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Thomas Jefferson National Accelerator Facility (JLab) refurbished the Large Aperture Solenoid Spectrometer (LASS) [1], 1.85 m bore solenoid, consisting of four superconducting coils to act as the principal analysis magnet for nuclear physics in the newly constructed, Hall D at JLab for the Glue Excitations Experiment (GlueX) [2]. The coils, built in 1971 at Stanford Linear Accelerator Center (SLAC) and used a second time for the Muon decays into Electron and GAmma ray (MEGA) Experiment [3] at Los Alamos, had electrical shorts and leaks to the insulating vacuum along with deteriorated superinsulation and instrumentation. Root cause diagnosis of the problems and the repair methods are described along with the measures used to qualify the vessels and piping within the Laboratory's Pressure Safety Program (mandated by 10CFR851). The extraordinary refrigerator operational methods used to utilize the obsolete cryogenic apparatus gathered for the off-line, single coil tests are described.

Two helium refrigerators, each rated for 3.5KW at 20K, are used at NASA's Johnson Space Center (JSC) in Building No. 32 to provide cryogenic-pumping within two large thermal-vacuum chambers. These refrigerators were originally commissioned in 1996. New changes to the controls of these refrigerators were recently completed. This paper describes some of the control issues that necessitated the controls change-over. It will describe the modifications and the new process control which allows the refrigerators to take advantage of the Ganni Cycle "floating pressure" control technology. The controls philosophy change-over to the floating pressure control technology was the first application on a helium gas refrigeration system. Previous implementations of the floating pressure technology have been on 4K liquefaction and refrigeration systems, which have stored liquid helium volumes that have level indications used for varying the pressure levels (charge) in the system for capacity modulation. The upgrades have greatly improved the performance, stability, and efficiency of these two refrigerators. The upgrades have also given the operators more information and details about the operational status of the main components (compressors, expanders etc.) of the refrigerators at all operating conditions (i.e. at various loads in the vacuum chambers). The performance data of the two systems, pre and post upgrading are presented.

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