Design and Operation of a 3-Crystal Multi-Volume Reflection Array for Loss Reduction in CERN SPS Slow Extraction
Abstract
The flux of protons slow-extracted from the CERN Super Proton Synchrotron (SPS) to the North Area experiments is limited by the induced radioactivity of the beam lost on the electrostatic septum during the third-integer resonant extraction. Crystal shadowing, in which a thin bent silicon crystal deflects the portion of beam that would otherwise impinge on the septum wires, has been in operation at the SPS since 2021, and a single crystal at a non-local position halved the extraction losses, short of the fourfold reduction that the activation budget of the future high-intensity operation calls for. In this paper, the design, deployment and operation of a 3-crystal Multi-Volume Reflection Array (MVRA) at the non-local position in the fourth long straight section (LSS4) are presented. The array geometry was designed by Multi-Fidelity Bayesian Optimization (MFBO), combining a multilayer-perceptron surrogate with full particle tracking, which predicts a fourfold loss reduction with three crystals and up to tenfold with an ideally aligned array of four to five. Measurements with beam confirm the prediction: at its optimum the as-built array reduces the losses by the predicted factor of four, and beam dynamics simulations matched to the measured position and angle scans reproduce them to a few per cent. The array has since been used in physics production, held on its operating plateau by an extremum-seeking controller, with a median loss reduction by a factor of 3.6 over 0.1 million extraction cycles relative to the pre-shadowing baseline. The remaining margin to the target, the limitations of the present installation and the upgrade to a 4- or 5-crystal array are discussed.
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BibTeX
@misc{velotti2026design,
title = {{Design and Operation of a 3-Crystal Multi-Volume Reflection Array for Loss Reduction in CERN SPS Slow Extraction}},
author = {F. M. Velotti and M. Bay and T. Calvet and M. Calviani and G. Daher and L. S. Esposito and M. A. Fraser and B. Goddard and M. Howes and V. Kain and E. Matheson and A. Perillo Marcone and R. Seidenbinder},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.23823},
eprint = {2609.23823},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.23823},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Design and Operation of a 3-Crystal Multi-Volume Reflection Array for Loss Reduction in CERN SPS Slow Extraction
AU - F. M. Velotti
AU - M. Bay
AU - T. Calvet
AU - M. Calviani
AU - G. Daher
AU - L. S. Esposito
AU - M. A. Fraser
AU - B. Goddard
AU - M. Howes
AU - V. Kain
AU - E. Matheson
AU - A. Perillo Marcone
AU - R. Seidenbinder
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.23823
UR - https://arxiv.org/abs/2609.23823
AB - The flux of protons slow-extracted from the CERN Super Proton Synchrotron (SPS) to the North Area experiments is limited by the induced radioactivity of the beam lost on the electrostatic septum during the third-integer resonant extraction. Crystal shadowing, in which a thin bent silicon crystal deflects the portion of beam that would otherwise impinge on the septum wires, has been in operation at the SPS since 2021, and a single crystal at a non-local position halved the extraction losses, short of the fourfold reduction that the activation budget of the future high-intensity operation calls for. In this paper, the design, deployment and operation of a 3-crystal Multi-Volume Reflection Array (MVRA) at the non-local position in the fourth long straight section (LSS4) are presented. The array geometry was designed by Multi-Fidelity Bayesian Optimization (MFBO), combining a multilayer-perceptron surrogate with full particle tracking, which predicts a fourfold loss reduction with three crystals and up to tenfold with an ideally aligned array of four to five. Measurements with beam confirm the prediction: at its optimum the as-built array reduces the losses by the predicted factor of four, and beam dynamics simulations matched to the measured position and angle scans reproduce them to a few per cent. The array has since been used in physics production, held on its operating plateau by an extremum-seeking controller, with a median loss reduction by a factor of 3.6 over 0.1 million extraction cycles relative to the pre-shadowing baseline. The remaining margin to the target, the limitations of the present installation and the upgrade to a 4- or 5-crystal array are discussed.
ER -