Particle-In-Cell/Monte Carlo Simulation of Ion Back BomBardment in a High Average Current RF Photo-Gun Page: 3 of 13
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
III. COMPUTATIONAL MODEL
We have used the IMPACT-T code as the computational tool in this study. The IMPACT-
T code is a self-consistent three-dimensional quasi-static particle-in-cell (PIC) code to simulate
charged particle transport in DC or RF photoinjectors . Here, a bunch of photo-electrons
emitted from the cathode will be transported through the injector subject to both the external
accelerating/focusing forces and self-consistent space-charge forces. The electron beam collides
with the background residual gas and produces ions through the electron-impact ionization. The
production of ions is simulated using an Monte Carlo method with an ionization probability of the
residual gas. Here, the probability of production of an ion by an electron impact ionization at each
time step is given by :
pi = 1 - exp(-ngasovdt) (10)
where ingas is the density of background residual gas, a is the electron impact ionization cross
section, v is the relative speed between the electron and the gas molecule, and dt is the time step
size. Given the ionization probability for an electron, a uniformly distributed random number r is
generated. If r < pi, the ionization occurs, an ion particle is generated. Once an ion is generated,
it will be subject to both the external forces of accelerating/focusing fields and the space-charge
forces from the electrons. The space-charge forces among the ions and the space-charge forces on
the electrons from the ions are neglected given the fact that the number of ions is much less than
the number of electrons. The initial spatial location of the ion is assumed to be the same as that
of the electron. The initial velocity of the ion is sampled from a Gaussian distribution with a
given initial gas temperature. Here, we also neglect the detailed momentum exchange between the
electron and the ion given the fact that this is small compared with the large external accelerating
IV. SIMULATION OF ION BACK BOMBARDMENT IN THE VHF RF GUN
Using the above particle-in-cell/Monte Carlo method, we have studied the ion back bombard-
ment in the VHF RF photo-gun that is being developed at Lawrence Berkeley National Labora-
tory . This VHF photo-gun is designed to provide a high average current electron beam with
a good beam quality to support the soft X-ray FEL array planned at Berkeley . The charge of
electron beam from a single laser pulse can be as high as 1 nC with a repetition rate of 1 MHz.
The normal conducting RF gun cavity is designed with a nominal 187 MHz frequency to combine
the advantage of DC gun for high repetition rate and that of RF gun for higher accelerating field.
The maximum field inside the gun is about 20 MV/m that provides the final electron beam energy
around 750 keV. In the current study of the ion back bombardment inside this RF photo-gun, we
have assumed a group of nominal parameters listed in the following table:
Table I: Nominal parameters of the 1/2 cell VHF gun.
Beam energy (keV) 750
Peak field (MV/m) 20
RF frequncy (MHz) 187
Charge/bunch (nC) 0.8
Repitition rate (MHz) 1
Initial pulse length (ps) 100
Initial pulse radius (mm) 1
Initial emittance (mm-mrad) 0.25
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
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/m1/3/: accessed June 17, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.