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Superconducting radio-frequency electron guns are viewed by many as the preferred technology for generating the high-quality, high-current beams needed for future high power-free-electron lasers and energy recovery linacs. All previous guns of this type have employed elliptical cavities, but there are potential advantages associated with other geometries. Here we describe the design, commissioning, and initial results from a superconducting radio-frequency electron gun employing a quarter-wave resonator configuration, the first such device to be built and tested. In initial operation, the gun has generated beams with bunch charge is excess in 78 pC, energy of 469 keV, and normalized rms emittances of about 4.9 um. Currently, bunch charge is limited by the available drive laser energy, and beam energy is limited by x-ray production and the available rf power. No fundamental limits on beam charge or energy have been encountered, and no high-field quenching events have been observed.

The Naval Postgraduate School (NPS) Beam Physics Laboratory, Niowave, Inc., and The Boeing Company have completed construction of a superconducting 500 MHz quarter-wave gun and photocathode drive laser system. This prototype gun went from conception to initial operation in just under one calendar year. Such rapid progress is due in part to the decision to develop the gun as a prototype, deliberately omitting some features, such as tuners and a cathode loadlock, desired for a linac beam source. This will enable validation of the basic concept for the gun, including high-charge bunch dynamics, as rapidly as possible, with lessons learned applied to the next generation gun. This paper presents results from initial testing of the gun, technical challenges of the prototype design, and improvements that would enhance capabilities in future versions of this novel design.

The Naval Postgraduate School (NPS) has begun the design and assembly of the NPS Free-Electron Laser (NPS-FEL). As part of this effort, the original DC gunbased injector system from the Stanford Superconducting Accelerator has been moved to NPS and is being refurbished and upgraded to operate as a photoinjector. Design work has begun on a new, SRF, quarter-wave resonator based cavity that can serve as either an energy booster or photocathode gun. The overall NPS-FEL design parameters are for 40- MeV beam energy, 1 nC bunch charge, and 1 mA average beam current, built as an energy-recovery linac in its final configuration [1]. As we move towards this goal, the injector system will be incrementally upgraded to add photocathode capability, have a higher final beam energy, and improve the beam brightness, to meet the needs of the overall experimental program.