New Target Viewing System (TVS2) InstalledJuly, 2008
A new Target Viewing System, a project under development at the University of Rochester’s Laboratory for Laser Energetics (LLE) since 2003, has been recently installed on the OMEGA target chamber. This system is a major redesign and upgrade of the existing TVS system. This multi-year project drew on the expertise of many disciplines at LLE, including optical design and fabrication, mechanical engineering, controls engineering, and software development. Its primary functions are to serve as a diagnostic tool and to accurately position targets within the target chamber. The TVS2 project evolved from the need to characterize the stability of cryogenic targets while compensating for the optical characteristics of the windows in the cryogenic shroud. The inner surface of this shroud is approximately 18 K, while the outer surface is at room temperature. It intercepts heat so that the target remains frozen until the laser is fired.
TVS2 has five cameras in each of two viewing axes. The views from these two axes allow the positioning of the target in three dimensions. Cameras are paired between axes and each pair is synchronized to an independent pulsed illumination source. These cameras include high-speed video, coarse and fine optical resolution, and a “smart camera”. Key features include adjustable focus and the ability to operate during an actual shot to acquire images just prior to the firing of the OMEGA laser. The “smart cameras” can perform real-time image processing for target detection, which streamlines system operation by eliminating many time-consuming setup and alignment tasks.
The installation took place over a period of two days and required a team of a dozen technicians to complete the four major assemblies, two of them weighing in at nearly one ton. Despite this enormous weight, once the TVS2 was initially installed in the OMEGA chamber, it was within one millimeter of its desired alignment.
Cryogenic target positioning, on the other hand, must be accurate to within ten microns in order to yield effective experimental results. The early success of the TVS2 included its ability to immediately see and identify the vibration of diagnostics in the chamber. The mechanical design of the system is robust. Its optics include primary mirrors that were manufactured at LLE in about six months, at less than one-quarter of the cost quoted by outside vendors. In addition, the optical manufacturing tolerances exceeded those quoted by other manufacturers. Fabrication of these mirrors required LLE to develop new metrology techniques to characterize the optics prior to performing magnetorheological finishing (MRF). The technique of MRF, using a magnetic-field-stiffened ribbon of fluid to polish a work piece, was developed by and has been studied extensively at LLE. Our engineers used extensive optical modeling and finite-element analysis to verify the designs before fabrication. Preliminary observations, validated upon installation, indicated that all design specifications were met.
This new system will help understand errors in target positioning from many contributing sources, including mechanical vibrations induced by rotating machinery, vacuum systems, and the removal of the cryogenic shroud. The TVS2 allows measurement of target position and vibration until a few milliseconds prior to the laser firing. Historically, scientists were unable to observe the target position for approximately three minutes prior to the firing of the laser. In addition, the TVS2 allows scientists to observe the ice-layer roughness within cryogenic targets from two additional views. With these additional views, it will now be possible to more accurately estimate the ice-layer uniformity at the time the laser is fired.