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105th Impedance Working Group meeting (dedicated to SY-BI)
Present: Rama Calaga, Miguel Díaz Zúmel, Jonathan Emery, Wolfgang Hofle, Ivan Karpov, Patrick Krkotic, Michal Krupa, Nicolas Mounet, Michela Neroni, Chiara Pasquino, Hermann Pommerenke, James Storey, Harry Sullivan, Carlo Zannini.
Excused: Christine, Sergio
Impedance studies on HL-BPMs (Michela Neroni, Chiara Antuono)
Michela explains that four of these components will be installed, per beam. She simulated assuming the matching condition on the ports.
In the original design, Michela observed a narrowband contribution at around 2.6GHz, that seems to be a trapped mode between the tapers.
She proposes an alternative solution, consisting of a straight tube, removing the intermediate tapers. In the alternative solution, the impedance peak is reduced by a factor 9.
The new model is also preferred for longitudinal stability.
Michela concludes that the new model shows a much-reduced impedance. This new model would increase the Im(Z)/n of the LHC by 0.06%, with respect to the present total impedance.
Discussion
Michal confirms that the new model is now the baseline.
Rama asks if the length of the noise feedback striplines of 120 mm is fixed. Wolfgang mentions that modifying the length may result in an asymmetric spectrum, hence complicating the measurements. Rama and Wolfgang will follow up with Michal about the striplines length.
Carlo explains that Chiara A. simulated the transverse impedance and confirmed that the impact of the device on transverse stability is marginal.
SPS BGI (Carlo on behalf of Jintao)
Carlo explains that Jintao did some simulations of the SPS BGI. Last year, a leakage field during AWAKE beams caused perturbations in the measurements. Problems were encountered as well in the MD of 25/3/2025. A possible solution is to densify the metal grid that shields the detectors. Jintao computed the field leakage as a function of the grid spacing.
A successful shielding solution, installed in June TS, is so far working correctly.
Power loss map in HL-LHC Wire Scanner (Harry Sullivan)
Harry explains that, during the design of the WS for HL-LHC, several modifications were applied to lower the beam impedance.
He has processed the 3D power dissipation data to do thermal simulations in ANSYS, and documented this workflow.
Most of the dissipated power is located in the absorbers.
He had done steady state simulations with ANSYS, to compute the temperatures at each location. The steady state simulation should represent the worst case scenario (they assume continuous heating).
Conclusions:
· The heating of the device seems acceptable
· Temperature measurements will be done during operation (in the wire and in the outside of the chamber)
Discussion
Jonathan points out that they are still pending of which measurements devices should be implemented (temperature probes, antenna). He reminds that the device is subjected to a bake-out. Regarding the antenna, he would need to know which device would be connected to it and how. Harry and Jonathan mention that a coupler could protrude into the chamber; however, this would require operating the scanner with a reduced stroke. This would not be ideal but it could be a compromise if deemed a priority to include the coupler.
Carlo points out that it would be already interesting to have the prototype installed during 2026 run, even without the antenna. Before LS3, a high intensity MD will be done, which would give precious information about the behavior of the component.
Carlo reminds of the necessity of doing bench tests (i.e.: impedance) to fully validate the device. Rama also suggests that during the impedance measurements, a feedthrough could be installed and tested.
Minutes taken by Michela and Miguel
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