In the 25 years since LLNL broke ground on the National Ignition Facility (NIF)—a cornerstone of the National Nuclear Security Administration’s Stockpile Stewardship Program—the NIF and Photon Science (NIF&PS) Directorate has steadily pushed the boundaries of laser physics, nonlinear optics, and photonics in service of inertial confinement fusion experiments and advanced photon sources development.
Beyond the 192 beamlines of the primary laser facility with its world-record energy and fusion yields, NIF&PS capabilities have expanded to include chirped-pulse amplification, kilojoule petawatt-class short-pulse systems to generate hard x-ray radiographic probes, high-average power lasers as secondary sources for generation or wake field particle acceleration along directed energy applications, and other scientific pursuits.
The Computing Directorate partners with NIF&PS on numerous projects and technologies from diagnostic measurements and high-performance control systems to data analysis, information technology infrastructure, and scientific simulation code development. “This strong collaboration enables cutting-edge science, particularly with regard to physics-based modeling and simulation,” says physicist Jean-Michel Di Nicola.
One crucial capability for NIF&PS is the Virtual Beamline (VBL) laser simulation code. It can model all the major laser physics and technology involved in the design optimization, commissioning, and operations of advanced laser architectures from tabletop to NIF-scale. VBL provides researchers with high-fidelity models and high-resolution calculations of laser performance predictions—including for the entire NIF laser system, the Advanced Radiographic Capability, parts of the High-Repetition-Rate Advanced Petawatt Laser System that was delivered to the Czech Republic, and the Optical Science Laser.
After more than two decades of experimentally verified physics and computing enhancements, this unique workhorse code is wrapping up another major milestone: migration from Java to C++ with a flurry of user interface and optimization features, as well as laser physics and high-resolution enhancements, thanks to parallel execution on Livermore Computing supercomputers.
“VBL is key to researchers’ ability to design architectures and experiments and deliver results,” explains software engineering manager Kathleen McCandless, who serves as Computing’s group leader for Diagnostics and Laser Performance. “The team is looking forward to deploying the new functionality into production.”
New Requirements, New Approach
Like other mission-driven codes at the Laboratory, VBL’s capabilities have changed over time to support additional users and new applications. It has grown to model lasers with monochromatic to broadband spectra, various beam geometries, and vastly different pulse durations from the femtosecond (10–15) to nanosecond (10–9) regime. VBL also covers—for all these regimes—laser amplification with wavelength-dependent emission cross section, nonlinear effects including Kerr self-focusing and frequency conversion, laser damage limits, and more.
Today, VBL is the physics engine used “under the hood” hundreds of times every day by the Laser Performance and Operations Model (LPOM) software as users set up their NIF experiments.
Increasing demands and scope prompted a thorough examination of VBL’s original approach. “Users needed enhanced physics models that were more computationally intensive and at higher resolutions, which means using the Lab’s supercomputing capabilities. We couldn’t run VBL Java in parallel on high-performance machines without rearchitecting it,” recalls McCandless.
With funding from the Laboratory’s Institutional Scientific Capability Portfolio (ISCP), the VBL team decided to rewrite the mature Java code base in favor of a faster, more flexible programming paradigm. The C++ programming language minimizes memory movement while the code is running, improves performance, interfaces more directly with the computing hardware it runs on, and provides more control.
With many prior years invested in Java, the team considered binding the two programming languages, but doing so could have made the code buggy and unstable. Ultimately, the stakes were too high to gamble with a patchwork solution. McCandless points out, “Our requirements are vast. We must make sure performance is optimized and calculations are accurate. We don’t want to be responsible for holding up a NIF experiment or introducing errors when designing new advanced architectures.”
Minor Name Change, Major Possibilities
While the team has delivered multiple major releases of upgraded VBL code since 2017, a new VBL++ (pronounced vee-bee-ell-plus-plus) is slated for LPOM production deployment in 2022. For portability to present and future high performance computers, including classified computing systems, the code leverages Livermore’s RAJA software framework. A wider array of physics calculations within VBL++ is now possible thanks to integration of another LLNL-developed software library called SUNDIALS, which provides solvers for differential algebraic and ordinary differential equations.
VBL++ has a robust interface that allows users to optimize the parameters of their laser architecture; import external files from finite element analysis codes to account for stresses and thermal effects inducing birefringence; resolve the impact of optical component imperfections; and perform an inverse solve to determine the input low-power pulse shape that will achieve the high-power request on target. Depending on the application’s computational requirements, laser physicists can run VBL++ using a graphical user interface on a laptop or submit batch processes on a Livermore supercomputer with thousands of cores.
All of these features are covered in user documentation, including video and live training sessions for LLNL scientists. “Our goal is to conduct broad training across the Lab to continue growing this expertise, incorporate more models for emerging needs, and engage more users and physicists in the project,” says McCandless, noting Livermore’s trusted reputation in laser physics modeling and high performance computing among the scientific community. Also on the project’s horizon are requirements to accommodate still more physics and refined amplifier models, such as for short-pulse and high-repetition-rate laser architectures.
Such advanced—and competitive—capabilities are brought to bear on upcoming new projects and collaborations. Di Nicola, the current project leader for VBL++, points out that ISCP investment demonstrates the importance of thinking broadly as laser science, nonlinear optics, and photonics enter a new era—one in which the code’s engineering and computational capabilities can drastically reduce the risks, development cycle, and cost of new laser architectures.
He explains, “We are building a multipurpose simulation code for customers with emerging and diverse laser designs, technologies, and applications.” McCandless adds, “VBL has endured because no other product can handle everything that it can. Future laser systems will be very different from what we have today, and VBL++ will help the Lab meet those challenges.”
Along with McCandless and Di Nicola, the VBL++ team now includes physics lead Samuel Schrauth; Francis Morrissey, Tom Lanier, and other physicists from the NIF&PS Laser Modeling and Analysis Group; software architect Tom Epperly; and two former interns hired during the pandemic: River Aden and Jordan Penner. Special thanks go to Barry Fishler, who previously served as software engineering manager, and Jarom Nelson.