1. For Discussion Possible Beam Dynamics Issues in ILC downstream of Damping Ring
LCWS2015
K. Kubo

2. Contents
Low emittance preservation (luminosity achievement) scenario in TDR
Other issues require more studies?
Alignment
Beam Tuning Procedure at Interaction Point ?
Questions/discussions

3. Luminosity reduction due to Misalignment (Vertical Emittance Increase)
Dispersion
Misaligned quadrupoles + energy spread
Acc. cavity tilts + energy spread
x − y coupling
Rotated quadrupoles
vertically misaligned sextupoles (Final Focus)
Single-bunch wakefields
Acc. Cavity
Beam pipe, collimator (BDS, e+ source undulator)
Multi-bunch wakefields

4. Cures for misalignment -1
Dispersion
Non acceleration region（RTML, BDS）
Kick minimization
Use steering magnets to make overall transverse kick minimum
Dispersion knob
Adjust skew quads at H-dispersion, looking at dispersion downstream.
Acceleration region （Bunch Compressors, Main Linac）
Dispersion Free Steering, Dispersion Matching Steering
Use steering to make dispersion matched to design at all BPMs
X-y coupling
Use skew quads or moves of sextupoles (coupling knob)

5. Cures for misalignment -2
Single bunch Wakefield
Cavities
Initial alignment (0.3 mm accuracy w.r.t .cryo-module)
Collimator (BDS)
Sufficiently long tapers
Beam pipe of undulator for e+ production (Aperture 5.85 mm)
Beam pipe on movers (at least some part of pipe)
Multi-bunch Wakefield
HOM coupler (impedance Q_ext of HOMs should be similar to that in TESLA TDR)
Frequency spread
Frequencies of important modes should be “detuned” ~0.001, cavity by cavity

6. RTML upstream Bunch Compressor ～5 nm vertical emittance growth
(20 nm from DR)
Bunch Compressor
～1 nm
vertical emittance growth
～4 nm
(no Cryomodule pitch optimization)
（Same as Main Linac ）

7. Coupler kicks in Bunch Compressor
Possible problem from asymmetrically mounted couplers (Input and HOM) on cavity
Coupler RF kick
Transverse kick by fundamental mode RF
Different kicks for head and tail of each bunch
Emittance growth, in addition to orbit change
Coupler Wakefield
Transverse kick by HOM, even if beam go through center of cavity
Small effect in Main Linac thanks to short bunch length
Much more important in BCs
upstream
downstream
RF Input
HOM
HOM

8. Coupler kicks in Bunch Compressor
There were great efforts. Found present design of cavities and couplers would not cause serious problem.
But, we should be very careful.
We have to rely on cancellation between upstream kicks and downstream kicks.
Are we convinced?
Can we confirm this experimentally?
upstream
downstream
RF Input
HOM
HOM

9. Cryo-module pitch adjustment (remote movers)
Mainly for correction of coupler kicks
V-emittance growth ~4nm (no pitch control)  1 nm
Cryo-Module
Adjust the vertical angle measuring emittance downstream.
For example
Range ~ 0.3 mm
Step ~ 10 micron
All 3 modules in BC1, 4 modules in BC2 (out of 48)
(Based on study by A.Latina)
Design started at KEK (H. Hayano)

10. Main Linac ~4 nm vertical emittance growth
BPM calibration error 10% required in TDR.
But better performance is desirable.
Long Range Wakefield of Cavities:
Frequencies should be “detuned” ~0.001, cavity by cavity

11. BDS  Design luminosity
Luminosity measurement: 1% accuracy. Need fast measurement.
 Discussion on luminosity tuning at IP later

12. Luminosity reduction due to dynamic errors
Position change (vibration) of magnets and cavities
Magnet strength change
Cavity field change
Stray field (magnetic field from outside) 10

13. Tolerances in ML Main Linac
Expected orbit jitter at the end of Main Linac
 Need post ML intra-pulse feedback

14. Tolerances RTML and BDS
RTML, BDS
Bunch compressor

15. Stray Field in Return Line
Changing magnetic fields change orbit
Important in Return Line (long beam line with weak focusing strength)　
Cause Emittance growth in Turnaround (important in vertical)
(10 nT  ~1-sigma orbit jitter  emittance ~10% increase) (very rough estimation. Depend on time and space distributions of the fields.)
Orbit Feed-forward will correct the effects after Turnaround
Orbit change originated in long RL
feed-forward
In Turnaround, vertical dispersion  emittance growth
Past measurements suggested high frequency components would be ~nT, which seemed OK. But is this realistic enough?
We may have fast kickers to correct fixed orbit change (same for different pulses, bunch by bunch different).

16. Cavity to cavity voltage change in a pulse
Cavity tilt + voltage change  transverse orbit change
Requirement
Cavity tilt （alignment）　0.3 mrad (RMS)
Voltage change　1% (RMS)
Experimentally demonstrated in FLASH

17. Alignment
Almost all simulations assumed random independent misalignment, or simple model.
Actual alignment will not make such simple distribution.
Some efforts existed, but?

18. Luminosity simulation at IP
Most simulations for dynamic errors start from good beam condition and simulate degradations, then cures.
In reality, we will start with bad beam. (Huge beam size and almost zero luminosity.)
What is the procedure of beam tuning?

19. Example of simulation using Ground motion model A,B,C,K (TDR Part 1 p162)
(motion in BDS is dominant)
Luminosity from 1st bunch is small.
10% Luminosity reduction in the case Model C
Ground motion model A,B,C:
International Linear Collider Technical Review Committee, Second Report, SLAC-R-606, 2003.

20. IP Tuning　procedure　1
Beam size is estimated based on Luminosity monitor
Beam-beam offset is estimated based on beam–beam kick
Tuning with one beam
No information of beam at IP
Rely on measurements and tuning upstream
Need to achieve reasonably small beam size and offset for luminosity measurement and beam-beam kick measurement.
Are we convinced that it will be achieved?
We will not have beam size monitor at IP and additional independent QD0?
Do we need to confirm and practice tuning before installation of physics detector?
I guess answer was no. (? Cost?)

21. IP Tuning 2 Two Beam Tuning, 1st stage
Large beam size (> position jitter)
Measure “max luminosity ” by scanning beam position
Adjust beam size knobs and increase “max luminosity”
Averaging luminosity of many pulses.
Achieved beam size will be comparable to position jitter.
Cannot be smaller than position jitter.
Reducing position jitter is very important.
Jitter determines achieved beam size in this stage.

22. IP Tuning 3 Designing the procedure is not trivial at all.
Two Beam Tuning 2nd stage
Small beam size (< pulse to pulse position jitter)
Scan beam position in a Pulse and measure “max luminosity”
Luminosities should be measured for many small parts of one pulse.
Adjust beam size knobs to increase “max luminosity”
Luminosity should be measured in short time.
Part of one pulse.
Luminosity
Relative position (in one pulse)
Designing the procedure is not trivial at all.

23. Discussion -1 Alignment Engineering model?
Coupler kicks (Bunch compressor)
Is RF calculation accurate enough? Can be confirmed?
Remote mover system for tilt control?
Stray magnetic field (RMTL)
Are we convinced it is not serious?
Fast orbit control? (change in one pulse, but fixed for pulse by pulse)
BPM in ML calibration error
Can be dominant source of emittance growth in ML

24. Discussion -2 Other Issues? Beam Tuning Procedure at Interaction Point
Fast luminosity measurement is critically important
Reducing position jitter (without feedback at IP) is important (Need post ML fast feedback)
Detailed procedure?
Tuning before detector installation?
Other Issues?