THERMAL EMITTANCE MEASUREMENT DESIGN FOR DIAMOND SECONDARY EMISSION. Page: 3 of 5
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THERMAL EMITTANCE MEASUREMENT DESIGN FOR DIAMOND
Q. Wu#, I. Ben-Zvi, A. Burrill, X. Chang, D. Kayran, T. Rao, J. Smedley, BNL, Upton, NY 11973,
Thermal emittance is a very important characteristic of
cathodes. A carefully designed method of measuring the
thermal emittance of secondary emission from diamond is
presented. Comparison of possible schemes is carried out
by simulation, and the most accessible and accurate
method and values are chosen. Systematic errors can be
controlled and maintained at small values, and are
carefully evaluated. Aberration and limitations of all
equipment are taken into account.
The ultimate emittance of a laser-photocathode gun is
given by the thermal emittance. As laser shaping
techniques improve, this limit is becoming more
important. The thermal emittance of positive electron
affinity photocathodes in terms of the thermal electron
energy is a large fraction of an electron volt, depending on
the type of photocathode. The diamond amplified
photocathode  provides a high gain  and robust
performance. In addition, being a negative electron
affinity (NEA) cathode, it promises to deliver a very small
thermal emittance. Theoretical expectations as well as
measurements in the reflection mode point towards a
temperature less than 0.leV. This was never measured in a
transmission mode. Therefore the aim of this work is to
measure the thermal emittance of hydrogenated NEA
The thermal energy of the secondary emission beam
depends on the characteristic of the diamond sample, the
properties of the hydrogenated surface and the electric
field in the diamond, and yet it is expected that the
thermal energy is not a strong function of the field. The
measurement system should have the ability to measure
this small emittance.
The design is shown in Figure 1. The beam coming out
of the diamond NEA surface is accelerated towards an
anode. The anode has a small hole, to collimate an output
beam. The output beam is focused on a screen by a
solenoid. The main idea of the measurement is to
compensate the strong linear defocusing effect at the
anode by applying a linear focusing lens and measure the
waist spot size downstream. The waist spot size is only a
function of the thermal emittance and is less affected by
the other components. Therefore we can compare the
measurement with simulation. It can also be calibrated
with known thermal emittance electron source such as a
*Work supported by National Science Foundation, the U.S. Department
of Energy, and the Office of Naval Research.
diamond anode focusing lens scrcmer
Figure 1: Setup design for thermal energy measurement of
diamond secondary emission
Common to all possible focusing schemes for thermal
energy measurement is the application of a small aperture
on the anode plate to limit the beam size, such that the
beam size after a drift distance is measurable. In addition,
the field applied for acceleration is constrained by the
need to obtain a good gain in the diamond amplifier. The
schematic layout of the cathode and anode is shown in
Figure 2: Acceleration of the secondary electrons by
applying high voltage across
The aperture in the anode applies a defocusing force on
the electron beam, which is mainly a linear force with a
small component of nonlinear aberration. A larger
aperture will lead to a stronger aberration, and will cause
increase in the beam spot size on the screen. Space charge
is another issue in increasing the spot size. A small
aperture in the anode also leads to a lower current with
smaller space charge effect, increasing the accuracy of the
measurement. On the other hand this will make the
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WU,Q.; BEN-ZVI, I.; BURRILL, A.; CHANG, X.; KAYRAN, D.; RAO, T. et al. THERMAL EMITTANCE MEASUREMENT DESIGN FOR DIAMOND SECONDARY EMISSION., article, June 25, 2007; United States. (https://digital.library.unt.edu/ark:/67531/metadc897372/m1/3/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.