Beam measurements of the SPS longitudinal impedance s

Beam measurements of the SPS longitudinal impedance s Injector MD days – 23 & 24/03/2017 A. Lasheen Acknowledgments: T. Bohl, S. Hancock, E. Shaposhnikova SPS, PS and PSB OP teams, MD coordinators, SPS Impedance team

Introduction 72 bunches 2

Quadrupole frequency measurements 3

Quadrupole frequency shift § Measurements were usually done in the past by scanning the intensity keeping the same average bunch length (not straightforward in practice…) § The shift with intensity (slope b) depends on the effective reactive impedance § The innovation to the method in 2016 consisted in scanning a broad range in bunch length and intensity, instead of scanning only the intensity § The data is organised in categories of bunch length, and fitted linearly 4

Machine configurations PSB C 02 RF voltage Adjusting longitudinal emittance C 16 RF voltage Adjusting intensity PS Settings for the PS radial pick-ups depending on the bunch intensity § PSB (LHCINDIV clone): set the bunch parameters (intensity and longitudinal emittance) from the RF program § PS (LHCINDIV clone): adjust PAX. PLIPROG to cope with the broad range of bunch intensities § SPS (flat bottom MD cycle, MD 1 -MD 2): adjust the injection and reference phase to minimize dipole oscillations 5

Measured SPS equivalent impedance Q 20 optics Analytical evaluation Measurements in Q 26 optics § The dependence of the effective impedance with bunch length is the signature of the frequency dependence of the machine impedance § The measured pattern of the bunch length dependence of the effective impedance is the same as expected in theory § Obtained with: Obtained in measurements for different categories of bunch lengths 6

Comparison of measurements with simulations Q 20 optics Q 26 optics § Simulations were done with BLon. D and the present SPS impedance model, using the machine parameters and bunch profiles from measurements Ø Reasonable agreement, in pattern and amplitude Ø Deviations may indicate some missing impedance source 7

Adding a single resonator impedance 8

MD conditions and experience § 5 days requested for MDs (>5 days assigned) § 2 effective days of measurements with ideal conditions § Q 26 data (13/06/2016) Ø 300 cycles acquired Ø 7. 5 hours Ø No major interruption § Q 20 data (05/07/2016) Ø 720 cycles acquired Ø 12. 75 hours Ø No major interruption § The main limitation is the lack of automation for the acquisition (MATLAB script on the computer in the BA 3 Faraday cage, required to click on the acquire button for each cycle) § The adjustments of the machine parameters along the injector chain were made very simple thanks to the help of T. Bohl, S. Hancock and the OP crew 9

Conclusions and plans for 2017 § The results of this MD in comparison with particle simulations shows that the present SPS impedance model is a reasonable representation of the reality (for the reactive part) § The longitudinal space charge impedance was shown to be an important contribution at flat bottom, a further study could consist in measuring the synchrotron frequency shift to evaluate in measurements the longitudinal space charge effects § Longitudinal instability studies along the ramp for a single and multi bunch were performed in 2015 but could not be continued due to the beam dump limitation in 2016 § Longitudinal instability studies for single and multi-bunch cases will be continued in 2017 10