Design of a low emittance electron gun with GV/m high field

The design of the electron gun for generating a low-emittance beam is presented. This gun employs a diode for generating the beam and a 1.6-cell RF cavity for accelerating up to relativistic energies. The goal of this gun is to generate a beam of ≈1nC and 100 A peak current with a normalized rms transverse emittance of below 1πμm. In the diode region, pulsed voltage with a pulse duration of ≈1ns and voltage of 2 MV is applied to the 2-mm-gap between cathode and anode, generating a field gradient of ≈1GV/m. The combination of the high-field diode and the RF cavity can reduce space charge effects on emittance to a greater extent than an RF gun and solenoid field system. A Cs-Te was selected to match the requirement of a photocathode with high quantum efficiency (QE) since the laser power available to us at KEK-ATF gives ≈5μJ/pulse at a UV wavelength of 266 nm. A diamond is used for the substrate to allow UV light to be injected from the back of the cathode. To estimate the generated beam emittance, a simulation was performed using the general particle tracer (GPT) code. Details of the design and simulations are presented.