The shaping is effective for pre-compensation of gain profile non-uniformities to achieve a flat output profile. To properly shape the beam profile for high-energy amplification, we use the apodization method, which involves a serrated aperture and a vacuum spatial filter. As Figure 3 shows, a serrated aperture is mounted on an x, y stage to position in the beam centroid. A 4-f relay system images the beam from the serrated aperture plane (position a) to the plane of the 25 mm amplifier (position c). A mode camera is set up to image the amplifier plane to study the beam shape with respect to the change of aperture or pinhole size.
Figure 4 shows the serrated aperture that we currently choose to put in the beamline. It is a steel plate with 90 teeth of ID 8.57 mm and OD 10.57 mm. The modulated near-field image (Fig. 4-a lower) shows the teeth biting into the edge of the beam. Figure 3 (b) shows the unshaped beam profiles (without serrated aperture and pinhole) imaged at the plane of the 25 mm amplifier. The beam width increases from 18 mm to 19.8 mm when the 25 mm rod is pumped. It is clear that The beam has some vertical stripes in the beam caused by a crystal defect from the YFE front end. After shaping, the beam profile is much smoother and smaller (figure 4-c). The beam size increases from 15 mm to 16 mm with pumping.
The current serrated aperture of teeth ID 8.57 mm is close to the YFE beam size 9.5 mm. The current pinhole size is 0.75mm in diameter. For further shaping, we have other serrated apertures and pinholes of smaller IDs as backups, which will further reduce the beam size and output energy.
Figure 3. The apodization method involves a serrated aperture (red circle a) and a vacuum spatial filter to remove modulated high-frequency mode through a pinhole of ID 0.75 mm (red circle b). The 4-f relay system (L1 and L2) images the plane a to plane c. A camera is set up to image the beam at plane c. This image is used to study the beam shape with respect to various serrated apertures.
Figure 4. The comparison of beam profiles with and without spatial shaping: (a) the serrated aperture drawing (upper) that is currently used in the beamline and the modulated beam profile (lower). (b) the original, unshaped beam profile (upper) and the unshaped beam profile being pumped by the 25 mm amplifier (lower). (c) the shaped beam profile (upper) and the shaped beam profile being pumped by the 25 mm amplifier (lower).
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.
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