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Sasha NovokhatskiSLAC, Stanford University

Machine-Detector Interface Joint Session April 22, 2005

“HOM Impedance in Vacuum

Designs”

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Luminosity and wake fields

• We need high current beams of very short bunches to achieve super high luminosity

• These beams carry high intensity electromagnetic fields.

• Any geometric disturbance or even surface roughness of a beam pipe leads to diffraction of these fields.

• The diffracted fields can propagate free in the

beam pipe. We call these field wake fields.

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Wake fields

• High frequency part of wake fields can penetrate through small holes of shielded fingers to bellows or through RF screens to vacuum pumps.

• These fields can also go outside vacuum chamber through heating wires of NEG pumps or through pump high voltage or BPM connectors.

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Wake fields

• In a time wake fields are absorbed in conducting chamber walls.

• Main effect from wake fields is temperature rise of different vacuum chamber elements, like shielded bellows, vacuum valves and pumps. In this case wake fields transfer energy to resonance High Order Modes (HOMs) excited in closed volumes.

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Wake fields

• The amplitude of the HOM electric field can rich the breakdown limit and bring damage to the metal surface

• Other effect can be the interaction of escaped (from the vacuum chamber) short wake field pulses with detector electronics.

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Resistive-wall wake fields

• Other type of wake fields is excited due to finite conductivity of vacuum chamber walls.

• Resistive-wall wake fields give temperature rise mainly to chamber walls.

• In all cases beams lose energy for wake field production. This energy has to be restored in RF cavities.

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Wake field Evidence from PEP-II

• Shielded fingers of some vacuum valves were destroyed by breakdowns of intensive HOMs excited in a valve cavity.

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• All shielded bellows in LER and HER rings have fans for air cooling to avoid high temperature rise.

• All chambers have water cooling against resistive-wall wake fields.

Wake field Evidence from PEP-II

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HER “resonance” bellows

Resonance at HER current of 1300 mA

Temperature difference9072QUA – 9062QUA

Resonance 1-2 degrees F

~dZ~100 micronQ=10^3

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HOM leaking from TSP heater connector

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Effect of absorberinstalled in

antechamber

Temperature

LER current

Nov. 2002-July 2004

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HOM Power in absorber

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The source of HOM power: Collimators

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Beams passing by collimators generate dipole

and quadruple modes.

These modes can easily penetrate though shielded fingers S. Weathersby

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HOM Power from collimators goes downstream

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Hottest Bellows 2012 takes HOM power from four Y and X

Collimators

Y and X collimators

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Collimator Loss Factor

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Bunch length dependencestraight section collimator

y = 24.755x-2.0628

y = 3.8x-1

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14

Bunch length [mm]

Lo

ss

fa

cto

r [V

/pC

]Loss factor [V/pC] Loss factor [V/pC]

Power (Loss factor [V/pC]) Power (Loss factor [V/pC])

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Collimator HOM Power

2 bP k IP ar ameter s P E P - I I Super B

B unc h l ength [mm] = 13 1. 8

Los s f ac tor [V / pC ]= 0. 125560171 2. 11111111

LE R c ur r ent [A ] 2. 4 23

B unc h s pac i ng [ns ec ] 4. 2 1. 05

P ower l os s (pul s e) [kW] 3. 04 1172. 62

Low HOM type collimators are needed for super B

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Special absorber device to capture

collimator HOMsRed line shows absorption in ceramic tiles

S. Weathersby

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Wake in IP region of PEP-IISimulation model

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PEP-II Vertex Bellows

Bel

low

sC

avity

bunch field

‘‘Mode Converter”

S. Ecklund measured 500 W dissipated in

vertex bellows

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Field leakage though bellows fingers

Will be captured by ceramic absorbing tiles in

the new vertex bellows design

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10 kW HOM power absorbed in ceramic tiles of Q2-

bellows in PEP-II

Stan Ecklund measurements

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Loss factor for PEP-II IR

Bunch length dependence

changes from

(14-8 mm)

to

-3/2 (6-1 mm)

PEP-II Interaction regionLoss factor and approximations

y = 38.575x-1.967

y = 17.379x-1.4934

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14

Bunch length [mm]

Lo

ss

fa

cto

r [V

/pC

]

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IP HOM Power

2 2

Incoherent Pow

(

er

) ( )b e eP k I I

P ar ameter s P E P - I I Super B

B unc h l ength [mm] = 13 1. 8

Los s f ac tor [V / pC ]= 0. 248 7. 224

LE R c ur r ent [A ] 2. 4 23

HE R c ur r ent [A ] 1. 5 10

B unc h s pac i ng [ns ec ] 4. 2 1. 05

P ower l os s (pul s e) [kW] 8. 36 4771. 34

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Additional beam energy loss due to “Cherenkov” radiation

in open ceramic pipes.

02

02

1

2

loss factor 2

loss factor 2

awhen s

cZ L sK

a

when s

cZ LK

a

Loss factorHEACC’92page 537

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Additional beam power loss in Q2-bellows

PEP-II Super Beps= 30 30L [mm] = 59.2 59.2 Bunch length [mm] = 13 1.8s= 5.385165 5.385165check s/sigma <1 or >1 0.414243 2.991758

Loss factor [V/pC]= 0.069179 0.296

LER current [A] 2.4 23HER current [A] 1.5 10Bunch pattenrt by2 T [nsec] 4.2 1.05

Power loss [kW] (incoherent) 2.3273 195.49

Using this formula for Q2

No open ceramics for Super B!

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RF screens.NEG chamber and a vacuum

pump flangeTemperature rise in NEG chambersdue to HOM heating changed the vacuum.

M. Sullivan attached a lot of thermocouples to NEG chambers to understand the

problem.

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RF antenna in a pump HV connector

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Antenna in other pump

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Q-value estimation

0

5

10

15

20

25

30

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Delta Frequency [MHz]

Sp

ectr

um

F=4.046119 GHz Breit-Vigner Q=6100

02468

101214161820

-1 -0.5 0 0.5 1

Delta frequency [MHz]

Sp

ectr

um

F=5.236GHz Breit-Vigner Q=12000

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Moveable collimator changes HOM spectrum in near pump

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RF screens and coupling

• Screen impedance scales with frequency (inversely to bunch length)

• Holes must be 5 times smaller than for PEP-II, or two times thicker

• Decreasing Q-value of a bellow or NEG cavity by placing absorber

• Low Q additionally decreases field coupling.

10

0

1 022

1 0

0 1

20 1

1

1

in

4(1 )

( )

4 /

(1 / )

cav

waveguide

cav inc inc

cav inc

Reflection from cavity

ZQQ

Z Q

Power a cavity

Q QP P P

Q Q

Q QP P

Q Q

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Resistive Wall Wakefield Losses

1/3

2 00

0

3/ 2

0 02

2 when 1

0.24

z

z

ss a

Z

Z c sK

a

Loss factor asymptotic (M. Sands, K. Bane)

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Resistive Wall Wakefield Power

pipe Radius [m] 4.50E-02 4.50E-02 4.50E-02Material Cu Al SSresistivity [Ohm m] 1.69E-08 2.86E-08 7.14E-07S0 [m] 5.67E-05 6.75E-05 1.97E-04

bunch length [m] 1.80E-03 1.80E-03 1.80E-03loss factor [V/pC] 0.005 0.006 0.032Bunch spacing [nsec] 1.05 1.05 1.05beam current [A] 23 23 23power [kW/m] 2.894 3.757 18.786

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Comparison of 2.5, 1, and 0.5 cm pipes.

pipe Radius [m] 2.50E-02 1.00E-02 5.00E-03

Material Cu Cu Curesistivity [Ohm m] 1.69E-08 1.69E-08 1.69E-08S0 [m] 3.83E-05 2.08E-05 1.31E-05

bunch length [m] 1.80E-03 1.80E-03 1.80E-03Loss factor 0.009 0.022 0.045Bunch spacing [nsec] 1.05 1.05 1.05beam current [A] 23 23 23power [kW/m] 5.209 13.022 26.045

This is only resistive-wall power!

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Total HOM Loss Estimation

LER HER SumCavity loss [MW] 4.35 1.65 6.00

Loss factor [V/ pC] 0.28 0.28Number of cavities 28 56Resisstive Wall [MW] 7.46 1.41 8.87

Al (R-45mm) 13.43 13.43IP region 4.01 0.76 4.77

Loss factor 7.22 7.22Other: collimators, feedback…[MW] 1.11 0.21 1.32

Loss factor 2 2

Currents 23 10Total Sum for HOMs 16.94 4.02 20.96

Comparison withSR power [MW] 17.48 35.5 52.98

loss per turn [MeV] 0.76 3.55Final sum [MW] 34.42 39.52 73.94

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DESY newsThis thorough analysis led to the conclusion that the backgrounds generated by protons lost in the interaction-region beam correlated with the poor initial vacuum conditions in the new system in the presence of the positron beam. The vacuum recovery was also slowed down by considerable thermal desorption of synchrotron radiation masks inside the beam pipe close to the experiments. This was due to higher-order mode heating at injection energy when the bunches are short.

F. Willeke

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Summary

• Electron and positron bunches generate electromagnetic fields at any discontinuity of vacuum chamber

• These fields can travel long distance and penetrate inside bellows, pumps and vacuum valves.

• Vacuum chamber must be optimized for minimum wake loss