Speckles, or “hot spots,” in a group of NIF laser beams can reflect off the wall of the hohlraum and be directed to the target capsule; the process is called “glint” because it represents a flash of light from a shiny surface. Researchers believe this laser pre-pulse and early-time laser reflection could cause capsule imprint, initiate the growth of perturbations, enhance fuel/capsule mix, and cause unwanted early-time shocks in NIF indirect-drive implosion experiments.
In a Review of Scientific Instruments paper published online on Aug. 1, LLNL researchers reported that the addition of a simple high-dynamic range optical signal detector to the widely used Velocity Interferometer System for Any Reflector (VISAR) diagnostic has made it possible to detect laser pre-pulse and laser glint that transmits through the capsule.
The first measurements showed evidence of laser pre-pulse and possible light reflection off the hohlraum wall and onto the capsule in gas-filled and near-vacuum hohlraums. The researchers said radiation hydrodynamic simulations of the early-time laser power showed that in gas-filled hohlraums there is a short period of time (about 0.2 nanoseconds) in which glint can reach the capsule without absorption in the hohlraum gas/plasma. Modeling also shows that near-vacuum targets are prone to glint throughout the laser pulse.
“Further tests will aim to determine the signal origin,” they said, “as well as use measurements of capsule transmission to 351-nanometer (ultraviolet) light to quantify the level of pre-pulse and glint.” The first of two two-shock glint re-emit experiments, conducted later in August to assess glint strength in a gas-filled high-density carbon (diamond) hohlraum, saw no evidence of glint. The second experiment will test for glint in a near-vacuum hohlraum.
Lead author John Moody was joined on the paper by LLNL colleagues Todd Clancy, Gene Frieders, Peter Celliers, Joseph Ralph, and David Turnbull.
Mike Dunne, NIF&PS director of laser fusion energy, gave the opening plenary talk at the SPIE Optics+Photonics conference in San Diego on Aug. 17. The annual conference is the largest international, multidisciplinary optical sciences and technology meeting in North America.
During his presentation, given to a standing-room-only audience of about 650, Dunne discussed the current status of the campaign to achieve ignition, presenting the most recent experimental results and analysis. He also discussed upcoming challenges, and how success could motivate future development of a source of electrical power from laser-driven fusion. For more details, see the article in Optics.org. In addition to Dunne’s talk, LLNL was represented by a number of researchers giving presentations, running symposia and serving on organizing committees.