Design of Laser-Excited Needle-RF-Gun

Abstract

A new-type of laser-excited RF-gun using a needle photocathode is proposed. Design studies of the gun performances are executed with numerical calculations for electric field of the tip of the needle photocathode, laser heating and Joule heating of the tip, and beam parameters of a photoelectron beam in the RF-gun cavity. This calculation assumes a 2856 MHz RF-gun system that has the maximum field of 40 MV/m and a mode-locked pulsed-laser at a wavelength of 351 nm with a micropulse duration of 10 ps. Calculation results show that, for the optimized tip radius of 8.8 µm, a peak photocurrent reaches up to a space-charge limited current of ∼90 A with a pulse width of ∼6 ps and an energy spread of ∼1% at the cavity exit using an optimized laser with a peak intensity of 330 MW/cm2 and a laser injection phase of 20 deg. A preliminary experiment of a needle-RF-gun is also performed.