09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Bunch Emission Simulation for the PITZ*
Electron Gun Using CST Particle StudioTM
Ye Chen, Erion Gjonaj, Wolfgang Müller,Thomas Weiland
DESY-TUD Meeting 09.08.2013
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Contents
Introduction CST field simulation
- Eigenmode simulation for Gun 4.3 cavity
- Solenoids simulation
CST PIC simulation - Modified simulation model
- ASTRA particles import
- Simulation results
Discussion- Cathode studies
Next steps
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Introduction
Motivation Main tasks
- 3D CST field simulations (Gun 4.1/4.3 cavity, Solenoids)
- 3D CST beam dynamic simulations• for different bunch charges
• with homogeneous/inhomogeneous particle distributions
• convergence study and comparisons to ASTRA
- Cathode studies• Influences from materials, non-uniformities, ……on beam qualities
- Emittance study
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST Field Simulation
Simulation resultsMode π mode
Field Ratio 1.04
Frequency 1.3019 GHz
Eigenmode Calculations
0 50 100 150 200 250 300-1.5
-1
-0.5
0
0.5
1 x 107
z/mm
Ez/
(V/m
)
Accelerating Ez field along z-axis
z
Ez
Simulation Model for Gun 4.3
Geometry Settings/mm
55 100
180.64
179.90
20
100
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST Field (Solenoids)
Simulation• Pos. of Main = 276 mm• Pos. of Bucking = -172 mm• Curr. of Main = 375 A• Curr. of Bucking = -31 A• Bzmax ≈ 0.2279 T• Bz(0,0,0) ≈10-7 T
Geometrical Settings/cm
Simulation Modelfor Solenoids
0 0.2 0.4 0.6 0.8 1 1.20
0.2
0.4
0.6
0.8
1 Longitudinal B field along z-axis
Bz
z
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
PIC Simulation Model
• Min. mesh step= 0.01mm• Meshcell numbers: up to 1000M• Including PIC position monitor, phase-space monitors for momentum,
energy, velocity… , 2D particle monitors and particle import interfaces
electron bunch
2D Particle Monitors: transversal/longitudinal
Particle Import Interface
Local Mesh Refinement
CST PIC Simulation
Bunch Parameters & Fields Data
• Bunch radius = 0.4 mm• Bunch charge = -1 nC• Bunch length = 21.5 ps• Rise/Fall time = 2 ps• Macro particles = 500 k• Cavity frequency = 1.30
GHz• Ez at cathode = 60.58 MV/m• Field ratio = 1.04• Bzmax = 0.2279 T
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST PIC Simulation
Am
plitu
de o
f Ez
z
Imported longitudinal electric field along z-axis for PIC simullation
field interpolation at the cathode plane
Problem description- mesh resolution difference in
the cathode region between eigenmode simulation and PIC simulation can lead to field interpolation at the cathode plane
- field interpolation within the first meshcell between PEC and vacuum
Solutions- keep the mesh resolution same,
but very mesh-consuming
- modify PIC simulation model
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST PIC Simulation
Goal• to improve the accuracy of the
field solution within a short distance from the cathode plane at z = 0
Implementation• send positrons & electrons at
the same time
• all velocity directions reversed
• keep field ratio same
-300 -200 -100 0 100 200 300-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Longitudinal E field in the mirrored cavity
z
Ez
electron bunch
positron bunch
Mirrored gun model for PIC
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
0 0.25 0.5 0.75 1 1.25 1.50
0.5
1
1.5
2
2.5
3
3.5
4
z/m
X rms/m
m
Note that,• simulations with both of the models showed trends of convergence• better convergence rate with the mirrored model
4
3.5
3
2.5
2
1.5
1
0.5
0
40
35
30
25
20
15
10
5
00 250 500 750 1000 1250 1500
Xrm
s /mm
Discrepancy /%
z /mm
CST-1, ∆z≈0.075mmCST-2, ∆z≈0.05mmCST-3, ∆z≈0.03mm, with original model
ASTRA SimulationDiscrepancy with ASTRA for CST-3
CST-5, ∆z≈0.015mmCST-4, ∆z≈0.03mm, with mirrored model
Discrepancy for CST-3
ASTRACST-1
CST-2 CST-3
CST-5CST-4
horizontal rms size of the beam along z-axis (Gun4.1)
Discrepancy with ASTRA for CST-5Discrepancy for CST-5
CST PIC Simulation
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Lt=21.5E-3nsrt=2E-3nsLE=0.00055keVsig_x=sig_y=0.4mmQ =1nCIpart=500,000
Species=‘electrons’Dist_z=‘p’Dist_pz=‘i’Dist_y=Dist_x=‘r’Dist_px=Dist_py=‘r’Ref_zpos=0.0m
Input Data for ASTRA:
ASTRA Particle Import
Particles z=z0, tє(t0,t1)
Astra2CST
Particles t=t0, zє(z0,z1)
Particle Import Interface(CST-PS)
CST PIC Simulation
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST PIC Simulation
0 50 100 150 200 250 3000
1
2
3
4
5
6
7
8
9
10
z/mm
Eki
n/M
eV
CST-PIC simulation, z=0.01mm
ASTRA simulation
average energy of the beam along z-axis
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST PIC Simulation
0 50 100 150 200 250 300 350 400 450 5000
1
2
3
4
5
z/mm
Xrm
s/m
m
0 50 100 150 200 250 300 350 400 450 5000
5
10
15
20
25
Dis
crep
ancy
/%
CST-PIC simulation, z=0.01mmASTRA simulation
horizontal rms size of the beam along z-axis
0 50 100 150 200 250 3000
40
80
120
160
z/mm
E
/keV
0 50 100 150 200 250 3000
20
40
60
80
Dis
crep
ancy
/%
CST PIC simulation, z=0.01mmASTRA simulation
beam energy spread along z-axis
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
CST PIC Simulation
0
1
2
3
4
z/mm
z/mm
CST-PIC simulation, z=0.01mm
ASTRA simulation
0 50 100 150 200 250 3000
10
20
30
40
Dis
crep
ancy
/%
bunch length of the beam along z-axis
0 50 100 150 200 250 300 3500
2.5
5
7.5
10
z/mm
x,no
rm
0
20
40
60
80
Dis
crep
ancy
/%
CST-PIC simulation, z=0.01mmASTRA simulation
horizontal normalized emittance of the beam along z-axis
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Discussion Cathode Studies
Surface impedance:
Frequency-dependent isotropic surface impedance model
1Z ω
2Z ω
Gun cavity material
Cathode material 1Z ω
1Z ω
2Z ω
Gun 4.3 Cavity
cathode plane at z = 0
y
z
Z s=(1+ j)√ωμ2 σ
σ : conductivity, ω: angular frequency
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Cathode Studies
Space charge field vs. time in correspondence to various conductivities of cathode material
SPC
H F
ield
• with bunch parameters: -1nC, 0.4mm(radius), 500k(particle numbers), 2ps/21.5ps\2ps• by using the same mesh resolution• during propagation time up to 80ps• at the same location, z=5mm
Simulation performed
Time /ps
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Summary & Plans
Summary Field simulations for gun 4.1 & 4.3 done, desired fields produced CST PIC results (1nC) on beam energy and spread, beam size, bunch length and
beam emittance obtained, compared to ASTRA. The discrepancy with ASTRA is about 10%, 5%, 9% and 20%, respectively.
Simulations on cathode study showed the influence of the cathode material on the space charge field.
Plans Perform PIC simulations
- for various bunch charges
- with inhomogeneous particle distributions Further study on the influence of cathode material on the beam qualities
09. August 2013 | TU Darmstadt | Fachbereich 18 | Institut Theorie Elektromagnetischer Felder | Ye Chen |
Thanks for your attention!
