Study of the Heat Load in the LHC

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The Large Hadron Collider

Study of the Heat Load due to the Electron Cloud in the LHC and in Higher-Luminosity LHC Extension

Electron Cloud with Inverted Beam Screens

We report the results of computer simulations studying the effect of wrongly oriented LHC beam screens on the local electron-cloud heat load and density. At 3.5 or 7-TeV energy and for maximum secondary emission-yield values below 1.5, with the inverted sawtooth orientation about ten times higher heat load is expected than for the standard orientation, and the wrongly oriented sawtooth chambers could lead to a local heat-load bottleneck during the process of surface conditioning at 25-ns bunch spacing. The available cooling margin can be significantly increased by correcting the sawtooth orientations at least for two dipole magnets in LHC arc cells 26 and 32 R3, in order that there be no half-cell cooling loop containing more than one inverted screen

Electron Cloud with Inverted Beam Screens

We report the results of computer simulations studying the effect of wrongly oriented LHC beam screens on the local electron-cloud heat load and density. At 3.5 or 7-TeV energy and for maximum secondary emission-yield values below 1.5, with the inverted sawtooth orientation about ten times higher heat load is expected than for the standard orientation, and the wrongly oriented sawtooth chambers could lead to a local heatload bottleneck during the process of surface conditioning at 25-ns bunch spacing. The available cooling margin can be significantly increased by correcting the sawtooth orientations at least for two dipole magnets in LHC arc cells 26 and 32 R3, in order that there be no half-cell cooling loop containing more than one inverted screen

Simulation of Electron-Cloud Build-Up for the Cold Arcs of the LHC and Comparison with Measured Data

The electron cloud generated by synchrotron radiation or residual gas ionization is a concern for LHC operation and performance. We report the results of simulations studies which examine the electron cloud build-up, at injection energy, 3.5 TeV for various operation parameters. In particular, we determine the value of the secondary emission yield corresponding to the multipacting threshold, and investigate the electron density, and heat as a function of bunch intensity for dipoles and field-free regions. We also include a comparison between simulations results and measured heat-load data from the LHC scrubbing runs in 2011

Synchrotron-Radiation Photon Distributions for Highest Energy Circular Colliders

At high energies, beam-induced synchrotron radiation is an important source of heating, beam-related vacuum pressure increase, and primary photoelectrons, which can give rise to an electron cloud. The photon distribution along the beam pipe wall is a key input to codes such as ECLOUD and PyECLOUD, which model the electron cloud build-up. For future high-energy colliders, like TLEP or SHE-LHC, photon stops and antechambers are considered in order to facilitate cooling and vacuum pressure control. We use the Synrad3D code developed at Cornell to simulate the photon distribution for the LHC

First electron-cloud studies at the Large Hadron Collider

During the beam commissioning of the Large Hadron Collider (LHC) [LHC Design Report No. CERN-2004-003-V-1, 2004 [http://cds.cern.ch/record/782076?ln=en]; O. Brüning, H. Burkhardt, and S. Myers, Prog. Part. Nucl. Phys. 67, 705 (2012)PPNPDB0146-641010.1016/j.ppnp.2012.03.001] with 150, 75, 50, and 25-ns bunch spacing, important electron-cloud effects, like pressure rise, cryogenic heat load, beam instabilities, or emittance growth, were observed. Methods have been developed to infer different key beam-pipe surface parameters by benchmarking simulations and pressure rise as well as heat-load observations. These methods allow us to monitor the scrubbing process, i.e., the reduction of the secondary emission yield as a function of time, in order to decide on the most appropriate strategies for machine operation. To better understand the influence of electron clouds on the beam dynamics, simulations have been carried out to examine both the coherent and the incoherent effects on the beam. In this paper we present the methodology and first results for the scrubbing monitoring process at the LHC. We also review simulated instability thresholds and tune footprints for beams of different emittance, interacting with an electron cloud in field-free or dipole regions

Study of longitudinal mechanisms and correlations in the definition of RFQ transverse acceptance

The Radio Frequency Quadrupole (RFQ) is a linear accelerator that focuses, bunches, and accelerates a continuous input of charged particles while preserving the beam emittance. This paper focuses on the study of the transverse acceptance of an RFQ and how this concept can be used in the design of frontend structures. A simple and fast system to qualify a source and low energy transfer line has been developed in terms of the number of particles delivered in the RFQ acceptance. Multi-particle simulation results show a dependence of the RFQ transverse acceptance on the particle phase in the radio frequency period. The usually referred-to acceptance value is, in fact, just an average value over the 360° phase range, whereas a modulation has been found between more and less favorable phase values, with different patterns depending on the specific structure. We use as a study case three RFQs designed and operated at CERN to investigate such correlations