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Minutes of #15 E-Beam Meeting (draft)

The meeting was held on Zoom on 20/06/2022 - See indico

Introduction (Adriana Rossi)

Despite the HEL project is presently frozen, CERN will continue all the R&D program foreseen at CERN, which includes the design, prototyping and tests of:

  • gun and collector
  • instrumentation (BPM and BGC gas monitor, the latter with Liverpool and UK-CI)
  • gun pulse generator

At a later stage, we might also look at the control system.

Electron Beam Tests Stand: experimental results and e-beam simulations (Sameed Muhammed)

The conclusions from the present results gathered by Sameed et al. at the CERN HEL test stand are:

  1. The test stand is in good track. At the moment there is a gun, e-beam diagnostics (Faraday Cup and YAG screen), isolation valve and collector (not cooled), with the gun negatively biased, and the rest of the system at ground. In particular:

    • the gun has been debugged
    • we had troubles with the Marx Generator that was supposed to provide a 200ns rise + 1us min - 100um max pulse, still being resolved
    • the commissioning of the magnets is completed
    • we are in the process of installing the interlock system (water cooling, HV, magnet temperature, etc.)
    • the automation of the instrumentation is being implemented
    • measurements with both YAG screen (size of the image to be calibrated) and FC could be performed manually, at the moment we might need to bake to reduce the FC outgassing.

  2. We could measure the total current (up to ~5 A at the collector - see logbook) but:

    • we seem to have a slightly higher current than measured by Giulio. This could be due to a different cathode temperature?
    • and the I(V) curve does not follow Child's law, so it could be that we have unaccounted losses.

    The measurements will therefore be repeated.

  3. We could play with the magnetic field (see images at the YAG screen on logbook) for the moment only at low current (max 0.1A, instead of 5A) because the Marx Generator used to produce the beam pulse has a lower limit on the pulse duration of 1 us, which, for nominal current, would produce a too powerful beam that would risk to damage the YAG screen. Main observations:

    • we can compress the size by about a factor of 2
    • we observed a movement of the e-beam as we were compressing, indicating a mis-alignment of the gun with the B solenoid field. we will try and see how to adjust the magnet alignment (we know we are no better than 1 or 2 mm at the moment) and check if the gun is not hanging. we also have H/V correctors in case of need, in each solenoid.

Simulation for the HEL collector design were also presented, showing that we need to shorten the collector 'neck' to be able to squeeze the beam through, and that the iron shielding needs to be reduced in length. Plus we looked at electrostatic simulation and notice that we might have some spots where the field is too high. Work is still ongoing for simulations with the complete HEL collector geometry.

Future plans:

  • complete interlock installation and commissioning and data acquisition (+automatic scan) from the FC
  • install (summer/fall) the HEL BPM prototype, plus a BTV screen at the position of the BGC to be able to image the beam at the location where the BGC will be tested. Proceed to tests
  • replace the BTV with the BGC and test.
  • in parallel build the collector prototype to be install and tested next year, and I do not know when yet the pulse generator (based on Behlke switches, by SY-ABT).