Wolfgang Höfle

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world׳s first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

Testing beam-induced quench levels of LHC superconducting magnets

In the years 2009-2013 the Large Hadron Collider (LHC) has been operated with the top beam energies of 3.5 TeV and 4 TeV per proton (from 2012) instead of the nominal 7 TeV. The currents in the superconducting magnets were reduced accordingly. To date only seventeen beam-induced quenches have occurred; eight of them during specially designed quench tests, the others during injection. There has not been a single beam- induced quench during normal collider operation with stored beam. The conditions, however, are expected to become much more challenging after the long LHC shutdown. The magnets will be operating at near nominal currents, and in the presence of high energy and high intensity beams with a stored energy of up to 362 MJ per beam. In this paper we summarize our efforts to understand the quench levels of LHC superconducting magnets. We describe beam-loss events and dedicated experiments with beam, as well as the simulation methods used to reproduce the observable signals. The simulated energy deposition in the coils is compared to the quench levels predicted by electro-thermal models, thus allowing to validate and improve the models which are used to set beam-dump thresholds on beam-loss monitors for Run 2.

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. 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.

Nominal LHC beam instability observations in the CERN Proton Synchrotron

The nominal LHC beam has been produced successfully in the CERN proton synchrotron since 2003. However, after having restarted the CERN PS in spring 2006, the LHC beam was set-up and observed to be unstable on the 26 GeV/c extraction flat top. An intensive measurement campaign was made to understand the instability and to trace its source. This paper presents the observations, possible explanations and the necessary measures to be taken in order to avoid this instability in the future.

Benchmarking headtail with electron cloud instabilities observed in the LHC

After a successful scrubbing run in the beginning of 2011, the LHC can be presently operated with high intensity proton beams with 50 ns bunch spacing. However, strong electron cloud effects were observed during machine studies with the nominal beam with 25 ns bunch spacing. In particular, fast transverse instabilities were observed when attempting to inject trains of 48 bunches into the LHC for the first time. An analysis of the turn-by-turn bunch-bybunch data from the transverse damper pick-ups during these injection studies is presented, showing a clear signature of the electron cloud effect. These experimental observations are reproduced using numerical simulations: the electron distribution before each bunch passage is generated with PyECLOUD and used as input for a set of HEADTAIL simulations. This paper describes the simulation method as well as the sensitivity of the results to the initial conditions for the electron build-up. The potential of this type of simulations and their clear limitations on the other hand are discussed.

Control of Intra-Bunch Vertical Motion in the SPS with GHz Bandwidth Feedback

A GHz bandwidth vertical beam feedback system has been in development at the CERN SPS for control of unstable vertical beam motion in single bunch and bunch train configurations. We present measurements and recent studies of stable and unstable motion for intensities up to 2 × 1011 p/bunch [1]. The system has been operated at 3.2 GS/s with 16 samples across a 5-ns RF bucket (4.2 ns 3σ bunch at injection) and experimental results confirm damping of intra-bunch instabilities in Q20, Q22 and Q26 optics configurations. Instabilities with growth rates of 1/200 turns are well-controlled from injection, consistent with the achievable gains for the 2 installed stripline kickers with 1 kW broadband power. Measurements from multiple studies in single-bunch and bunch train configurations show achieved damping rates, control of multiple intra-bunch modes, behavior of the system at injection (including interaction with the existing vertical damper) and final damped noise floor. The work is motivated by anticipated intensity increases from the LIU and HL-LHC upgrade programs [2], and has included the development of a new 1 GHz bandwidth slotline kicker structure and associated amplifier system. TRANSVERSE WIDEBAND INTRA-BUNCH FEEDBACK DEMONSTRATION SYSTEM A single-bunch wideband digital feedback system was initially commissioned at the CERN SPS in November 2012 [3]. Over time the system has has been extended to include control for trains of 64 bunches [4] and configured with two 500-MHz bandwidth stripline kickers, each powered with 500W of broadband RF power. Over time the system has been used to study single bunch and multi-bunch beams at the SPS in Q26, Q20 and most recently Q22 optics. The essential goals in al these studies is to try to quantify the behavior of the feedback in terms of damping intra-bunch motion from impedance mechanisms, TMCI, or Ecloud mechanisms. ∗ Work supported by the U.S. Department of Energy under contract # DE-AC02-76SF00515, the US LHC Accelerator Research (LARP) program, the CERN LHC Injector Upgrade Project (LIU) and the US-Japan Cooperative Program in High Energy Physics. Figure 1: Vertical motion signal showing 16 samples across the bunch for 20,000 turns. Positive feedback is applied from turns 3500 6500, there is no feedback applied after turn 6500. The bunch motion is excited and continues without being damped.

Recent Upgrades to the CERN SPS Wideband Intra-bunch Transverse Feedback Processor

BACKGROUND One possible limitation of high luminosity operation for the LHC is electron cloud induced instabilities (ECI) and transverse mode coupled bunch instabilities (TMCI) in the SPS [1]. Such instabilities increase with higher beam intensities and can ultimately limit luminosity in the LHC. Several schemes are being pursued [2] to mitigate this effect: changes to the SPS lattice, vacuum chamber coating and active feedback control. The last item has been the focus of our work. To develop a practical feedback control system, a research and development effort has been undertaken between CERN and SLAC. This effort is multifaceted and involves beam and system dynamics simulation and modelling, technology development and machine measurements. The technology development portion has produced a wide bandwidth 4GSa/s transverse feedback demonstration or “demo” system. Machine development studies involving single bunch beam dynamics have given encouraging results: control of low-order intrabunch (head-tail) mode instabilities has been shown [3]. Building on this, our efforts continue with further refinement of the models and development of advanced control algorithms; both driving improvements to the feedback demonstrator system. The overall demo system has already undergone multiple improvements [4]. Focusing on the feedback processor itself, we outline several upgrades already performed and others in progress or being planned. SYSTEM OVERVIEW The transverse feedback demo system is shown in Figure 1. Vertical motion of the beam is sensed by a stripline Beam Position Monitor (BPM) pickup. Preprocessing occurs in the Analog Front End, where an RF hybrid produces the displacement signal, from the four pickup signals. This signal is then amplified, filtered and equalized before being passed to the feedback processor.