Particle-In-Cell/Monte Carlo Simulation of Ion Back BomBardment in a High Average Current RF Photo-Gun Page: 4 of 13
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We have used the energy dependent electron impact ionization cross section for residual H2 gas,
H20 gas and CH4 gas that are commonly observed in most RF photo-guns [10, 19]. Figure 1 shows
the electron impact ionization cross section for those three species gases [20-22]. It is seen that the
ionization cross section reaches the maximum value around 50 eV of electron kinetic energy and
falls off quickly after 100 eV for all three species of gas. Among these three species, the CH4 gas
has the largest electron ionization cross section followed by the H20 gas. The H2 has the smallest
ionization cross section, which is more than a factor of two less than the other two gas species.
A. Effects of gas pressure
In this study, we have assumed a background residual gas pressure of 10-6 Torr for most
simulations. This is significantly higher than the vacuum gas pressure in the planned RF gun that
could be as low as 10-11 Torr. Using such an artificially higher pressure is to increase the numerical
statistics in the simulation since the production rate of ions is very low in the real RF gun and the
number of ions will be too small during the period of simulation time of about 50 pulses (i.e. 50 ps).
To check the effects of background gas pressure on the ion back bombardment, we ran simulations
using the nominal gas pressure of 10-6 Torr, gas pressure of 0.5 x 10-6 Torr, and of 2 x 10-6
Torr. Figure 2 shows the pulse averaged HZ ion line density and the ion energy distribution on
the photocathode for the three gas pressures. Figure 3 shows the pulse averaged HZ ion power
line density distribution on the photocathode for the three gas pressures. Here the ion line particle
density and power density of the half pressure case are multiplied by a factor of two while those
of the doubled pressure case are divided by a factor of two. It is seen that after those scalings,
all three cases show very close ion line density and power distributions. The ion kinetic energy
distribution on the photocathode is also very close except somewhat larger fluctuation for the half
pressure case due to the poorer numerical statistics. These results suggest that we could scale our
simulation results linearly down to 10-11 Torr without changing the details of ion distribution on
B. Effects of RF frequency
From previous theoretical discussions, we see that the frequency of RF field can have significant
impact on the ion particle dynamics inside the RF field. Figure 4 shows the pulse averaged total
HZ ion particle number back bombardment onto the cathode as a function of the RF frequency.
Here, we have assumed the same electric field profile for all different frequencies in order to focus
on the effect of the RF frequency. This will not be true in a real cavity in general but can still be a
valid approximation for some low frequency cavities. From the Figure 4, it is seen that the number
of ions back bombardment onto the cathode is significantly reduced inside the RF gun compared
with that inside the 0 frequency DC gun case. With the increase of RF frequency, the of number
of ions also decreases, which qualitatively agrees with the theoretical prediction. Figure 5 shows
the pulse averaged HZ ion particle line density distribution on the photocathode for different RF
frequencies inside the gun. For the DC gun case, it shows a high peak particle line density at
the center, which is consistent with the experimental observation . It also has a wider spread
of the distribution on the photocathode since all ions generated inside the gun will come back to
the cathode eventually. Those ions generated at larger radial amplitude contribute to larger radial
distribution on the cathode. The ion particle line densities in the RF guns are much less than
that in the DC gun with a factor of 4 to 7 lower peak value. The density distribution is also much
narrower on the cathode. This is because only the ions that are produced near the photocathode
can reach the cathode. Most ions produced inside the RF gun will drift downstream towards the
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Qiang, J. Particle-In-Cell/Monte Carlo Simulation of Ion Back BomBardment in a High Average Current RF Photo-Gun, article, October 17, 2009; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc928446/m1/4/: accessed June 16, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.