High Specific Power, Rapid Start-up 95 GHz Source Technology

Military applications of high power millimeter-wave directed energy weapons technologies require high field superconducting magnets that are incorporated in gyrotron devices. The magnets must be cooled to cryogenic temperatures to operate, giving rise to several technical challenges. Gyrotron magnets have traditionally required a relatively long time to cool-down (typically at least 24 hours) and require high input power compressors. The compressors are also bulky, heavy, and motion-sensitive, making them difficult to integrate into mobile platforms. Recent developments in high temperature superconducting magnets have aided, but not eliminated these problems. Present projections for second generation superconducting materials, YBCO, indicate the superconducting magnet will require cooling to approximately 50 Kelvin. This makes the use of several different refrigeration approaches technically feasible. To minimize cool-down time and overall system size and weight, the best refrigeration approach must be determined and refined for the mobile gyrotron application. A key operational parameter in military applications is field readiness, including time to cool the superconducting coils to their operating temperature. To date efforts have focused on standard Gifford-McMahon cryocoolers which are capable of very low temperatures. But G-M cryocoolers are hampered by motion sensitive compressors and high power draw (from low efficiency). Newer, oil-free compressors address the first issue, but refinement is needed to reduce size and weight, and to improve efficiency. A novel technique was identified and investigated in Phase I to address the weaknesses of G-M cryocoolers. Using a modified pulse tube refrigerator technology, it is possible to greatly enhance cryocooler efficiency, especially near room temperature. Variable inertance pulse tube refrigeration technology presents an exceptional opportunity to significantly advance gyrotron magnet technology by reducing power requirements, minimizing system size, and greatly reducing cool-down time.