Effect of kicker impedance on beam - longitudinal

1. Effect of kicker impedance on beam - longitudinal E. Shaposhnikova, LIU-SPS Review, 20.03.2013 In collaboration with T. Argyropoulos, T. Bohl, J.E. Muller, H. Timko + help in beam measurements from OP shifts, PS and PSB experts Acknowledgements: impedance calculations of C. Zannini

2. Outline • Present MKE kickers • Effect of serigraphy • Beam measurements • Open MKE kickers • Beam out • Beam in • Impedance • Beam energy dependence • Time scale

3. 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

4. SPS impedance budget(in single bunch simulations) • 200 MHz TW RF system: • Fundamental: Rsh=4.5 MΩ, Q=140, R/Q=32 kΩ • HOM: fr=629 MHz, Rsh=600 kΩ, Q=500, R/Q=1.2 kΩ • 800 MHz RF system: Q=300, R/Q=65 kΩ • 19 kickers (shielded & unshielded) + ZS impedance • Space charge: ImZ/n = - 1.0 Ω • “1.4 GHz impedance” Rsh > 30 kΩ

5. Synchrotron frequency shift • 1999-2001: SPS impedance reduction - factor 2.5 decrease in slope in agreement with reduction of ImZ/n from 12 to 5Ω • 2003-2006: impedance increase due to re-installation of 8 MKE – main contribution to longitudinal broad-band impedance budget • 2007-2013: impedance decrease • due to serigraphy of MKEs => seen in MDs in 2013

6. MKE kicker shielding Longitudinal Re[Z] Printed strips in MKE-L10 Imaginary part of Z Interdigital comb structure 20mm spacing surface discharge Transverse Re[Zh] F. Caspers, T. Kroyer, M. Barnes , E. Gaxiola et al.

7. Synchrotron frequency shift ? Difficult measurements: 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

8. Total SPS reactive impedance(can be measured via synchrotron frequency shift) Reactive impedance of all kickers in the SPS (without ZS), calculated by B. Salvant (using Tsutsui formula) → significant increase due to MKE installation Now we expect to be close to the 2001 situation 2013

9. Synchrotron frequency shift: preliminary results for 2013 The slope is smaller than in 2008 (-2 < b < -1) T. Argyropoulos et al

10. Measured and calculated slope |b| as a function of bunch length: -0.2Ω SC January 2013 space charge term is larger for bright bunches (-1 Ohm for round beam pipe and b/a=5) → similar results for 2001 and 2013 as expected!

11. Total reactive impedance Total ImZ/n for 2008 Total ImZ/n for 2012 MKE contribution above 50 MHz is small, but we need to verify the difference 2008  2013 due to kicker models

12. Contribution of present MKEs to impedance of all kickers C. Zannini MKEs (0.7Ω ): ~ 1/5 of ImZ/n=3.6Ω in range (50 – 500) MHz

13. Coupled bunch instability and loss of Landau damping Threshold impedances for nominal LHC intensity Minimum threshold on the flat top 8 MKEs with serigraphy: resonant peak at 48 MHz with Rsh=25 kΩ

14. Frequency dependence of threshold

15. Open MKELongitudinal impedance for beam out Case1: S2=0 cm Case2: S2=2 cm Case3: S2=3.8 cm S2 C. Zannini ~ 5 times more for 5 open MKEs, but still very small

16. Open MKE “beam in” Comparison with present MKE without serigraphy C. Zannini ~ 3 smaller ImZ/n for open MKE

17. Open MKE “beam in” Comparison with present MKE with serigraphy C. Zannini Open MKE: ~4 times higher ImZ/n ~5 times higher ReZ

18. Total longitudinal MKE impedance C. Zannini Open MKEs: ~3 times higher ImZ/n ~3 times higher ReZ

19. Longitudinal multi-bunch instabilitySingle 200 MHz RF system 50 ns beam, 1.6x1011 p/b 25 ns beam, 1.2x1011p/b T. Argyropoulos • Instability starts at energy ~ 1/Ntot => multi-bunch effect • But single batch with 2-3x1010 p/b is still unstable on flat top (450 GeV/c)

20. Open MKE kickers • No problem for “beam out” (much less than present MKEs with serigraphy) • “Beam in” – significant impedance, higher than now • “Beam in” is only 100 ms – is it long or short? • Minimum thresholds on flat top • Q20, 7 MV, 450 GeV: Ts=3.3 ms, dfs/fs=0.2 (2RF) => instability growth time > 1 ms should be damped due to synchrotron frequency spread (simple Sacherer’ criterion) • Beam quality is measured by the SPS BQM 50 ms before beam extraction => interlock

21. Double RF system and Q20 50 ns, 1.9x1011 p/b 25 ns, 1.2x1011 T. Argyropoulos => Still controlled emittance blow-up needed for stability

22. Summary Present kickers: • The positive effect of serigraphy on the present MKE kickers can be clearly seen from beam measurements. Contribution to the impedance budget is much smaller (~ 1/5 of ImZ/n of all kickers for f> 50 MHz and 1/2 below).However possible measures to reduce peak at 48 MHz should be studied New kickers: • Relative contribution of open MKE with “beam out” is very small in the present SPS impedance model • Effect of open MKE with “beam in” is more important than now • If beam is unstable – it will be extracted to the LHC? • We need to have a very accurate SPS impedance model to predict the effect of the open MKE impedance – ongoing benchmarking of impedance calculations with particle simulations and beam measurements