1. Status from the collimator impedance MD in the LHC
Collimation team: R. Assmann, R. Bruce, A. Rossi.
Operation team: G.H. Hemelsoet, W. Venturini, V. Kain , G. Crockford.
Impedance team (and friends): H. Burkhardt, W. Hofle, E. Métral, N. Mounet, B. Salvant.
Many thanks for their help to:
G. Arduini, M. Gasior, B. Goddard, S. Redaelli, F. Roncarolo, G. Rumolo, R. Steinhagen, D. Wollmann.
LCU meeting – June 10th 2010

2. This is a work in progress!!!

3. Agenda Context Objectives Methods MD results
Conclusions and next steps

4. Context: LHC impedance and collimators
LHC transverse impedance is predicted to be one of the potential limitations to reach nominal beam parameters for collisions at top energy (14 TeV/c).
LHC collimators are predicted to be the major contributors to the LHC transverse impedance at top energy. An upgrade of the collimation system is under study to reduce the impedance and improve the cleaning efficiency (Phase 2 collimation).
Impedance theories, EM simulations, RF bench measurements and MDs in the SPS with a prototype collimator were all consistent and showed that we seem to understand the impedance of a single collimator.
Now that beam is in the LHC, it is important to compare the LHC beam-based observations with predictions, in order to take decisions for the Phase 2.

5. Context: previous LHC observations and predictions with different intensities
Several measurements performed at injection by Stefano Redaelli et al and Brennan Goddard et al.
They monitored the tunes for bunches of different intensities (see here):
Qx~ and Qy~ /04/2010 (B2)
Qx~ and Qy~ /04/2010 (B1)
Our predictions of tune shift with HeadTail macroparticle simulations for nominal collimator settings and nominal beam parameters at 450 GeV/c were
Qx~ and Qy~
for Nb~ p/b
for Nb~ p/b
 these measurements seem to indicate that the impedance may be a factor 5 higher than predicted by the model…
 need for dedicated measurements, to try to record and control the beam parameters

6. Agenda Context Objectives Methods MD results
Conclusions and next steps

7. MD objectives
1) Assess the collimator impedance by (a) moving the jaws of a chosen set of LHC collimators
(b) monitoring the transverse coherent tune shifts, and other beam parameters.
2) Assess the impedance of the whole LHC machine by
(a) changing the beam intensity (through scraping)
We got 4 hours of MD time (May 28th, from 13:00 to 17:00).

8. Agenda Context Objectives Methods MD results
Conclusions and next steps

9. Methods LHC impedance model is calculated through ZBASE
Includes theoretical models of phase 1 collimators at desired settings, beam screens, warm pipe, MQW, MBW and a BB impedance (from design report).
Significant contributors could be missing (kickers, PIMS, etc.).
Tune shift predictions with LHC model
Impedance  Sacherer formulae for single bunch transverse tune shifts
Wake  Headtail macroparticle simulations  SUSSIX  transverse tune shifts MD
Record relevant beam parameters, in particular: intensity Nb, transverse tune shifts Q, bunch length L, as we expect

10. Agenda Summary Context Objectives Methods MD results
Moving IR 7 collimators from 5 sig to 15 sig
Moving injection protection collimators from nominal to retracted (TDIs+TCLIs)
Scraping in IR3
Conclusions and next steps

11. B2: Moving IR7 collimators

12. Summary for beam 2 (moving IR7)
Bunchlength=1.4 ns
Nb=
Collimator gap open at 15 sigma
Qy~
Qx<7 10-4
Collimator at 5 sig

13. B2: effect on vertical tune shift of moving IR7 collimators
4*Std(sussix)=
4*Std(fft)=
out (15 )
out
in (5 ) in
 The tune shift is correlated to the collimator position.
 Qy (meas.) ~
 Vertical tune shift prediction when moving IR7 from 15 to 5 (ZBASE model with measured collimator settings and Sacherer formula with measured beam settings): Qy (theory) ~

14. B2: effect on horizontal tune shift of moving IR7 collimators
Tune peaks?
Sideband -1
the H tune seems to jump from one peak to the next
 difficult to estimate the tune shift, but we can write Qx<7e-4.

15. B2: effect on horizontal tune shift of moving IR7 collimators
IR7 in
out in
- Very large vertical tune signal… Coupling?
- Many peaks around the tune make it difficult to analyze the horizontal tune.
- During the MD, Wolfgang may have had a cleaner signal from the feedback pickups. To be checked.

16. B2: Moving injection protection collimators

17. B2: effect on horizontal tune shift of moving injection protection collimators (TDI+TCLIs)
Qx~0
Qy~
TDI Collimator gap
 Correlation between the collimator gap and the vertical tune shift
 The horizontal tune switches to another peak when collimators are in. To be investigated in more detail.
Tune shift due to injection protection collimators from B2 measurements: Qy~ and Qx~0
Coarse extrapolation from nominal model (only TDI): Qy~ and Qx~0

18. B2: Scraping the beam in IR3

19. B2: effect of scraping in IR3
Tune shift with intensity:
Scraping was performed with one collimator in IR3, resulting in a large bunch length decrease.
Accounting for this bunch length decrease and comparing with Sacherer tune shift from the nominal 450 GeV/c impedance model (collimators at nominal settings in the model instead of in the measurements): Qy Qx
Bunch length
Intensity
From p to p, tune shifts are less than 10-3 and look similar in H and V

20. B2: effect of scraping in IR3
Vertical tune shift (beam 2)
Horizontal tune shift (beam 2)
At Nb= p/b, nominal model predicts Qy~ (5.9) and Qx~ (6.3)
measurements Qy~ and Qx~
Warning!!! Preliminary results obtained with nominal collimator settings!!!
The model is being refined now to account for exact collimator positions during the MD.

21. B2: Overinjection

22. Overinjection tune shift for beam 2
Overinjection tune shift is Qx~ and Qy~
(bunch length of 1.1 ns for high intensity, but not well defined for the pilot bunch)

23. Conclusions and next steps
The predictions with the impedance model from ZBASE and the measurements on 28th May 2010 seem reasonably consistent.
More detailed simulations with Headtail should be performed.
We need to work on getting a cleaner tune measurement
The injection protection collimators may have a slightly larger impedance than expected, and this has to be investigated in more details.
The ZBASE impedance model should improved (e.g. adding new kickers and other suspected sources of impedance).

24. Thank you for your attention!

25. B1: Moving IR7 collimators

26. B1: effect on tune shift of moving IR7 collimators
Moving IR7 collimators from 5 sigma to 15 sigma leads to a tune increase of:
1 or 2 e-4 in horizontal plane (the tune jump to -1/+1 sideband shadows the graph... to be filtered)
3e-4 in vertical plane (first guess)

27. B1: Moving injection protection collimators

28. B1: Moving injection protection collimators
Moving TDI collimators from 5 sigma to 15 sigma leads to a tune increase of:
2e-4 in vertical plane (first guess)
? in horizontal plane (the tune jump to +1 sideband shadows the graph... to be filtered)

29. B1: scraping in IR3

30. Scraping beam 1 from 0.91e11 p to 1e10p lead to a tune shift of 8e-4 in vertical plane
horizontal plane is too jumpy to tell. Deeper analysis is needed. (Tune shift range between 3e-4 and 10 e-4)

31. B1: overinjection

32. Overinjection tune shift for beam 1
Overinjection tune shift is Qx~ and Qy~
(bunch length of 1.1 ns for high intensity, but not well defined for the pilot bunch)