Auxiliary Target Chamber for Nova We have added a second target chamber to the Nova laser facility to increase its experimental capabilities and to allow more cost-effective use of the laser. We have effectively doubled the productivity of the N ova laser for a fraction of the cost of the original Nova project. For more information contact he basic Nova laser facility, 200-.um-diameter spot onto the target Greg Suski (415) 422-0'681. completed in December 1984, at the center of the chamber. T was constructed primarily to These major subsystems are perform experiments in weapons supported with other equipment to physics and inertial confinement maximize the effectiveness and fusion. In its initial configuration, 1 simplify the operation of Nova. We Nova was composed of two use an array of laser-energy and fundamental subsystems, the laser image sensors to measure energy and itself and the target area, which to assist in beam alignment at several includes the instrumented 4.6-m­ points along the laser chains. Target diameter vacuum target chamber. The diagnostic instruments, in many cases laser subsystem comprises ten chains uniquely designed for use on Nova, of cascaded amplifiers that collectively precisely measure the effects on generate approximately 100 TW of targets of the intense laser light during infrared (l.OS-.um) light in a 1-ns time intervals of less than a billionth pulse. Each of the ten chains produces of a second. A power-conditioning a 74-cm-diameter beam of infrared system includes a 60-MJ, 20-kV light at its output. Large-aperture, capacitor bank that energizes the laser computer-controlled mirrors steer amplifiers and other high-power, these beams toward the main target electo-optic devices. An integrated chamber, where they arrive in two computer-control system uses a opposed conical clusters of five beams network of more than 50 computers each. The infrared light is converted to to coordinate setup, operation, and green light (0.527.um) or ultraviolet diagnosis of the entire Nova system. light (0.3S1.um) by precision arrays Planning and construction of the of KDP (potassium dihydrogen Nova laser system took eight years phosphate) crystals placed in the and was completed in December 1984. beam paths just before the full­ By the end of February 1985, we had aperture focusing lenses. These lenses begun operation of Nova as the provide a vacuum-tight window into world's most powerful and best the chamber and focus the beam instrumented facility for studying the down to a precisely positioned interaction of high-intensity laser light 28 INERTIAL FUSION with targets. As Nova was nearing To make productive use of Nova's completion, we began to set our plans "excess" laser capacity, we added the for the highest priority experiments; it smaller Shiva target chamber (the soon became evident that the facility same one that had been used with was becoming overbooked by a Novette) to the Nova facility as an growing number of important target­ auxiliary chamber. This target experiment campaigns. chamber has allowed us to perform Our previous laser facility, Novette, experiments in one chamber while the had been shut down and its parts other is being prepared for different installed to complete the Nova experiments. The operation of the system. Novette had been assembled second chamber is fully integrated in 1982, "borrowing" two of Nova's into the Nova system. As a result, we ten laser chains, to perform target have effectively doubled our target experiments in a 2-m-diameter experiment capacity and flexibility, chamber salvaged from the Shiva and we are able to operate this second laser (dismantled in 1981). During chamber with only a modest increase Novette's relatively short lifetime, it in operational resources over those proved the value of a smaller, lower required for single chamber operation. power (relative to Nova's ten beams) We chose a Novette-like, two­ target facility for certain types of beam, target-illumination design for experiments. We knew that we could this target chamber. Given the success continue these important experiments of the soft-x-ray laser experiments using the full Nova system. However, during the final days of Novette some of these experiments did not operation, we elected to optimize the require the full power of all ten Nova initial configuration of the second laser beams, and to perform them in Nova target facility to support a Nova's large ten-beam chamber continuation of these studies. These would take away valuable system soft-x-ray laser experiments require time from the experiments that did that the high-intensity laser light be require its full capability. An example focused in a thin line (not a circular of a series of such experiments was spot). This creates an x-ray lasing the soft-x-ray laser campaign medium in the target with a performed with Novette in the sufficiently long path so that summer of 1984. This campaign significant gain can be measured. succeeded in producing the first Another important choice was the verifiable x-ray laser in a laboratory wavelength of light to be used for environment. As Novette was being target experiments. The frequency­ shut down, scientists were already conversion arrays for the ten-beam planning further experiments to chamber were designed to maximize expand their knowledge of soft-x-ray delivery of third-harmonic ultraviolet lasing and its future applications. light. 2 However, the soft-x-ray laser To address the demand for more experiments were planned for second­ experimental capacity, we investigated harmonic green light, so the the limitations on the effective target frequency-conversion arrays for the experiment rate for Nova and two-beam target chamber were determined that, in keeping with past optimized for green-light conversion, experience, target-related factors while still providing acceptable dominate the laser-related factors. conversion efficiency for ultraviolet We noted that target-experiment light. campaigns frequently require different calibrations and setups of diagnostic ystem Design Overview instrumentation and beam-focusing To minimize costs, the two­ arrangements. The full Nova laser can S beam target chamber was be readied for firing more quickly designed to make maximum use of than the diagnostics and targets can existing components. Major elements be installed and aligned. available from previous laser systems, 29 primarily Novette, were reused where and propagated in a similar fashion possible. These included the target into the chamber using two, full­ chamber itself (previously salvaged aperture, motorized mirror elements. from Shiva), the supporting space The 50-m beam path from the frame, parts of the target-chamber optical switch yard to the mirrors in vacuum system, safety system, and the two-beam chamber area is elements of the diagnostics systems. protected by beam tubes, which can We replicated Nova designs for be evacuated or gas-filled. The beam required new components wherever tubes serve two purposes. First, they possible instead of making costly provide insulation from the air redesigns to achieve only nominal turbulence in the class-lO 000 clean performance improvements. rooms; air turbulence can seriously We moved the Novette space frame degrade the quality and focusing to a new location in the laser bay characteristics of the high-intensity previously occupied by Shiva to serve laser beams. Second, they preserve the as the support frame for the target quality of the laser light over long chamber and the optics located in the paths. Recent experiments have beam lines near the chamber. The verified that high-intensity light Shiva space frame, which still propagating through long paths of occupied much of the Shiva laser bay, nitrogen can be scattered into different was removed. The Novette frame was wavelengths through a process called cut into seven segments, installed in stimulated Raman scattering. This the Shiva laser bay, and rewelded scattering reduces the efficiency of our using low-distortion welding frequency-conversion arrays, resulting techniques. We added mirror supports in less focusable power on target at on this frame and in the Nova the desired wavelengths. Since switchyard to control beam divergence nitrogen makes up approximately and pointing. These mirrors and their 80% of our normal atmosphere, we electronically controlled gimbals are designed the beam tubes to be able to identical to those designed for Nova. hold a vacuum or to be filled with a We diverted two of Nova's beam nonscattering gas like argon or neon, lines from the optical switch yard area as necessary. into what once was the Shiva laser bay. This eliminated the need for onstruction and additional construction and left room Activation for a third and larger chamber in the C In early September 1984, we Shiv a target area, should a future began work to design and install the need arise. We chose two Nova beam auxiliary target chamber for the Nova lines whose optical path lengths to the laser facility. We had to meet a very ten-beam chamber corresponded with tight schedule (we wanted to begin the path lengths to the two-beam x-ray laser experiments with the Nova chamber. Thus the light pulses would laser in September 1985), and we arrive, in synchronization, at the could not delay completion of the center of either chamber without time­ Nova facility itself (planned for consuming adjustment of path length December 1984) or the experimental when operations were switched from program that was to start soon one chamber to the other. thereafter. In the two-beam chamber area, We were able to complete the one beam line is reflected off a addition of the two-beam chamber, motorized mirror into one end of the while meeting these criteria, through two-beam chamber, after passing close coordination with the activities through alignment, diagnostics, of the Nova construction and frequency-conversion, and final­ activation personnel.