eirini koukovini-platia epfl, cern effect of impedance and coatings in the clic damping rings 1 3rd...

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

2

• 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


4

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


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


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


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

3rd Low Emittance Ring workshop - 8th-10th July Oxford9

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


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


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


• 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



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


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?


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?)


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


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

eirini koukovini-platia epfl, cern effect of impedance and coatings in the clic damping rings 1 3rd...

Documents

function of impedance

resistive wall impedance

available impedance

remaining impedance

machine impedance budget

oxford slide

rd low emittance ring

pipe vertical impedance