Energy production by laser-triggered fusion just came one step closer to reality with the efforts of Ryosuke Kodama and his fast-igniter consortium at Osaka University, in collaboration with a team of British scientists from CLRC Rutherford Appleton Laboratory (Chilton, UK), Oxford University (Oxford, UK), Imperial College (London, UK), and the University of York (York, UK). The group demonstrated proof of concept of the fast-ignition technique for inertial confinement fusion (ICF).
Conventional ICF is based on the concept of igniting a target pellet of deuterium tritium by compressing it with high-energy laser beams applied isotropically. The technique is theoretically viable but requires enormous amounts of energy; hence the construction of projects like Laser Megajoules (Bordeaux, France) and the National Ignition Facility (NIF; Livermore, CA). The newer fast ignition technique involves a two-part process of compression followed by ignition with a fast-heating pulse. Using a novel deuterated polystyrene target embedded with a gold cone that provided a clear path for the fast-heating pulse that ignites the target, Kodama and colleagues were able to use a lower energy petawatt laser to trigger fusion ignition, paving the way for the possibility of eventual laser fusion energy sources.
Key to Kodama's fast-ignitor geometry and successful fast-heating experiments is a powerful petawatt (PW) laser that is an upgrade of Osaka University's existing petawatt module laser.
"[For the PW laser], we kept the main amplifier system as it was and upgraded the front end, the compressor, and the focusing optics," says Hiroyuki Shiraga of Kodama's group. "We enlarged the beam size to 50 cm in order to extract 1 PW, or 500 J in 0.5 ps."
The experiment also used the University's GEKKO XII laser. The GEKKO XII is a high-power neodymium-doped glass laser system operating at 1053 nm. Both the PW laser and the GEKKO XII are seeded with a 1-nJ pulse. In the GEKKO XII, oscillator output of up to 10 mJ goes through a preamplifier chain, after which it is divided into the 12 beams, each of which is amplified and expanded to a clear aperture of 35 cm. The resultant 1.2 ns pulses correspond to a spatial length of 3 to 60 cm.
The front end referred to by Shiraga includes a neodymium-doped glass oscillator, a pulse stretcher, and an optically parametric chirped pulse amplifier (OPCPA) that provides 20 mJ of output with a 6 nm bandwidth. The OPCPA ensures that the pulse duration does not lengthen appreciably during the amplification process. "We transported the output beam to a target room, where we used a deformable mirror to compensate for any phase distortions on the wave front," Shiraga says.
In the fast-ignition process, the pulse that produces the spherical irradiance that compresses the target must be synchronized with the fast-heating pulse that ignites the target such that the fast-heating pulse impinges on the target while it is still compressed. The Kodama team used nine of the 12 GEKKO XII beam lines to provide the compression pulse. The PW laser provided the fast-heating pulse, as well as the seed pulse for beam lines generating the compression pulse. By seeding the GEKKO XII beams with part of the fast-heating pulse, they ensured the required synchronization. "As a result, then, the PW laser beam can illuminate the imploded core plasma we used in our experiment to within ±10 ps accuracy," Shiraga says.
The beam lines were important, but most critical was the design of the target, a shell 500 µm in diameter with walls 7-µm thick. "For the fast-heating pulse to drill its way in [to the target core] requires a lot of energy," says Henry Hutchinson, director of the Central Laser Facility at Rutherford Appleton. To circumvent this requirement, the team embedded a gold cone in the pellet, creating a conical vacancy on one side. "When you apply those compression pulses to it, all the bits move in radially and the vacancy where you placed the gold cone stays. So now you've got a cone giving you passage toward the center and you can therefore send a laser pulse through that hole without having the problem of drilling the hole, which would have needed a lot more power and energy." This was the innovation that allowed them to test ICF with existing lasers.
Using the nine GEKKO beamlines to provide 2.5 kJ each in 1.2-ns flattop pulses at 527 nm, Kodama's team imploded the target pellet. It collapsed into the tip of the cone, becoming a cloud of plasma. The team then sent a second, 0.5 PW fast-heating pulse through the gold cone to generate energetic electrons at the end of the cone. The cone focused the laser energy, causing the plasma to be heated and fusion reactions to occur in the compressed plasma between some of its deuterium nuclei.
"We detected neutron enhancement at about three orders of magnitude at 0.5 PW, compared with neutrons obtained without a heating pulse," says Shiraga. Specifically, they observed the enhancement during the stagnation time (the time during which the compressed plasma remains its high density state) of the imploded plasma (±40 ps). Such a long period could relax the short pulse requirements for the fast-heating laser. "The period necessary to heat the target sufficiently corresponds to the stagnation time and that suggests we could increase the heating laser's energy and use it to ignite fuel with a pulse of 10 to 20 ps or more," Shiraga continues. "I think that means we may possibly be able to ignite compressed deutrium-tritium plasma with fast-ignition petawatt lasers relatively inexpensively."
"This is regarded as a very important result worldwide because it shows rather efficient coupling of short pulse laser energy to the ignition spark," says Mike Key, director of petawatt science at Lawrence Livermore National Laboratory (Livermore, CA). "If this effect can be maintained on scaling up to ignition, all of the potential benefits of this new technique of fast ignition could be obtained, which may open the way toward inertial fusion energy."