In FY23 MEC scientists drove an operational improvement project for the long pulse laser energy upgrade to 100 joules to answer the FES notable outcome for MEC operations:
For the MEC, develop and commission a 100 Joule energy upgrade for nanosecond pulse beam delivery to expand the capability for tackling the challenges in the area of HED hydrodynamics, HED plasma physics, warm dense matter, and remain ahead of the dynamic compression science community.
MEC long-pulse laser system has been providing 60J, shapeable, 5-40ns pulses at 527nm. These laser pulses drive shocks on targets with pressure up to several Mbar enabling high-energy experiments through FEL x-ray diffraction and phase contrast imaging measurements. The purpose of this notable outcome was to upgrade the laser output energy to 100 joules, with all the other parameters remaining competitive, to drive the shocks to higher pressure states with significant increases in pulse-shaping flexibility and performance.
The MEC department took advantage of its expertise, coordinating and combining the efforts of the laser, instrument, and engineering teams, to achieve the project milestone in a limited time frame, despite the significant delays in progress due to the lab-wide safety stand-down activities after winter break. By mid-February 2023, the MEC team has already demonstrated 107.9 J total energy output at 527nm delivered to the target - see discussion of beam combiners. The final goal was mainly achieved by sufficiently extracting the energy stored in four 50-mm diameter Nd: glass amplifiers through reshaping and expansion of seed modal volume as shown in Figure 1. The previous beam diameter passing through the 50-mm amplifiers was under 32mm. Increasing the beam diameter from 32mm to 40mm would increase the modal volume by 40%. Through the combination of beam shaping and larger modal volume, we can increase our green pulse energy to >100 J without significantly altering the system architecture or the fluence of our current optics. By mid-February 2023, the upgraded beams have been delivered to the target chamber. The beam expanders prior to the chamber were modified from M=2 to 1.5 to maintain the same beam size and collimation in the chamber. The mode measurement done in the chamber presented a 72-mm diameter top-hat beam profile with excellent focus quality and a 10 ns square pulse shape (Fig. 2).
There are remaining tasks to complete including beam diagnostics recovery and upgrade (imager cameras installation, pulse-shape scope repairs, energy meter calibration) and pulse-shape finetuning. We estimate that the full system can be commissioned in two weeks of time and plan to be ready to turn over to experimental operations by mid-March 2023, well in advance of the first user experiment.
Figure 1. MEC long pulse laser energy upgrade has been achieved to reach 100J through reshaping and resizing the beam to sufficiently extract the stored energy in the 50mm Nd: glass amplifiers with a top-hat profile maintained. The spatial mode was reshaped at the output of the YFE front end using a serrated aperture and spatial filter. The truncated Gaussian mode is suitable to pre-compensate the radial gain profiles of the 25mm and 50mm amplifiers. The mode size was further increased through a relay imaging system. We adjusted the magnification from 2.5x to 3x such that the beam size increased by 1.2 times and fully filled the 50mm Nd-glass amplifiers. The resultant extracted energy increased exactly by 1.4 times as the gain calculation suggested.
Figure 2. The 100J upgraded spatial and temporal profiles: (a) the expanded beam image taken at the gate position prior to delivering into the chamber, (b) the horizontal lineout of image (a) shows 72 mm beam diameter with top-hat distribution, (c) the comparison of 10ns square pulse shape before (blue) and after the upgrade (red). (d) focus CPP 150 micron, 10 ns square pulse, intensity at plateau 3.3e+13 W/cm2
In summary, the MEC long pulse laser upgrade project has been executed and completed by the following efforts and investigations, which will be detailed in the sections II, III, IV, and V.
- Active shaping: For high-energy laser systems, top-hat beams are desired to optimize energy extraction. To generate top-hat beams, the MEC long pulse laser relies on the apodization method, which involves the spatial filtering of a modulated near-field beam. The spatial mode is modulated at the output of the YFE by a serrated aperture (Fig. 1). The truncated beam shapes are ideal for amplification to obtain top-hat beams. We investigated various serrated aperture sizes and the performance of resultant shapes.
- Modal gain study: Implementing mode imager cameras in various locations in the beamlines to systematically investigate the spatial profile of seed beam, radial gain, and amplified mode.
- Beam expansion: With shaped spatial mode and gain profile modeling, we were able to predict and adjust the magnification to maximize the energy extraction while minimizing the spatial clipping.
- Energy optimization and delivery: investigating the efficiency and conditions of existing optical components in the beam line to maximize throughput and remove loss: including SHG, thin film polarizer, irises, beam splitters, etc.
- Backup plans and parallel investigations for further energy and beam quality improvement