2. Experience and Perspectives of Transverse Feedback Systems for Scrubbing Thanks to: SPS OP crew and colleagues from BE-ABP and BE-RF, for their support and regular fruitful discussions Many thanks to colleagues in BE-RF-FB and BE-RF-CS for LS1 upgrades and to US-LARP collaboration for High Bandwidth Feedback R&D LIU-SPS Scrubbing Review, 08 September 2015 https://indico.cern.ch/event/433608/ Prepared by W. Hofle, G. Kotzian

3. Short Reminder of upgrade of existing damper in LS1 in the framework of the LIU project Difficulties setting up damper with Q20 optics: needed to explore new Pick-up processing schemes (PU phase advance not 90 degrees) Move to dedicated pick-ups (BPCR couplers for LHC type beams), due to future incompatibility of sharing pick-ups with SPS orbit system; needed a long shutdown (cabling) Goal: Deploy system similar to LHC ADT, with built-in bunch-by-bunch observation capabilities and modern signal processing adapted to 25 ns bunch spacing; normalisation with intensity Facing also obsolescence of controls, including function generators Need to damp SPS doublet beam (needed for LHC scrubbing), incompatible with existing damper hardware (blocking issue); old system: 120 MHz down conversion; new: 200 MHz and 40 MHz Following SPS studies prior to LS1 with existing SPS damper: Scrub pLHC 40 MHz 80 MHz 120 MHz 160 MHz 200 MHz 500 turns injection 5 ns bunchlets Scrub pLHC 40 MHz 80 MHz 120 MHz 160 MHz 200 MHz 120 MHz component jumps in phase by 180 o phase rel. to 40 MHz comp.; shift of phase of 40 MHz due to asym. splitting not displayed

4. SPS Damper – System Overview Kickers: E-field only (2xH, 2xV)

5. Experience with damper during scrubbing: DOUBLET BEAM Damping achieved during splitting at injection and throughout the cycle Extensive hardware analysis carried out (simulations of hardware along with measurement set-up in a lab test stand); possible performance limiting factors identified (rectification during winter shut-down being considered): a) Analog frontend (AFE) operating a Variable Gain Amplifier (VGA); mismatch in signal levels with ADC full scale level: required patching gains in a way that soils the noise figure b) 200 MHz Bandpass filter in place, required for LHC type beam (200 MHz RF frontend); THRU-line bypass (used to process 40 MHz signal) shows perturbing artefacts at 2.4ns resp. 4.8 ns  deteriorates doublet response (5 ns spaced)

6. Experience with damper during scrubbing: HIGH INTENSITY 25 ns Operational changes in per-bunch-intensity require frontend gain settings to be modified  if not done impact on resolution Feedback phase adjusted with FIR filters using beam transfer function measurement with VNA; optimisation done for operational tunes (0.13 / 0.18); sensitivity to tune variation Changes in total intensity and bunch intensity result in tune shifts  tune variation has impact on feedback phase Need to fine trim tunes also existed with system prior to LS1 Reduction of damping by 30% for tune shift of 0.025 in new system Aim for a processing scheme less sensitive to tune variation:  vector sum of pick-ups to be tested  check feasibility of the installation of additional pick-ups at correct phase (requires four pick-ups) for fixed tunes  test FIR and IIR filters with less tune sensitivity for phase adjustment

7. Example of improved digital filter Phase compensation for sidebands (Q20 optics) Q20 optics: Synchrotron tune high  sidebands spaced by large amount  can simulations help to qualify the digital filters we can propose  what type of simulations are needed ?  do we have to cover the synchrotron sidebands with existing feedback tune side-bands z-plane: Stability: inside unit circle gain This filter was not yet tested

8. Example of pLHC Damper action (injection of batch) Logbook: SPS [Thursday 09-Oct-2014 Night] 00:27 HORIZONTAL VERTICAL DAMPER OFF DAMPER ON Viewed by BI instrument (AutoQ application)

9. Instability viewed at end of batch with damper PU ~70 turns activity in the last injected batch (NB: roll-off of damper gain @ ~4.5 MHz) phase compensated To 20 MHz) 1 st batch 2 st batch 3 rd batch 4 th batch H-plane

10. High Bandwidth Feedback System R&D Aimed at vertical plane single bunch intra-bunch motion by ecloud instability Joint US-LARP effort (J. Fox et al.); numerous MDs with single bunches 2 strip-line kickers in place (GHz bandwidth); slotted kicker under development Signal processing @4 GS/s is being prepared for damping on bunch trains Analog Front End Analog Back End Signal Processing BPMKicker Power Amp ADC DAC Beam transverse position pre-processed sampled position “slices” calculated correction data correction signal pre-distortiondrive signal

11. Slotted Kicker action on beam Beam trajectory Horizontal deflection, 1800 mm downstream, to a 50 ns long rectangular kicker pulse Leading edge of the deflected beam response response faster than 5 ns kicker can target individual bunches @ 5 ns spacing ! value beyond high bandwidth feedback 1 ns Simulation: M. Wendt

12. Areas to improve existing feedbacks & upgrades  Tune range acceptance −Vector sum −Digital filters −New PU locations  Diagnostics (using built-in damper diagnostics and BI instruments) −Disentangle what is higher order mode instability inside the bunch and cannot be damped by the existing feedback from what can be damped  Optimise frequency response of existing system in range up to 20 MHz  Existing damper kicker system has its limitation −Roll-off of kick strength at 4.5 MHz −not optimal for single isolated bunches going unstable (V-plane) −not a lot of power available from the power system at 20 MHz (H-plane)  If more kick strength (+at higher frequencies) needed in H-plane or V-plane −New kickers for existing transverse feedback system −Launch adapted version of High Bandwidth Damper (H- or V-plane)

13. THANK YOU FOR YOUR ATTENTION!