Particle physics experiments

‘Twisted’ laser light experiments offer new insights into plasma physics

Andrew Longman aligns the COMET laser at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. Upcoming experiments using the COMET laser in early 2023 aim to create some of the strongest static magnetic fields ever generated by a laser.

Electromagnetic vortices occur naturally throughout the universe and have recently been observed in association with black holes. Over the past decade, scientists have sought methods to study how extremely powerful electromagnetic vortices interact with matter, particularly plasma, in a laboratory environment.

Plasma, known as the “fourth state of matter,” makes up nearly all of the observable matter in the universe and is made up of free-moving ions and electrons. Using high-intensity lasers to generate electromagnetic vortices has shown great promise and has the potential to unlock new physics when such beams interact with plasma.

Andrew Longman, Center for High Energy Density Science (HEDS) postdoctoral fellow for Lawrence Livermore National Laboratory (LLNL), proposes that spiral phase mirrors, when incorporated into a laser system, will allow scientists to “twist” the laser light and generate an optical effect. vortex. An optical vortex is best described as a beam with a helical wavefront, like a whirlpool but spinning at the speed of light.

Trip to Livermore

While pursuing his doctorate. in electrical and computer engineering at the University of Alberta, Longman gave a presentation at the University of Rochester on the use of spiral mirrors in high-power laser systems. His work caught the attention of several Lawrence Livermore scientists, including David Strozzi, who encouraged Longman to consider coming to the Laboratory. Longman’s unique research proposal on how to use these mirrors to “twist” laser light and create extreme magnetic fields – something that until now has been mostly theoretical – won him a two-year fellowship. at the HEDS Center of the LLNL.

“We launched the HEDS Center Postdoctoral Fellowship in 2019 with support from the Weapons Physics and Design Program of Weapons and Complex Integration,” said Deputy Director and HEDS Center Fellowship Coordinator, Félicie Albert. “Andrew exemplifies the quality of research and talent that this scholarship speaks of. The program gives our fellows the opportunity to pursue their own original research, utilize laboratory resources, and interact with other scientists. while benefiting our programs and strengthening the HEDS pipeline, one of our core competencies.

As a fellow, Longman had the opportunity to further his graduate research and improve the fabrication process of his spiral mirror design using the lab’s unique optical fabrication capabilities with magnetorheological finishing (MRF) technology. ) – a technique for polishing ultra-precise corrective topographic structures on optical surfaces.

swirl 102122
Optical swirls, as shown here, are best described as a beam with a helical wavefront. In Andrew Longman’s research, optical vortices are used to spin plasma at extremely high angular velocities, allowing the study of new physical phenomena. To generate these vortices at ultra-high intensities, Longman uses an off-axis spiral phase mirror – a mirror that has a spiral of deep wavelength (less than a thousandth of a millimeter) imprinted on its surface.

mirror mirror on the wall

The spiral mirror technology used to generate an optical vortex is relatively simple. “To make the specialized off-axis spiral phase mirrors, we used MRF techniques to imprint a deep wavelength (less than a thousandth of a millimeter) spiral onto the mirror surface,” Longman explained. . “As the laser beam reflects off the mirror, it picks up a ‘twist’ and can transfer that angular momentum to any type of target – solid, gas or plasma.” Hence, allowing researchers to drive helical plasma waves and currents capable of generating very strong magnetic fields, as well as trapping, guiding and accelerating particles that could enhance laser-plasma interactions.

When researching methods to generate high-power vortices, Longman had to consider the constraints associated with directly modifying the components of a given laser setup. The installation of specialized mirrors offered the most practical solution. Overall, Longman’s mirror design proved to be extremely successful, generating the highest intensity optical vortices ever produced. In fact, the design has now been implemented in approximately 10 laser installations around the world. Longman said, “It’s a great feeling to see something I’ve created used so widely.”

Humble beginnings

After years of research, Longman’s journey at LLNL has come full circle. His first introduction to LLNL and its facilities was in 2015 and 2016 as a graduate student when he assisted LLNL scientists Art Pak and Tony Link in a series of experiments at the Jupiter Laser Facility (JLF). Now, as a fellow, Longman will have the opportunity to lead an upcoming experiment at JLF, where he will use the finished spiral mirrors to “add a twist” to JLF’s COMET laser platform with the goal of generating and to measure extreme magnetic fields.

Reflecting on his experience at LLNL so far, Longman admitted that it was difficult to start in a new lab in the midst of a pandemic. “During the first year of my fellowship, I did not have the opportunity to interact regularly with the other scientists due to COVID restrictions, but now that I am on site more frequently, I can be more involved and have quality conversations with other scientists about their research. He added: “There is so much talent under one roof and the people here are so welcoming.”

Longman’s mentor, Pierre Michel, said, “Andrew came to the lab with a remarkable skill set, ranging from hands-on experimental expertise to advanced theory and simulation techniques. He independently conducts his own research project while collaborating with large teams, and he makes full use of the resources offered by the laboratory, from advanced manufacturing to large-scale computing. This makes him an ideal candidate for the HEDS fellowship and for the lab in general.

– Shelby Conn

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