July 21, 2022
The dedicated scientists, researchers, engineers, and technicians who work on Lawrence Livermore National Laboratory’s unique diode-pumped alkali laser (DPAL) technology have garnered high praise over the years.
The Lab Director’s Office has presented the DPAL team with an LLNL Science & Technology Award for a “a tour de force of physics and engineering” in achieving significant scientific advances in diverse areas of science, including atomic, molecular, and optical physics, and computational and experimental fluid dynamics for the previous generation system.
The DPAL project has benefited from LLNL’s ability to attract world-class experts who combine a diverse set of skills to work on novel, groundbreaking technology designed to enhance the nation’s safety and security.
“With DPAL, we’re building a system that is a little bit different from some of the other laser systems at the laboratory,” says DPAL Project Manager Jeff Horner, who is also chief engineer for the NIF & Photon Science principal associate directorate.
“We’re developing a new type of laser, so there’s a lot of creativity and attention to detail that’s required,” he says. “There’s a certain amount of a sense of mission that the team also embraces. This system could be very important to the government and people are passionate about that.”
Historically, gas lasers have been pumped, or energized, by direct electrical discharge or a chemical reaction, while solid-state lasers are pumped by flashlamps or semiconductor diode laser arrays. A diode-pumped alkali laser is a relatively new class of laser that is based on diode excitation of atomic alkali vapors that combines the strengths of both gas and solid-state lasers (see “Getting Laser Diodes to March in Lockstep”). LLNL is developing the technology under a contract with the Department of Defense Missile Defense Agency.
Because of the sensitive nature of their work, the team’s members don’t often receive much public recognition in ways other Lab researchers receive, such as with the publication of major scientific papers or presenting in major conferences. But their motivation isn’t in the accolades or individual accomplishments. Instead, their drive comes from combining their individual talents to help solve complex technological challenges that are part of a larger mission to serve the nation.
And the team has managed to maintain extraordinarily high levels of productivity even while navigating the protocols enacted to keep Lab employees safe during the COVID-19 pandemic. The type of in-lab activities required to refine the DPAL technology also meant less flexibility for the team to work remotely, yet they’ve consistently met their goals.
“We have an extremely driven and professional team,” says Dan Mason, who led the design of the current DPAL system.
This high bar of excellence was set during a storied history of laser development at LLNL that includes NIF, the world’s largest and highest-energy laser system, and the L3-High-Repetition-Rate Advanced Petawatt Laser System, built by LLNL for the ELI-Beamlines facility in the Czech Republic.
“We have a whole lot of really impressive people who learned to overcome the technical challenges associated with designing, building, and operating big lasers on NIF, and they’ve come over and applied that expertise to this project,” Mason says. “The DPAL work is just another example in this lab’s long history of building high complexity first-of-a-kind systems.”
The idea for the technology originated with now-retired LLNL senior scientist Bill Krupke, who in 1972 helped a team consolidate the Lab’s laser efforts into a single-focused program expanding the use of lasers for fusion weapons physics, fusion energy, military applications, and other functions.
But the technology couldn’t take off until the early 2000s with the availability of high-average-power laser diode arrays, “the kind of thing you would use in your CD player,” says DPAL Chief Scientist Ray Beach.
Krupke was instrumental in recognizing the potential for diodes as the excitation source for alkali vapors. Development started as a very small Laboratory Directed Research and Development (LDRD) project.
Beach says it’s been “very gratifying” to see the project succeed. “My real passion is to understand the physics of it,” he says.
Today, a team of about 50 people work on the project, which is part of the NIF & Photon Science Directorate. Over the years, contributions have come from a wide variety of departments, including infrastructure, designers, modelers, diagnostics, and operations.
“It’s like a mini NIF—we conduct shots from a similar control room with a shot director and system operators,” says Horner, who has helped build and develop multi-disciplinary teams working on projects like DPAL, HAPLS, NIF’s final optics, and target fabrication engineering. “The breadth of talent on the team is part of what make us be successful.”
The shot director also coordinates the work in the DPAL lab even when shots are not running. Members of the Laser Science & Technology and Systems Engineering group provide guidance for experimental planning, technical approach, and sponsor interactions.
The project also has specialists in adaptive optics, who put the finishing touches on a laser beam to refine its shape and make it uniform and collimated, and experts who lead DPAL’s laser experiment planning.
“Improving laser beam quality is a sometimes under-appreciated task in the development of high-power laser systems where total laser power frequently grabs the headlines,” says Doug Homoelle, who leads the DPAL laser diagnostics and adaptive optics system. “As an example, for certain applications, doubling the peak intensity of the focused laser through improved beam quality has the same impact as doubling the laser output power.”
At the heart of DPAL are the laser diodes that are unique to the technology. The NIF Optics & Target Materials Division of the Physical and Life Sciences Directorate has a special team that carries out the key role of assembling and testing those diodes before they go into the DPAL system.
The DPAL team has also been able to add key members like laser physicist Fran Morrissey, an expert in the specialized field of unstable resonators, which allow high-average-power lasers to operate while maintaining good beam quality.
Morrissey joined LLNL in 2016 from MIT’s Lincoln Laboratory, where he became aware of the work LLNL was doing with unstable resonators for DPAL. Because “nearly all lasers in use utilize stable resonators, often coupled with amplifiers” he says, he believed his rare skills in a specialized field of unstable resonator mode theory would be a better fit at LLNL.
The computational methods for modeling unstable resonators “are not only quite complicated, but also computationally intensive” he added, but he’s grateful for the chance to work at a lab such as LLNL that allows him to work with a dynamic team on such a specialized field.
“DPAL physics is not settled science, which makes it not only interesting but exciting. We’ve developed new models that extend to the state-of-the-art that haven’t really existed, as far as we know,” Morrissey says. “ Our capability to predict the laser output is impressive and is critical for enabling a design that can achieve high brightness.”
There are also the hands-on engineers and technicians, who take the concepts worked out by the scientists and designers and turn them into the hardware.
“This group is very good about integrating everybody into the decision processes,” says Larry Platz, an engineering technical associate/supervisor.
DPAL is a “constantly evolving project where every day is not the same,” Platz says. “It’s all R&D type stuff, so every time you turn that laser on or every time you do something new, you’re always learning new things.”
Every step of the process is crucial, such as the optics team being able to keep the optics used for DPAL pristine and free from any little defects or dust.
As with everything these days, software plays a huge role. The project has benefited from advanced computer modeling to calculate the physics involved and the use of computer-aided design tools to make sure the parts fit together as designed. NIF’s Integrated Computer Control System group developed software that makes sure operating systems run seamlessly.
“You can’t underestimate (the need to) have user-friendly ways of operating it and that takes a lot of effort and experience on the part of these software developers,” says Ben Haid, DPAL’s lead engineer.
Team members also acknowledged the behind-the-scenes contributions of employees who put together reports and slide shows to keep project sponsors and other stakeholders up to date on DPAL’s accomplishments, and the efforts of those who track purchasing orders and contracts to keep needed parts and supplies flowing and processed in time.
And there’s the facility managers who ensured that DPAL’s workspaces, which include high-power utilities and compressors, were built properly and keep operating smoothly.
A few DPAL team members have even gone into semi-retirement, yet still contribute to the project at least part-time.
“It’s more than just work, it’s a pastime,” Haid says. “It’s something they enjoy thinking about throughout their day. There’s been a number of people like that involved with DPAL over the years.”
Haid says he enjoys working on DPAL because it “promises to be a very useful technology.
“It’s not only a project that has a bright future, but it presents some unique challenges,” he says. “The combination of having the challenge, seeing the future, and being successful is very satisfying.”
Platz, who joined the project in 2016, works with a “vast and dynamic” team of members who come from different industries—from automotive to electronics to construction.
“Everybody chips in in their own way,” he says. “People are willing to take the time to teach each other so that everyone learns as you go, which makes the team better because you’re not always relying on only one person to do a specific thing.”
Mason joined LLNL 20 years ago as a NIF & PS Summer Scholar. Mason eventually became lead engineer on the HAPLS project and has used that experience on DPAL, where “the learning never stops.”
“I’ve spent most of in my career supporting NIF, which is a great big pulsed laser,” says Mason, DPAL’s former lead engineer. “These continuous wave lasers like DPAL have some similarities, but there are a whole host of interesting new engineering challenges that are very different than NIF and that’s one of the things that make it a really exciting project to work on.”
Sheldon Wu, the staff scientist in DPAL, came to LLNL in 2006 and is now a senior member of the team. Even after all these years, he calls DPAL “a very fascinating system.”
“I’m still very interested in the physics that went into developing this laser,” Wu says. “And we’re still discovering new effects.”
Lead DPAL operator Kurt Cutter says one of the strengths of the team is the direct line from the engineers to the scientists so they can work together. “It’s the key component to the success of DPAL,” he says. “And everybody’s on board with making sure everyone is successful.”
Cutter chuckled when he recalled being hired as a mechanical tech by the Lab almost 25 years ago and “I didn’t even know lasers were a thing.” But he became so interested after joining the diode development group that he took classes at Los Positas College and earned an associate degree in laser technology.
Now he’s enjoying a rewarding career as a laser electro optical technician.
“I never thought my career would take this path,” Cutter says.
“Congratulations to the DPAL team,” Horner says, commending the team members for their “incredible technical accomplishments” and their continuing efforts to cultivate team comradery.
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