Pulse contrast measurement on the NIF Advanced Radiographic Capability (ARC) laser system

Abstract

Accurate characterization of pulse contrast for high peak power lasers is critical to the success of experiments exploring inertial confinement fusion. The Advanced Radiographic Capability (ARC) laser at the National Ignition Facility (NIF) is a petawatt class laser system that produces pulses in the picosecond regime for the creation of diagnostic x-rays. ARC leverages four of the NIF’s beamlines for final amplification while implementing a separate front-end and pre-amplification stage, known as the High-Contrast ARC Front End (HCAFE). To characterize pulse contrast at the output of HCAFE, a means of measurement at long times (>500 ps) has been developed using a photodiode that has achieved a dynamic range of over 100 dB and 125 dB after deconvolution. Within hundreds of picoseconds of the main pulse, a commercial third-order cross-correlator (Amplitude Technologies Sequoia) is used to characterize the pulse contrast. Together, these diagnostics provide the necessary data for ensuring pulse contrast requirements can be met on ARC. Efforts were made to mitigate existing pre-pulses and to increase the stability of the system as a long-term operational companion to the NIF. We describe the development and testing of the photodiode diagnostic and the analysis of the data resulting from contrast measurements. Details are also provided regarding the identification and mitigation of pre-pulses within the HCAFE system.