Kevin Li

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.

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.

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.

Numerical Methods I & II

Numerical methods is a fundamental field of research and application in accelerator physics and beam dynamics. It is widely used to study intensity effects and limitations in accelerators and becomes more important still in regimes where the simple analytical models break down and experiments can not be easily conducted. In this course we will introduce some basic concepts of numerical methods used to study collective effects in circular accelerators. The course covers macroparticle models, the implementation of the beam dynamics and applications relevant to intensity effects and limitation. This includes basic tracking in the transverse and longitudinal planes, modeling of impedance effects and finally two stream effects such as electron clouds.

Effect of the LHC Beam Screen Baffle on the Electron Cloud Buildup

Electron Cloud (EC) has been identified as one of the major intensity-limiting factors in the CERN Large Hadron Collider (LHC). Due to the EC, an additional heat load is deposited on the perforated LHC beam screen, for which only a limited cooling capacity is available. In order to preserve the superconducting state of the magnets, pumping slots shields were added on the outer side of the beam screens. In the framework of the design of the beam screens of the new HL-LHC triplets, the impact of these shields on the multipacting process was studied with macroparticle simulations. For this purpose multiple new features had to be introduced in the PyECLOUD code. This contribution will describe the implemented simulation model and summarize the outcome of this study.

Beam Dynamics Observations of the 2015 High Intensity Scrubbing Runs at the Cern Sps

Beam quality degradation caused by electron cloud effects has been identified as one of the main performance limitations for high intensity LHC beams with 25 ns bunch spacing in the SPS. In view of the beam parameters targeted with the LHC injectors upgrade (LIU) project, about two weeks of SPS machine time in 2015 were devoted to dedicated scrubbing runs with high intensity LHC 25 ns and dedicated ’doublet’ beams in order to study the achievable reduction of e-cloud effects and quantify the consequent beam performance improvements. This paper describes the main observations concerning the coherent instabilities and beam dynamics limitations encountered as well as a detailed characterisation of the performance reach with the highest beam intensity presently available from the pre-injectors.