Cryogenic Systems Operator Eric Mertens takes final quality assurance photos of the target for the Jan. 15 layered deuterium-tritium high-picket experiment using adiabat shaping. (Credit: James Pryatel)
(Download Image)Fine-tuning NIF Laser Pulses
By reducing hydrodynamic instabilities and the mixing of capsule material into the hydrogen fuel, the recent "high-foot" (high initial laser energy) experiments on NIF achieved record neutron yields and showed signs of alpha-particle heating of the fuel, or "bootstrapping," a necessary step forward on the path to ignition. This was accomplished, however, by trading off fuel convergence and compressibility due to the resulting high entropy, or adiabat (internal energy) of the fuel—a potential challenge to pushing high-foot experiments to ignition conditions.
To address this challenge, LLNL researchers have tested an alternative adiabat tuning methodology that seeks to maintain the ablation front stability benefits of the high-foot design while placing the fuel on a lower adiabat to permit higher convergence and compression and potentially increased neutron production. This "high-picket" design involves an adjustment to the high-foot pulse known as "adiabat shaping," accomplished by lowering the laser power between the first and second of the three high-foot pulses (the "trough") so that the first shock remains strong initially but decays as it transits the ablator and enters the capsule fuel.
In a Physical Review E Rapid Communication published online on March 9, the researchers reported that capsule and hohlraum simulations indicate that hohlraum cooling is sufficient to launch decaying shocks that can lower fuel adiabat, but maintain hydrodynamic stability with adequate symmetry control, suggesting that adiabat shaping (a technique long proposed for direct drive) may be possible with indirect-drive implosions.
Initial experiments on a high-picket design with a lower trough and second pulse have demonstrated behavior largely consistent with the design simulations. For example, a Jan. 15 cryogenic deuterium-tritium experiment using the adiabat-shaped design showed enhanced compression and yield over a companion high-foot implosion without evidence of ablator-fuel mix.
Lead author J. Luc Peterson was joined on the paper by LLNL colleagues Laura Berzak Hopkins, Oggie Jones, and Dan Clark.
