Oct. 23, 2024
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New Plasma Switch Cleans the Pulses in High-Power Lasers

By Charlie Osolin

Today’s ultra-high-power, rapid-fire lasers promise breakthroughs in fields ranging from fusion energy to national security, laboratory astrophysics, medicine, and many industrial applications.

But all that power–petawatts (quadrillions of watts) and much more–can be hard to control. “Noise” in the laser pulses caused by delays in energizing the amplifiers and other factors can destroy delicate targets even before an experiment begins.

“It takes a few nanoseconds for an amplifier to power up,” said former Lawrence Livermore National Laboratory (LLNL) postdoc Matthew Edwards. "There can also be picosecond-long reflections in the system as the pulse is bouncing around inside the laser. You can end up with some light that takes a shorter path than the main pulse, and that can damage downstream components.”

“A contrast measurement like this requires tens of thousands of laser shots.”
—Matthew Edwards

To meet this challenge, Edwards and his collaborators have developed and tested an innovative plasma optic that “cleans” the laser pulse by diffracting away the potentially damaging pre-pulses, then rapidly switching to let the main pulse through to the target. The new plasma switch is described in a Physical Review Letters paper published online on Oct. 8 and highlighted in Physics.

“If this switch can be implemented in high-power laser systems,” said Edwards, now an assistant professor of mechanical engineering at Stanford University, “it could expand the range of possible experiments. In particular, it would allow running precise experiments that are very sensitive to the quality of the laser pulse that drives them.”

Photo of L3-HAPLS
Livermore delivered the High-Repetition-Rate Advanced Petawatt Laser System to the Extreme Light Infrastructure Beamlines facility in the Czech Republic in 2017, demonstrating the Laboratory’s expertise in high-repetition-rate petawatt laser construction. Credit: ELI Beamlines

Edwards was joined on the paper by co-author Julia Mikhailova, an associate professor in Princeton University’s Department of Mechanical and Aerospace Engineering; LLNL Laser-Plasma Interactions group leader Pierre Michel, with whom he shares a patent application for the switch; and other colleagues from LLNL, Stanford, and Princeton.

Working in Mikhailova’s laser lab at Princeton, the researchers created the new plasma optic, or ionization grating, by splitting a femtosecond (quadrillionth of a second) Ti:sapphire (titanium-sapphire) laser into multiple beams and firing them into a jet of carbon dioxide. The interaction of the lasers ionizes the gas and creates a plasma, a mixture of ions and free electrons.

Plasma Lab at Princeton University
Julia Mikhailova’s laboratory at Princeton University, where the plasma switch experiments were conducted.

“By using a gas cell target under vacuum, we were able to better control the grating properties,” Edwards said. “This allowed us to make a true plasma transmission grating for the first time.”

When the grating is off, the researchers said, light travels unaffected through the interaction region; but when the grating turns on, a substantial fraction of the light diffracts to a new direction. Switching between off and on occurs within a few hundred femtoseconds.

Temporal Contrast Improvement

Edwards said the experiment’s optical properties were measured “by capturing the diffracted beam and running it through relatively standard optical diagnostics–cameras to capture beam profile and focusability along with devices to measure pulse duration and most significantly, the improvement of the temporal contrast” between the cleaned and uncleaned pulses, which reached more than five orders of magnitude.

“Nondirectional light scattering depends on the initial gas distribution,” the researchers said, “and although it is difficult to estimate the degree to which (prepulse) can be suppressed, we anticipate that contrast improvements much greater than 105 are possible. Scaling this process to higher energy and higher efficiency, coupled with plausible increases in the contrast improvement, suggest a route to contrast cleaning for terawatt and petawatt lasers.”

Edwards said measuring the contrast of the diffracted beam was the most challenging aspect of the experiments. “This required significantly improving the efficiency, stability, and energy throughput of the plasma diffraction gratings over what had previously been achieved,” he said.

“A contrast measurement like this requires thousands of laser shots. The system runs at 10 Hz, so we can take 36,000 laser shots in an hour. Therefore, the grating must be stable for many hours while experiments are conducted at 10 Hz.

“This type of characterization had not previously been done on an ionization grating,” he added, “in part because the diffracted light produced from previous experiments was not sufficiently high quality.”

A Gratings Leader

LLNL is a long-time world leader in the development of state-of-the-art optical gratings, both solid-state and plasma. And plasma gratings play a key role in National Ignition Facility (NIF) experiments by helping direct incoming laser light to where it’s needed inside a NIF hohlraum to create a symmetrical implosion (see “How NIF Targets Work”). This diffractive-optics technique was instrumental in enabling NIF to repeatedly achieve ignition in support of the National Nuclear Security Administration (NNSA)’s science-based Stockpile Stewardship Program.

Because plasma optics are transient and generated from a gas, they can be formed many times per second, making the optical switch particularly well suited to petawatt-class high-repetition-rate laser systems. It could also potentially be applicable to high peak and average power lasers like LLNL’s Big Aperture Thulium (BAT) concept as well as potential inertial fusion energy systems.

“We are very happy with the results we have achieved thus far–it is a substantial advance," Edwards said, "but there is still a lot of work remaining to be done. We’d like to use this grating to run experiments that are sensitive to laser contrast–in particular, studies on relativistic harmonic generation and laser-driven particle acceleration.”

The research, which began in late 2019, was supported by LLNL’s Laboratory Directed Research and Development Program and grants from the National Science Foundation and NNSA. “I traveled from LLNL to Princeton to run the first experiments in (Mikhailova’s) lab,” Edwards said, “and her group has been central to the success of our experimental work in this area.”

Joining Edwards, Mikhailova, and Michel on the paper were Nuno Lemos from LLNL; Nicholas Fasano, Andreas Giakas, Michelle Wang, Jesse Griff-McMahon, and Anatoli Morozov from Princeton; and Victor Perez-Ramirez from Stanford.

More Information:

Greater than Five-Order-of-Magnitude Postcompression Temporal Contrast Improvement with an Ionization Plasma Grating, Physical Review Letters, October 8, 2024

Cleaning Intense Laser Pulses with Plasma,”Physics, October 8, 2024

Targeting Lasers as Sources,” Science & Technology Review, July-August, 2024

Researchers Design a Compact High-Power Laser Using Plasma Optics,” NIF & Photon Science News, August 18, 2022

Holographic Plasma Lenses for Ultra-High-Power Lasers,” NIF & Photon Science News, March 14, 2022

“Lasers Without Limits,” Science & Technology Review, July 2021

“Laser-driven plasma sources of intense, ultrafast, and coherent radiation,” Physics of Plasmas, January 11, 2021

Super-Fast, Super-Powerful Lasers Are About to Revolutionize Physics,” NIF & Photon Science News, March 19, 2020