In the first quarter of FY2016, the NIF facility commissioned the Advanced Radiographic Capability (ARC) short-pulse laser system. ARC high-energy backlighting capability is required for the Stockpile Stewardship Program (SSP)-High-Energy-Density (HED) and SSP-Inertial Confinement Fusion (ICF) programs. Two beams in a single NIF quad (Q35T) are converted into four ARC “beamlets”; the infrastructure to convert the other two beams in the quad to four more beamlets is partially installed in the NIF target bay. A layout of the ARC system is shown in Figure 4-14. Conversion of NIF beamlines to high-intensity, picosecond operation requires three top-level changes to the existing NIF beam lines:
- Addition of the High Contrast ARC Front End (HCAFE), which provides dual regenerative amplifiers for split beam injection.
- Improved amplifier isolation for backscatter protection.
- Pulse compression in the NIF Target Bay and final focusing optics.
The HCAFE uses short pulse optical parametric amplifier technology to meet the high temporal contrast requirement of 80 dB for t < -200 ps before the main pulse (70 dB at TCC). The dual amplifiers on the ARC Dual Regen Table and split beam injection produce two beamlets that can be independently timed (0–20 ns currently) and pointed (0–1 mm currently) to a target. Pointing between the B353 and B354 pair of beamlets is currently ± 50 mm. The short-pulse output of the compressor is diagnosed using the ARC Diagnostic Table (ADT) during system shots. The ADT is capable of measuring key short-pulse laser performance metrics (near-field and far-field intensity patterns, pre-pulse levels, spectrum and energy in the compressed pulse, etc.) on system shots. The measured focal spot performance goal for a 1.5 kJ beamlet with 30 ps FWHM Gaussian pulse is 50% of the energy with ≥1017 W/cm2 irradiance and contained in 150 µm spot size.
Currently, ARC is commissioned at 30 ps pulses and 1 kJ per beamlet. As ARC optics are upgraded and laser performance optimized, the maximum energy will increase to 1.5 kJ at 30 ps pulse length. As new user requirements are developed for future missions, ARC will be commissioned at different operational parameters (pulse length, beamlet timing and pointing, etc.).
Figure 4-14. Advanced Radiographic Capability (ARC) system in the NIF Target bay. The ARC beamlets (shown in red) enter the chamber from ports near the chamber’s equator.