PREVIOUSLY Livermore’s Petawatt laser was built into a beamline of the Nova laser. Today, the Advanced Radiographic Capability (ARC) laser follows the same “laser within a laser” strategy. The most energetic short-pulse laser in the world, ARC resides within the National Ignition Facility (NIF)—the most energetic of all the world’s lasers. Both operate simultaneously during experiments, and synchronizing the enormous amounts of energy released by the billionths-of-a-second NIF with the trillionths-of-a-second ARC is nothing short of an operational wonder.
A major challenge for researchers using NIF to study high-energy-density (HED) plasmas and inertial confinement fusion (ICF) for the Stockpile Stewardship Program has been examining the inside of the target to ensure that implosion is occurring in a highly spherical manner. (See S&TR, July 2015, Stockpile Stewardship at 20 Years.) The greater the asymmetries and shape swings as the target compresses, the more performance is compromised. Different Laboratory missions require different target materials, and instabilities can adversely impact experiments in a multitude of ways, ways, including reduced fusion yield in the case of ICF. As they strive to improve symmetry and robustness, researchers require a diagnostic tool that can reveal the shape of the target as it compresses, and increasingly ARC is being used to unlock these never-before-measured extremes. “ARC opens up an avenue that we didn’t previously have at NIF,” says David Martinez, a Livermore physicist researching HED science and who heads the team developing ARC for HED backlighting. “As we use NIF’s main laser to drive our targets to higher densities, pressures, and compressions, we now have in ARC a diagnostic that can see through these objects, look at perturbations, and probe further in time.”
X rays image a target’s interior in a manner similar to a doctor taking an image of a patient’s body. To block the x rays from parts of the body not being examined, the doctor has the patient wear a lead-filled apron. However, if lead can stop x rays during a doctor’s visit, how can NIF image the interior of something far denser? For many applications, NIF accomplishes this feat by diverting a small subset of its main laser beams from the experimental target to a separate metal foil target for generating probing x rays of much higher energy. Yet even these NIF-driven x rays are not of high enough energy or flux to probe the target during the final stages of implosion or to see details in lightweight materials such as hydrogen isotopes, an important fuel for ICF. ARC seeks to overcome these limitations.
Since being commissioned in December 2015, ARC has been used in at least 64 shots, including platform-development firings. Beyond backlighting applications, ARC enables exciting new and diverse research into exotic matter and particle generation. Potential studies range from better understanding the nature of the most energetic events in the universe to improving methods for treating cancer.