The next generation of accelerator-based light sources requires short bunches of electrons to be produced at a high repetition rate with low emittance (a key measure of beam quality based on spot size and angular divergence). Photoinjectors meet this requirement by producing electrons when a photocathode, typically a piece of metal or semiconductor material, is illuminated by a short-pulsed laser.
Semiconductor photocathodes can provide high beam current because of their good quantum efficiency (the number of electrons emitted per incident photon), but exhibit relatively slow temporal response. Metal photocathodes give much faster response times, allowing shorter pulses, but with reduced quantum efficiency. In ASTeC, research into photocathode materials is undertaken by the Accelerator Physics group in conjunction with colleagues in the Vacuum Science group.
The Accelerator Physics group has taken the lead in the provision of GaAs photocathodes for the ALICE accelerator. In-situ preparation and quantum efficiency mapping are routinely undertaken and group members are also proactive in the analysis of used photocathodes, to gain greater understanding of the relationship between materials properties and performance.
ASTeC scientists have led the design of an XHV three-chamber preparation system for GaAs photocathodes in collaboration with the Institute of Semiconductor Physics in Novisibirsk, Russia. Along with photocathode preparation, the system also provides facilities for quantum efficiency measurements and has recently been upgraded to allow linear profile scans. Quantum efficiencies over 20% at 635 nm wavelength have been achieved for semiconductor photocathodes prepared in this system.
The members of the group are currently commissioning the Transverse Energy Spread Spectrometer (TESS), an instrument designed to accurately measure the 3D energy distribution of electrons emitted from photocathode materials at both room and liquid nitrogen temperatures.
The spectrometer will be used to carry out fundamental studies of electron beam properties as a function of cathode preparation procedure and its exposure to common contaminants within an accelerator vacuum environment. Initial work will focus on GaAs samples fabricated in the photocathode preparation system, but it is hoped to expand these studies to other photocathode materials, particularly metals in the near future.
In addition, the group also directs and sponsors research into metal photocathodes for normal-conducting RF guns such as that currently being installed in the VELA. A surface characterisation facility with X-ray and Ultra-violet Photoelectron Spectroscopy (XPS and UPS) and Atomic Force Microscopy (AFM) is being established within the vacuum science laboratory.
This facility will be used to evaluate the effectiveness of various preparation strategies on the cleanliness and morphology of metal photocathodes and the consequent effect on their performance. Wavelength-dependent quantum efficiency measurements will also be possible using a tunable monochromated light source.
The group will play a key role in the commissioning of the VELA photoinjector and the copper photocathode on which it relies. In the initial set-up of this facility, the photocathode will be fixed so that only in-situ cleaning and conditioning will be possible. However, staff are currently designing a second RF gun with interchangeable photocathodes and a transfer system which will allow further experimentation with a variety of metal photocathodes and different preparation procedures.