eirini koukovini-platia epfl, cern effect of impedance and coatings in the clic damping rings 1 3rd...
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Eirini Koukovini-PlatiaEPFL, CERN
Effect of impedance and coatings in the CLIC damping rings
1 3rd Low Emittance Ring workshop - 8th-10th July Oxford
CLIC DR parameters
Introduction Damping Rings
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• Small emittance, short bunch length and high current
• Rise to collective effects which can degrade the beam quality
• Their study and control will be crucial
3rd Low Emittance Ring workshop - 8th-10th July Oxford
Impedance budget and coating
Focus on single bunch instabilities driven by impedance Define an impedance budget To suppress some of the collective effects, coating will be
used Positron Damping Ring (PDR): electron-cloud effects
amorphous carbon (aC) Electron Damping Ring (EDR): fast ion instabilities need for
ultra-low vacuum pressure Non-Evaporable Getter (NEG)
Techniques to fight electron cloud or have good vacuum do not come for free and can be serious impedance sources
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Resistive Wall Impedance: Various options for the pipe
• Vertical impedance in the wigglers (3 TeV option) for different materials
Þ Coating is “transparent” up to ~10 GHz
Þ But at higher frequencies some narrow peaks appear
ÞAbove 10 GHz the impact of coating is quite significant
N. Mounet, LER Workshop, January 2010
Resonance peak of ≈1MW/m at almost 1THz for C- coated Cu
Single bunch simulations without space charge to define the instability thresholds
HEADTAIL code Simulates single/multi bunch collective phenomena associated
with impedances Computes the evolution of the bunch by bunch centroid as a
function of time ImpedanceWake2D
Computes the longitudinal and transverse wake functions of multilayer structures, cylindrical or flat
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Goal Estimate the available impedance budget for the
various elements to be installed after known impedance sources such us the broad-band resonator, the resistive wall and the kickers are considered
QD
Estimating the impedance budget with a 4-kick approximation
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• A uniform coating of NEG, 2μm thickness, on stainless steel pipe• The contributions from the resistive wall of the beam chamber were
singled out for both the arc dipoles and the wigglers
Straight section:13 FODO
ARC (9mm round):DS1- 47 TME cells - DS2
2nd kick resistive wall from the arcs3rd kick wigglers 4th kick rest of the FODO
QFwigglerwiggler
FODO cell 9mm radius
round 6mm radius, flat
1st kick broadband resonator
3rd Low Emittance Ring workshop - 8th-10th July Oxford
TMCI 15 MΩ/m
Estimating the machine impedance budget with a 4-kick approximation – single bunch simulations
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Mode spectrum of the horizontal and vertical coherent motion as a function of impedance (applying an FFT to the HEADTAIL output)
TMCI 4 MΩ/m
3rd Low Emittance Ring workshop - 8th-10th July Oxford
For zero chromaticity, the (remaining) impedance budget is estimated at 4 MΩ/m
For positive chromaticity, the impedance budget is estimated now at 1 MΩ/m
ξx 0.055
ξy 0.057
Head-tail instability
• No mode coupling •Higher TMCI thresholds
6-kick study- adding stripline kickers 2 kickers in the DR (injection&extraction) Calculate the wake function, and add this wake as a kick in our
impedance model Idea: Use the shortest bunch length possible in the CST simulation
so that the wake function (input for HEADTAIL) could be approximated with the wake potential (output of CST)
•Bunch of 0.2 mm•3 lines per wavelength•~150 million meshcells•time consuming simulation•Untrustworthy results
R=25mm, Aperture=12mm/20mm, h=20mm, Thickness=4mm
CST simulation
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Design from Carolina Belver-Aguilar, IFIC
•Use at least 10 lines per wavelength•1 billion meshcells!•Limit for CST
Next steps on the stripline kickers study
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Convergence study with numerical parameters showed that the wake potential from CST in this frequency range (requires a very dense meshing and very long computational times) can not be just used as the wake function (for HEADTAIL simulations)
Further work needs to be done to understand and possibly overcome this problem. Benchmark of CST with other codes (such as the ImpedanceWake2D for resistive wall problems and GDfidl for geometric impedances) has also begun
Try ABCI (Azimuthal Beam Cavity Interaction) Calculations of wake fields, wake potentials, loss factors
and impedance
Instabilities studies for the
CLIC DR
Code calculating wake fields/ impedances
• Characterize the electrical conductivity of NEG• Unknown values of the conductivity in this frequency range
EM properties of NEG and aC
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• Need to characterize the properties of the coating materials at high frequencies (CLIC), i.e. 500 GHz
Motivation for the material EM properties characterization
3rd Low Emittance Ring workshop - 8th-10th July Oxford
Material EM properties characterization
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ÞEstablishing and testing a method to measure the properties of NEG
ÞCombine experimental results with CST simulations to characterize the electrical conductivity of NEG
ÞPowerful tool for this kind of measurements
Material propertie
sσ, ε, μ
)(21 S
CST MWS simulation
50 cm Cu wg9-12 GHz
Calculation of the
wake fields
Study of instabilities with
HEADTAIL
)21,( SfIntersection
Experimental method
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Waveguide Method First tested at low frequencies, from 9-12 GHz Use of a standard X-band waveguide, 50 cm
length Network analyzer Measurement of the transmission coefficient S21
Experimental Method (I)
Experimental setup
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• Signals traveling in the waveguide experience loss due to the conductor resistance
• S21 is related to the loss suffered in the transmission from one port to the other
• Cu is a very good conductor and the losses are small
• S21 is related to the material conductivity
Copper waveguide First test: a pure copper (Cu) X band waveguide Measure the S21 from 9-12 GHz
Experimental Method (II)
|S21| for a Cu waveguide
3rd Low Emittance Ring workshop - 8th-10th July Oxford
Result from the intersection of measurements with CST MWS simulations
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• Cu conductivity was estimated within the same order of magnitude with the known value
• Average is 5.91x107 S/m• Good agreement with the
known value of 5.8x107 S/m• The attenuation is very
sensitive to the errors because of the small losses (high conductivity of Cu)
• Despite this, the results were encouraging to continue with a coated waveguide
Conductivity of Cu
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NEG coated Cu waveguide Same Cu waveguide used before is now coated with NEG
Coating procedure Elemental wires intertwisted together produce a thin Ti-Zr-
V film by magnetron sputtering Coating was targeted to be as thick as possible (9 µm
from first x-rays results)
NEG coating of the Cu waveguide
3rd Low Emittance Ring workshop - 8th-10th July Oxford
• S21 results indicate that the skin depth is small enough compared to the coating thickness
• Allows the EM interaction with the NEG
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NEG coated Cu waveguide Measure the S21 from 9-12
GHz
Comparison of Cu and NEG coated one
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• Upper limit for the conductivity of NEG in this frequency range (assume 100 μm thickness)
Conductivity of NEG
•Is this frequency dependent behavior physical? •Repeat measurements with a spare Cu waveguide to check reproducibility
•The non uniformity of the coating might impose uncertainties in the characterization
X-Ray Fluorescence
Second Cu waveguide
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• Small difference observed in the S21 measurement between the 2 waveguides
• How much does this affect the conductivity estimation?• 20% difference
• Observing the same frequency dependence behavior
• Waiting to coat at max thickness this waveguide to check
Why all this trouble?Effect of NEG conductivity on the budget
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Pipe material/ coating Chromaticity Impedance budget
(MΩ/m)
ss and NEG 2μm (σNEG = 106 S/m)
0 4
ξx = 0.055/ξy = 0.057 1
ss and NEG 2μm (σNEG =1.6 106 S/m)
0 5
ξx = 0.055/ξy = 0.057 1
ss/ Cu 10 μm / NEG 2 μm (σNEG =1.6 106 S/m)
0 5
ξx = 0.055/ξy = 0.057 2
The characterization of NEG properties is important in the high frequency regime and affects the instability thresholds predictions
3rd Low Emittance Ring workshop - 8th-10th July Oxford
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Measurements with the stainless steel waveguide- accuracy of the method
X band stainless steel (ss) waveguide of 50 cm length
Define the accuracy of the experimental method
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Results with the stainless steel waveguideMeasurements and CST
Intersection with CST
Average σ=0.75 106 S/m while expected DC value is 1.3 106 S/m (40% error)Problem in the measurement or the model?
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Results are in very good agreement, repeated the procedure 3 times
Further checks for errors during the measurements or the waveguide itself
Measurements output
Repeat 3 times
Problem of the waveguide?
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Add aluminum foil along the connection to ensure good contact and any extra losses (the material is not welded there but two different pieces hold together with screws) Results are in very good agreement
• Seems the experimental results can be trusted
• Need to work on the model (try another code? Analytical calculation of S21?)
3rd Low Emittance Ring workshop - 8th-10th July Oxford
Further checks on this method Work on the model Think of other possible methods? Before going up to 700 GHz, should be fun…
1. Coating and material properties characterization are important in this frequency regime
2. Stripline kickers and RF cavities to be added to complete further the impedance budget study
3. Properties at 750 GHz? (EPFL Network Analyzer)
Other tasks ongoing…
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1. Longitudinal radiation damping being implemented. Tests to be done in the near future. Next step, transverse damping
2. Implementation of space charge in HEADTAIL. Modifying the code to give multiple kicks for space charge along the lattice of the ring. Tests to be done. 1. Study the effect of space charge with impedance in the
TMCI threshold (moving to higher thresholds?)2. Study the tolerable space charge tune spread in
combination with the newly implemented radiation damping modules
3rd Low Emittance Ring workshop - 8th-10th July Oxford
Summary
Acknowledgements
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C. Zannini N. Biancacci K. Li G. De Michele N. Mounet P. Costa Pinto G.Rumolo M. Taborelli
Thank you for your attention!
The goal is to operate at 0 chromaticity which allows for a larger impedance budget (7 MΩ/m)
But since chromaticity will be slightly positive, a lower impedance budget has to be considered, 4 ΜΩ/m
SPS, 7 km, 20 MΩ/m
Chromaticity
ξx/ ξy
Impedance threshold MΩ/m
x y
0/ 0 18 7
0.018/ 0.019
6.5 6
0.055/ 0.057
4 4
0.093/ 0.096
5 3
-0.018/ -0.019
4 5
-0.055/ -0.057
2 2
-0.093/ -0.096
2 2
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• BROADBAND MODEL• Chromaticity make the modes move
less, therefore it helps to avoid coupling (move to a higher threshold)
• Still some modes can get unstable due to impedance
• As the chromaticity is increased, higher order modes are excited (less effect on the bunch)
Backup slides
3D EM Simulations and measurements (I) X band Cu waveguide, εr=µr=1, σ is the (unknown) scanned
parameter For each frequency from 9-12 GHz, the output result is the
S21 coefficient as a function of conductivity Combine with the measurement results σ as a function of
frequency
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Example at 10 GHzIntersection of the simulation results with the measurement point of intersection defines the conductivity
Value of conductivity
•The 2 integration methods in Gdfidl give different results between them•Not in agreement with CST either
Try a simple structure for resistive wall compare CST, Gdfidl, ImpedanceWake2D, analytical
Conductivity: 1.3e6 S/m0.2mm bunch length
Factor of 4-4.5 difference between CST and ImpedanceWake2D. Gdfidl is in even worse agreement.
Analytical and ImpedanceWake2D are in very good agreement