Comparison of laser energy absorbed over time by the gold "bubble" from the hohlraum walls in a cylindrical hohlraum (left) and an I-Raum (right). By 6 nanoseconds, the laser energy absorbed by the bubble has increased to 40 percent in the cylinder and 10 percent in the I-Raum; and at 7 ns, close to the end of the laser pulse for the cylinder, the bubble absorbs 82 percent of the inner-cone energy in the cylinder, resulting in an uneven distribution of energy, but only 37 percent in the I-Raum.
(Download Image)Improving the Energy Balance in NIF Hohlraums
Imagine trying to bake a cake in an oven with uneven heating. Chances are you’ll get a cake with the top undercooked and the bottom overcooked, or vice versa. The same principle applies to NIF hohlraums and targets; if the x-ray energy generated in the hohlraum by NIF’s lasers is uneven, the target capsule’s implosion won’t be symmetrical and implosion performance will suffer.
Recent inertial confinement fusion (ICF) experiments on NIF have shown just such energy imbalances between the inner and outer cones of the laser beams caused by the growth of a "bubble" of hohlraum wall material (hohlraums are made of gold or depleted uranium). The bubble, a result of the higher-intensity outer-cone beams hitting the hohlraum wall, absorbs energy from the inner-cone beams and causes an oblate, or "pancaked," implosion that limits implosion performance.
To deal with this problem, LLNL researchers have designed a new shaped hohlraum called the "I-Raum" that shows promise of enhancing the energy yield from NIF implosions by equalizing the energy deposited by the laser beams on the walls of the hohlraum. The innovative design, aimed at controlling and maintaining implosion symmetry for as long as possible, was described in a Physics of Plasmas paper published online on Jan. 22.
The researchers said the absorption of the inner cone beams by the "bubble" reduces the laser energy reaching the hohlraum equator during the later stages of the laser pulse. The new hohlraum is designed to reduce the bubble’s impact by adding a recessed pocket at the location where the outer cones hit the hohlraum wall.
"This recessed pocket displaces the bubble radially outward," they said, "reducing the inward penetration of the bubble at all times throughout the implosion and increasing the time for inner beam propagation by approximately one nanosecond (billionth of a second). This increased laser propagation time allows one to drive a larger capsule, which absorbs more energy and is predicted to improve implosion performance by as much as a factor of eight in neutron yield."
The new design is based on a June 2017 NIF shot which produced a record neutron yield. The expansion rate and absorption of laser energy by the bubble was quantified for both cylindrical and shaped hohlraums, and the predicted performance was compared. The design has not yet been fine-tuned, the researchers said, "which would be expected to increase the performance further. Future work is ongoing to design an initial series of tuning experiments to establish the initial shock timing, symmetry, laser backscatter, etc. These experiments will be essential for quantifying the potential benefits of this design."
Joining lead author Harry Robey on the paper were LLNL colleagues Laura Berzak Hopkins, Jose Milovich, and Nathan Meezan.
