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

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

Abort Gap Cleaning using the Transverse Feedback System: Simulation and Measurements in the SPS for the LHC Beam Dump System

The critical and delicate process of dumping the beams of the LHC requires very low particle densities within the $3 \mu$s of the dump kicker rising edge. High beam population in this so-called 'abort gap' might cause magnet quenches or even damage. Constant refilling due to diffusion processes is expected which will be counter-acted by an active abort gap cleaning system employing the transverse feedback kickers. In order to assess the feasibility and performance of such an abort gap cleaning system, simulations and measurements with beam in the SPS have been performed. Here we report on the results of these studies

Chromaticity Measurements via RF Phase Modulation and Continuous Tune Tracking

Chromaticity diagnostics with high time resolution is of paramount importance for the control of the dynamic events in various accelerators, in particular for the LHC collider. This paper describes the possibility of measuring the machine chromaticity via RF phase modulation and continuous tune tracking. The RF phase modulation can be done at much higher frequencies than a classical RF frequency variation and thus, allows chromaticity measurements with a time resolution below the second. The paper describes the general measurement principle and discusses in detail open questions, which still have to be addressed experimentally. First results from machine measurements in the CERN SPS on beam stability during RF phase modulation are presented

Tune Measurements in the SPS as a Multicycling Machine

Throughout the operation cycle of the SPS different particles are accelerated : High intensity protons, leptons and heavy ions. For each particle type a measurement of the betatron tunes along the acceleration cycle is required. The paper describes the different excitation and data analysis methods used in order to minimize beam blow-up during the measurements (protons) or in order to optimize the time resolution (leptons). Measurement examples are given