Giovanni Iadarola

Beam Instabilities in Hadron Synchrotrons

Beam instabilities cover a wide range of effects in particle accelerators and they have been the subjects of intense research for several decades. As the machines performance was pushed new mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. The aim of this paper is to review the main mechanisms, discussing in particular the recent developments of beam instability theories and simulations.

PyECLOUD and build-up simulations at CERN

PyECLOUD is a newly developed code for the simulation of the electron cloud (EC) build-up in particle accelerators. Almost entirely written in Python, it is mostly based on the physical models already used in the ECLOUD code but, thanks to the implementation of new optimized algorithms, it exhibits a significantly improved performance in accuracy, speed, reliability and flexibility. Such new features of PyECLOUD have been already broadly exploited to study EC observations in the Large Hadron Collider (LHC) and its injector chain as well as for the extrapolation to high luminosity upgrade scenarios.

PICs: what do we gain in beam performance

The beam parameters in the LHC resulting from the Performance Improvement Consolidation (PIC) activities presented in (1)(2) will be briefly recalled and motivated assuming that LINAC4 will be operational as PS-Booster Injector. The corresponding limitations in the LHC are outlined. Based on the above performance an estimate of the LHC yearly integrated luminosity will be provided. The evaluation of the need and extent of the performance and reliability improvement for some of the PIC items might imply additional information: the necessary machine studies and the specific operational experience required during Run 2 will be summarized.

Synchronous Phase Shift at LHC

The electron cloud in vacuum pipes of accelerators of positively charged particle beams causes a beam energy loss which could be estimated from the synchronous phase. Measurements done with beams of 75 ns, 50 ns, and 25 ns bunch spacing in the LHC for some fills in 2010 and 2011 show that the average energy loss depends on the total beam intensity in the ring. Later measurements during the scrubbing run with 50 ns beams show the reduction of the electron cloud due to scrubbing. Finally, measurements of the individual bunch phase give us information about the electron cloud build-up inside the batch and from batch to batch.

Electron cloud in the CERN accelerators (PS, SPS, LHC)

Several indicators have pointed to the presence of an Electron Cloud (EC) in some of the CERN accelerators, when operating with closely spaced bunched beams. In particular, spurious signals on the pick ups used for beam detection, pressure rise and beam instabilities were observed at the Proton Synchrotron (PS) during the last stage of preparation of the beams for the Large Hadron Collider (LHC), as well as at the Super Proton Synchrotron (SPS). Since the LHC has started operation in 2009, typical electron cloud phenomena have appeared also in this machine, when running with trains of closely packed bunches (i.e. with spacings below 150ns). Beside the above mentioned indicators, other typical signatures were seen in this machine (due to its operation mode and/or more refined detection possibilities), like heat load in the cold dipoles, bunch dependent emittance growth and degraded lifetime in store and bunch-by-bunch stable phase shift to compensate for the energy loss due to the electron cloud. An overview of the electron cloud status in the different CERN machines (PS, SPS, LHC) will be presented in this paper, with a special emphasis on the dangers for future operation with more intense beams and the necessary countermeasures to mitigate or suppress the effect.

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.

SIMULATION OF ELECTRON-CLOUD HEAT LOAD FOR THE COLD ARCS OF THE LARGE HADRON COLLIDER

The heat load due to the electron cloud in the Large Hadron Collider (LHC) cold arcs is a concern for its performance near and beyond nominal beam current. We report the results of simulation studies, which examine the electron-cloud induced heat load for different values of low-energy electron reflectivity and secondary emission yield at injection energy, as well as at beam energies of 4 TeV and 7 TeV, for two different bunch spacing: 25 ns and 50 ns. Benchmarking the simulations against heatload observations at different beam energies and bunch spacings allows an estimate of the secondary emission yield in the cold arcs of the LHC and of its evolution as a function of time.

High Luminosity LHC: Challenges and plans

The Large Hadron Collider (LHC) is one of the largest scientific instruments ever built. Since opening up a new energy frontier for exploration in 2010, it has gathered a global user community working in fundamental particle physics and the physics of hadronic matter at extreme temperature and density. To sustain and extend its discovery potential, the LHC will undergo a major upgrade in the 2020s. This will increase its rate of collisions by a factor of five beyond the original design value and the integrated luminosity by a factor ten. The new configuration, known as High Luminosity LHC (HL-LHC), will rely on a number of key innovations that push accelerator technology beyond its present limits. Among these are cutting-edge 11–12 T superconducting magnets, including Nb3Sn-based magnets never used in accelerators before, compact superconducting cavities for longitudinal beam rotation, new technology and physical processes for beam collimation. The dynamics of the HL-LHC beams will be also particularly challenging and this aspect is the main focus of this paper.

Dependence of E-Cloud on the Longitudinal Bunch Profile: Studies in the PS & Extension to the HL-LHC

Recent studies have shown that the prospects for significantly increasing bunch intensities in the LHC for the luminosity upgrade (HL-LHC) may be severely limited by the available cryogenic cooling capacity and the electron-cloud (EC) driven beam instability. However, it is planned that during the HL-LHC era the bunch intensities in the LHC will go up by nearly a factor of two compared to the LHC-design values. This motivates the exploration of additional EC mitigation techniques that can be adopted in addition to those already in place. Preliminary simulations indicated that long flat bunches can be beneficial over Gaussian bunches to reduce the EC build up. Rigorous studies using realistic bunch profiles have never been done. Therefore, we have undertaken an in-depth investigation in the CERN 26 GeV PS to see if we can validate the previous findings and, in particular, if flattening the bunch can mitigate the EC. Here we present the results from dedicated EC measurements in the PS using a variety of bunch shapes and a comparison with simulations. Finally, we investigate if reshaping the bunch profiles using a 2nd harmonic rf cavity can mitigate EC in the HL-LHC.

SPS Scrubbing Run in 2014

Yearly machine scrubbing has been applied in the SPS since 2002 in order to reduce the amount of electron cloud in the machine and permit smooth operation with 25 ns beams. While a quick scrubbing is usually necessary to recover performance after any extended technical stop due to in vacuum deconditioning, a longer period needs to be envisaged when the machine stop is long and a large fraction of the machine is exposed to air. Therefore, the restart of the SPS after LS1 will offer a unique opportunity to qualify the machine degradation due to a long stop as well as quantify length and efficiency of a scrubbing run to recover the previous performance and possibly extend it to higher intensity beams. This information will be the key input to decide on the upgrade strategy for the SPS, as it will show whether the SPS can be operated with scrubbing also for future intensities or electron cloud needs to be actively suppressed through a-C coating. Goals, requirements (in terms of beam and instrumentation) and a possible planning of the SPS scrubbing run in 2014 will be covered by this presentation. In this context, we will also describe the doublet beam, which can be potentially used for enhancing the scrubbing efficiency.