First Emission of Novel Photocathode Gun Gated by Laser-induced Schottky-effect

A laser-induced Schottky-effect-gated photocathode gun has been in use since 2006. This new type of gun utilizes a laser’s coherency to create a compact laser source using Z-polarization of the IR laser on the cathode. This Z-polarization scheme reduces the laser pulse energy by reducing the cathode work function due to the Schottky effect. Before this epoch-making scheme emerged as a concept, photocathode guns had never utilized a laser’s coherency. A hollow laser incidence scheme is applied with a hollow convex lens that is focused after passing the beam through a radial polarizer. According to our calculations (convex lens: NA=0.15; 60% hollow ratio), a Z-field of 1 GV/m needs 1.26 MW at peak power for the fundamental wavelength (792 nm). Therefore, we expect that this laser-induced Schottky emission requires just a compact femtosecond laser oscillator. We observed the first emission with a hollow laser incidence scheme (copper cathode illuminated by THG: 264 nm as a pilot experiment). It has a net charge of 21 pC with a 100-fs laser pulse (pulse energy: 2.5 μJ; spot diameter: 200 μm). However, we could not measure a significant difference between radial and azimuthal polarizations with focusing. We will use a hollow axicon lens in a vacuum to make the laser spot larger. The maximum cathode surface field is 97 MV/m. Relaxing the space-charge limit at the cathode, it generates a higher charge to measure the difference. This new scheme of gun will be investigated on several metal photocathode materials by comparing radial and azimuthal polarizations at 264, 396, and 792 nm. This is not only for development of an electron gun, but also to study cathode response and fundamental behaviors of electrons at the cathode. If we cannot observe any Z-field enhancement of electron emission with these laser wavelengths, we will prepare femtosecond laser halfcycle pulses (optical certification) up to mid-IR wavelength to illuminate photocathode metals.