Impedance of SPS travelling wave cavities (200 MHz)
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Impedance of SPS travelling wave cavities (200 MHz)
A. Grudiev, E. Métral, B. Salvant, E. Shaposhnikova, B. Spataro
Acknowledgments:
Erk Jensen, Eric Montesinos, Rolf Wegner
Impedance meeting – March 3rd 2010
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This is a work in progress…
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Agenda
• Context
• EM Simulations to get the transverse wakes– Particle Studio– Try with ABCI
• Effect of these transverse wakes on beam dynamics
• Future plans
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Context: SPS impedance model• A potential way to improve the LHC performance is to increase the intensity in the SPS
• Fast vertical instability is one of the limitations to increase the intensity in the SPS
many arguments point towards a Transverse Mode Coupling Instability
• Need to identify the sources of transverse impedance in the SPS to be able to find more solutions to increase the threshold of this instability
current impedance model (kickers, beam pipe, BPHs, BPVs)explains ~60% of the tune shift with intensity and instability thresholds when used in HEADTAIL beam dynamics simulations.
(direct space charge is not included in these simulations).
Other potential sources? RF cavities septa pumping ports
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Context: SPS 200 MHz TW cavities
• 4 cavities– ACTA: 4*11 cells (x~ 31m, y ~ 76 m)
– ACTB: 4*11 cells (x ~84 m, y~27 m)
– ACTC: 5*11 cells (x ~25 m, y~91 m)
– ACTD: 5*11 cells (x ~85 m, y~27 m)
Example: ACTB
4 m 18.4 m
For HEADTAIL,to lump all these impedances in the same point with x = y =40 m, need to apply a factor 24to the wake of a single cell
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Context: previous studies
• Resonant impedances in the SPS (T. Linnecar and E. Shaposhnikova) SL-Note 96-49 RF and G. Dome (CERN-SPS/ARF/77-11)
Longitudinal and transverse parameters of known impedances in the SPS
SW and TW cavities, kickers, septa, pumping ports included
Longitudinal
Transverse(dipolar most likely)
From SL-Note 96-49 RF
From SL-Note 96-49 RF
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Context: previous studies
• Mafia Electromagnetic simulations by Bruno Spataro– Simulations of 9 cells instead of 11 (very large structure) and checked linearity with cells
of the low frequency part
-1000
-500
0
500
1000
1500
2000
0 0.5 1 1.5 2 2.5 3 3.5 4
Vertical transverse real part of the coupling impedance as function of the frequency estimated at 1.5 cm off
set along the vertical coordinate ( 9stems)
Re[Z](Ohm)
Freq[GHz]
Vertical
-1500
-1000
-500
0
500
1000
0 0.5 1 1.5 2 2.5 3 3.5 4
Vertical imaginary part of the couplingimpedance estimated at 1.5 cm off seton the vertical coordinate (9 stems)
Im[Z](Ohm)
Freq[GHz]
Vertical
Im[Z(f=0)] / m = 15 K/ m @ 1.5 cm on the vertical coordinate
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Simulations with CST Particle Studio
(1) input file of Bruno was translated.(2) Sharp edges were Blended (small impact)(3) the couplers on each side were added(4) cavity was rotated by 90 degrees around z (to be checked with Bruno)(5) bunch length=2 cm rms (~6 million mesh cells)
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Transverse wakes (displacement=1 cm)
s (cm)
Wy
(V/p
C)
Wx
(V/p
C)
s (cm)
s (cm)
Wdi
p (V
/pC
)
Wqu
ad (
V/p
C)
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Transverse impedance (displacement=1 cm)
s (cm)
Re(
Zy)
(O
hm)
f (GHz)
Im(Z
y) (
Ohm
)
Re(
Zx)
(O
hm)
Im(Z
x) (
Ohm
)
f (GHz)
f (GHz) f (GHz)
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Remarks• Very similar results than Bruno for the vertical dipolar impedance.
• Quadrupolar peaks are located at the same frequencies as the longitudinal resonances (200 MHz and 630 MHz)
noise or true signal?
• Im(Zx)=Im(Zy)~200 at 1 cm
Im(Ztransdipolar) ~ 0.5 M/m for all the cavities
• Source bunch length is large
DFT/iDFT to get wake function is more tricky than for kickers
use of wake potential directly
widens artificially the wake imported into HEADTAIL
need to think of other solutions: 2D codes or GdfidL first try with ABCI (2D code)
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First try with ABCI
Rotational symmetry implies that the rods cannot be accounted for
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First try with ABCI
200 MHz???
Bunch length=10 cm (rms)
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Back to Particle Studio…
Same structure as ABCIOriginal structure from Bruno
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Back to Particle Studio…
s (cm)
Wz
(V/p
C)
s (cm)
Zlo
ng (
Ohm
)
Longitudinal wake Longitudinal impedance
(1) CST PS and ABCI are giving very similar results (hopefully!!!)(2) Maybe we should stick to 3D simulations in this case…
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Effect on beam dynamics• Wakes to be input into Headtail
s (m)
s (m)
s (m)
s (m)
Wx
dipo
lar
(V/p
Cm
m)
Wy
dipo
lar
(V/p
Cm
m)
Wx
quad
(V
/pC
mm
)
Wx
quad
(V
/pC
mm
)
Without cavities
With cavities
2 cm rms bunch length is probably too large need to find tricks or change code
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Mode spectrum with HEADTAIL accounting for the current model
Threshold seen at 1.1 1011 p/b
Seems to be between modes -1 and -2
More to be done…
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Future plans• Get the low currents to obtain the tune shift
• Reduce the bunch length to obtain a better wake over 1 m– Try with GdfidL
• Include other RF cavities, septa and pumping ports in the model
• Refine existing models (in particular the kickers)