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    New LHC BGI dipole magnets - kick-off discussion with TE-MSC & BE-ABP

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    Present: Massimo Giovannozzi (BE-ABP), Philip Schwarz (TE-MSC), Hampus Sandberg (BE-BI), James Storey (BE-BI) 

    New LHC BGI dipole magnets (James)

    LHC BGI instruments operated during Run1 & 2

    • Magnetic field provided by a doublet of 0.2T dipole magnets. 
    • Performance below E=4TeV okay, but above this energy B-field strength is insufficient to compensate for the   effect of bunch space charge on trajectory of ionisation electrons to the detection plane and consequently the measured beam profile is distorted.
    • Technical problems:
      • Beam induced heating damaged the in-vacuum optical components;
        • Instruments were removed in EYETS 16/17.
      • Ageing of the MCP / phosphor and intensified cameras;
      • Not an independent measurement, since optical spread must be determined by cross-calibration to the wire scanner.

    Recent BGI developments

    • Development of LIU-PS BGI based on the detection of ionisation electrons directly inside the beam-pipe vacuum with a Timepix3 hybrid pixel detector (HPD).
      • Removes need for MCP / phosphor / intensified camera.
      • Independent measurement, since no optical point spread
      • Aperture of vacuum tank - and hence magnet - much reduced since less space needed for HPD c.f. MCP & associated optical components.
    • Development of Virtual-IPM by the IPMSim collaboration, which facilitates accurate simulations of BGI instruments.

    New non-invasive beam profile monitor for (HL-)-LHC

    • Context - recommendation of LHC beam size review to produce a TDR for a new LHC BGI, in time for a final decision on a future non-invasive beam profile monitor at the end of 2021.
    • New LHC BGI will be based on the LIU-PS BGI design.
    • Virtual-IPM simulations suggest that beam profiles can be measured accurately at 7TeV if the B-field strength is increased from 0.2T to 0.6T.
    • Tentative requirement for magnet:
      • Aperture = 118mm minimum;
      • In a good field region (14mm (along beam axis) x 14mm (measurement axis) x 40mm (drift axis):
        • B-field = 0.6 T;
        • Uniform to 1 part in 10^3.

    Discussion 

    General questions from Massimo :

    • Will the instrument provide bunch-by-bunch measurements?
      • Yes.
    • Will it work for HL-LHC?
      • Virtual-IPM simulations performed for HL-LHC beam parameters; need for 0.6T dipole magnetic field for accurate beam profile measurements.
    • Will a final decision be made between BGI and BGV?
      • LHC beam size review recommended to produce TDR's for both the BGI and BGV instruments, in time for a final decision to be made in 2021.

    Location of the BGI instruments

    • Massimo raises the point that we should look into optimising the location of the BGI instruments to maximise optical beta, since this would also help to minimise the affect of the bunch space charge. This could help reduce the B-field strength required for accurate beam profile measurements at flat-top and could thus reduce the cost of the magnets. Philip comments to keep in mind the space available for the magnets.  Action: Massimo will contact Riccardo to investigate ( Post meeting update - Riccardo will have a look for an optimised location by the end of June 2020 ).  
    • Massimo points out that a beam pipe aperture of 60mm - which is assumed In the tentative magnet specification - is already less than the nominal 80mm beam pipe aperture. James discussed this with Riccardo last year, who indicated a minimum beam aperture of 50mm should be possible - to be confirmed by Riccardo in the context of location optimisation.

    LHC aspects of the magnet specification

    • Massimo comments that it should be possible to use a doublet magnet configuration, since compared to the PS there are more orbit correctors & the beam is more rigid. Doublet configuration should be considered if the complexity / cost is less than a triplet configuration. Philip comments that since the triplet configuration is preferable - as it has the minimal affect on the beam -  that he will start design work this configuration.
    • Massimo comments that the field quality should be similar to other warm magnets in the LHC.  Action: Philip will check the field quality specification of other LHC warm magnets.
    • Massimo makes the point that until the location is optimised - which could affect the required B-field and minimum aperture - it would be better not to develop the details of any design too much.
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