1. Longitudinal impedance of the SPS
E. Shaposhnikova,
LIU-SPS BD WG,
Most of measurements done with T. Bohl, T. Linnecar, J. Tuckmantel + help from OP shifts, PS and PSB experts
6/9/2018

2. Longitudinal impedance
Broad-band → single bunch effects
Narrow-band → coupled bunch instabilities
(2Q/ωr vs bunch spacing)
Reactive part (broad-band and narrow-band) → loss of Landau damping, instability threshold
Resistive part → instability growth rate, element heating, outgassing
6/9/2018

3. SPS impedance budget Main known contributors (2012):
200 MHz TW RF system
800 MHz RF system
19 kickers (shielded & unshielded)
HOM at 629 MHz in 200 MHz RF system
6/9/2018

4. SPS longitudinal impedance from beam measurements
Single bunch:
synchrotron frequency shift with intensity → ImZeff
stable (synchronous) phase shift with intensity → ReZeff
bunch lengthening with intensity → ImZeff below instability threshold and Zeff above
spectrum of long bunches with RF off → resonant impedances with high R/Q and low Q (resonant frequency)
Multi-bunches
unstable beam spectrum → mode number (narrow-band impedance)
6/9/2018

5. Synchrotron frequency shift
: SPS impedance reduction in preparation for nominal LHC beam - factor 2.5
decrease in slope (in agreement
with reduction of ImZ/n from 12 to 5 Ohm)
: impedance increase due to re-installation of 8 MKE – main contribution to longitudinal broad-band impedance budget
6/9/2018

6. Synchrotron frequency shift?
Increase of the slope was measured in 2007 instead of expected reduction
due to the removal of one MKE kicker and shielding of another + 2cells
6/9/2018

7. Synchrotron frequency shift: |b| measurements in 2007-2008
E. S. et al., PAC’09
6/9/2018

8. Quadrupole frequency as a function of intensity and bunch length
6/9/2018

9. SPS kicker impedances E. Metral, talk at SPSU SG 21.08.2007
Theory: Tsutsui, CERN-SL
J. Uythoven, 2006
6/9/2018

10. Synchrotron frequency shift: reactive impedance
Reactive impedance of all kickers in the SPS (without ZS), B. Salvant
(Tsutsui formula),
→ Significant increase due to MKE impedance
6/9/2018

11. Synchrotron frequency shift: reactive impedance
6/9/2018

12. Measured and calculated slope |b| as a function of bunch length:
→ not much room for extra impedance, unless space charge term is larger (-1 Ohm for round beam pipe and b/a=5); for SC -
MD at 14 GeV/c, Note
space ch. 0.2 Ohm
6/9/2018

13. Synchrotron frequency shift: measurements and ESME simulations
Simulations with SPS impedance
Only reactive part for kickers - if not → dipole instability (damped by phase loop)
MD2, 2008
MD1, 2008
good agreement in general,
look for details (shape)
6/9/2018

14. Synchronous phase shift and energy loss
Raw data, 26 GeV/c, 2003
Energy loss /turn and particle
the loss factor for the Gaussian bunch
E. S. et al, EPAC’04
6/9/2018

15. Synchronous phase shift and energy loss
SPS resistive impedance budget
comparison with measurements
→ no room for extra impedances also (at least in 2003)
6/9/2018

16. MKE kicker shielding M. Barnes, talk at SPSU SG, 19.05.2009
Only 1 magnet is without serigraphy now (2012)
6/9/2018

17. MKE kicker shielding
T. Kroyer, F. Caspers, E. Gaxiola, AB-Note ,
Longitudinal and Transverse Wire Measurements for the Evaluation
of Impedance Reduction Measures on the MKE Extraction Kickers
6/9/2018

18. Bunch lengthening (at 600 ms, 900 kV, ε = 0.15 eVs, 26 GeV/c)
1999 – bunch lengthening due to microwave instability
1999/2000 – pumping port shielding: factor 7 decrease in slope
bunch lengthening due to potential well distortion (Im Z/n)
2007 – no microwave instability and bunch lengthening is similar to 2001 (why?)
1999 ○
2001
T. Bohl et al.,
AB-Note
6/9/2018

19. Bunch lengthening (at 600 ms, V=900 kV, 26 GeV/c)
2001
1999
1999
2001
2003
2006
2006
2006
2007
2007
2007
2007
→ due to initial bunch length measurements are not always directly comparable
2007
6/9/2018

20. Pumping port shielding
6/9/2018

21. Pumping ports: unstable bunch spectra
1999
2001 – after shielding
PU resonances
PRL 1997
6/9/2018

22. Unstable bunch spectra - 2007
Projection
Contour plot
N = 8x1010 ?
T. Bohl, 2007
=> Similar measurements with Q20?
6/9/2018

23. Unstable bunch spectra - 2001
N = 2x1010
N = 8 x1010
No 400 MHz peak after removal of MKE and MKP
Shielding of MSE and MST didn’t help
T. Bohl, 2001
6/9/2018

24. Narrow band impedance (?) – multi-bunch instability
Threshold: single batch with 2x1010/bunch (2006, 3x before) is unstable at the end of the ramp
Possible source: fundamental or HOMs of
200 MHz (629, 912 MHz) or 800 MHz (RF systems
Cures:
- FB, FF, longitudinal damper for 200 MHz
- 800 MHz RF system in bunch shortening mode through the cycle
- controlled longitudinal emittance blow-up (0.35 → 0.42 eVs → 0.63 eVs)
6/9/2018

25. Longitudinal impedance measurements in 2012
Predictions based on the longitudinal SPS impedance model were in the past in good agreement with different single bunch measurements (probing both ReZ and ImZ)
Do we have enough changes to see the difference in comparison with 2008 (other 5.5 MKEs have serigraphy)?
quadrupole frequency and synchronous phase shifts -> demanding longitudinal bunch parameters -> a lot of preparation in injectors
-> 2-4 sessions of 8 hours with single bunch of variable intensity
Unstable bunch spectra (long PS bunches – special settings) with Q20 and Q26 – for μw instability
Main intensity limitation comes from multi-bunch instability, the source is still unknown -> new ideas/measurements?
6/9/2018