Lawrence Livermore National Laboratory's groundbreaking achievement of fusion ignition is often described as "historic" - and for good reason. The scientific breakthrough came after over 60 years of dogged persistence. But what were the pivotal events and daily challenges that filled those years, from before the invention of the laser to ignition on the National Ignition Facility in 2022?
LLNL physicist Mordecai D. ("Mordy") Rosen fills in those gaps in a new paper, "The Long Road to Ignition: An Eyewitness Account," published in Physics of Plasmas. He blends rigorous technical details, history and his personal experiences.
"As scientists, we acknowledge, in brief, that we stand on the shoulders of giants," he said. "I wanted to put some meat on those bare bones and describe the contributions of so many."
Over the course of his career, Rosen encountered many of those giants, like Edward Teller, Hans Bethe, Dick Garwin, Marshall Rosenbluth, Carl Haussman, Mike May and Claire Max. He came to LLNL in 1976, after completing his Ph.D. in physics at Princeton. His doctoral work was on magnetic fusion, but he soon shifted to target design for inertial confinement fusion (ICF) experiments.
"I was excited by the possibility that as a target designer, I could put 'a new tokamak' in front of the high-tech and expensive element of the ICF program, namely, the laser, every single day," Rosen said. "This insight served me well throughout my career and kept my work fresh, diverse and exciting for nearly 50 years."
He was hired by John Nuckolls, associate program leader in the Laser Fusion Program at the time and later a Lab director, who first theorized that lasers could be used to induce fusion in a laboratory setting. Rosen went on to lead X-Division and fulfillment of the multi-faceted Nova Technical Contract, a requirement for approval of NIF. He has continued to play a key role in hohlraum and capsule design.
The paper reviews the long journey that culminated in ignition, including the early origins of LLNL's Laser Program and key developments such as the paradigm-shifting birth of high-energy-density physics studies with lasers, changes in laser wavelength and the development of key diagnostics and computer codes. It describes the evolution of the ultimately successful ignition target design - including changes in the hohlraum design, laser pulse shapes and materials and size of the capsule.
"This evolution embodies what I call 'ICF's superpower', namely the ability to innovate target design choices to circumvent the roadblocks Mother Nature put in our way," he said.
Rosen shares pivotal moments, like how Lyman Spitzer drew inspiration from a ski lift for the first stellarator, which eventually became the Princeton Plasma Physics Laboratory, conversations with Edward Teller and how the 1980 Greenville fault earthquake drove development of free-floating laser tables at LLNL.
"It brings me great joy to share this history," Rosen said. "All of it - the mistakes and setbacks, the many talented colleagues who contributed over decades and of course the incredible success we achieved. I hope we can continue to apply the lessons learned on this journey to future endeavors."
