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Optimization of electron gun

Katarina Radinović Kapralović
Center for talented youth Belgrade II, Belgrade, katarina.radinovic.k@gmail.com

1. Introduction electrons with initial energy of 0.1eV from Y=0.5mm and
Z=0mm (Xis the axial direction).
In atomic phisics measurements that utilize crossed beam
experiments with electrons as projectiles, it is very important to
have a well-defined both the geometry and the energy of the
electron beam. If the experiment does not require a very high
energy electron resolution, electron gun can be used as an
electron source. Most commonly, the electron gun consists of a
flat /or a hairpin) cathode that emits electrons through thermo-
electron emission, as well as of a few additional cylindrical Figure1(left) - 3Dmodel image from SIMON8[1] of electron
cathodes. The latter are used to achieve a precise beam gun and geometry of its electrodes.
geometry, a good focusing at a desired distance and for setting Figure2(right) - Potential energy (grey) from SIMON8[1] of the
the electron beam energy. A difference of the potencial of the electron gun and electron beam trace (black).
cathode and of the last electrode defines the electron beam
energy, while the energy resolution is defined by a thermal Pulsed mode of operation was achieved by applying a time
distribution of electrons emerging from the cathode (usualy dependent voltage on the Wenelt electrode E1.
about 0.5eV). Furthermore, various applications and time of
flight dependent experiments require an usage of a pulsed We analyzed Pierce’s electron gun design method using the
electron beam, in order to sequence the measurements. properties of complex functions and Laplace equation to find
the optimized electron gun and electrodes shape.
2. Methods

In this study we have made a model in SIMON8[1] of an
electron gun. We present the results of a 1MHz pulsed
operation mode, with 5ns, 10ns and 20ns pulse widths. We
have performed voltage optimization for the range of (1-
1000)eV electron energy, allowing theoretically to preserve the
focal point at a fixed distance of 40mm from the exit aperture,
in the continuous mode of operation, and with an influence of
the Coulomb repulsion between electrons. Furthermore, we
descrebed Pierce’s electron gun design method in order to
obtain optimized electron gun and electrodes shape.
Figure3 - Pulsed voltage time shape of one full period of 1MHz
3. Results pulse on electrode E2. Voltages b and a are arbitrary on and off
trigger voltages respectively.Pulse with and slope grades are
The first step in the simulation is to define a desired electron defined with simulation variables t 1 , t 2 , t 3 and t 4 .
optics geometry and its electric potentials via special
programming code, which creates geometry file. The program 4. Conclusion
then solves the Laplace equation for the electric potential and Program SIMON8[1] was used for simulation of a commercial
calculates the electric fiels defined by gradient of that potential electron gun working in both continuous and pulsed mode. We
by using a method of finite differences. This process gives the investigated a possibility of obtaining a fixed focal point in a
solution for the electric field in an empty space between wide energy range by tuning only one electrode potential. Also,
electrodes. After having defined the electric field, desired we presented in this study a possibility of turning from the
changed particle initial conditions can be defined and the continuous into pulsed mode of operation by setting a pulsed
program solves differential equations of motion. In the final voltage on the Wenelt electrode E2, down to nanosecond pulse
step, the program displays particle trajectories. All remaining width and 1MHz repetition period. Having analyzed Pierce’s
electrodes have fixed voltages, except for the E1 (in the pulse electron gun design method, we obtained one type of
mode). All electrode voltages are set relative to the cathode, in cylindrical gun that we can designe analitically.
order to keep the electron gun’s primary electron extraction
lenses at nearly the same focusing capabilities, while changing Refrences
through electron energies. [1] M. Lj. Ranković, M. Čeklić and A. R. Milosavljević:
The voltage on the electrode E4 was optimized via special Optimization of electron gun in continuous and pulsed operation
iterative LUA programming code in SIMON8[1], in order to modes. (2013)
obtain minimal possible beam radius at the distance of [2]Stanley Hamphries, Jr.: Charged Particle Beams (2002),
d=40mm, from the last electrode E5. This is simulated for two
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