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Multi-Petawatt Laser Overcomes Power Limitations of Conventional Solid-State Optical Gratings

Researchers at the Lawrence Livermore National Laboratory (LLNL) have created a portable multi-petawatt laser that circumvents the power constraints of traditional solid-state optical gratings by using plasma transmission gratings. The concept might make it possible to build an ultrafast laser up to 1,000 times more potent than comparable existing lasers.

The L3 HAPLS at ELI Beamlines Research Center in the Czech Republic. Image Credit: ELI Beamlines.

Diffraction gratings are essential for chirped-pulse amplification (CPA), a process used in petawatt (quadrillion-watt) lasers to extend, amplify, and compress a high-energy laser pulse without harming optical components. The Advanced Radiographic Capability of the National Ignition Facility and the Nova Laser, the first petawatt laser in the world, both depend on CPA, which received the 2018 Nobel Prize in Physics.

Plasma gratings “allow us to deliver a lot more power for the same size grating” because they have a damage threshold that is several orders of magnitude higher than conventional reflection gratings, according to Matthew Edwards, a former LLNL postdoc and co-author of the study.

The study was published on August 9, 2022, in the Physical Review Applied journal. The study describes the new design. Pierre Michel, the leader of the Laser-Plasma Interactions Group, collaborated with Edwards on the research.

Glass focusing optics for powerful lasers must be large to avoid damage. The laser energy is spread out to keep local intensity low. Because the plasma resists optical damage better than a piece of glass, for example, we can imagine building a laser that produces hundreds or thousands of times as much power as a current system without making that system bigger.

Matthew Edwards, Study Co-Author and Assistant Professor, Mechanical Engineering, Stanford University

LLNL, which has developed high-energy laser systems for 50 years, has also long been a pioneer in the design and manufacture of the largest diffraction gratings in the world, such as the gold gratings used in the 1990s to power 500-joule petawatt pulses on the Nova laser.

Next-generation multi-petawatt and exawatt (1,000-petawatt) lasers would need even larger gratings to get around the limitations on maximum fluence (energy density) imposed by traditional solid optics.

According to Edwards, a plasma laser consisting of ions and free electrons is “well suited to a relatively high-repetition-rate, high-average-power laser." For instance, the new design would enable the deployment of a laser system with 100 times the peak power comparable in size to the L3 HAPLS (High-Repetition-Rate Advanced Petawatt Laser System) at ELI Beamlines in the Czech Republic.

HAPLS, which was developed and built by LLNL and delivered to ELI Beamlines in 2017, was intended to generate 30 joules of energy in a 30-femtosecond (quadrillionth of a second) pulse duration, which is equivalent to a petawatt and to do so at 10 Hertz (10 pulses per second).

If you imagine trying to build HAPLS with 100 times the peak power at the same repetition rate, that is the sort of system where this would be most suitable. The grating can be remade at a very high repetition rate, so we think that 10 Hertz operation is possible with this type of design. However, it would not be suitable for a high-average-power continuous-wave laser.

Matthew Edwards, Study Co-Author and Assistant Professor, Mechanical Engineering, Stanford University

The difficulties of producing an adequately homogeneous big plasma and the complexity of nonlinear plasma wave dynamics have prevented plasma optics from being utilized for pulse compression at high power, the researchers noted, even though they have been successfully used in plasma mirrors.

It has proven difficult to get plasmas to do what you want them to do. It’s difficult to make them sufficiently homogenous, to get the temperature and density variations to be small enough, and so on. We’re aiming for a design where that kind of inhomogeneity is as small a problem as possible for the overall system—the design should be very tolerant to imperfections in the plasma that you use.

Matthew Edwards, Study Co-Author and Assistant Professor, Mechanical Engineering, Stanford University

The researchers’ findings were based on simulations performed using the EPOCH particle-in-cell (PIC) algorithm. “We expect that this approach is capable of providing a degree of stability not accessible with other plasma-based compression mechanisms, and may prove more feasible to build in practice,” the researchers added.

The novel design “needs only gas as the initial medium, is robust to variations in plasma conditions, and minimizes the plasma volume to make sufficient uniformity practical. By using achievable plasma parameters and avoiding solid-density plasma and solid-state optics, this approach offers a feasible path toward the next generation of high-power laser,” added the researchers.

The study was partly supported by LLNL’s Laboratory Directed Research and Development (LDRD) Program.

Journal Reference:

Edwards, M. R. & Michel, P. (2022) Plasma Transmission Gratings for Compression of High-Intensity Laser Pulses. Physical Review Applied.


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