Brian Dance
A recent major upgrade to the neodymium:glass "Vulcan" laser at the Rutherford Appleton Laboratory (RAL; Oxon, England) has increased the power it can deliver by a factor of ten. It is claimed that the laser now produces a higher focused intensity than any other laser in the world. Pulses of 500-fs duration with energies of 500 J can be produced, corresponding to a power level of 1 petawatt (1015 W) on target. The chief scientific advisor to the United Kingdom government, David King, who is head of the Office of Science and Technology, inaugurated the upgraded laser in April. The inauguration formed part of the 25th anniversary celebrations of the formation of the Central Laser Facility at RAL.
The Vulcan petawatt laser system has been constructed as a national user facility for U.K. academic users and international collaborators. It will be used for a wide range of ultrahigh-intensity studies. These will include plasma behavior in the extreme conditions normally found only in the interiors of stars where density, temperature, and magnetic field can be extremely high. New phenomena, such as pion formation, may be observed. Studies will be made of the inertial confinement approach to controlled thermonuclear power by the so-called fast-ignition technique first demonstrated by U.K. and Japanese workers.
The laser pulse is focused by adaptive optics that correct wavefront errors. The beam can be focused to form a near-diffraction-limited focal spot only 10 μm in diameter to provide a peak irradiance on the target surface of 1021 W/cm2. If a beam of this intensity strikes a solid target, it produces a plasma that interacts with the incoming laser beam. The use of a thick target of high atomic number results in a narrow beam of electrons of giga-electron-volt energies that collide with target atoms to form energetic gamma and x-ray photons; this is a possible new x-ray source. Radioisotopes can also be made. Chris Edwards, project manager for the Vulcan petawatt upgrade, said, "This level of performance will open up new regimes of plasma physics to the U.K. scientific user community and its international collaborators, including photon-induced nuclear reactions, new schemes for the acceleration of charged particles, and further opportunities to study fusion reactions."
Chirped-pulse amplification is used in Vulcan to stretch low-intensity pulses from a few hundred femtoseconds to a few hundred picoseconds so that they can be safely amplified before they are recompressed. The technique enables pulse intensities on the target to be up to a thousand times greater than would otherwise be possible. The design of the two large ultraclean vacuum vessels for the petawatt target area has been a major engineering project.
The 71-ton compressor chamber has three main sections. Two cylindrical vessels at either end hold two 940-mm-diameter pulse-compressor gratings separated by 13 m (see figure). The gratings were fabricated at Lawrence Livermore National Laboratory (LLNL; Livermore, CA). The interaction vessel is 5 x 3 x 2 m and weighs 60 tons. In use, it will be surrounded by 70 tons of lead, aluminum, and polythene for radiation shielding.
The upgrade involved boosting the laser output by the addition of three 208-mm-aperture amplifiers driven by four pairs of 25-mm-diameter flashlamps pulsed at 20 kV. Isolation is provided by a 208-mm-aperture Faraday rotator. A new clean-room facility has been established for handling and testing large-aperture optics and for servicing disc amplifiers. The upgrade includes the addition of a third target station to Vulcan for experiments involving very high fields, while additional beams can be added in the future for plasma probing and heating.
Funding of $7 million for the three-year upgrade came from a grant from the Engineering and Physical Sciences Research Council of the British government. LLNL provided specialized laser components from its dismantled Nova laser in exchange for beam time on Vulcan; for example, the Vulcan capacitor bank was reconstructed from more than 300 high-energy-density capacitors from Nova. All major components, including capacitors, triggers, and charging units, were preassembled and tested off-line to minimize disruption to other work with Vulcan.
Users will have access to the upgraded facility from the last quarter of this year. Rutherford Appleton Laboratory provides university users with facilities that would be far too costly for each university to obtain. It is hoped that the facility can be made available to European researchers through the Large Scale Facility access program.
Brian Dance is a freelance writer based in Alcester, England.