Controlling Backscatter Damage to NIF Optics
March 9, 2022
A major cost of operating NIF is mitigating damage to optics as the laser beams propagate to the target.
Stimulated Brillouin scattering (SBS), in which light travels back up the beamline from the target with nearly the same wavelength as the incoming light, has the potential to cause additional damage. SBS results from the interaction between the laser light and the plasma ion acoustic wave, or sound wave.
SBS is a threshold phenomenon, and the ability to simulate the “turn-on” threshold is limited. Therefore, an expert group reviews each experiment for SBS, sometimes recommending “walk-up” shots to approach the threshold at a rate that prevents significant optic damage. These shots add cost and take time.
A Physics of Plasmas paper by LLNL physicist Joseph Ralph and scientist Andreas Kemp focuses on a single approach to controlling SBS by tailoring the target material.
“In inertial confinement fusion (ICF), we use hohlraums to trap laser light and convert it to x rays that bathe the capsule located in the center and cause it to implode,” Ralph said. “We need the hohlraum wall material to have a high-Z (atomic number) to confine the x rays. Typically, gold or gold-lined depleted uranium are used for the hohlraum walls, but in principle this could be any high-Z material.”
This image shows the reduction of stimulated Brillouin scattering (SBS) backscatter at peak laser power in two laser plasma interaction (LPI) simulations. Left: a hydrodynamic simulation of the hohlraum with the green outlining the wall material boundary as it expands inward. The NIF lasers enter from the top and bottom. The target capsule is in the center. Center: a simulation showing bright SBS in a standard hohlraum. Right: a simulation showing diminished SBS (in red) when using a new tantalum pentoxide (Ta2O5) liner. Kemp ran a series of computer simulations with the backscatter code pF3D and compared the modeled backscatter from a tantalum pentoxide (Ta2O5)-lined versus pure gold hohlraum. Using the plasma conditions provided by the standard hydrodynamics simulations of NIF hohlraums, the pF3D simulations confirmed theoretical predictions. The simulations were performed before the actual experiment.
“This result is exciting because it provides us an extra tool to control potentially detrimental levels of SBS backscatter in future hohlraum experiments,” Kemp said. “It also demonstrates our ability to predict and control SBS backscatter using basic plasma physics.”
High-Z materials also produce much higher SBS than low-Z materials. This is because the heavier high-Z ions do not move as fast as low-Z ions when they get hot, and don’t damp the acoustic wave.
“Like surfers trying catch a wave in the ocean, the surfers need to swim with the wave in order to catch it, thereby gaining energy from the wave,” Ralph said. “In these experiments, we used Ta2O5 as a liner. Here, the hot lower-Z oxygen ions damp out the acoustic wave of the higher-Z tantalum atoms. When the (lower-Z) oxygen atoms heat up, some of them ‘surf’ the ion acoustic wave. As the oxygen ions gain energy the wave loses energy, reducing the amplitude and resulting in reduced SBS.”
The research was conducted using simulations and experiments. The simulations showed a clear reduction in SBS of the outer laser beams when using Ta2O5 compared to pure gold hohlraums. Experiments conducted on NIF, the world’s largest and highest-energy laser, using targets similar to those used in ICF, showed a similar reduction in SBS.
Researchers measured the SBS using drive diagnostics and the full-aperture backscatter system (FABS). The hohlraum wall was found to expand inward at a faster rate when using Ta2O5, resulting in a more oblate implosion shape. While this is not ideal, techniques such as cross-beam energy transfer and target geometries such the I-Raum that can help mitigate the shape asymmetry.
“These results show promise for controlling SBS in future experiments, especially as we increase the laser power and energy,” Ralph said.
In addition to Ralph and Kemp, co-authors include Nathan Meezan, Robert Berger, David Strozzi, Brian MacGowan, Otto Landen, Nuno Lemos, Mikhail Belyaev, Monika Biener, Debbie Callahan, Tom Chapman, Laurent Divol, Denise Hinkel, John Moody, Abbas Nikroo, Oggie Jones, Stefano Schiaffino (deceased), Michael Stadermann, and Pierre Michel.
—Michael Padilla
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