Friday, October 1, 2004, 10:30 AM
(Note different time!!!)
Simulation, Measurement and Analysis of Photoemission
from Dispenser Cathodes, Metals and Coated Materials
Naval Research Laboratory
Photoemission cathodes are important sources for electron beams, from Free
Electron Lasers (FELs) to accelerator applications, due to the high quality
electron beams that can be achieved. Metallic photocathodes are rugged and
have fast response times, but low QE and require UV illumination. Direct
band-gap p-type semiconductors have the highest QE and operate at longer
wavelengths, but are chemically reactive and easily poisoned by H20 and CO2,
damaged by back ion bombardment, and (for NEA III-V photocathodes) insufficiently
responsive for pulse shaping in an rf injector due to their long emission time.
The ideal photocathode will have a high QE at the longest possible wavelength,
be capable of in situ repair or rehabilitation, and demonstrate good lifetime.
To meet the particular needs of a megawatt (MW) class FEL, a photocathode must
produce 1 nC of charge in a 10-50 ps pulse every nanosecond (ns) (100 A peak
and 1 A average current) in applied fields of 10-50 MV/m and background pressures
of 0.01 mTorr – and to do so for several seconds.
Even if such a photocathode were available, making predictions of its performance
is a complex challenge. Useful models of photoemission must account for cathode
surface conditions and material properties, as well as drive laser parameters.
We shall report on our efforts to develop dispenser photocathodes and to model
photoemission from them: they are rich in physics as a combination of field,
thermal, and photoemission effects contribute to the electron emission process.
We shall relate the models to our measurements as well as studies in the literature.
The model accounts for surface conditions (coating, field enhancement, reflectivity),
laser parameters (duration, intensity, wavelength), and material characteristics
(reflectivity, laser penetration depth, scattering rates) to predict current
distribution and quantum efficiency. While we focus on dispenser photocathodes in
particular, as they introduce complications such as coverage non-uniformity, field
enhancement, and work function variation with degree of coverage by coatings such
as barium and cesium, we shall also discuss other photocathode configurations.
For more information, please contact
Dr. Alex Bogacz,
Chair of CASA Seminar Committee
Updated May 11, 2016
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