RSI note revised highlighted

advertisement
Note: Charateristic beam parameter for the line electron gun
M. Iqbal1, 2a), G. U. Islam1, Z. Zhou2 and Y. Chi2
1Centre
for High Energy Physics, University of the Punjab, Lahore 45590, Pakistan
of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
2Institute
(Received XXXXX; accepted XXXXX; published online XXXXX)
(Dates appearing here are provided by the Editorial Office)
We have optimized the beam parameters of line source electron gun using Stanford Linear Accelerator Centre electron
beam trajectory program (EGUN), utilizing electrostatic focusing only. We measured minimum beam diameter as 0.5 mm that
corresponds to power density of 68.9 kW/cm2 at 13.5 mm in the post anode region which is more than two-fold (33 kW/cm2),
of the previously reported results. The gun was operated for the validation of the theoretical results and found in good
agreement. The gun is now without any magnetic and electrostatic focusing thus much simpler and more powerful.
An electron gun with a long filament (line source) is
indispensable for processing of large area surfaces. It
provides a homogenous emission over a long period of
time, across a length of several centimeters which causes
the uniform evaporation. It produces a higher emission
beam current and adequate resolution in the narrow
direction1-2 which is termed as characteristic parameter that
determines the quality of the source in use. High emission
current density is responsible for producing high beam
current that eventually produced high power density at the
target. In this work, we optimized the beam and then
calculated the characteristic parameter for the line gun
using EGUN software3 followed by experimental
validation of the results. We have removed the magnetic
focusing from the actual assembly and utilized the self
focusing of the gun. A detailed description of the design,
performance and thermal analysis of the actual gun has
already been reported earlier 4-6.
The gun was simulated with parameters listed in table 1,
and the applied potentials to different electrodes are given
in table 2.
TABLE 2. Potential applied to electrodes for the two configurations.
Electrodes
Optimized gun
Actual gun
Cathode (kV)
Focusing Electrodes
(kV)
Anode (kV)
Target (kV)
-10
-10
-10
-10
0
0
0
0
The obtained electron beam trajectories for the
actual configuration using electrostatic focusing only are
shown in figure 1. The figure shows the beam diameter at
13.5 mm form anode surface is 5.8 mm.
TABLE 1. Parameters fixed for beam optimization.
Parameters
Cathode to anode
distance (mm)
Anode slit spacing
(mm)
Focusing electrode
spacing (mm)
Anode to focusing
electrodes distance
(mm)
Cathode to focusing
electrodes edge
distance (mm)
a)
Actual gun
configuration
parameters
8
Optimized gun
configuration
parameters
6
7.5
2.5
6
4.3
4
3
4
3
Author to whom correspondence should be addressed.
Electronic mail: [email protected] [email protected]
FIG 1. Electron beam trajectories of the actual gun.
Then, we calculated electron beams trajectories
for the optimized configuration and presented in figure 2.
The beam focuses in the post anode region at 13.5 mm
from upper side of the anode surface where beam diameter
was measured to be 0.50 mm.
Finally, the gun was installed in a vacuum chamber under
pressure of 10-6 mbar for experimental validation of the
theoretical results. Using the optimized parameters and
acceleration potential of 10 kV, a uniform beam spot of
diameter 0.62 mm was observed at the target that can be
seen in figure 6. The measured beam current was 5 A.
FIG 4: Beam profile of line beam electron gun at the target.
FIG 2. Electron beam trajectories of the optimized gun.
Emission current densities as a function of beam radius for
the both configurations are shown in figure 3. The profiles
are plot normalized to 1.0 with the peaks, as shown at the
top of both plots, with values of 0.000689 A/(mesh unit)2
and 0.0001 A/(mesh unit)2 respectively. Our mesh unit
(mu) is equal to 0.1 mm. So, by applying the mesh scale to
put the plots in real dimensions, we obtained maximum
emission current density equal to 6.89 A/cm2 for optimized
and 1 A/cm2 for the actual configuration. The average
values for these emission current densities are equal to 0.5
A/cm2 and 0.37 A/cm2 respectively. The two peaks in the
actual configuration are due to the electron trajectories
concentrations which are condensed at the edges and weak
in the center of the beam profile. However, in the
optimized configuration; these trajectories are concentrated
at the center of the beam profile reducing its size from 5.8
mm to 0.5 mm. Consequently, the calculated maximum
power density was 68.9 kW/cm2 at 13.5 mm away from
anode, where the beam diameter was 0.5 mm.
FIG 3: Current densities profile for the two configurations as a function
of beam radius.
Therefore, the measured value of power density
was 67.2 kW/cm2. This value is close to our theoretical
value of 68.9 kW/cm2 and is more than two times of the
previous reference5.
Thus, with the help of simulation by EGUN
software, a sharp focused beam is obtained that enhanced
the power of the gun more than two times. The obtained
beam profile shows the maximum focusibilty, constancy
and uniformity through-out the emission surface of the
source. The gun is without any electrostatic and magnetic
focussing that requires extra power supplies, hence much
simpler and powerful now. Besides metallurgical
applications, the gun is useful for thermal barrier coatings
and electron accelerators for fundamental and applied
research.
1
K.B. Thakur, G.K.Sahu, R.V.Tamhankar, and P.Kartik, Rev. Sci.
Instrum. 72, 207 (2001).
2
A. D. Brodie and W. C Nixon, Microelectronics Engineering, 6, 111
(1987).
3
W. B. Herrmannsfeldt, SLAC-331-UC-28, 1 (1988).
4
M. Iqbal, K. Masood, M. Rafiq, M.A. Chaudhry, and F. Aleem, Rev Sci.
Instrum.74, 4616 (2003).
5
M. Iqbal and F. Aleem, Rev. Sci. Instrum.77, 106101 (2006).
M. Iqbal, A.Wasy, M. A. K. Lodhi, Rev. Sci. Instrum. 84, 056113
(2013).
6
Download