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 Prepared by W. Hofle, G. Kotzian

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

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

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)

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 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

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

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)

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

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 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

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 5 ns spacing ! value beyond high bandwidth feedback 1 ns Simulation: M. Wendt

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)

THANK YOU FOR YOUR ATTENTION!