Studies of Linear and Nonlinear Photoelectric Emission for Advanced Accelerator Applications

Various electron emission properties of accelerator photocathodes were studied using short pulse lasers. The quantum efficiencies (QE’s) of copper and magnesium were compared before and after above-damage threshold laser cleaning. Changes in the emission properties of copper photocathodes used in the UCLA RF photoinjector gun were observed by mapping the QE's of the cathode surfaces. The electron yield of thin copper films resulting from back-side illumination was measured. Multiphoton (nonlinear) emission was studied as a means of creating ultrashort electron bunches, and an RF gun Parmela simulation investigated the broadening of such pulses as they propagate down the beam line. I. COPPER AND MAGNESIUM QUANTUM EFFICIENCY MEASUREMENTS In a radio-frequency (RF) photoinjector gun, short pulse electron beams are created using a laser pulse incident on a photocathode placed in a high RF field accelerating structure. The laser is synchronized with the RF cycle to achieve maximum acceleration and minimum energy spread of the electron bunch. For high peak currents, a photocathode material should be chosen which has a high quantum efficiency at the laser wavelength. It must also be robust to withstand the high RF fields and high incident laser intensity, and should have a smooth, nonreactive surface for uniform emission over long periods of operation. Copper and magnesium are two materials commonly used in these devices.1,2 The work function of copper is 4.6 eV and thus ultra-violet (UV) laser pulses are required to achieve linear photoemission. In the UCLA RF gun1 these pulses are created by frequency quadrupling an amplified Nd:YAG laser pulse to obtain 266 nm (4.7 eV photon energy) radiation. Magnesium has a 3.7 eV work function and therefore may exhibit a higher electron yield than copper. However, Mg is more reactive than Cu and subject to greater surface contamination which can hinder photoemission. A DC field test gun was devised to measure the QE's of copper and magnesium photocathodes. Laser pulses of 266 nm wavelength, 50 ps pulsewidth, and 3 mm spot size were sent through a hollow anode onto the cathode sample at normal incidence. The anode was biased at +5 kV and located 3 mm from the cathode, resulting in an electric field of 1.6 MeV/m. In the actual RF gun the electric field is 100 MeV/m, which enhances the electron emission via the Schottky lowering of the potential barrier. No such enhancement was observed in the DC gun. Thus the QE of a photocathode in the RF gun is typically an order of magnitude greater than that of the same cathode measured in the DC gun. The anode-cathode system of the DC gun was enclosed in a vacuum system at a pressure of 10 torr. The energy of each laser pulse was measured on line by a photodiode placed behind a UV mirror in the laser line. The emitted charge was measured by a charge preamplifier placed across a 1 MΩ load resistor.