Ultrafast Laser Laboratory

Novel Acceleration Concepts Photoemission

PHOTOCATHODE RESEARCH

In the past decade, RF injectors have been used as high brightness electron sources for free electron laser and accelerator research. A novel component in this injector is the photocathode, which is incorporated into the RF cavity and acts as the source of the electrons. The ease with which the electron bunch parameters such as the charge, the current, the current density, and the spatial and the temporal profile could be modified is a major advantage of these injectors. The complexity of the injector is determined primarily by the choice of the photocathode material and the laser system that drives the photocathode. The photocathode materials that have been used so far could be broadly categorized into two types, namely cesiated metals and semiconductors or simple metals. The cesiated materials typically have a very large quantum yield and hence require a simpler laser system to drive it. This advantage is offset by the delicate nature of the material. Its susceptibility to contamination reduces the lifetime to a few days or hours and necessitates use of complex preparation techniques and vacuum levels exceeding 10-9 Torr. The metal cathode, on the other hand, is relatively insensitive to contamination and has a very long lifetime, but has low quantum yield.

The research at the Instrumentation Division has focused on various methods to improve the quantum yield of metal photocathodes. A wide variety of techniques such as in situ surface ablation, energy transfer via surface phonons or multi photon process, optical field enhancement, surface field enhancement, and Schottky effect have been tested and shown to improve the quantum yield. More than a dozen metals have been tested for their photoelectric properties as well as laser induced damage properties.

Quantum efficiencies of various metal photocathodes were measured in the linear and the nonlinear photoemission regimes as shown in the graph. It is expected that the nonlinear process will be more advantageous at  high intensities before laser damage initiated. Behavior of these metals under RF fields has been investigated in an ongoing experimental program at the Accelerator Test Facility in collaboration with scientists from NSLS. Recent experiments with bulk Mg, ion sputtered Mg and bulk Cu indicate that current densities of 40 kA/mm2 could be obtained without damaging the metal surface. This is the highest current density reported so far from macroscopic metal electron emitters. We have also established a preparation technique for reliable cathode performance. Click here for more photocathode information.

Currently, our laboratory is the only facility where metal photocathode research is being done for this application. These results are used extensively world wide in choosing the cathode materials. A number of research academic and commercial institutions such as UCLA, CERN, Argonne National laboratory, MIT, Grumman-Northrup have been using metal cathodes based on this research.

GENERATION OF HIGH BRIGHTNESS ELECTRON BEAMS

The future electron colliders require bunches of very high brightness electron beams to provide the high luminosity beams required by these machines.  The short wavelength Free Electron Lasers (FEL) also require electron beams with high current and emittance comparable to the wavelength. The research in the Instrumentation Division in the generation of such high brightness electron beams follow two parallel paths, RF injectors and pulsed power guns.

In a typical electron beam, if the transport at high energy is designed carefully, the emittance and the brightness of the electron beam is dominated by its momentum and energy distribution at the source, namely the cathode. The electron emission from the cathode is modified by the field seen by the electrons within the cathode. This field is a combination of the surface field due to the applied RF and the space charge field due to the electrons in the vicinity of the cathode. The velocity distribution of the emitted electrons and hence the transport of the electrons are also affected by this dynamic field. An ongoing experimental program that investigates the electron emission and the properties of the electrons at the cathode in a RF injector is in place at the ATF. This is a collaborative effort between scientists from Instrumentation Division and the NSLS.

At the ATF, the emitted charge was measured as a function of the RF phase at which the laser illuminated the cathode. A dynamic model that takes into account the variation of the field in the emission regime and its impact on both emission and transport of subsequent electrons is currently being developed. 

When completed, this model would provide the temporal shape and momentum distribution of the electrons at the source, and can be used to optimize the laser parameters to reduce the longitudinal emittance. With this dynamic model, the electrons can be characterized accurately for the first time at their source. These characteristics can be used as the input parameter for the beam transport, and optimal parameters for the laser beam can be determined. The emittance growth in the injector could be minimized with such a laser beam and the brightness of the electron beam could be improved significantly. From a practical point of view, this research has also led to the development of surface preparation technique for achieving highest quantum yield and reliable performance from the cathode.

For more information or preprint request contact Triveni Rao