Beam-induced surface modifications as a critical source of heat loads in the Large Hadron Collider

Valentine Petit,Giovanni Iadarola,Paolo Chiggiato,Mauro Taborelli,Danilo Andrea Zanin,Marcel Himmerlich,Holger Neupert

doi:10.1038/s42005-021-00698-x

Valentine Petit, Giovanni Iadarola + Show 5 more

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https://doi.org/10.1038/s42005-021-00698-x

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Abstract

Beam-induced heat loads on the cryogenic regions of the Large Hadron Collider (LHC) exhibit a wide and unexpected dispersion along the accelerator, with potential impact on the performance of its High-Luminosity upgrade. Studies related the heat load source to the avalanche multiplication of electrons at the surface of the beam vacuum chamber, a phenomenon known as electron could build-up. Here, we demonstrate that the topmost copper surface of beam pipes extracted from a low heat load region of the LHC consists of native Cu2O, while the pipe surface from a high heat load region had been oxidized to CuO during LHC operation and maintenance cycles. Experiments show that this process increases the secondary electron yield and inhibits efficient surface conditioning, thus enhancing the electron cloud intensity during LHC operation. This study relates the abnormal LHC heat loads to beam-induced surface modifications of its beam pipes, enabling the development of curative solutions to overcome this critical limitation.

Highlights

Beam-induced heat loads on the cryogenic regions of the Large Hadron Collider (LHC) exhibit a wide and unexpected dispersion along the accelerator, with potential impact on the performance of its High-Luminosity upgrade
Electron clouds developing in modern particle accelerators[1,2] and resulting from the exponential multiplication of electrons in the vacuum beam pipe[3] have been identified as a critical source of beam instabilities[4,5], pressure rises[6,7], and heat load on the cryogenic system[8,9] of the Large Hadron Collider (LHC) at CERN
After a warm-up of the concerned LHC arcs from cryogenic to room temperature and venting of the beam pipes to atmospheric pressure with a high-purity mixture of nitrogen-oxygen (80%20%), the two beam lines were cut at the dipole extremities and the open ends were immediately closed with Viton®-sealed caps

Summary

Introduction

Beam-induced heat loads on the cryogenic regions of the Large Hadron Collider (LHC) exhibit a wide and unexpected dispersion along the accelerator, with potential impact on the performance of its High-Luminosity upgrade. We demonstrate that the topmost copper surface of beam pipes extracted from a low heat load region of the LHC consists of native Cu2O, while the pipe surface from a high heat load region had been oxidized to CuO during LHC operation and maintenance cycles Experiments show that this process increases the secondary electron yield and inhibits efficient surface conditioning, enhancing the electron cloud intensity during LHC operation. The observed heat load scattering corresponds well to the one obtained from electron cloud build-up simulations, assuming different SEY in the different parts of the LHC ring, as confirmed by independent measurements with different beam configurations[23,27] Under this hypothesis, the heat load spread reflects a strong dispersion of electron cloud intensity along the ring, possibly due to a modification of the surface properties of some beam pipes and in particular of their SEY. This work aims to assess this hypothesis, as a first step in solving this critical issue

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