Fusion optics research targets higher laser shot rates

Overcoming some of the challenges of engineering optics for laser inertial confinement fusion (ICF) could ensure the UK retains a leading position in the field of nuclear fusion, Professor David Neely from the Science and Technology Facility Council’s Central Laser Facility in the UK has commented, speaking at the Photonex conference.

He said at Photonex, which was held 15 to 16 October in Coventry, UK, that anticipating difficulties in producing optics for higher shot rates expected at laser fusion facilities in the future, and dealing with these problems now can offer large rewards for industry and research.

Facilities such as the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) in the USA, and the soon-to-be-completed Laser Mégajoule in France aim to test if laser ICF can be a viable energy source.

Neely commented: ‘There isn’t the money at the moment in the UK to build a system that can produce megajoule energies; ultimately, we are trying to look at the next step on.’

The problem is that the thermal effects in the optics induced by the high laser energies mean that laser shots can only be carried out a few times a day to allow the glass to cool down.

Talking to Electro Optics, he said: ‘To commercialise [fusion] technology, we need to go from shots that are hours apart to multi-hertz [repetition rates]. This would mean the number of shots goes up by orders of magnitude. However, this means increased flux and the optics have to be able to withstand the increased loading.’

The team at the Central Laser Facility mostly use mirrors to direct the laser beam in high-power laser experiments. Neely commented: ‘We typically observe surface damage instead of bulk damage [in the optics] and more so at higher energies, as you would expect. But there also seems to be different damage from short and long pulses. Also, parts of the mirror will survive and other parts will suffer; it seems that the damage is initiated at certain points. Some parts of the mirror have damage initiation sites and the suspicion at the minute is that these are caused by imperfections or inclusions in the glass or optics.’

Neely’s team is manufacturing a debris shield for the laser fusion optics using a membrane in order to overcome some of the damage caused by the high-power laser beams. He said: ‘We have looked into using a debris shield made of Mylar film. If you have an ultra-thin optic, you don’t have to worry about non-linear phase accumulation through it and you can get almost perfect optical transmission through it. In fact the optical quality is so good that we could put it on two rollers and let it take a few shots; once it has collected some debris, you simply roll the next part into the beam path. It works brilliantly.’

He pointed out that a roll of Mylar costs roughly £10 and as the team are using a small 50mm beam they can get roughly 100 different shot locations from it.

‘The use of membrane optics is currently an idea that we are trying to get people interested in, not a funded project as of yet,’ Neely said. ‘We have been doing simple tests with students just stretching membranes and we get beautiful transmission quality, quite adequate for our experiments. While we only produce as we need them, this could be a way of mass producing large flat optics. We can make really cheap optics, but they only become cheap when you need them in bulk.’

‘It’s only when we have to start running these things at tens of hertz that we will have to deal with these issues,’ he added. The CLF's diode pumped 100J system will operate at 10Hz and Neely commented that these systems are going to come more online over the next few years.

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