The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is on target to demonstrate “breakeven”–creating as much fusion-energy output as laser-energy input. The NIF laser will compress a tiny sphere of hydrogen isotopes with 1.8 MJ of light in a 20 ns pulse, packing the isotopes so tightly that they fuse together.
If laser fusion is ever to generate power for civilian consumption, the laser will have to deliver pulses nearly 100,000 times faster than NIF–a rate of perhaps 10 shots per second as opposed to NIF’s several shots a day.
The Mercury laser (named after the Roman messenger god) is intended to lead the way to a 10-shots-per-second, electrically efficient driver laser for commercial laser fusion. While the Mercury laser will generate only a small fraction (1/30,000) of the peak power of NIF, it operates at a higher average power.
One significant difference is that, unlike the flashlamp-pumped NIF, Mercury is pumped by highly efficient laser diodes. Mercury is a prototype laser capable of scaling in aperture and energy to a NIF-like beamline with greater electrical efficiency, while still running at a repetition rate 100,000 times greater. While the goal of NIF is to achieve fusion breakeven, the goal of the Mercury laser system is the demonstration of a reliable diode-pumped solid-state laser system capable of scaling in aperture to the equivalent energy of a single one of NIF’s 192 beamlines. Diode pumping reduces the heat deposited into the Mercury laser, while gas cooling allows the residual heat to be efficiently removed.
To date, Mercury has operated for more than 300,000 shots at greater than 50 J at a repetition rate of 10 shots per second. Ultimately, Mercury’s goal is to generate 100 J pulses at this rate.
The amplifiers are face-cooled with high-pressure helium gas, removing approximately 3 W/cm2 of heat with minimal thermal wavefront distortions. Helium is chosen for two unique properties: its low refractive index and its high thermal conductivity. One advantage of gas cooling is that the technology is scalable–once a system design has been formulated to remove a certain amount of heat per square centimeter, it is possible to scale in aperture, allowing a small-energy (100 J) laser system to operate with the same characteristics as a large-aperture system with 100 times more energy. This heat-removal method is applicable not only to the laser gain medium, but also to the frequency-conversion system, in which the IR laser emission is converted to green or UV light.