Table top, pulsed, relativistic electron gun with GV/m gradient Page: 4 of 9
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Table Top, Pulsed, Relativistic Electron Gun with GV/m Gradient
T. Srinivasan-Rao, and J. Smedley
Brookhaven National Laboratory, Upton, NY, 11973
ABSTRACT
We present the design and performance characteristics of a compact, high voltage pulser
with 150 ps rise time, 0.2 to 2 ns adjustable flat top and up to 1 MV amplitude on a 80 Ohm load or up to
0.5 MV on a 20 Ohm load, at 1 Hz repetition rate. Combination of a laser triggered SF6 and a liquid gap is
used to form the fast rising pulse and maintain a low jitter between the laser, external trigger, and the high
voltage pulsed output. The dark current and breakdown studies with this pulse applied between the
electrodes of a diode indicate that fields up to 1 GV/m could be supported by stainless steel and copper
cathodes without breaking down. The dark current from a conditioned cathode in a background pressure of
10" Torr is below the detection limit of 0.5 mA of our system. Photoemission studies had been conducted
with 300 kV applied between copper cathode and stainless steel anode seperated by 2 mm. KrF laser of 5
eV photon energy and 20 ns FWHM was used to irradiate the cathode. In these preliminary
measuremements, 3 nC charge and corresponding quantum efficiency of 3.5x104 have been obtained.
Future plans include increasing the gradient to GV/m range, decreasing the laser pulse duration to ps and
subps range and increasing the electron energy to a few MeV.
INTRODUCTION
In the past decade, there has been extensive research [1] in the development of low emittance,
high brightness electron injectors for linear collider and free electron laser applications. RF injectors with a
few nC charge in a few ps, with an emittance of-1-5 t mm mrad are operational in a number of facilities
[2-4]. The frequency of the RF and the design of the cavity are chosen to minimize the RF and space
charge effects on the electron bunch so that low emittance, high brightness electron beam could be
generated. Minimization of RF effects on emittance growth require a low RF frequency while minimizing
the space charge effects require high field and hence high RF frequency. The design is hence a compromise
between these two conflicting requirements. Some of these limitations could be overcome by using a large
pulsed electric field at the cathode rather than a RF field. An added advantage of these high fields on metal
surface is the lowering of the work function due to Schottky effect. The change in the work function is
given by [5]
p 4 o (pE) a (1)
where e is the charge of the electron, so is the dielectric constant of free space, E is the applied field and Q
is the field enhancement.
For a field of 1 GV/m, and a field enhancement factor of 3 [5], the change in the work function
can be calculated to be ~2 eV. This opens up the possibility of using either the infrared or visible radiation
to overcome the work function and to extract the electrons from low workfunction metals such as yttrium
or magnesium. The complexity and cost of the laser system associated with the photocathode is then
significantly reduced.1
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Srinivasan-Rao, T. & Smedley, J. Table top, pulsed, relativistic electron gun with GV/m gradient, article, October 1, 1996; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc680454/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.