1. Observing Transverse Modes shifting and Coupling in the SPS : MDs in the SPS
G. Arduini, R. Calaga, E. Metral, G. Papotti, G. Rumolo, B. Salvant, R. Tomas
PhD on improving the model of the SPS impedance.
Acknowledments: T. Bohl, E. Chapochnikova, R. Jones, R. Steinhagen, OP team
LIS meeting - Oct

2. Agenda
Context
MD settings
MD results
What we do next?

3. Context
Headtail Simulations and MOSES calculations predict a transverse mode coupling instability for the SPS beam at low longitudinal emittance
Objectives:
Can we see modes shifting in the machine?
TMCI threshold for 2007?

4. Agenda
Context
MD settings
MD results
What we do next?

5. Issues for the measurements
Need of high number of turns before decoherence
 correct non linear chromaticity and amplitude detuning with octupoles (also reduces Landau damping…)
Need of both position and momentum signals
 find 2 pickups in the same region (ideally, phase=90° between them)

6. MD settings Parallel MD on August 2nd Quick MD on August 27th
Beam current [  ] p/b
Low chromaticity in the vertical plane (-0.2-0.25), high chromaticity in the horizontal plane
RF Voltage very high at injection (3 MV instead of 700 kV in 2006), as in transverse impedance MD.
Observation of the beam behaviour mainly with SPS Qmeter and Faraday Cage Transverse BPM after kicking in the vertical plane, about 900 ms after injection (both in section 3)
As a consequence, no bunch length measurement…
Quick MD on August 27th
Beam current [  ] p/b
Chromaticity at 0.05 (H)/0.05 (V)
find the threshold instability

7. Agenda Context MD settings MD results What we do next?
August 2nd, 2007
August 27th, 2007
What we do next?

8. MD on August, 2nd 2007 Conclusions Measurements
Very long decoherence time (more than 4000 turns)
…However…
Large longitudinal oscillations due to longitudinal bunch mismatch
TMCI threshold measurement should be performed right at injection

9. Aug 2nd : Comparison with July 27 MD
SPS Transverse Impedance measurement on July 27, 2007
TMCI MD on August 2, 2007
Slope =
H. Burkhardt et al (APC 17/08/2007)
Slope consistent with transverse impedance measurement on July 27, 2007

10. Aug 2nd : Comparison with simulations
Headtail simulations (Z=10 MOhm/m)
Measurements (Zmeasured=20MOhm/m) x y
Round Chamber x y
Flat Chamber

11. Agenda Context MD settings MD results What we do next?
August 2nd, 2007
August 27th, 2007
What we do next?

12. MD on August, 27th 2007 BCT BPM V BPM H
Vertical BPM signal affected by saturation?

13. MD on August, 27th 2007 (6/3)
Measurements in 2007 (Sussix frequency analysis BPM H)

14. MD on August, 27th 2007
Measurements in 2007 (horizontal)

15. MD on August, 27th 2007 Faraday cage sum signal
Headtail monitor sum signal
Ralph confirmed that there is a 1.35 GHz reflexion present on the headtail monitor signal

16. Agenda
Context
Issues for the measurement
MD results
What to do next?

17. What to do next? Conditions were not ideal in both MDs in August
In case of another MD:
Possibility to use both BBQ pickups in the same plane (270° apart)  Rhodri and Ralph
Check vertical Qmeter pickup
Make sure that the bunch is matched to the bucket (bunch length measurements in the Faraday Cage)
First find the TMCI threshold
Then observe the effect of correcting non linear chromaticity on the transverse modes