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 third-integer resonant extraction. Crystal shadowing, in which a thin bent silicon crystal deflects the protons that would otherwise hit the septum wires, has been in operation at the SPS since 2021; a single crystal at a non-local position halved the extraction losses relative to the local crystal alone, short of the fourfold reduction that future high-intensity operation requires. 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; it predicts a fourfold loss reduction with three crystals and up to tenfold with an ideally aligned array of four to five. At its optimum the as-built array reaches, and at its best point exceeds, the predicted factor of four, and beam dynamics simulations matched to the measured position and angle scans reproduce their shape to within 0.04 of the crystals-retracted loss. In physics production an extremum-seeking controller has held the array on its operating plateau, with a median loss reduction by a factor of 3.6 over 1.14 x 10^5 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.

arXiv comment23 pages, 14 figures, 8 tables