RF Gun Photo-Emission Model for Metal Cathodes Including Time Dependent Emission Page: 2 of 15
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RF GUN PHOTO-EMISSION MODEL FOR METAL CATHODES
INCLUDING TIME DEPENDENT EMISSION
J.F. SCHMERGE, J.E. CLENDENIN, D.H. DOWELL, AND S.M. GIERMAN
SLAC, Stanford University, 2575 Sand Hill Rd
Menlo Park, CA 94025, USA
The quantum efficiency from a metal cathode is strongly dependent on the field at the cathode
due to the Schottky effect. Since the field is time dependent the quantum efficiency is also
time dependent. Thus the laser pulse shape used to generate electrons in a photocathode rf gun
is not the same as the electron bunch shape. In addition since the thermal emittance and
quantum efficiency are related, the thermal emittance is also time dependent.
The emission from a photo-cathode rf gun depends on both the rf field and the
incident laser pulse. In metal cathodes the rf field and image charges affects
emission through the Schottky effect. All of these fields have fast time
dependencies which must be accounted for to get an accurate model of the
emitted electron pulse.
This paper describes the main time dependent effects and provides simple
mathematical models to predict experimentally observed phenomena. The
results apply to metal cathodes where the Schottky effect leads to an effective
lowering of the work function. For those applications which require a flat top
temporal electron beam, the time dependent cathode emission will lead to the
use of drive laser pulses with asymmetric temporal shapes. In addition to the
time dependent quantum efficiency (QE), it will be shown that the same time
dependent fields result in a time dependent thermal emittance.
Typically one of the first measurements performed while commissioning a
photo-cathode gun is the measurement of charge vs laser phase. This type of
measurement demonstrates the time dependent emission process due to the
Schottky effect. All measurements reported here are conducted with a
transverse flat-top laser profile.
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Schmerge, J.F.; Clendenin, J.E.; Dowell, D.H.; Gierman, S.M. & /SLAC. RF Gun Photo-Emission Model for Metal Cathodes Including Time Dependent Emission, article, February 24, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc876628/m1/2/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.