Transverse beam stability with low-impedance collimators in the High-Luminosity Large Hadron Collider: Status and challenges

Abstract

The High-Luminosity LHC Project aims to increase the integrated luminosity that will be collected by the Large Hadron Collider for the needs of the high energy physics frontier by the end of its Run 3 by more than a factor ten. This will require doubling the beam intensity, and in order to ensure coherent stability until the brighter beams are put in collision, the transverse impedance of the machine has to be reduced. As the major portion of the ring impedance is generated by its collimation system, several low resistivity jaw materials have been considered to lower the collimator impedance and a special collimator has been built and installed in the machine to study their effect. In order to assess the performance of each material we performed a series of tune shift measurements with LHC beams. The results show a significant reduction of the resistive wall tune shift with novel materials, in good agreement with the impedance model and the bench impedance and resistivity measurements. The largest improvement is obtained with a molybdenum coating of a molybdenum-graphite jaw. This coating, applied to the most critical collimators, is estimated to lower the machine impedance by up to 30% and the stabilizing Landau octupole threshold by up to 240 A after accounting for uncertainties of the model and other destabilising effects. 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

Overview of material choices for HL-LHC collimators

“…Finally, collimator jaw operates in UHV, with a limited pumping speed. The outgassing of a collimator must be lower than 2•10-7 mbar•l/s [12]. Considering the contribution of the tank, the total outgassing of the absorber materials has to be lower than 5•10-8 mbar•l/s.…”

“…Considering the contribution of the tank, the total outgassing of the absorber materials has to be lower than 5•10-8 mbar•l/s. The residual gas test must comply with the specification [12].…”

Resistivity Characterization of Molybdenum-Coated Graphite-Based Substrates for High-Luminosity LHC Collimators

“…The estimates assume a single-beam stability threshold with a factor 2 safety margin to account for additional destabilizing effects ranging from the imperfections of the impedance model and the optics to hardware noise. More details on the estimates can be found in [24,26].…”

Energy loss reduction of a charge moving through an anisotropic plasma-like medium

“…Typically, the large portion of parasitic effects in accelerators and colliders is connected with its collimation system [1,2]. Therefore, new materials are considered for traditional collimator assembly [2] and alternative dielectric-based collimation systems are also discussed [3,4].…”

“…Typically, the large portion of parasitic effects in accelerators and colliders is connected with its collimation system [1,2]. Therefore, new materials are considered for traditional collimator assembly [2] and alternative dielectric-based collimation systems are also discussed [3,4]. Moreover, the successful usage of hollow electron lenses (low-energy hollow electron beams) for efficient halo removal of intense high-energy beams in storage rings and colliders should be particularly mentioned [5][6][7].…”