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

Abstract

The High-Luminosity Large Hadron Collider (HL-LHC) project aims at extending the operability of the LHC by another decade and increasing by more than a factor of ten the integrated luminosity that the LHC will have collected by the end of Run 3. This will require doubling the beam intensity and reducing the transverse beam size compared to those of the LHC design. The higher beam brightness poses new challenges for machine safety, due to the large energy of 700 MJ stored in the beams, and for beam stability, mainly due to the collimator contribution to the total LHC beam coupling impedance. A rich research program was therefore started to identify suitable materials and collimator designs, not only fulfilling impedance reduction requirements but also granting adequate beam-cleaning and robustness against failures. The use of thin molybdenum coatings on a molybdenum–graphite substrate has been identified as the most promising solution to meet both collimation and impedance requirements, and it is now the baseline choice of the HL-LHC project. In this work we present the main results of the coating characterization, in particular addressing the impact of coating microstructure on the electrical resistivity with different techniques, from Direct Current (DC) to GHz frequency range.

Design and characterisation of frequency selective conductive materials for electromagnetic fields control

“…To carry out the measurements the Eddy current technique [37][38][39][40][41][42][43][44] has been used. The Eddy currents are induced in the foils by an excitation coil; the experimental set-up for this technique and the idealised geometry are indicated in Fig.…”

“…The plastic housing was oriented so that samples could be placed on-top of the coil, to avoid moving the apparatus between reference and sample measurements. Eddy currents are induced in the foils with a circular drive coil, and the impedance to these currents measured using equipment such as a VNA or LCR meter; this impedance has a predictable dependence on the conductivity of the sample 44 . Above some frequency, the skin effect confines these currents to a thin layer of the sample material [35][36][37] , and the measured impedances are consistent with that of an infinitely thick sample.…”

High intensity proton beam impact at 440 GeV/c on Mo and Cu coated CfC/graphite and SiC/SiC absorbers for beam intercepting devices

“…2 and 3. coated with with molybdenum, a promising material for impedance reduction with a higher melting point than the copper -as installed in the HL-LHC collimators [23].…”

“…Before applying the coating the substrates were cleaned in an ultrasonic bath and subsequently fired in order to remove impurities and hence to ensure a good adherence. In addition, the substrates were baked-out at 950 • 𝐶 under vacuum for a minimum of 2 hours with the aim of reducing out-gassing into the primary vacuum during the experiment [23].…”

“…Several techniques have been proposed in the literature -such as the Four-point method, Eddy Current Testing and Resonant Cavity Method [48][49][50]. The Resonant Cavity method has been found to be accurate and especially suitable for thin coating films on a substrate and was therefore employed in the present work [23].…”