1. Electromagnetic field simulations for accelerator optimization
Alexej Grudiev
CERN, BE-RF
1st oPAC Workshop: Grand Challenges in Accelerator Optimization,
26-27 June 2013, CERN

2. Outline
A review of tools for electromagnetic field simulations used at CERN will be given.
A number of examples of its usage will be presented covering several areas of accelerator component design and optimization both RF and non-RF equipment:
accelerating cavities,
collimation devices,
etc.
A brief review of CLIC main linac RF frequency and accelerating gradient optimization will be given as an example of incorporating electromagnetic field simulation into a global optimization process including both
constraints coming from the beam dynamics simulations and
the empirical RF constraints related to the high gradient linac operation.

3. Packages for computer simulations of electromagnetic EM fields and more
CST
HFSS
ACE3P
GdfidL

4. CST Studio Suite CST STUDIO SUITE: - CST MWS - CST DS - CST EMS
- CST PS
- CST MPS
- CST PCBS
- CST CS
- CST MICROSTRIPES
- Antenna Magus

5. CST: All you need in one package
Powerful and user-friendly Input:
Probably the best time domain (TD) solver for wakefields or beam coupling impedance calculations (MAFIA)
Beta < 1
Finite Conductivity walls
Once geometry input is done it can be used both for TD and FD simulations
Moreover using Design Studio (DS) it can be combined with the other studios for multiphysics and integrated electronics simulation, but this is relatively fresh fields of expertise for CST
Accelerator physics oriented post processor, especially in MWS and PS
Enormous progress over the last few years compared to the competitors.
Courtesy of Igor Syratchev
An example of what can be solved easily on a standard PC

6. CST (examples) Two examples of what can be solved on bigger PC:
128 GB of RAM and 24 CPUs
CLIC accelerating structure from Cu with
HOM damping loads from SiC
(frequency dependent lossy material)
Giovanni De Michele

7. CST (examples) Wx [V/pC] Transverse wake at offset of 0.5 mm Zx [Ω]
s [mm]
Transverse beam couping impedance at offset of 0.5 mm
f [GHz]

8. CST (examples)
LHC TDI 5m long with ferrite
Benoit Salvant

9. CST MWS: Example. S-parameters in CLIC Crab cavity
Mesh view
Praveen Ambattu

10. CST: Shortcomings
Cartesian mesh: Especially in FD can results to less accurate calculations of frequency, Q-factor, surface fields compared to tetrahedral mesh (HFSS, ACE3P). => if possible use tetrahedral mesh which became available recently and essentially gives the same results as FEM codes (HFSS, ACE3P).
Boundary conditions can be set only in Cartesian planes
No Field Calculator (HFSS)
...

11. HFSS: Still an excellent tool for FD
High-Performance Electronic Design
Ansoft Designer
ANSYS HFSS
ANSYS Q3D Extractor
ANSYS SIwave
ANSYS TPA
Electromechanical Design
ANSYS Multiphysics
ANSYS Maxwell
ANSYS Simplorer
ANSYS PExprt
ANSYS RMxprt
Product options
AnsoftLinks for ECAD
AnsoftLinks for MCAD
ANSYS Distributed Solve
ANSYS Full-Wave SPICE
ANSYS Optimetrics
ANSYS ParICs
HFSS was and I think still is superior tool for FD simulations both S-pars and eigenmode, though CST shows significant progress in the recent years
Automatic generation and refinement of tetrahedral mesh
Most complete list of boundary conditions which can be applied on any surface
Ansoft Designer allows to co-simulate the pick-up (antenna), cables plus electronics and together with versatile Optimetrics optimise the design of the whole device
Recently HFSS became an integral part of ANSYS – reference tool for thermo-mechanical simulations -> multiphysics
REcently time-dependent solver has been released

12. HFSS (examples, eigenmode)
LHC TDI 5m long beam dump:
One of the most dangerous eigenmodes at GHz, Q = 873,
Tetrahedral mesh with mixed order (0th , 1st , 2nd) elements: Ntetr =
Solution obtained on a workstation with 128 GB of RAM,

13. HFSS (example, S-parameters)
Port excitation
Incident plane wave excitation
O. Kononenko
O. Kononenko
Inverse FFT

14. HFSS example

15. HFSS: shortcomings No possibility to simulate particles
Automatic mesh is not always perfect, but it has improved after adoption by ANSYS
TD and multiphysics are only recently implemented, but thermo-mechanics from ANSYS is a reference by itself
...

16. GdfidL: Parallel and easy to use tool
The GdfidL Electromagnetic Field simulator
GdfidL computes electromagnetic fields in 3D-structures using parallel or scalar computers.
GdfidL computes
Time dependent fields in lossfree or lossy structures. The fields may be excited by
port modes,
relativistic line charges.
Resonant fields in lossfree or lossy structures.
The postprocessor computes from these results eg. Scattering parameters, wake potentials, Q-values and shunt impedances.
Features
GdfidL computes only in the field carrying parts of the computational volume.
GdfidL uses generalised diagonal fillings to approximate the material distribution. This reduces eg. the frequency error by about a factor of ten.
For eigenvalue computations, GdfidL allows periodic boundary conditions in all three cartesian directions simultaneously.
GdfidL runs on parallel and serial computers. GdfidL also runs on clusters of workstations.
Availability
GdfidL only runs on UNIX-like operating systems.

17. GdfidL (example) CLIC accelerating structure from Cu with
HOM damping loads from SiC
(frequency dependent properties)

18. GdfidL: shortcomings Available only under UNIX-like systems
Geometry input is limited. Often other 3D input tools have to be used.
...

19. ACE3P

21. ACE3P: example waveguide CLIC two-beam module rf circuit AS AS PETS
Arno Candel et. al., SLAC-PUB-14439

22. ACE3P: shortcomings
Very complex package to use. It is not user-friendly at all and requires a lots of time to invest before it can be used efficiently
It is not a commercial product -> no manual reference, limited tech support. No it is an open source.
...

23. Summary for the EM simulation tools
CST
GdfidL
ANSYS HFSS
ACE3P
Larger objects in TD
Better FD calculations,
3D EM + circuit co-simulation,
RF + thermal + structural
Accurate solution for very larger objects in TD and FD

24. CLIC main linac accelerating structure optimization.
Brief Review of what was done back in 2007

25. General layout of CLIC at 3 TeV
More on CLIC :

26. Optimization procedure
<Ea>, f, ∆φ, <a>, da, d1, d2 BD
Bunch population
Cell parameters N
Q, R/Q, vg, Es/Ea, Hs/Ea
Q1, A1, f1
Structure
parameters
Bunch
separation BD Ns
Ls, Nb
η, Pin, Esmax, ∆Tmax rf
constraints
Cost function
minimization
YES NO

27. Optimization parameter space
All structure parameters are variable:
<Eacc> = 90 – 150 MV/m,
f = 10 – 30 GHz,
Δφ = 120o, 150o,
<a>/λ= ,
Δa/<a> = 0.01 – 0.6,
d1/λ= , d2 > d1
Ls = 100 – 1000 mm.
N structures: 7 14 2 24 60 61 4

28. Structure parameter calculation
From 3D simulation (few hours) to an adequate model (few seconds)
Dipole mode: Ns I N
Fundamental mode:
P(z)
η, Pin, Esmax, ∆Tmax

29. Cell parameter calculation
Single cell parameter interpolation
Q, R/Q, vg, Es/Ea, Hs/Ea
a/λ
0.1
0.25
0.4
d/λ
WDS 2 cells

30. Optimization constraints
Beam dynamics (BD) constraints based on the simulation of the main linac, BDS and beam-beam collision at the IP:
N – bunch population depends on <a>/λ, Δa/<a>, f and <Ea> because of short-range wakes
Ns – bunch separation depends on the long-range dipole wake and is determined by the condition:
Wt,2 · N / Ea < 10 V/pC/mm/m · 4x109 / 150 MV/m
D. Schulte
RF breakdown and pulsed surface heating (rf) constraints:
ΔTmax(Hsurfmax, tp) < 56 K
Esurfmax < 380 MV/m
Pintp1/3/Cin = 18 X-band

31. Optimizing Figure of Merit
Luminosity per linac input power:
Collision energy is constant
Figure of Merit (FoM = ηLbx/N)

32. Ci = Excel{fr; Ep; tp; Ea ; Ls ; f ; Δφ}
Parametric Cost Model
Total cost = Investment cost + Electricity cost for 10 years
Ct = Ci + Ce
Ci = Excel{fr; Ep; tp; Ea ; Ls ; f ; Δφ}
Repetition frequency;
Pulse energy;
Pulse length;
Accelerating gradient;
Structure length (couplers included);
Operating frequency;
rf phase advance per cell
Ce = ( /FoM)/12 [a.u.]
Figure of Merit (ηL/N) in a.u. (the same as before)
[a.u.]=[1e34/bx/m2•%/1e9]
Hans Braun, 2006

33. CLIC performance and cost versus gradient
Ecms = 3 TeV L(1%) = cm-2s-1
Performance
Cost
New
Previous
New
Optimum
Previous
Performance increases with lower accelerating gradient (mainly due to higher efficiency)
Flat cost variation in 100 to 130 MV/m with a minimum around 120 MV/m

34. CLIC performance and cost versus frequency
Ecms = 3 TeV L(1%) = cm-2s-1
Performance
Cost
New
Optimum
Previous
New
Optimum
Previous
Maximum Performance around 14 GHz
Flat cost variation in 12 to 16 GHz frequency range with a minimum around 14 GHz