(Left) The target configuration for diamond (HDC) experiments on NIF. The hohlraum is 9.425 millimeters long and 5.75 mm in diameter. The 192 laser beams enter the hohlraum through two laser entrance holes 3.101 mm in diameter. The high-density carbon capsule is located at the center of the hohlraum and has a shell thickness of 76 microns. (Right) X-ray radiograph of the imploding HDC capsule. Late in time (top), the capsule self-emission dominates the recorded signal.
(Download Image)Diamond Capsules Produce Promising Results
Most NIF inertial confinement fusion (ICF) experiments to date have used plastic target capsules to contain the hydrogen fuel. When the plastic is heated by x-rays generated inside a NIF hohlraum, it ablates, or blows off, causing a rocket-like implosion that compresses and heats the fuel until fusion is achieved.
One possible alternative to plastic is high-density carbon (HDC), a nanocrystalline diamond that has a number of advantages over plastic. Because HDC has a higher density, the same initial outer diameter results in a thinner initial shell and a larger initial inner diameter. The thinner shell leads to less absolute inward motion during shock compression, which produces an ablation front at larger radius when the peak of the drive and acceleration begins.
This larger radius and therefore larger ablation surface area results in higher efficiency and more absorbed energy compared to plastic. This allows HDC designs to reach ignition with a higher adiabat (internal capsule energy) compared to plastic ignition designs, or to achieve higher gains for a similar adiabatic. The larger initial inner radius also leads to an increase in fuel and hot spot mass for the same initial fill density. HDC also can be polished to a much higher surface quality than plastic.
Possible disadvantages include solid or partial melting of the HDC during compression, which may provide microstructures that seed hydrodynamic instability growth; also, reduced ablation-front scale lengths could produce larger overall instability growth leading to a mixing of ablator material into the hot spot.
In an article in Physical Review E published online on Feb. 25, LLNL researchers reported on the results of NIF implosion experiments using HDC capsules with laser pulse energies of 1.3 megajoules and peak power reaching 360 terawatts.
Five indirect-drive ICF implosions were conducted with 76-micron-thick HDC capsules using a four-shock laser pulse optimized for HDC. The experiments demonstrated good laser-to-target coupling (about 90 percent) and excellent nuclear performance. A deuterium and tritium gas-filled HDC capsule implosion produced a neutron yield in excess of 1.6×1015 (1.6 quadrillion), approximately twice the yield of similarly filled plastic ablator capsules.
“These experiments confirm HDC as a viable material for ICF implosions and open the path for higher-velocity layered (cryogenic) implosions,” the researchers said. “Future experiments are planned with HDC capsules with the goal of reaching the alpha-heating regime, where alpha-particle deposition significantly enhances the neutron yield production, and eventually ignition.”
Lead author Steven Ross was joined on the paper by LLNL colleagues Darwin Ho, Jose Milovich, Tilo Döppner, James McNaney, Andrew MacPhee, Alex Hamza, Juergen Biener, Harry Robey, Eddie Dewald, Riccardo Tommasini, Laurent Divol, Sebastien Le Pape, Laura Berzak Hopkins, Peter Celliers, Nino Landen, Nathan Meezan, and Andy Mackinnon.
