Towards a Transverse Feedback System and Damper for the SPS in the LHC Era

The SPS will serve as injector for the LHC, accelerating up to 4 x 10^13 protons per cycle from 26 GeV/c to 450 GeV/c. The transverse feedback system (damper) is essential for keeping the transverse emittance blowup within the limits fixed for the LHC injector chain. The fast filamentation requires rapid damping of any injection errors. Injection errors are the combined result of steering errors and ripples on the magnet power supplies in the transfer line as well as from the PS extraction kicker and the SPS injection kicker. Besides damping injection oscillations the damper will also provide transverse feedback to stabilise the beam against the resistive wall coupled bunch instability. The required bandwidth, kick strength and power bandwidth (rise time) were discussed during the 1996 Montreux "Workshop on High Brightness Beams for Large Hadron Colliders" in the working group on "Active Emittance Control". In the present report the requirements for the damper are summarised and the development of a system to meet these specifications, based on the existing hardware, is described

Transverse feeback systems in the LHC and its injector: projected performance and upgrade paths

Transverse feedback systems are essential to preserve the small transverse emittances throughout the injector chain and in LHC itself. The striving for higher brilliance beams will put increased demands on the transverse feedback systems in the future. Possible upgrades of the LHC damper will address the low noise performance that is essential for operation in coast, while for the injectors a new generation of sophisticated digital electronics will replace the analogue signal processing as it is still employed in the PS booster today. A particular challenge for the smaller accelerators is the large frequency swing not present in the LHC

Digital Signal Processing for the Multi-Bunch LHC Transverse Feedback System

For the LHC a VME card has been developed that contains all functionalities for transverse damping, diagnostics and controlled bunch by bunch excitation. It receives the normalized bunch by bunch position from two pick-ups via Gigabit Serial Links (SERDES). A Stratix II FPGA is responsible for resynchronising the two data streams to the bunch-synchronous clock domain (40.08 MHz) and then applying all the digital signal processing: In addition to the classic functionalities (gain balance, rejection of closed orbit, pick-up combinations, one-turn delay) it contains 3- turn Hilbert filters for phase adjustment with a single pickup scheme, a phase equalizer to correct for the non-linear phase response of the power amplifier and an interpolator to double the processing frequency followed by a low-pass filter to precisely control the bandwidth. Using two clock domains in the FPGA the phase of the feedback loop can be adjusted with a resolution of 10 ps. Built-in diagnostic memory (observation and post-mortem) and excitation memory for setting-up are also included. The card receives functions to continuously adjust its parameters as required during injection, ramping and physics

Emittance Growth at LHC Injection from SPS and LHC Kicker Ripple

Fast pulsed kicker magnets are used to extract beams from the SPS and inject them into the LHC. The kickers exhibit time-varying structure in the pulse shape which translates into small offsets with respect to the closed orbit at LHC injection. The LHC damper systems will be used to damp out the resulting betatron oscillations, to keep the growth in the transverse emittance within specification. This paper describes the results of the measurements of the kicker ripple for the two systems, both in the laboratory and with beam, and presents the simulated performance of the transverse damper in terms of beam emittance growth. The implications for LHC operation are discussed

Transverse Behaviour of the LHC Proton Beam in the SPS: an Update

During the 1999 SPS run, strong transverse instabilities were observed with the LHC beam [1]. Both the instability characteristics and the identical threshold current as for beam-induced electron multipacting led to consider the interaction of the beam with the electron cloud as a likely source. In 2000, we have measured the dependence of beam motion, beam loss, and emittance growth on bunch intensity, number of bunches, octupole strength, chromaticity, and gaps in the bunch train. We report on these recent studies and compare the beam observations with simulations of electron cloud build up and electron-induced single-bunch instabilities

Initial Results of Simulation of a Damping System for Electron Cloud-Driven Instabilities in the CERN SPS

Single and multi-bunch instabilities on bunch trains driven by electron clouds have been observed in the CERN SPS for some years. In this paper, we present initial results to implement a damping system in a computer simulation of a single bunch vertical instability using the HEADTAIL code. The code simulates the interaction between a proton bunch and a uniform electron cloud that has built up inside of the beam pipe. In all simulations we use typical SPS parameter sets for three different values of the beam momentum : 26 GeV/c, 55 GeV/c and 120 GeV/c. The feedback is implemented as a corrective kick calculated from the vertical centroid of each slice of the electron bunch with a one turn delay. The bandwidth of the feedback is varied by filtering the slice information along the bunch. Initial results indicate that the instability can be damped with a minimum bandwidth of 300 MHz with a relatively high gain

Controlled Transverse Emittance Blow-up in the CERN SPS

For several years, a large variety of beams have been prepared in the LHC injectors, such as single-bunch and multi-bunch beams, with 25 ns, 50 ns and 75 ns bunch spacings, nominal and intermediate intensities per bunch. As compared to the nominal LHC beam (i.e. with nominal bunch intensity and 25 ns spacing) the other beams can be produced with lower transverse emittances. Beams of low transverse emittances are of interest during the commissioning phase for aperture considerations and because of the reduced long-range beam-beam effects. On the other hand machine protection considerations might lead to prefer nominal transverse emittances for safe machine operations. The purpose of this paper is to present the results of controlled transverse emittance blow-ups using the transverse feedback and octupoles. The procedures tested in the SPS in 2008 allow to tune the transverse emittances up to nominal values at SPS extraction