Ultrafast Laser Laboratory
Novel Acceleration Concepts Photoemission
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
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
Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST