AROUND THE LAB
Beam Delay Extender to Support Hydro Growth Radiography Experiments
In February 2021, Los Alamos National Laboratory (LANL) made a request to LLE to add an additional 4 ns of beam delay to ten specific beamlines on OMEGA in support of the Cylindrical Deceleration-Phase Rayleigh–Taylor (CYLDRT) Campaign. This campaign was comprised of a series of experiments using x-ray radiography to image unstable hydrodynamic growth using cylindrical targets. The proposed capability, also endorsed by the Omega Laser Facility Users Group, would increase the temporal diagnostic window available when conducting single-driver experiments on the 60-beam OMEGA laser. While the capability would be valuable to many users, John Kline, the Fusion Energy Sciences Program Manager at LANL, provided the comments below to describe the importance to the LANL campaigns:
“We are requesting modifications to OMEGA to allow for backlighter delays to extend beyond the current limits. A longer backlighter delay enables the cylindrical implosion experiments to extend the present work into the re-shock regime. Currently, we have successfully utilized OMEGA to capture unprecedented high-quality direct measurements of instability growth in convergent geometry but cannot measure times late enough into the implosion to capture the effect of the rebounding shock on the instability growth. This is an important area of research to both support the advancement of ICF (inertial confinement fusion) and validate our turbulent-mixing models. Such a long-duration backlighter will also enable measurements later in time for platforms such as shock and shear to capture the more-turbulent phase of hydrodynamic instabilities, which takes time to develop.”
A recent publication from LANL was also highlighted by Kline, detailing the cylindrical implosion work spanning the range of experiments accessible on OMEGA and the National Ignition Facility (NIF): The manuscript, “Hydro-Scaling of Direct-Drive Cylindrical Implosions at the OMEGA and the National Ignition Facility,” by S. Palaniyappan appeared in Physics of Plasmas. The manuscript describes hydrodynamic scaling of deceleration-phase Rayleigh–Taylor experiments between the Omega and the NIF Laser Facilities. Experiments that directly scale between the two facilities are rather unique, enabling this platform to benefit from the NIF’s experimental size and energy and Omega’s higher shot rate and experimental configuration flexibility. Cylindrical implosions have the advantage of providing direct observation of instability growth due to Rayleigh–Taylor and Richtmyer–Meshkov instabilities while retaining the effects of convergence. Control of hydrodynamic instabilities plays a central role for the success of ICF. Thus, these experiments enable an excellent path to validating codes with the goal of utilizing the NIF to achieve convergence ratios of 10 to 15, directly relevant for the inner shell of a double shell and for laser direct drive, as well as moving to more-relevant regimes for laser indirect drive.
With its promise for helping to advance high-energy-density physics, inertial confinement fusion, laboratory astrophysics, and plasma astrophysics, the contract for the beam delay extender (BDE) was awarded in October 2021 with the cost paid for by general funds available from the Laboratory for Laser Energetics.
The top-level requirements for the project included:
- A 4-ns delay path to be added to Beamlines 10, 15, 16, 17, 20, 28, 31, 33, 35, and 37. The design shall be capable of future deployment in all 60 beamlines.
- The new system must not interfere with preventive and corrective maintenance that occurs in and around its placement within the Laser Bay.
- The ability to perform position sensing to determine if the system is in or out of the beamline.
- Provide in-situ storage of components when not in use.
- Maintain pointing requirements consistent with random dynamic vibration (RDV) and long-term stability:
- doubling the existing OMEGA system pointing performance = 2.1 µrad (requirement presented in Conceptual Design Review);
- 28-µrad RDV required for the CYLDRT Campaign (too loose for operational consistency, future campaigns, pinholes);
- the final, conservative model result of ~10 µrad meets the experimental and operational requirements;
- the long-term drift specification defined on OMEGA as less than 0.3 µrad/h at 1σ.
A commercial, off-the-shelf, OptoSigma mirror mount for the BDE was tested on the OMEGA EP Large Optic Test Facility interferometer, which is recognized as a very stable system. The long-term pointing drift met the requirement of <0.3 µrad/h. The optimal location for the BDE assembly was chosen to be in the area between the Stage-E single-segmented amplifier (E-SSA) and the Stage-E spatial filter (E-SF). Several factors led to this choice, including:
- Allowing access for maintenance activities (amplifiers and infrared OMEGA Transport Instrumentation System)
- Providing a linearly polarized beam
- Creating a favorable beam size compared to downstream options
- Optical analysis confirming the E-SF input location
- The proposed space avoiding the ghost zone and allowing for personnel to work on E-SF and E-SSA.
The project’s mechanical design provided for ease of assembly; the complete structure simply bolts together while siting them easily within the defined space envelopes. All limit switch wires are connected to the Local Operating Network rack under Cluster 1. Wires pass through “bridges” between structures, while the upper shelf equipment can be accessed using a manlift. Side shear panels can be removed if necessary. Additional fastener holes were added to the BDE to accommodate an LLE-designed replacement tip–tilt mount.
Random vibration analysis was used to optimize and predict the effect of the BDE on OMEGA pointing. The process was started by measuring the floor vibrations in the OMEGA Laser Bay. The OptoSigma mount was modeled and correlated with a modal test. The OptoSigma mount was mounted on the Omega Laser Bay floor and the dynamic tilt was measured/compared to analysis predictions. The BDE structure was modeled using a lumped-mass representation of the fixed mirrors and OptoSigma mounts. A structure/mechanism design was iterated to maximize performance. Full models of the structure/mechanism, fixed-mirror mount, and OptoSigma mount were combined, and the final performance prediction was calculated.
The BDE structure provides a stable and safe support for the optics mounted on it. Each BDE beam path uses two OptoSigma mirror mounts and two LLE designed fixed mirror mounts to achieve the 4-ns delay. The OptoSigma mirrors are mounted to a carriage on high-precision, linear rails that can be manually moved from the storage position to the shot position. Thorough structure design and analysis predict acceptable overall pointing errors while demonstrating high factors in ensuring safety.
The shot date for the first experiment with the completely built and aligned BDE structure will take place on 24 August 2023. A high degree of planning, coordination, and expertise by adept individuals has been required to ensure the success of this structure. The list of these individuals making up the BDE Project Team include: Project Manager, Ray Huff; Principal Investigator, Joe Kwiatkowski; Mechanical Engineering: Milt Shoup, Ryan Fairbanks, and Matt Soffa; Installation and Rigging: Mark Romanofsky; Optical Manufacturing: Amy Rigatti; Optical Engineering: Dave Weiner; and Controls: Mike Hofer.