Energy transfer between lasers in low-gas-fill-density hohlraums

Abstract

We investigate cross-beam energy transfer (CBET), where power is transferred from one laser beam to another via a shared ion acoustic wave in hohlraums with low-gas-fill density as a tool for late-time symmetry control for long-pulse (greater than $10\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$) inertial confinement fusion (ICF) and laboratory astrophysics experiments. We show that the radiation drive symmetry can be controlled and accurately predicted during the foot of the pulse (until the rise to peak power), which is important for mitigating areal density variations in the compressed fuel in ICF implosions. We also show that the effective inner-beam drive after CBET is much greater than observed in previous high-gas-filled-hohlraum experiments, which is thought to be a result of less inverse bremsstrahlung absorption of the incident laser light and reduced (by more than 10 times) stimulated Raman scattering (and Langmuir wave heating). With the inferred level of inner-beam drive after transfer, we estimate that more than $1.25$ times larger plastic capsules could be fielded in this platform with sufficient laser-beam propagation to the waist of the hohlraum. We also estimate that a full-scale plastic capsule, $1100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in capsule radius, would require $\ensuremath{\sim}1--2\phantom{\rule{0.16em}{0ex}}\AA{}$ of $1\ensuremath{\omega}$ wavelength separation between the outer and inner beams to achieve a symmetric implosion in this platform.