High-Speed Photo-Modulated Spindt Cathode for FELs

SRI International is integrating a Spindt-type field-emission source with a high-mobility GaN photoconducting semiconductor switch (PCSS) to develop cathodes capable of producing Ampere-level currents in the picosecond pulse regime. Semi-insulating Fe-doped GaN was selected for this application because of its high breakdown voltage, low resistance, and high carrier mobility. Both thin-film and bulk GaN are being investigated. Measurements of GaN carrier lifetime have shown that switch response times of 3-50 ps should be possible. Initial PCSS measurements with a moderate power femtosecond laser operating between 400-530 nm are limited by instrumentation but have shown sub-100 ps switch rise times. Subnanosecond field-emission current modulation has also been measured. A radiofrequency (RF) cavity based streak camera is being developed to measure the electron beam's pulse width.