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Faulty Connections in the BPM and their effect on the Beam

This summer student project explores the effect of faulty terminations on Beam Position Monitor (BPM) connections in particle accelerators. The study involves building and measuring model BPMs, comparing different terminations, and investigating the implications. Detailed discussions on accurate model creation, measurements setup, S-parameters analysis, and Wakefield solver usage are covered. The challenges faced during measurements, solutions like SMA calibration, and comparison of results in frequency and time domains are also addressed. The study highlights discrepancies between simulations and measurements, leading to valuable insights for improving BPM stability in accelerator systems.

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Faulty Connections in the BPM and their effect on the Beam

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  1. Faulty Connections in the BPM and their effect on the Beam Summer Student Project 3.6. – 23.8.2019 Otto Ellonen Supervisors: Benoit Salvant & Carlo Zannini Special thanks to Joel Daricou & Christian Boccard

  2. Table of Contents • Introduction to project • Building of the Model and Measurements of the DUT • Comparisons and shorting of a BPM • What next?

  3. Introduction – background for thisproject • In the BE – Newsletterissue 24 in december 2018 it wasintroducedthatsomeinstabilites in the PSB extractionkickerwerecausedbyunmatched termination [1] • LEIR hadthesamekind of a problemwhichwasmade bythefaultyterminations of striplinepickups [2] • Furtherstudy of faultyterminationswassuggested

  4. Introduction - Main Features • Producing an accurate model of the test bench BPM • Measurements regarding the test bench BPM • Comparison between the model and the measurements • What effects do different terminations have? • Further investigation (Wakefield solver, debug etc.)

  5. The Most Accurate Model with parameterization • Made from scratch (again) • Followed the technical drawings much more carefully so the geometry would be almost identical to DUT • Fully parameterized • Nearly the same results with the measurements

  6. Most Accurate Model S-Parameters • Compared to SMA-Measurements this model gives satisfactory results when all ports are matched • Can be modified for the purpose of studying shorting/open of the BPM

  7. Most Accurate Model

  8. Measurements setup and complications

  9. General Information • The first measurements were done with the 4-Port VNA (Vector Network Analyzer) by connecting cables to all ports of the DUT • Electronic Calibration kit was used to calibrate the cables • The focus was on the scattering parameters s11 and s21

  10. Setup

  11. VNA Measurement results • With the VNA, S-parameters of the DUT were measured and then compared with the CST Model • The first measurements gave good approximations, some problems were involved

  12. Problems involving the measurements • In order to connect the cables to DUT the adapters had to be removed • This caused (small) errors due to adapters being attached during the calibration • The calibration could not be done without the adapters with the kit

  13. Solution: SMA-calibration/measurements • In order to calibrate the cables without the adapters, SMA measurements were done • The SMA measurement results differ a bit from the original measurements and have more or less the same pattern as the simulations in the Most Accurate Model

  14. Solution: SMA-calibration/measurements

  15. Problems involving the measurements • Another problem is that the inner cavity is not 100% firmly in place • If the DUT is moved or shaken the cavity might move a bit which results in an asymmetric device  The CST Model does not match in that case

  16. Shorting of the BPM and comparison

  17. CST Model – Frequency Domain • The shorting of one of the ports in frequency domain is easily done by using the Schematic tool

  18. CST Model – Time Domain • In Time Domain we can not use the schematic (gives very different results) • The surrounding PEC-background with the electric boundary condition will short it! • There is also an option to use the lumped elements

  19. Comparison between Frequency Domain and Time Domain

  20. Comparison between Frequency Domain and Time Domain • The schematic, modelled and lumped elements version of the short produce the same kind of ‘spike’ at the same frequency • Problem: Sometimes there is no spike in the simulations • Carlo suggested that because of that particular behavior and the very sharp spike, it might be a numerical error in CST  in reality there might not be a spike at that frequency

  21. Comparisons and results • In this section the most important comparisons between the model and measurements up to date will be introduced. • This includes at the moment the following comparisons and results: • S-parameters when all ports are matched • S-parameters when one port (BPM) is shorted • Wakefield results

  22. Matched case between the Simulation and Measurements

  23. Matched case between the Simulation and Measurements

  24. Shorted Case between the Simulation and Measurement • With the PEC short there is no spike in the lower frequency range but the SMA has one at about 0.36 GHz • Taking into account the previous results the waveforms begin to match each other at 0.6 GHz, earlier than that they are not identical

  25. Wakefield solver – Cross-section of the model used

  26. Wakefield Solver – Technical difficulties • The hexahedral mesh problem: staircase modes • The region between the button and the frame is filled with a very thin layer of air (0,5mm) • Because of the curvature form of the BPM, some of the mesh cells in the boundary were considered PEC (Staircase)  it shorted the BPM with the frame. • Solution: Air gap was made wider, no changes to the output of the model

  27. Wakefield Solver – ResultsWake Impedance Comparison Beam propagation along the x-axis! Wake Length=10000

  28. Wakefield Solver – ResultsWake Impedance Comparison

  29. Wakefield Solver – ResultsWake Potential Comparison • Matched Case Shorted Case 

  30. What next? • Figuring out the problematic parts e.g. why do the simulation and measurements differ in the frequency range of 0 – 0.7 GHz - Problem in the Model? - Problem in the Measurements/DUT? • Analyzing Wakefield results • Wire model and comparison between Wakefield results • Playing with schematics in the wire model

  31. THANK YOU FOR YOUR ATTENTION AND FOR THE PAST 11 WEEKS!

  32. References • [1] ”Identification of instability sources in PSB and LEIR” http://cds.cern.ch/record/2651040/files/BE-newsletter_2018_edition24.pdf • [2] ”Identification of the source of the LEIR vertical instability” https://indico.cern.ch/event/773528/contributions/3213859/attachments/1757462/2849952/ABP_info_meting_22112018_NB.pdf • Otherpapersworththeread: • https://indico.cern.ch/event/773228/contributions/3219667/attachments/1754421/2843873/PSB_EKFandHInstability.pdf • https://indico.cern.ch/event/826121/contributions/3456252/attachments/1866003/3068409/KOUKOVINI_ABPinfmeeting_20062019.pdf

  33. Backup slides -Simulation of thicker air gap in frequency domain • In order to check how much does the thicker air gap influence the final results we simulate the same model as in wakefield solver in frequency domain and compare the results to the measurements • This gives an image on how much does the behavior change  not that much

  34. Backup slides - 3D-CST Simulation Model • Model made from scratch with the help of technical drawings and the actual DUT

  35. Backup slides - Simplified Model • Coarse model just with the basic geometry • No detailed materials • All ports are matched with 50Ω • Parts excluded: • Plexiglas rings • Proper BPM design • Proper Connector design

  36. Backup slides - Simplified Model S-Parameters • The S-parameters, especially the gave promising results • The frequencies of the transmissions are almost identical • The on the other hand was not looking good

  37. Backup slides - Modified Model • More detailed and accurate structure • Different materials chosen to match the materials of the DUT • Added missing parts

  38. Backup slides - Modified Model – Details

  39. Backup slides - Modified Model S-Parameters • Much better results than the simplified model • Waveform is almost identical to the measured ones • Problem: the waveform is now shifted approx. 50-100 MHz to the right • Changing different things in the model for example the width of the connector had no major impact on the shift so another model was produced

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