Mercury: A Diode-Pumped Solid-State Laser
Concurrent with the development of the National Ignition Facility and its goal of achieving thermonuclear burn was another ambitious Livermore laser project named Mercury.
Mercury is a single-beam laser system that developed capabilities that will build on NIF’s accomplishments. As currently designed, NIF’s 192 beams can fire simultaneously only once every few hours. After each shot, the thousands of optics must be given a chance to cool down to ensure that they can operate correctly for the next shot.
Mercury developed a method of continuously cooling the optics, while at the same time allowing the laser to fire rapidly over extended periods. The technology propels high-velocity helium gas across the optics to keep them cool, while laser pulses pass through the optics at a sustained rate of ten shots a second.
Unlike NIF, which uses seven-foot tall flashlamps to energize the laser amplifiers, Mercury relied on diode lasers—similar to those in commercial read/write CD players—which give off one-third as much heat as flashlamps. Mercury’s beam was amplified as it passed through slabs of specially grown ytterbium-strontium flouroapatite crystals, as opposed to NIF’s neodymium-doped phosphate laser glass. More advanced amplifier media, such as transparent ceramics, are also being developed.
As of mid-2008, Mercury was able to run continuously for several hours (300,000 shots), firing ten times a second at more than 50 joules per shot, each shot lasting just 15 nanoseconds (billionths of a second).
The project, which began in 1996 and was initially funded through LLNL’s Laboratory Directed Research and Development (LDRD) program, received three R&D 100 Awards, the most recent for developing a unique frequency conversion crystal. Earlier awards were for the original design of Mercury’s diode array and for its Pockels cell, a unique light-switching technology (see Optical Switch).
The long-term goal is a laser system capable of producing NIF’s energy output, with Mercury’s ability to rapidly fire shots, and with innovations that will enable the project’s ultimate goal: the rapid ignition of targets for electrical power generation (see Energy for the Future).
