side absorb the laser energy and transfer the shock to the diamond.
In the design for the Japanese, a zinc X-ray production foil will be used. ‘Different materials emit X-rays at different energies, tailoring the diagnostic to the information you need. The power of the laser will also affect the thickness of the foil,’ adds Spindloe.
Research into ICF and fast ignition as sources of energy will continue over the next three years as part of a new European High Power Laser Energy Research project.