Three-wavelength scheme to optimize hohlraum coupling on the National Ignition Facility

Abstract

By using three tunable wavelengths on different cones of laser beams on the National Ignition Facility, numerical simulations show that the energy transfer between beams can be tuned to redistribute the energy within the cones of beams most prone to backscatter instabilities. These radiative hydrodynamics and laser-plasma interaction simulations have been tested against large scale hohlraum experiments with two tunable wavelengths, and reproduce the hohlraum energetics and symmetry. Using a third wavelength provides a greater level of control of the laser energy distribution and coupling in the hohlraum, and could significantly reduce stimulated Raman scattering losses and increase the hohlraum radiation drive while maintaining a good implosion symmetry.

A plasma amplifier to combine multiple beams at NIF

“…The need for accurate prediction of the effects of wavelength tuning on implosion symmetry has built on early models of the linear and non-linear CBET processes observed in experiments specifically designed to study CBET [20][21][22] and produced new models used describe the effect in the fusion target experiments at the NIF. [20][21][22][23][24][25][26][27] These models have shown encouraging correspondence with observations, both in early experiments, 9,10,13,14 and in more recent experiments at the NIF [15][16][17] and have undergone their most precise validation yet in recent experiments with smaller lasers and precise measurements. 28 The development of a robust technology of controlling optical power flow in plasmas now benefits from careful comparison of these models to the existing data base.…”

The role of a detailed configuration accounting (DCA) atomic physics package in explaining the energy balance in ignition-scale hohlraums

“…Thus, the HFM had made some bold predictions, which appear to have come true. In addition, in this context of the Michel et al [32] work, we can now also explain the single remaining discrepancy on the long list of Section 3, namely the observed drop in T drive with Dl. The total SRS increased with Dl, but, as we now believe, it was lost from the hohlraum by exiting the hohlraum in the unmonitored 23.5 inner beam lines.…”

“…A breakthrough in confirming this prediction came when Divol et al [32] re-interpreted the hard x-ray spectrum, not in terms of a single 30 keV T hot-e and f hot-e , but rather as a 2-temperature distribution:…”

“…For example, incorporating a suggestion by Moses, Michel et al [32] have calculated a Dl 30À23.5 that transfers laser power from the 23.5 inner beams, which have proven to be more prone to SRS, to the more benign 30 inner beams. This is a possible method of reducing SRS losses and reducing the level of hot electrons that they create, as we progress to the 1.3 MJ class ignition experiments.…”

Demonstration of High Performance in Layered Deuterium-Tritium Capsule Implosions in Uranium Hohlraums at the National Ignition Facility

“…To counteract this trend, crossbeam energy transfer (CBET) from the outer to the inner beams can, within a certain limit, be enhanced by increasing the wavelength separation Δλ between these laser beam groups [17][18][19]. On the other hand, we want to keep Δλ low to minimize backscatter and hot electron generation through stimulated Raman scattering (SRS) [20]. Higher-Z (than Au) hohlraums are predicted to help increase the capsule drive at a given P laser and provide a more symmetric hohlraum drive [4].…”