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Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing
D H Edgell1, P B Radha1, J Katz1
1Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623-1299, USA.
Physical Review Letters
|August 30, 2021
Summary
Distributed polarization rotators (DPRs) in OMEGA laser-direct-drive implosions cause significant light nonuniformity. New optics reducing polarization offsets can greatly improve implosion symmetry and performance.
Area of Science:
- Laser-driven inertial confinement fusion
- Plasma physics
- Optical engineering
Background:
- OMEGA laser facility uses 60 beams for symmetric implosions.
- Distributed polarization rotators (DPRs) smooth laser beams via orthogonal polarizations.
- Observed scattered light shows unexpected asymmetry and high variation.
Purpose of the Study:
- Quantify scattered-light nonuniformity from individual beams.
- Identify the source of enhanced nonuniformity in OMEGA implosions.
- Investigate the impact of DPRs on implosion symmetry.
Main Methods:
- Developed a novel instrument to measure intensity and polarization of scattered light from individual beams.
- Modeled on-target polarization offsets using a 3D cross-beam energy transfer (CBET) code.
- Analyzed time-integrated nonuniformity in laser absorption and scattered light.
Main Results:
- Identified DPRs as the primary source of enhanced scattered-light nonuniformity.
- Asymmetric intensity and polarization measurements correlate with DPR-induced polarization offsets.
- CBET modeling explains the observed asymmetric measurements.
- Predicted significant time-integrated nonuniformity dominated by low spherical harmonic modes.
Conclusions:
- DPRs introduce significant nonuniformity in laser-direct-drive implosions.
- Polarization offsets from DPRs are a critical factor in OMEGA implosion asymmetry.
- Replacing DPRs with new optics offering smaller offsets is predicted to reduce nonuniformity significantly.
