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.
Highlights
The LHC was designed with a very efficient and robust 3-stage collimation system [1,2,3] put in place to protect the accelerator against regular and accidental beam losses
In this work we have summarized the resistivity measurements performed on Mo-coated samples of different graphitic materials foreseen or being considered for the impedance reduction program of the High-Luminosity Large Hadron Collider (HL-LHC) collimators
We have characterized both bulk and coated materials with different techniques: the Direct Current (DC) four-probes method, the low-frequency Eddy Current Testing (ECT), and RF resistivity measurements with a cylindrical resonator working in the H011 mode
Summary
The LHC was designed with a very efficient and robust 3-stage collimation system [1,2,3] put in place to protect the accelerator against regular and accidental beam losses. In the LHC, the beam stability is currently ensured by the octupole magnets and by the transverse feedback The former induces a spread in betatron frequency proportional to particle amplitude to provide Landau damping to all impedance-driven unstable modes [12], while the latter provides active bunch-by-bunch correction kicks that damps unstable low-frequency coherent coupled-bunch oscillations. Given the collimation contribution to the total machine impedance, the main mitigation measure is to reduce the collimator impedance by lowering the resistivity of the jaws with alternative substrate materials and coatings. For this reason, we extensively investigate the possible alternatives and characterize their electrical resistivity to meet the required beam stability target. We conclude with the present status and plans for coating fabrication for the ongoing LHC collimator replacement
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