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101st Impedance Working Group meeting (Roman pots)

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6/R-012 - conference room (CERN)

6/R-012 - conference room

CERN

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Carlo Zannini (CERN), Christine Vollinger (CERN)
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Present: Chiara Antuono, Joao Bento, Hikmet Bursali, Federico Carra, Elena de la Fuente García, Miguel Díaz Zúmel, Diogo F.G. Esteves, Giorgia Favia, Dora Gibellieri, Patrick Krkotic, Elena Macchia, Nicolas Mounet, Michela Neroni, Chiara Pasquino, Leonardo Sito, Wilhelmus Vollenberg, Christine Völlinger, Carlo Zannini.

Excused: Sergio Calatroni

ACTIONS to be followed-up for Roman Pots (CTPPS):

  • To check if cooling/contact with the flange of ferrite can be improved so to have more control of temperature (Leonardo Sito)
  • To check longitudinal stability (Ivan Karpov)
  • To check impact of 12 non-colliding bunches (transverse: Chiara Antuono, Nicolas Mounet)

CTPPS Roman Pot – Impedance Studies for HL (Leonardo Sito)

1. Introduction

  • CTPPS Roman Pots (PPS): detect forward protons near the beam at IP5 (CMS–TOTEM).

  • PPS operated successfully in Runs 2 and 3.

  • PPS2 = upgraded version for HL-LHC (Run 4–6).

  • HL-LHC requires new impedance studies due to higher beam intensity and shorter bunches.

2. Simulation models

  • Used simplified EM models (no flanges, metal-filled geometry, parametric ferrite housing).

  • Consistency checks with mesh convergence and solver comparisons confirmed reliability.

  • Studied models:

    • V4: no outer groove.

    • V5.3: large outer groove.

    • V6: narrow outer groove.

3. Longitudinal Impedance Studies

  • Measurements and simulations (single-wire and CST) were performed.

  • Results:

    • V5.3 and V6 have reduced impedance peaks compared to V4.

    • New modes appear below 1.4 GHz in grooved designs.

  • Power loss computations (HL beam, 1 mm pot-beam distance):

    • Total power deposition ≈ 148–160 W.

    • Most power dissipates in the ferrite ring (≈ 126–136 W).

4. Thermal Studies

  • Without cooling: ferrite temperature rises to ~424 °C, above the Curie temperature (~375 °C).

  • With cooling channels: reduced to ~365 °C, still close to critical limits.

  • Strong dependence on thermal contact conductance between ferrite and steel flange.

  • Comparison with Run 3:

    • Flange temperatures measured at ~55 °C.

    • Simulations indicate actual ferrite may exceed 100 °C → discrepancy highlights need for direct ferrite temperature monitoring.

  • Proposed solution:

    • Install in-vacuum fiber optic temperature sensors (radiation-hard, small size, already used in CMS).

5. Transverse Impedance Studies

  • Simulated for parking (40 mm) and operational positions (down to 1 mm).

  • Impact on beam stability is minor:

    • Increase of octupolar threshold is < 0.5% in x and < 0.8% in y.

  • Consideration of 12 non-colliding bunches during collision still under study.

6. Conclusions

  1. Multiple pot designs (V4, V5.3, V6) were studied for HL-LHC.

  2. Risk of ferrite overheating with 1 mm distance pot-beam → requires improved thermal contact, stronger cooling, and direct temperature monitoring.

  3. Transverse impedance not a major concern as impact on beam stability is minimal. More considerations on non-colliding bunches are under study.

  4. Further R&D needed on thermal management and monitoring systems for PPS2.

AoBs
Spare MKP-L Magnet Measurements (Miguel Diaz, Carlo Zannini, Giorgia Favia)

  • Magnet setup: One tank with four modules.

  • History: Low-impedance MKP-L with beam screen and finger conductors installed in Dec 2022.

  • Results:

    • Old MKP-L (2022) without alumina chamber showed broadband behaviour → large heating.

    • With alumina chamber (2025), broadband impedance reduced drastically, lowering heating.

    • However, serigraphy on alumina chamber introduces low-frequency resonances, though far from critical SPS harmonics (40 MHz).

    • Transverse measurements also show low-frequency resonances.

    • Very good agreement between installed MKP-L and spare magnet → spare is validated, no expected limitations.

      Discussion:

      • Christine asked about reducing resonances from serigraphy.

      • Carlo: Major effort already invested in reducing heating; stability with these resonances has been checked → no issue expected with LIU beam.

PSB RF Bypasses (Joao Bento)

  • Inspection (24 June):

    • Issue at 11L1.R2 bypass → open circuit (high impedance) → risk of sparking.

    • Temporary fix: bypass was short-circuited (no vacuum opening possible).

      Discussion:

      Markus: Opening vacuum for a fix in YETS25/26 may not be feasible (YETS will be kept very short).

LHC WS CST Model (Hikmet Bursali)

  • Hikmet asks about the coating layer of alumina absorbers with titanium.

  • Patrick suggests that titanium layer would be the preferred solution and no issue is expected.

  • Hikmet will update the model accordingly.

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