Longitudinal impedance of the SPS E. Shaposhnikova, LIU-SPS BD WG, 22.03.2012 Most of measurements done with T. Bohl, T. Linnecar, J. Tuckmantel + help from OP shifts, PS and PSB experts 6/9/2018

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

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

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

Synchrotron frequency shift 1999-2001: 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) 2003-2006: impedance increase due to re-installation of 8 MKE – main contribution to longitudinal broad-band impedance budget 6/9/2018

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

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

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

SPS kicker impedances E. Metral, talk at SPSU SG 21.08.2007 Theory: Tsutsui, CERN-SL-2000-004 J. Uythoven, 2006 6/9/2018

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

Synchrotron frequency shift: reactive impedance 6/9/2018

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-2004-047 space ch. 0.2 Ohm 6/9/2018

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

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

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

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

MKE kicker shielding T. Kroyer, F. Caspers, E. Gaxiola, AB-Note-2007-028 , Longitudinal and Transverse Wire Measurements for the Evaluation of Impedance Reduction Measures on the MKE Extraction Kickers 6/9/2018

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 2001 - bunch lengthening due to potential well distortion (Im Z/n) 2007 – no microwave instability and bunch lengthening is similar to 2001 (why?) 1999 ○ - 2007 2001 T. Bohl et al., AB-Note-2008-022 6/9/2018

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

Pumping port shielding 6/9/2018

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

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

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

Narrow band impedance (?) – multi-bunch instability Threshold: single batch with 2x1010/bunch (2006, 3x1010 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

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