Transverse Beam Stability with Low-Impedance Collimators in the High Luminosity Large Hadron Collider: Status and Challenges

TL;DR

The paper discusses the development and testing of low-impedance collimators with novel materials, notably molybdenum coatings, to enhance transverse beam stability in the HL-LHC upgrade by reducing impedance and increasing stability thresholds.

Contribution

It introduces a new collimator design with molybdenum coatings that significantly reduces impedance and improves beam stability thresholds in the HL-LHC.

Findings

Molybdenum-Graphite jaws with 5 μm molybdenum coating reduce impedance by up to 30%.

Coating four key collimators increases the Landau octupole threshold by up to 120 A.

Implementation of low-impedance collimators is planned during the Long Shutdown 2.

Abstract

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will double its beam intensity for the needs of High Energy Physics frontier. In order to ensure coherent stability until the beams are put in collision, the transverse impedance has to be reduced. As the major portion of the ring impedance is supplied by its collimation system, several low resistivity jaw materials have been proposed to lower the collimator impedance and a special collimator has been built and installed in the machine to study their effect. The results show a significant reduction of the resistive wall tune shift with novel materials, in agreement with the impedance model and the bench impedance and resistivity measurements. The largest improvement is obtained with a 5 {\mu}m Molybdenum coating of a Molybdenum-Graphite jaw. This coating can lower the machine impedance by up to 30% and the stabilizing Landau octupole threshold by up to 120 A. The collimators to be upgraded have been chosen based on the improvement of the octupole threshold, as well as the tolerance to steady state losses and failure scenarios. A half of the overall improvement can be obtained by coating the jaws of a subset of 4 out of 11 collimators identified as the highest contributors to machine impedance. This subset of low-impedance collimators is being installed during the Long Shutdown 2 in 2019-2020.