RF Gun Photo-Emission Model for Metal Cathodes Including Time Dependent Emission Page: 3 of 15
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A typical measurement performed at the SLAC Gun Test Facility (GTF) is
shown in Figure 1 where the measured QE is plotted as a function of the laser
phase for two different metal cathodes'. The QE is defined as the ratio of the
number of electrons emitted to the number of incident photons. The charge is
measured on a Faraday cup approximately 80 cm from the gun exit with a
solenoid immediately downstream of the gun that has been adjusted to
maximize the collected charge. Dark current is subtracted in the measurement
to determine the photo emitted charge contribution. The laser energy is
measured with a joule meter monitoring the energy downstream of a 1%
Figure 1. The measured QE as a function of laser phase for both Cu and Mg cathodes. A laser phase
of 00 is defined as the zero crossing of the rf field at the cathode. The peak field at the cathode was
110 MV/m for the Cu cathode and 90 MV/m for the Mg cathode.
The Schottky scan in Figure 1 clearly demonstrates the importance of the rf
field in the emission process. Equally important but not shown in Figure 1 is
the effect of image charge. To demonstrate this effect the charge is measured as
a function of laser energy. Figure 2 shows the result of such a measurement
performed on the same Cu cathode but at a later time. Here the QE can be seen
to be laser energy dependent. The linear fit is plotted and results in a QE of 6.6
10-5 which is roughly a factor of two higher than the QE shown in Figure 1.
Other measurements on this and other Cu cathodes at the GTF have resulted in
QEs ranging from 0.6-6 105 for an applied field of 120 MV/m and 30 laser
phase. The reason for the variation of the QE has not been determined but it is
suspected due to surface contaminants.
* Mg *
0 20 40 60 80 100 120
Laser Phase (0)
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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/m1/3/: accessed February 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.