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107th Impedance Working Group (FCC absorbers, MKP-S, LHC BWS)

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774/1-017 - BE CONFERENCE ROOM (CERN)

774/1-017 - BE CONFERENCE ROOM

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Minutes of 107th Impedance Working Group Meeting

Present: Chiara Antuono, Miguel Díaz Zúmel, Dora Gibellieri, Patrick Krkotic, Elena Macchia, Nicolas Mounet, Michela Neroni, Benoît Salvant, Harry Sullivan, Christine Völlinger, Carlo Zannini.                                                                                                                                                                                           

 Minutes taken by Miguel and Chiara.

FCC-ee Synchrotron Radiation Absorbers (Patrick)

Goal: start conversations about simulation, studies and design of absorbers

Some photons from synchrotron radiation penetrate the copper chamber, so absorbers are needed to stop those.

Absorber has a particular geometry, with tilt and taper-transition, with impact on impedance.

 

There were discussions about how close to the beam the absorbers should be. If they are too close, the RF current on the copper chamber, that induced by the beam, is too high.

The hybrid solution is now the baseline, where only the 2 first absorbers are 4mm closer to the beam.

 

MKPS extraction kicker

Carlo presented a summary of the MKPS kicker at the SPS, showing the beneficial impact of serigraphy.

For the MKEs he shows that the serigraphy can work up to a certain frequency, can induced a lambda-quarter resonances which is close to a beam line. Carlo shows that if the serigraphy is shorter one can shift the mode inducing a factor 2 reduction in heating observed in 2017 SPS scrubbing. After the improvement of the extraction kicker, the focus was on the injection kicker (MKP-L) which also show observation of heating. From the model we expect a factor 3 higher in beam induced power wrt the MKPS.

From 2021 was clear that it was not possible to run in that scenario given the ferrite temperature.

The solution with longitudinal serigraphy exists with 4,5 and 6 stripes. The impedance was found to be consistent also with bench measurements.

However, a trade-off between impedance mitigation and design contrains (kicker field quality, aperture limitations and ceramic robustness). The strategy was to keep the narrow impedance far enough from spectrum line, since stability is not an issue.

A final model has been found in simulations and compares well with bench measurmenets.

After installation in the SPS the MKPL is below the MKPs heating, which is exactly the opposite wrt the modifications, the new bottleneck is the MKPs, which was faced experimentally since also new beam parameters have been tested.

 

The MKPs started to be a limitations during the 2023 scrubbing run, which had to be modulated due to the MKPs heating.

Would the MKP-S limit the HL-LHC operation?

Which are the options to mitigate ?

Beam offset, serigraphy and active cooling.

 

Carlo shows the temperature behaviour (interlock at 60 deg.) shwing that already in MD2 prep and LIU MD the MKP-S was a limitation. MD was stopped after 4h to avoid excessive heating, therefore for the high intensity studies it is.

 

Carlo shows prediction done for HL-LHC

He considers also a 1h of LHC filling every 8h to have the correct the baseline temperature.

The baseline temperature is extremely higher, showing high probability to reach high temperature for the HL-LHC era and therefore be a limitation.

 

Carlo shows that without serigraphy the MKE would have been already a limitation in 2025 and it was essential to identify mitigations and therefore it is crucial to implement also in the MKP-s.

 

Concerning the mitigation strategy:

 

-offsetting beam, actually offsetting the kicker: it was investigated in MD with a beam bump and the predictions about heating and temperature reduction were confirmed. However, due to aperture constrains and offset constrains can only be done at flatbottom and it is not a viable option at the moment.

 

-Serigraphy: if you put already the MKP-L serigraphy the improvement is clear and heating would go down to the current MKP-L level However for aperture constrains is not feasible.

 

-Active cooling is not compatible for LS3 and might work for LS4. To be further discussed.

 

Discussion

Patrick asks about the extrapolation of temperature. What is the first flat part why it is lower (at the beginning) . Carlo says that is the beam commissioning so you do not have the trains, that is why is very low.

 

Christine asks about the body of the serigraphy and if it was the reason to have a ceramic body. CArlo says yes, indeed for the MKP-s the cells are very short and if you put the serigraphy directly on ferrite it would not have a strong effect because is it not enough, therefore you need a ceramic body. She adds that in that case you have aperture limitations nd indeed Carlo confirms.

Nicolas asks if the same temperature model can be done on the MKI-cool in the LHC, since we observed it to become already a limitation during MDs at the LHC and Carlo answers yes.

 

AOB

Michela summarizes what was done for the HL-LHC wire scanners and show the ECR comment proposal for the installation of the prototype in the LHC during the YETS in Beam1. All the comments and results were already discussed in previous IWG meetings.

Carlo asks when the measurement are planned and Henry answer that it will be probably mid Nov, however Christine asks after CAs, 1st week for Dec, but they device has to go to vacuum testing before installations so it has to be checked.

 

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