High-Fidelity RF Gun Simulations with the Parallel 3D Finite Element Particle-In-Cell Code Pic3P Page: 4 of 5
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For Pic3P simulations, a conformal, unstructured 3D (1/4) mesh model with 305k
tetrahedral elements is used, with mesh refinement along the center of the beam pipe.
High fidelity cavity mode fields are obtained with the parallel FE frequency domain code
Omega3P and directly loaded into Pic3P as drive fields. Figure 2a) shows a comparison
of transverse emittance results by the different codes and Figure 2b) shows the parallel
scalability of Pic3P.
LCLS RF Gun: Normalized Transverse RMS Emittance Pic3P parallel scaling on Franklin (Cray XT4)
6 no solenoid PARMNELA ideal speedup- -
Pic2P and MAFIA 2D --- 9 Linear Solver (CG) speedup "
5 Pic3P 3D full domain 2M DO~s - 8PI3speu x
Pic3P 3D causal window 200k DOFs O s
- 4 g
E 3 5
w 2 - 3
1 120 MV/m, Q=1nC, 10 ps flat-top, r=1mm - 2 2M DOFs, 500k particles
DOFs: Field degrees of freedom % 1 plC workload: <30
0 2 4 6 8 10 0 16 32 48 64 80 96 112 128
Bunch Position <Z> / cm CPUs
FIGURE 2. (a) Comparison of normalized transverse RMS emittance as a function of beam position in
the LCLS RF gun as calculated with PARMELA, Pic2P and MAFIA 2D (both agree), and Pic3P, where
the causal moving window technique reduces the problem size by one order of magnitude.
(b) Parallel scalability of Pic3P for the full domain simulation with 2M DOFs.
Excellent agreement between 3D results from Pic3P and the 2D results from Pic2P
and MAFIA is found, as expected from the high cylindrical symmetry in the fields and
the convergence behavior of the codes. PARMELA results differ as space-charge effects
are significant, presumably because wakefield and retardation effects are ignored, as
detailed in a previous study .
Pic3P allows large-scale 3D PIC simulations of RF guns with realistic particle dis-
tributions, including intra-bunch effects. Starting with an initial particle distribution ob-
tained from measurements, another LCLS RF gun simulation was performed. Figure 3a)
shows a snapshot of the scattered fields during bunch transit. Figure 3b) shows the trans-
verse and longitudinal phase phase of the particle bunch near the exit of the gun.
The first successful implementation of a parallel Finite Element 3D electromagnetic
PIC code is presented. The new code Pic3P was used to model space-charge effects
in the LCLS RF gun from first principles, including wakefield and retardation effects.
It employs state-of-the-art parallel Finite Element methods on conformal, unstructured
meshes with unconditionally stable time integration and self-consistent higher-order
particle-field coupling. In combination with novel causal moving window techniques
and dynamic load balancing, Pic3P allows unprecedented accurate and efficient simu-
lations of low-emittance electron injectors, aiding the design and operation of the next
generation of light sources and colliders.
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Candel, A; Kabel, A.; Lee, L.; Li, Z.; Limborg, C.; Ng, C. et al. High-Fidelity RF Gun Simulations with the Parallel 3D Finite Element Particle-In-Cell Code Pic3P, article, June 19, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc926096/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.