Performance of the Photocathode Gun for the TJNAF FEL

(MO-3-03)

T.Siggins, C.Sinclair, D.Bullard, A.Grippo, J.Gubeli, G.Krafft  

Thomas Jefferson National Accelerator Facility(Jefferson Lab)

The performance of the DC photocathode gun has met the design specifications for the 1kW IRFEL demo at TJNAF. This gun has shown the ability to deliver high current, high charge beam with outstanding lifetimes. The GaAs photocathode has delivered 135pC per bunch at a bunch repetition rate of 37.425 MHz, corresponding to 5mA. In a recent cathode lifetime measurement 20 hours of CW beam was delivered with an average current of 3.1mA and 211 Coulombs of total charge from a 0.283 cm² illuminated spot. The cathode showed a 1/e lifetime of 58 hours and a 1/e extracted charge lifetime of 618 Coulombs. We have achieved this performance increase by adjusting our GaAs cleaning methods and by using high voltage processing techniques for the electrode structure. The limiting factor in lifetime though comes from ion back bombardment. The gun is currently run at 320 kV with a gradient of 3.9 MV/m at the cathode surface. Higher DC voltage operation is prevented by field emission. However, working at higher voltage would result in improved electron beam emittance. We currently have a program underway to study materials for improved high voltage standoff characteristics.