Unabsorbed light beamlets for diagnosing cross-beam energy transfer

Area of Science:

• Plasma Physics
• Fusion Energy
• Optical Diagnostics

Background:

• Cross-beam energy transfer (CBET) significantly impacts laser energy deposition in direct-drive inertial confinement fusion (ICF) implosions.
• Understanding CBET is crucial for optimizing ICF performance and achieving ignition.
• Previous diagnostics lacked the resolution to probe CBET effects at specific locations within laser beam profiles.

Purpose of the Study:

• To introduce and validate a novel diagnostic for investigating CBET during direct-drive implosions on the OMEGA laser facility.
• To analyze the spatial and temporal characteristics of CBET and its influence on laser energy.
• To provide the first experimental evidence of polarization rotation induced by CBET in this regime.

Main Methods:

• Utilizing a gated optical imager to capture images of unabsorbed laser light from individual beamlets.
• Employing a Wollaston prism to split each beamlet's light into two orthogonal polarizations for detailed analysis.
• Recording images in two distinct 200-picosecond time windows to capture dynamic CBET effects.

Main Results:

• The diagnostic successfully imaged unabsorbed light from individual beamlets, revealing spatial variations in intensity due to CBET.
• Analysis of polarization states provided insights into the interaction mechanisms within the plasma.
• The study presents the first direct evidence of polarization rotation occurring as a result of CBET during direct-drive implosions.

Conclusions:

• The new diagnostic is effective in spatially and temporally resolving CBET effects in direct-drive ICF.
• Observed polarization rotation offers new avenues for understanding and potentially mitigating CBET.
• This work advances the capability to diagnose and control laser-plasma interactions critical for fusion energy development.