World’s Highest Gain High-Power Laser Amplifier

The Vulcan laser target area at the Central Laser Facility, with the Raman amplification setup. (Source: U. Strathclyde)

Researchers from Strath­clyde demonstrated the feasi­bility of using plasma to amplify short laser pulses of pico­joule-level energy up to 100 milli­joules, which is an ampli­fication of more than eight orders of magni­tude. They used 150 J pulses from the powerful Vulcan laser system at the Science and Tech­nology Faci­lities Council’s Central Laser Faci­lity (CLF). Over the course of two pio­neering expe­riments at the CLF, the scientists worked closely with CLF staff to adapt the Vulcan laser in order that two different colour lasers could exchange energy in a plasma.  The measured gain coef­ficient is more than 100 times larger than achievable from existing high power laser system ampli­fiers based on solid-state media.

Dino Jaro­szynski of Strath­clyde’s Department of Physics led the research. He said: “Raman ampli­fication in plasma is a fasci­nating concept that combines the ideas of Nobel Physics laureate CV Raman with plasma, optical and laser physics.” Here, a relatively long, high-energy laser pulse is made to collide in plasma with a short, very low energy pulse. At the point where they collide they produce a beat wave, much like that of two colliding water waves. The light pressure of the beat pattern drives plasma electrons into a regular pattern or echelon that mimics the beat wave.  This multi-layer echelon acts as a very high reflec­tivity, time-varying mirror that sweeps up the energy of the high energy pulse reflecting it into the low energy pulse, thus amplifying the low energy pulse and compressing its energy into an ultra-short duration pulse of light.

“Our results are very significant in that they demonstrate the flexi­bility of the plasma medium as a very high gain amplifier medium. We also show that the efficiency of the amplifier can be quite large, at least 10 %, which is unpre­cedented and can be increased further. However, it also shows what still needs to be understood and controlled in order to achieve a single stage high-gain, high-effi­ciency amplifier module“, Jaro­szynski said. “One example of the challenges that we still face is how to deal with ampli­fication of noise  produced by random plasma fluc­tuations, which is exa­cerbated by the extremely high gain. This leads to undesirable channels for the energy to go. We are making excellent progress and believe that we are in an excellent position to solve these problems in our next experi­mental campaigns.”

Gregory Vieux who led the research team working at the CLF, said: “Plasma is a very attrac­tive medium to work with. It has no damage threshold since it is already a fully broken-down medium, therefore we can use it to amplify short laser pulses without the need for stretching and re-compres­sing. Another advantage is that further compres­sion during the ampli­fication is theore­tically possible. This could pave the way for the develop­ment of the next generation of laser systems delivering ultra-intense and ultra-short pulses and at a fraction of the cost of existing lasers. Still, we are not quite there yet. The scheme relies on controlling the Raman insta­bility. It has such a large growth factor that it can develop and grow from small plasma fluc­tuations.”

Laser ampli­fiers are devices that amplify light. In those that are familiar to us, this is done by syn­chronising the light emission from electrons in atoms or solid state matter, to make it coherent, which is a necessary step to achieving very high powers. However, very high power lasers at the frontier of tech­nology are limited by damage to their optical components and ampli­fying media. This makes them very large and very expensive.

Plasma offers a way around this limi­tation because it is very robust and resistant to damage. By har­nessing waves in plasma we can dramatically reduce the size of laser amplifiers while providing a route to much higher peak powers than possible now, exceeding the petawatt range to possibly reach exawatts. This is a very worthy goal because very intense laser pulses can be used for funda­mental studies, such as acce­lerating particles, helping drive nuclear fusion or even extrac­ting particles from vacuum and recreating the condi­tions inside stars or the primordial condition of the universe in the labora­tory.

The highest power lasers in the world will be available for use at three research centres that are part of the European Extreme Light Infra­structure (ELI) project. This €850m project is dedicated to the study of light-matter inter­actions at the highest inten­sities and shortest time scales. These lasers will lead to new science and technology that could, for example, transform our under­standing of high field physics and result in new radio­therapy moda­lities for the treatment of cancer.  There is a need to reduce the cost of laser tech­nology, which plasma could offer. Plasma may be a route to higher powers to go beyond those available at ELI to reach exawatt powers. (Source: U. Strathclyde)

Reference: G. Vieux et al.: An ultra-high gain and efficient amplifier based on Raman amplification in plasma, Sci. Rep. 72399 (2017); DOI: 10.1038/s41598-017-01783-4

Link: Dept. of Physics, University of Strathclyde, Glasgow, UK • Extreme Light Infrastructure ELI, EU consortium

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