M3 Snowmass Working Group on Linear Colliders – Part II · • Single bunch BBU solved in SLC in mid-80’s • Need to measure/model wakefields – Quasi-static emittance dilutions

M3 Snowmass Working Group on Linear Colliders – Part II

Reinhard Brinkmann, Tor Raubenheimer, and Nobu Toge

M3 Working Group on Linear Colliders

Outline

• Superconducting rf system– Test facilities– Gradient!

• Linac beam dynamics– Wakefield control– Emittance control– Beam-beam issues

• Operational issues (integrated luminosity is what counts!)– Commissioning– Machine protection issues– Reliability issues

• Personal comments


TESLA Test Facility• Operating since 1997

– 7000 hrs at ~ 1Hz with two 8-cavity modules– Delivering beam for SASE FEL

• Good for operational discipline– bad for machine development!• 17 MV/m typical gradient

– Some dedicated TESLA-type operation• Measured HOMs• Demonstrated beam loading compensation• Gradients up to 23 MV/m (TESLA-500 goal) with single module

operation


TESLA Test Facility


Gradient Achievement!

Yield with Eacc > 23 MV/m in 3rd production are ~ 90%


Super-Structures

• Super-structure will increase filling factor from 74% to 79%– TESLA-500 gradient would be 22 MV/m– TESLA-800 gradient would be 35 MV/m

• Super-structures reduce number of couplers by 50% and HOM couplers by 25%

• 2x7 super-structure to be tested next year and 2x9 afterwards


Electropolishing versus Etching

1st TESLA 9-cell cavity reached ~ 30 MV/m


RF System Tests• Test superstructure concept with 2x7 cavities in 2002• Build 2 more 8-cavity cryo-modules for TTF-2 (5 total)• Build one 12-cavity TDR-style module in ~ 2004

TTF-2 to be commissioned in 2003


Linac Dynamics• Two separate issues: Beam BreakUp (BBU) and ‘static’

alignment or emittance dilutions– BBU quasi-exponential amplification of incoming trajectory errors

• Well understood and well simulated!• Multi-bunch BBU seen in 60’s in SLAC linac• Single bunch BBU solved in SLC in mid-80’s• Need to measure/model wakefields

– Quasi-static emittance dilutions• Cavity alignment• Magnet alignment• Rf deflections• Stray fields• Use beam-based alignment!• Techniques developed and tested at SLC, FFTB, ASSET, and

elsewhere!

Snowmass 2001 Jacek Sekutowicz for the TESLA Collaboration

4

HOM couplers have been designed and positioned to fulfill previous BBU limit !!!!!

Does the HOM coupler design meet requirements of the new limit ?

Damping of the first two transversal passbands:

Cryomodule #1 (measurements by G. Kreps)

1

10

100

1000

10000

1,65

16

1,65

16

1,67

38

1,67

45

1,70

08

1,70

18

1,72

84

1,72

93

1,75

77

1,75

77

1,78

28

1,78

65

1,79

38

1,79

38

1,83

69

1,83

69

1,85

36

1,85

39

1,86

47

1,86

47

1,87

39

1,87

42

1,87

96

1,87

99

C#1 C#2C#3 C#4C#5 C#6C#7 C#8New BBU-Limit

Z=(R/Q)Qext

[kΩ/cm^2]

f [GHz]

Snowmass 2001 Jacek Sekutowicz for the TESLA Collaboration

14

Summary of HOM damping in the TTF linac: • Very good damping of the first two dipole passbands (TE111, TM110, 36 modes/cavity),

• Good damping of the monopole passband (TM011),

• Modes in the 5-th passband untrapped and Qs lowered , at least, by a factor of 400,

• Damping of 3-rd passband ( dipole at 2.58 GHz with sw pattern in the interconnection)

should be improved to keep Q below the BBU limit independent from production tolerances.

This can be easily done (M. Dohlus and V. Kaluzny ) if angular position of the HOM couplers

will be properly chosen.


NLC Wakefield Measurements

• Cavities excitied with beam in ASSET facility– Resolution is roughly 10x better than tolerances

• Model agrees well with measurements– Construction errors prevent attaining ideal wakefield but sufficient!


Wakefield Summary• Wakefields have been measured in the TTF and the

ASSET facility at SLAC using beam– Both wakefields are larger than design although sufficient

• NLC errors were due to known construction errors• TESLA cavity errors were due to calculation errors

– Both cases are not ‘final’ prototype cavities• NLC prototype cavity in 2003 and TESLA in 2004(?)

– Devil is in the details!

• NLC aims to measure ‘final’ cavity prototype in 1.5 yrs– Must develop high gradient structure with low group velocity and

wakefield control

• TESLA will choose between 2x9 superstructure and present single cavity design– 2x7 superstructure to be tested next year and 2x9 to follow


Beam-Based Alignment (ε Tuning)• To preserve emittance must correct net effect of

individual dilution sources

• ‘Local’ correction - directly correct dilution sources– Beam-based alignment – tested SLC; FFTB; other beam lines– Most robust solution / least sensitive to energy or strength errors

• ‘Quasi-Local’ correction - correct dilution effects over short distance, i.e. betatron wavelength– Dispersion-Free steering – tested in SLC; LEP; other rings– Based on ‘measurements’ of dilution / sensitive to systematics

• ‘Global’ correction - tune emittance using direct εdiagnostics– Directly corrects desired quantity / sensitive to phase advance –

tested SLC


FFTB Quadrupole Alignment

• Used quadrupole shunting technique– Fit residuals ranged from 2 µm to 30 µm at the end of the beam line

• FFTB optics poorly designed for beam-based alignment• Ran out of BPMs to measure deflected trajectory!

– Dispersion measurements show errors in 1st two regions< 7 µm after alignment

• Confirms technique

– NLC designed for BBAwith better diagnosticsand smoother optics

• Would expect a factor of2 ~ 3 improvement

• Other techniques asbackup


Rf Cavity Alignment

• NLC structures (cavities) must be aligned to beam within 10 µm rms for 20% ∆ε– Every structure has two rf-BPMs with better than 2 µm accuracy– Short-range wakefields depend on average of structure offset – Average position of the 6 structures on an rf girder and move girder end-

points with remotely controlled movers

• TESLA cavities must be aligned with 500 µm rms for 15% ∆ε– Achieved +/- 250 µm alignment within cryostat– But effects add à tolerance for 12 cavities in cryostat ~ 140 µm– Effect is worst at ¼λβ = 150 m à tolerance for cryostats ~ 45 µm– Either add read-backs on HOM dampers and steer beam to center of

cavities or use global emittance bumps like those used in SLC to cancel dilutions


Beam-Beam Issues

• High disruption à single bunch kink instability– Sensitive to IP position and angle offsets (IP feedback)– Sensitive to position correlations along the bunch, i.e. ∆ε– Fractional luminosity decrease is much larger for correlated errors

such as those from the linac or bunch compressor

– Effect can be reduced by decreasing bunch length but this increases beamstrahlung energy spread

– Smaller fractional effect for large emittance dilutions and smaller disruption – initial calcs. suggest smaller problem in NLC design

Uncorr. ∆ε Corr. ∆ε Ldesign (∆ε = 50%) 3.4x1034 L0 (∆ε = 0% i.e. from DR) 4.1x1034 4.1x1034 Lsim (∆ε = 10%) 3.9x1034 3.2x1034 Lsim (∆ε = 20%) 3.7x1034 2.7x1034

Simulation by R. Brinkmann including IP feedback tuning


Luminosity: only few x104 larger than SLC!

• Luminosity increase from two effects:– Increased beam power from long bunch trains

• SLC: 120 Hz x 1 bunch @ 3.5x1010

• NLC: 120 Hz x 190 bunches @ 0.75x1010à 200x• TESLA: 5 Hz x 2820 bunches @ 2.0x1010à 340x• Control of long-range wakefields is essential to assure multi-bunch

– Larger beam cross-sectional densities: N / (σx σy)• SLC: 3.5x1010 x 1.6 µm x 0.7 µm (FFTB: 0.6x1010 x 1.7 µm x 0.06 µm)• NLC: 0.75x1010 x 250 nm x 2.7 nm à 360x SLC• TESLA: 2.0x1010 x 550 nm x 5 nm à 230x SLC• Factor of 5 from energy (adiabatic damping) and factor of 10 from stronger

focusing (similar to Final Focus Test Beam) but smaller bunch charge• Factor of 15 ~ 30 from decrease in beam emittance

Dyxcms

b HN

EP

Lσσπ 4

2 =D

yx

brep HNnf

Lσσπ

2

4 =


Commissioning and Luminosity

• Linear colliders are very complicated accelerators• Design luminosity values have little margin

– Will take time to attain these values!

• Commissioning will likely begin during ‘construction’– For example, NLC injector complex, i.e. sources, damping rings,

and bunch compressors can be commissioned 1~2 years before the linacs are finished

– Do not yet have detailed commissioning plans but difficult to plan anyway!

• Simplistic estimates of commissioning impact were provided to E3 working group– Integrated luminosity after 3 years at 500 GeV: 250 ~ 350 fb-1


Machine Protection Issues

• Single bunches will likely damage any material at the end of the linac or in the beam delivery– Complicated turn-on process to prevent damage– Complicated MPS system with diagnostics on many components

• Anything that can change from pulse-to-pulse

– Some impact on operationnot yet fully quantified

Damage from 13 pC/µm2


Reliability Issues

• Essential to understand!– Significant limitation in SLC operation

• Would take 3 ~ 4 times the length of each down time to recover luminosity!

• New LC are being designed to avoid known problems– Multiple (redundant) power supplies– Overhead in klystron / modulator populations– Redundant electrical / cooling systems– Still need detailed analysis

• Must qualify reliability of all components, especially those in the tunnel!


Personal Opinion: XFEL

• First thought of in ’92 (C. Pelegrini and H. Winick)– Convergence of LC technology; rf guns; undulators; star wars

• No fundamental advantage of different technologies– TTF FEL and APS FEL LCLS and TESLA XFEL

• Great idea however do we/they really want a combined fac.?– Cost sharing is minimal (new sources; new compressors; only share 5%

of linac) and operating expertise can be transferred!– Experimental requirements very different: users need few hours of

beam time– Real operational issues in sharing linacs and tunnels

• Build user facilities at radiation sources: SSRL at SLAC, APS at Argonne, HASYLAB at DESY


Summary

• TESLA rf system in good shape!– 500 GeV system is nearly ready – 800 GeV will be tested in few years!

• No showstoppers!– Many outstanding problems in both designs– Prototype hardware still needs demonstration– Most failures can be worked around

• Example: damping ring space charge – double number of bunches and decrease charge per bunch by 2 – use two rings if kickers don’t work!

• At this point the designs will reach conclusion faster than any political process – Believe this is true for both NLC/JLC and TESLA designs– Time to start process


Summary

• ICFA panel to report on technology in ~ 1 year– Likely public meetings to discuss progress– For example: LC2002 in January at SLAC

• LC accelerator community is tight– This meeting has been fantastic– Great discussions (nobody managed to actually finish a talk)– Good friends based on mutual respect

• We want (need) you to get involved and help understand technology limitations and choices

Documents

M3 Snowmass Working Group on Linear Colliders – Part II · • Single bunch BBU solved in SLC in mid-80’s • Need to measure/model wakefields – Quasi-static emittance dilutions