Electromagnetic field simulations
for accelerator optimization
Alexej Grudiev
CERN, BE-RF
1st oPAC Workshop: Grand Challenges in Accelerator Optimization,
26-27 June 2013, CERN
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.
Packages for computer simulations of
electromagnetic EM fields and more
CST
GdfidL
HFSS
ACE3P
CST Studio Suite
CST STUDIO SUITE:
- CST MWS
- CST DS
- CST EMS
- CST PS
- CST MPS
- CST PCBS
- CST CS
- CST MICROSTRIPES
- Antenna Magus
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
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
Wx [V/pC]
CST (examples)
Transverse wake at
offset of 0.5 mm
Zx [Ω]
s [mm]
Transverse beam
couping impedance at
offset of 0.5 mm
f [GHz]
CST (examples)
LHC TDI 5m long with ferrite
Benoit Salvant
CST MWS: Example. S-parameters in CLIC Crab cavity
Mesh view
Praveen Ambattu
CST: Shortcomings
1. 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).
2. Boundary conditions can be set only in Cartesian planes
3. No Field Calculator (HFSS)
4. ...
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 pickup (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
HFSS (examples, eigenmode)
LHC TDI 5m long beam dump:
One of the most dangerous eigenmodes at 1.227 GHz, Q = 873,
Tetrahedral mesh with mixed order (0th , 1st , 2nd) elements: Ntetr = 1404891
Solution obtained on a workstation with 128 GB of RAM,
HFSS (example, S-parameters)
Incident plane wave
excitation
Port excitation
0.8
80
4
O. Kononenko
1
1
60
0.4
40
0.2
20
/s]
2
0.6
-1/2
2
Time Response [V W
3
Beam Impedance [V / A]
Frequency Response [kV W
-1/2
]
O. Kononenko
3
Inverse FFT
0
11.5
11.6
11.7
11.8
11.9
12
12.1
Frequency [GHz]
12.2
12.3
12.4
0
12.5
0
0
20
40
60
80
100
120
Time [ns]
140
160
180
0
200
Wake Potential [V / pC]
4
HFSS example
HFSS: shortcomings
1. No possibility to simulate particles
2. Automatic mesh is not always perfect, but it
has improved after adoption by ANSYS
3. TD and multiphysics are only recently
implemented, but thermo-mechanics from
ANSYS is a reference by itself
4. ...
GdfidL: Parallel and easy to use tool
bruns@gdfidl.de
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.
GdfidL (example)
CLIC accelerating structure from Cu with
HOM damping loads from SiC
(frequency dependent properties)
GdfidL: shortcomings
1. Available only under UNIX-like systems
2. Geometry input is limited. Often other 3D
input tools have to be used.
3. ...
ACE3P
waveguide
ACE3P: example
CLIC two-beam
module rf circuit
AS
AS
PETS
Arno Candel et. al.,
SLAC-PUB-14439
ACE3P: shortcomings
• Very complex package to use. It is not userfriendly 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.
• ...
Summary for the EM simulation tools
1.
2.
CST
Larger objects in TD
Better FD calculations,
3D EM + circuit co-simulation,
RF + thermal + structural
GdfidL
ANSYS HFSS
Accurate solution for very larger
objects in TD and FD
3.
ACE3P
CLIC
CLIC main linac accelerating structure
optimization.
Brief Review of what was done back in 2007
Alexej Grudiev, CLIC main linac structure optimization.
General layout of CLIC at 3 TeV
CLIC
More on CLIC : http://clic-study.org/
Alexej Grudiev, CLIC main linac structure optimization.
Optimization procedure
CLIC
<Ea>, f, ∆φ, <a>, da, d1, d2
BD
Bunch population
N
Ls, Nb
Cell parameters
Q, R/Q, vg, Es/Ea, Hs/Ea
Structure
parameters
Ns
Q1, A1, f1
Bunch
separation
η, Pin, Esmax, ∆Tmax
rf
constraints
NO
Alexej Grudiev, CLIC main linac structure optimization.
YES
Cost function
minimization
BD
Optimization parameter space
CLIC
All structure parameters are variable:
<Eacc> = 90 – 150 MV/m,
f = 10 – 30 GHz,
o
o
Δφ = 120 , 150 ,
<a>/λ= 0.09 - 0.21,
Δa/<a> = 0.01 – 0.6,
d1/λ= 0.025 - 0.1, d2 > d1
Ls = 100 – 1000 mm.
Alexej Grudiev, CLIC main linac structure optimization.
N structures:
7
14
2
24
60
61
4
-------------68.866.560
Structure parameter calculation
CLIC
From 3D simulation (few hours) to an adequate model (few seconds)
Dipole mode:
Wt 
N cells
 A'1i e
i 1

1i t
2 Q1i
sin( 1i t )
Ns
I
N
Fundamental mode:
dP

 R'

P
I P2
dz
Qv g
vg Q
1
P(z)
η, Pin, Esmax, ∆Tmax
Alexej Grudiev, CLIC main linac structure optimization.
Cell parameter calculation
CLIC
Single cell parameter interpolation
Q, R/Q, vg, Es/Ea, Hs/Ea
a/λ
0.7
1.5
2.3
0.1
d/λ
a1 , d1
0.25
a2 , d 2
0.4
WDS 2 cells
Alexej Grudiev, CLIC main linac structure optimization.
Optimization constraints
CLIC
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 MW·ns1/3/mm @ X-band
Alexej Grudiev, CLIC main linac structure optimization.
Optimizing Figure of Merit
CLIC
Luminosity per linac input power:
Lb N b f rep
Lb
L
1




Pl e Ec N N b f rep e Ecm N

Collision energy is constant
Figure of Merit (FoM = ηLbx/N)
Alexej Grudiev, CLIC main linac structure optimization.
Parametric Cost Model
CLIC
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 = (0.1011+7.1484/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
Alexej Grudiev, CLIC main linac structure optimization.
CLIC performance and cost versus gradient
CLIC
Ecms = 3 TeV
Performance
New
Previous
L(1%) = 2.0 1034 cm-2s-1
Cost
New
Optimum
• 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
Alexej Grudiev, CLIC main linac structure optimization.
Previous
CLIC performance and cost versus frequency
CLIC
Ecms = 3 TeV
L(1%) = 2.0 1034 cm-2s-1
Performance
New Optimum
Cost
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
Alexej Grudiev, CLIC main linac structure optimization.