UK Central Laser Facility raises damage threshold with plasma optics

During an optical metrology conference at the Photonex trade fair, Professor David Neely from the UK's Central Laser Facility described the latest work for increasing the damage thresholds of some of the highest intensity lasers in Europe. Jessica Rowbury reports 

The laser damage threshold of the optics is a major limiting factor for large laser facilities when sources operate at hundreds of petawatts of power. Professor David Neely and his team at the UK’s Central Laser Facility are working on improving the thermal limitations of the optics by using plasma mirrors and by employing a pre-pulse mechanism.

Neely described the work during a talk at the Photonex trade show, held in Coventry on the 14 and 15 October.

The Central Laser Facility provides research lasers covering a broad range of scientific studies. Over the last decade or so, scientists at CLP have pushed the boundaries of laser technology to reach higher powers, shorter pulse lengths and faster repetition rates. However, the damage threshold of the optics is currently limiting the performance of the lasers.

‘At the minute what we’re having to do is operate roughly at about a quarter of the ultimate damage threshold of these optics,’ Neely explained in his talk.

The most powerful laser at the facility is a 100 petawatt source. The number of shots CLP fires from this laser has gone from about 5,000 a year 15 years ago to what Neely expects to be about 5,000 a day next year. ‘We have gone from…shots per hour [because of the thermal limitations of the optics], to shots per tens of seconds, and we believe that by the end of this year or beginning of next year we’ll be running at tens of hertz,’ he said.

With laser powers set to increase next year, the primary focus of Neely and his colleagues at CLP has been to try and improve the damage threshold of the optics.

‘We’ve looked at the limits of material damage, and what happens is that after you damage the material, you form plasma,’ Neely remarked. ‘So, we’ve actually tried to turn this round to ask if we can actually use plasma optics as the next generation [of high-power optics].’

To generate plasma, a laser beam is focused onto a substrate, typically a piece of glass. The normal behaviour would be that the laser would pass through the piece of glass with a small amount of light being reflected from it. But if the intensity of the laser beam hitting the glass is above 1013W/cm2, a plasma is formed, which increases the reflectivity of the optic to 75 per cent.

CLP tried to increase further the reflectivity of the plasma, but realised that they should be concentrating on the absorption. ‘We were actually asking the wrong question – you do not optimise the reflectivity, you minimise the absorption.’

By increasing the density gradient of the plasma, the electric field absorbs a lot less energy. ‘Then it [the optical material] goes from about 75 per cent reflectivity to about 97 per cent reflectivity,’ he said.

Another way CLP has increased the damage threshold is through the use of a ‘pre-pulse’, a pulse fired onto the optic before the main beam. It was found that without the pre-pulse, the reflectivity was around 75-80 per cent, but with it the reflectivity reached 96 per cent.

He showed an example of a piece of glass being hit with around 500 joules of energy, with and without a pre-pulse. ‘Because there is about a quarter of the energy deposited [without the pre-pulse], there is a shockwave which just blows the glass apart,’ he explained. ‘If I add the pre-pulse at the same energy, there is less energy absorbed, so it reduces the debris – it’s much more effective.

‘We now have a method of controlling and tuning these plasma mirrors,’ he added.

The facility’s work on plasma optics has meant that in the last year CLP has managed to dramatically increase the reflectivity of its mirrors used in the high-power laser systems. ‘To do the interaction we have to use two plasma mirrors and we have realised that we can take these optics from 50 per cent to delivering about 92 per cent reflectivity.

‘So we have really taken these plasma optics from something that you have to use, to being something that is actually quite valuable,’ he said.

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