wakefields and impedances part ii · 2019. 2. 28. · martin dohlus rainer wanzenberg cas nov. 2,...
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Wakefields and ImpedancesPart II
Martin DohlusRainer Wanzenberg
CASNov. 2, 2015
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computer programs
time domain solver for short range wakes
eigenmode solver for long range interaction
in principle, resolution and numerical dispersion, indirect wake integration, extrapolation to wake function, incomplete overview of codes
modal part of wake and long range wake
splitting into resonant part and resttwo particle interaction per modeazimuthal symmetry: monopole and dipole modeslongitudinal wakes and impedancesmodal and total loss parameterdipole wakes, damping and detuningequivalent circuit modellosses
in principle, problems without/with losses, perturbation methods, example, notation
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computer programs:
time‐domain solverfor short range wakes
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in principle
EB
JBE
curl
curl 111
dtddtd
211111
curlcurl
nnn
tE
JBBE
BE
or leap frog schemes
source term (the beam)
initial fields 1
BE
spatial discretization (resolution ~ )+ recursion in time:
boundary conditions: perfect electric/magnetic conducting“beam boundary”waveguidefinite conductivity
tnt n
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fixed volume
bunch enters and exits domain through boundary “beam boundary”
excited EM fields propagate through boundary “waveguide boundary”“open boundary”
decay of fields can be observed“middle” range wakes
window (moving mesh)
beam, waveguide & open boundariesare not required
bunch starts with surrounding fieldshort and ultra‐short range wakes
Ez component
energy density
W
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wake integration:
resolution and numerical dispersion Ez component
spatial resolution: cavity geometry and bunchmesh has to be fine compared to bunch
z
accuracy and stability: tc
propagation of waves: numerical dispersion error2numerical c
1st generation of wake‐field codes: FDTD (finite‐differences‐time domain) methods; criterion:with L >> W , the length of interaction
2nd generation: codes without dispersion error in z‐direction; for instance DG (discrete Galerkin) methods; see table
Lz32
W
z
2/
2/,,,,
L
LdzczszEdzczszE
errornumericalphase, cv
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indirect wake integration
long catch‐up distance22
2 dd
bzc
example: monopole wake of pillbox cavity sWsbWsW 0
||0
||0
|| ,0,,0,0,0,0,0,0
b
2
2,,0,,,0,
g
gzz dzczazbEdzczazbE
short integration path
this simple method is not always applicable, but there are schemes that are!f.i. use decomposition into waveguide modes in beam pipes
see:I. Zagorodnov: Indirect Methods for Wake Potential Integration, Phys. Rev. ST Accel. Beams 9, (2006).Henke, W. Bruns: Calculation of Wake Potentials in General 3D Structures, Proc. of EPAC’06, Edinburgh, UK (2006), WEPCH110.E. Gjonaj, T. Lau, T. Weiland, R. Wanzenberg: Computation of Short Range Wake Fields with PBCI, BD‐Newsletter No 45.
g
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20 years
5 years
table from: S. Schnepp, W. Ackermann, E. Arevalo, E. Gjonaj, T. Weiland: Large Scale 3D Wakefield Simulations with PBCI, ILC wakefield workshop at SLAC 2007
ABCI, Yong Ho Chin, KEK http://abci.kek.jp/abci.htmEcho 2D, Igor Zagorodnov, DESY http://www.desy.de/~zagor/WakefieldCode_ECHOz/
an (incomplete) survey of available codes
FDTD = finite differences time d.DG = discontinuous GalerkinIW = indirect wake integrationLC = large finite conductivity
Tim
e1980
2007
2002
FDTD
FDTD
FDTD
FDTD
IW
(IW)
(IW)
IW
IW
IW
IW
LC
LC
LC
DG
DG
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NOVO: beam‐pipe with random surface roughnessM. Timm, PhD Thesis (2000); S. Novokhatsky
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wakefield accelerator
de‐chirper de‐chirper
FELs: chirp (longitudinal energy distribution with slope) is needed for bunch compression; it can be reduced (afterwards) by a de‐chirper
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bunch
-20 -10 0 10 20 Ez / [kV /m]1mm step
FD analysis reconstruction
PBCI: diagnostic cross in PITZ (injector section)
b) quasi‐analytic field propagation in long intermediate pipes
from: E. Gjonaj, et. al.: Large Scale Parallel Wake Field Computations with PBCI, ICAP 2006
a) full time domain calculation: effect of a small step before the cross
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Echo: ultra short‐range wake in module with TESLA‐cavities
geometry
module 0.12:1
contribution of 3rd module (with 8 cavities)
L 0 .. 25 m for module 1 & 2, 25 .. 37 m for module 3
= 700 ... 50 m
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extrapolation: bunch‐length 0contribution of 3rd module (with 8 cavities)
longitudinal wake (monopole)
transverse wake (dipole)
from: The Short‐Range Transverse Wake Function for TESLA Accelerating StructureT. Weiland, I. Zagorodnov, TESLA Report 2003‐19
0|| exp ssAsw
11 exp11 ssssBsw
asymptotic model:
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computer programs:
eigenmode solverfor long range interaction
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eigenmode solver, in principle
EB
BE
ˆcurl
ˆcurlˆˆ
ˆ11 i
curl‐curl equation
case “no losses”
EE ˆcurlcurlˆ ˆ 112 (similar for B)
eigenvalue eigenvector
problem: curl‐curl operator has an (in)finite number of “static” eigenvalues but we are interested in “dynamic” solutions
0 ˆ 2 0ˆcurl E
0 ˆ 0ˆcurl E
EB
JBE
curl
curl 111
dtddtd
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trick: modify curl‐curl equation so that “dynamic” solutions are not changed, but eigenvalues of “static” solutions are shifted from zero; this is done by adding the grad‐div equation
EEE ˆdivgradˆcurlcurlˆ ~ 112 f
loss‐free problems: there are very effective simultaneous eigenmode solvers; usually they find a specified number of lowest eigen‐solutions and distinguish between static and dynamic modes
~
shifted eigenvalues:
0(density increases with frequency)
“static” and “dynamic”
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lossy eigenmode problems: f.i. Jacobi‐Davidson eigenvalue solver in the complex plane; modes are iterative searched, one‐by‐one large numerical effort
pictures from W. Ackermann and Cong Liu, TEMF‐TU‐Darmstadt
example: eigenmodes in TESLA cavity
complex frequency plane
Re{f} / GHz
Im2Re
Q
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example: eigenmodes in TESLA cavity
losses by waveguide modes above fc
accellerating mode
1st and 2nd dipole band(pairs of modes)
pictures from W. Ackermannand Cong Liu, TEMF‐TU‐Darmstadt
log(Q) of monopole/dipole/qudrupole/sextupole‐modes
1stcutoff freq
uency of beam‐pipe
2ndcutoff freq
uency
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example: eigenmodes in TESLA cavity
pictures from W. Ackermann and Cong Liu, TEMF‐TU‐Darmstadt
accelerating mode
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example: eigenmodes in TESLA cavity
pictures from W. Ackermann and Cong Liu, TEMF‐TU‐Darmstadt
a pair of “dipole” modes
not quite perpendicular!
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example: accelerating mode in 3.9 GHz superconducting cavity
pictures from E. Gjonaj, TEMF‐TU‐Darmstadt
upstream downstream
electric fieldstrength
magnetic fieldstrength
thermal quench
multipacting
EM fields in HOM couplers
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example: trapped modes in 3.9 GHz superconducting cavity
pictures from J. Sekutovicz, TEMF‐TU‐Darmstadt
azimuthal geometry (without dampers), monopole modessome (of many) trapped modes
f/f0 = 2.94
f/f0 = 2.96
f/f0 = 3.66
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perturbation methodssurface losses
waveguide ports
[1] N. Kroll, D. Yu: Computer determination of the external Q and resonant frequency of waveguide loaded cavities,SLAC‐PUB‐5171, Jan 1990
[2] P. Balleyguier: A straightforward method for cavity external Q computation, Particle Accelerators, Vol. 57, p113‐127, 1997
PWQ ˆˆˆˆ
power‐loss method: with AAHE dHZdP 2surface
* ˆRe21ˆˆRe
21ˆ
2
Re surface Z conductivity
Kroll‐Yu method: calculate for resonator with perfect reflection in wave port, for different reference planes (length L to the port) resonance frequency and quality; (it is more than a pert. method and works even for low Q)
L
Balleyguier method: calculate eigenmodes for two different boundary conditions (E=0, H=0); superimpose these modes to get the travelling wave in the waveguide; power flow through waveguide
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there are many eigenmodes “” with eigenvalues , spatial fields ,and mode amplitudes
rEˆ rB
ˆ
for simplicity we skip the index, but write every quantity with “hat”
complex eigenvalues: with
ˆˆ1ˆ i decay time
ˆˆ
resonance frequency
2ˆˆˆ
Q quality factor
notation
tieatt ˆ
ˆˆ
ˆRe,,
rBrE
rBrE
a
EM field energy, without losses
WadVtrtrWEMˆˆ,,
21 2212 BE
dVBdVEW 212 ˆ21ˆ
21ˆ
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unexpected resonance in a klystron instability
gap
from: B. Krietenstein, K. Ko, T. Lee, U. Becker, T. Weiland, M. Dohlus: Spurious Oscillations in high Power Klystrons, SLAC‐PUB‐9957
consider all resonances!
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modal part of wakeand long range wake
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splitting into resonant part and rest
syxyxsyxyxsyxyx ,,,,,,,,ˆ,,,, 2211rest22112211 www
this splitting is unique if the modes are fully excited (the source particle “1” is not longer in interaction with the field of the mode) and the mode is just ringing
csieyxyxLsyxyx /ˆ22112211 ,,,ˆRe,,,,ˆ fw
the modal part contributes essentially to long range interactions (from bunch to bunch); usually the bunch distance is larger than L, the length of the field of the mode
for ultra‐relativistic bunches the wake function is causaland it is useful to define:
0w 0,,,, 2211 syxyx
0w 0,,,, 2211rest syxyx
0w 0,,,,ˆ 2211 syxyx
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closed cavities without losses
it can be shown* that the expansion into an infinite set of modes is complete, and:
no beam pipes, perfect conductivity
0,,,, 2211rest syxyxw
csieyxyxshsyxyx 22112211 ,,,ˆRe,,,,ˆ fw
otherwise2
0for 10for 0
ss
shwith
* see: T. Weiland, R. Wanzenberg: Wake Fields and Impedances, DESY M91‐06
therefore csieyxyxshsyxyx 22112211 ,,,ˆRe,,,, fw
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two particle interaction per modefor simplicity we choose zero offset of both particles and skip the offset coordinates
particle 1 travels alone through the cavity and excites the mode
energy loss of the particle 0ˆˆ||
21
211 wqkqW
mode rings with complex amplitude kqA ˆ~ 11
particle 2 travels alone through the cavity, but shifted in time
energy loss of the particle 0ˆˆ||
22
222 wqkqW
mode rings with different phase csiekqA 22
ˆ~
cst
kwith an unknown constant
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0ˆˆˆ0ˆ ||22||12||21||
2121 wqswqqswqqwqWW
both particles together, in any distance
energy loss
mode keqqAAA csi ˆ~ ˆ2121
kqcskqqkqW ˆˆcosˆ2ˆ 2221
21mode
2mode ~ AW
with energy conservation follows0mode21 WWW
kw ˆ0ˆ||
cskswsw cosˆ2ˆˆ ||||
with (defined) causality for ultra‐relativistic bunches
shcsksw ˆcosˆˆ ||
otherwise2
0for 10for 0
ss
sh
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modal loss parameter
swqqwqWW ||12||2121 ˆ0ˆ
particle 1 travels through the cavity and excites the mode
kqW ˆ21mode
particle 2 is a test charge (q20)
voltage observed by test particle
cskq cosˆ21kqV ˆ21mode
mode
2mode
4ˆ
WVk modal loss parameter
with dVBdVEW 212mode
ˆ21ˆ
21ˆ and dzezEV czi
z~
mode ,0,0ˆ
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two particle interaction per mode, more general
arbitrary offset of source particle (index=1) and test particle (index=2)
csieyxvyxvshsyxyxw 22||11
*||2211|| ,ˆ,ˆRe,,,,ˆ
W
dzezyxEyxvczi
z
ˆ2
,,ˆ,ˆ
~
||
with
modal loss‐parameters and voltages are post‐processing results from eigenmodesolvers; transverse wake functions are
yxvy
ciyxvy ,ˆˆ
,ˆ ||
csi
yy
csixx
eyxvyxvshsyxyxw
eyxvyxvshsyxyxw
ˆ2211
*||2211
ˆ2211
*||2211
,ˆ,ˆRe,,,,ˆ
,ˆ,ˆRe,,,,ˆ
the transverse voltages vx, vy are either calculated directly (as v||), or with help of the Panofsky‐Wenzel theorem:
yxvx
ciyxvx ,ˆˆ
,ˆ ||
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azimuthal symmetry
longitudinal field (of TM modes):
mm
zrEE mzz sin
cos,ˆˆ
for m > 0 there are always pairs of modes (with cos(m) and sin(m))
m = 0 monopole modes1 dipole modes2 sextupole modes…
with
mm
W
dzezrEyxvczim
z
sincos
ˆ2
,ˆ,ˆ
~
||
ultra‐relativistic case:
mm
rvyxvv mm
sincos
ˆ,ˆ0ˆ ||||2
2ˆ:ˆ mm vk
can be chosen real mv
normalized loss‐parameter:
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monopole modes: constvyxv 0|| ˆ,ˆ
0,ˆ
ˆcosˆ,ˆ
0
00||
sw
cskshsw
dipole modes:
yx
vrvyxv 11|| ˆ
sincos
ˆ,ˆ
csykcshsyxyxw
csxkcshsyxyxw
csyyxxkshsyxyxw
y
x
ˆsinˆˆ
,,,,ˆ
ˆsinˆˆ
,,,,ˆ
ˆcosˆ,,,,ˆ
11
22111
11
22111
21211
22111
||
the pair of modes is combined to
longitudinal kick is second order;transverse kick depends linear on the offset of the source particle,it does not depend on the offset of the test particle
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longitudinal wakes and impedances
pictures from: Zagorodnov, T. Weiland, M.Dohlus: Wake Fields Generated by the LOLA‐IV Structure and the 3rd Harmonic Section in TTF‐II, TESLA Report 2004‐01 and T. Weiland, R. Wanzenberg: Wake Fields and Impedances, DESY M91‐06
longitudinal wake for a multicell structure (transverse deflecting cavity LOLA)
real part of impedance, in principle
mm 1z
few cells of LOLA
1000 ... 50, ,25μm z
dashed curve = mode expansiondashed curve = extrapolation 0
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modal and total loss parameters for finite bunch length
swswsw ,,ˆ, rest||,|||| splitted longitudinal wake function:
calculation by time‐domain wake code gives wake potential:
duusuwsW ,, ||||
total loss parameter: dsssWk ,||tot, and
with normalized line charge density , in particular s 221 2exp2 ss
0,||tot wk
modal loss parameters with: cskshsw cosˆ,ˆ ||
otherwise2
0for 10for 0
ss
shand
uscudusdskk
ˆcosˆ2ˆ
0
222ˆexpˆˆ ckk
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travelling wave accelerator structure
one periode of a„disc loaded“ structure:
these structures are tapered, individual parameters for each cell: a, b, t, …
the parameters per period can be tuned to fulfill several conditions simultaneously, as resonance frequency and phase advance of the fundamental mode and frequency and loss‐parameter of the 1st and 2nd dipole band
beam
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(de)tuning of dipole modes:2 modes, with nearly the same strength (k(1))
4 modes, „detuned“ k, distribution
15 modes
detuned coherence
recoherence
adjust k and
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damping and detuning
15 modes with damping
from: R. Jones, N. Kroll, R. Miller, R. Ruth, W. Wang: Advanced damped detuned structure development at SLAC, PAC 1997
a multi‐cell structure with damping and detuning (=DDS)
cell
waveguides(to absorber)
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s/m
recoherence in detunedstructure with natural losses
natural + external damping
a multi‐cell structure with damping and detuning (=DDS)
from: R. Jones, N. Kroll, R. Miller, R. Ruth, W. Wang: Advanced damped detuned structure development at SLAC, PAC 1997
calculationmeasurement● ● ●
transverse long‐range wake is reducedby two orders of magnitude, in comparison to the short range wake
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equivalent circuit model for longitudinal wake
ˆcosˆ20 tktV
i(t)
V(t)
R
LC
kC2
1
ˆ
ˆ2
no losses:
tqti bunch current:
with
voltage:
the voltage V(t) induced by a series of particles i(t) = q1(t t1) + q2(t t2) + … corresponds to voltage observed by a test particle at time t(t, t1, t2, … corresponds to time in a certain reference plane)
with losses:
ˆ
ˆˆ2ˆˆˆˆ QkLQC
QR
longitudinal impedance (per mode):
11 ˆˆ
ˆ1ˆ
ˆˆ211ˆ
QiQk
RCi
LiIVZ
shunt impedance
this is the Fourier‐transform of , as defined in part 1 ˆ sw
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(angular) resonance frequency
ˆˆ2ˆ QkR
loss parameter
quality
shunt impedance
„R/Q“
2ˆ4ˆˆ VWk
Q
some important cavity parameters
ˆ2ˆˆ kQR
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losses (absorption by R)
„single“ bunch losses
fully resonant losses
TkqPkqWˆˆˆ
2
12
1 decay time is large compared to bunch distance T:
RIRIPr 2~21 22 for and : 2ˆ nT T
2ˆ
ˆ22
1
1ˆT
T
e
eTqkP
in general
ˆˆ1ˆ iwith and
i(t)
V(t)
2ˆˆˆ
Q
TP 2
1
t
TmIIti 2cos 2
I~
tcIRtu os 2 rr Tm 2
V~
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weak decay
22
2ˆ
ˆ22
)1(ˆ
1
1ˆ1
1ˆ
iT
T
eT
RIe
eTqkP
resonant losses phase dependentterm
the phase dependent term
2,ˆmod T
T
2, 5, 50RI
Pˆ2
with
it is not very likely to excite a sharp resonance, and it is very difficult to determine the phase , but …
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2ˆT
T single bunch losses P1 T losses may be resonant
2ˆT
spectrallines
ofthebe
am
if the probability for is equally distributed or
the exact frequency of parasitic, unwanted resonances is usually not known and may depend on geometric parameters that are not exactly determined (f.i. length of bellows)
12
ˆ
T
ˆ2)1(
ˆ1
21
2
0
2T
eT
d
i
1
22 )1(
ˆ1 Pe
TRIP i
with
the worst case losses are resonant; in best case the losses are much smaller than P1, but typically:
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acknowledgement
We thank Wolfgang Ackermann, Erion Gjonai, Torsten Limbergand Igor Zagorodnov for their interest and stimulating
discussions.