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Minutes of E-Beam Meeting #8 (draft)

The meeting was held at CERN (864/2-B14) on 15/01/2020 - See indico

Participants

J. Cenede, R. Corsini, A. Frassier, D. Gamba, L. V. Joergensen, S. Mazzoni, D. Mirarchi, V. Kain, A. Kolehmainen, A. Pikin, L. Ponce, S. Radaelli, A. Rossi, S. Sadovich, O. Sedlacek, L. Soby, G. Tranquille, R. Veness, M. Wendt.

Actions

  • G. Tranquille, A. Frassier: implement hardware needed for e^- beam intensity modulation in all e-coolers.
  • L. Soby: finalize implementation of acquisition chain for e- beam orbit in all e-coolers.
  • L. Soby: investigated on charging-up effect of e-cooler BPMs.
  • D. Gamba, L. Soby: verify if using the RF frequency as e^- intensity modulation is the best option.
  • c, L. Soby: investigate if a lock-in acquisition system for e-cooler BPMs could be useful.
  • L. Soby: provide an estimate of BPM resolution in the different systems.

Diagnostics of BNL e-lenses (A. Pikin)

Main design characteristics of e-lens device installed at BNL are:

  • 6 T superconducting main solenoid
  • symmetrical structure between the two injecting and extracting arms (given tight margins, not to miss the collector)
  • vacuum gate valves between gun (and collector) and beam transport arms
  • magnet on gun split in 2 to be able to change beam size without affecting beam dynamics
  • drift tube electrodes inside main solenoids to create an axial potential to evacuate residual-gas-interaction-generated ions
    • Alexander commented that this turned out to be fundamental only for DC beam operation
    • He also warned that those electrodes can charge up, giving undesired effects
    • Note that passing beam can induce AC potential =>> grounded with capacitance to leave DC potential
  • before the collector, a strong solenoidal field is used to squeeze the beam into the smaller aperture of the collector entrance
    • together with an electrostatic reflector in the middle of the collector, it also allows to better spread e^- beam on collector cylindrical side surface
    • smaller entrance aperture is beneficial to reduce vacuum conductance toward the accelerator
    • vacuum ion pump behind the collector with aperture to pump larger than entrance.

The beam profile can be measured just before the collector using:

  • an insertable YAG screen
    • made of 100 um thick glass with 100 nm coating
    • choice of coating material looking at the energy spectra of backscattered e^-
    • single shot acquisition, but only for ~10 \mus-long pulses, above which the screen is destroyed
    • experience with scintillator not positive due to limitation on power that can be deposited. Now instead of 1 scintillator there is a revolver with severals. Software gives distribution also on 3 lines, 3rd line not on x/y to see if beam uniform.
  • a 0.2 mm diameter pinhole detector
    • "multishot" measurement: it requires scanning the beam by upstream steering to measure different point in the profile
      • A single measurement can take about 26 seconds
    • extremely important was to screen the lead from being reached by parasitic electrons which would disturb the acquisition

Details of both profile monitors are available in IBIC2014, paper MOPF08.

A so called P. Thieberger's detector is installed off-axis next to the e^- gun and is used to optimize the overlap between circulating ion beam and electron beam.

  • This monitor measure the ~1 MeV back-scattered e^-, generated from the interaction with the ions.
  • Due to the presence of the toroid magnets, the back-scattered electron arrive at the gun off-axis.
  • The e^- energy allows to put the detector outside the vacuum chamber.
  • The overlap between circulating and e^- beam is optimized by maximizing the signal detected by such a monitor (sensitivity of a fraction of mm).

Details of such a monitor can be found in BNL-94132-2011-CP.

Discussion

  • It seems not possible to optimize the angular alignment between the two beams with such a P. Thieberger's detector, however, from experience, this doesn't seems to be a major issue.
  • S. Radaelli remarked that we should consider if/how to add a P. Thieberger's detector for the HL-LHC Hollow Electron Lens.
    • A. Kolehmainen asked to provide him with an estimate of the size of such a monitor, and he will try to see if it could fit in the present design.
  • D. Mirarchi showed a draft paper (submitted to PRAB) on recent MDs at BNL(?):
    • several measurement have been performed with P. Thieberger's detector with a hollow electron beam.
    • measurements show that for a hollow e^- beam the minimum signal region is pretty wide, making such a monitor probably not good enough for use during operation, but only for beam setup.
  • M. Wendt commented that with normal BPMs one should get better than 0.1 mm precision, which is what a P. Thieberger's detector is capable of providing.
    • R. Corsini stressed that having an absolute calibration for two very different kind of beam could be tricky.
    • More simulations are needed to evaluate the expected resolution of BPMs.

Beam gas jet monitor (R. Veness)

The present baseline profile monitor for both e^- and ion beam in the HL-LHC Electron Lens is based on a Beam-Gas Curtain (BGC) monitor. The monitor is based on Beam Induced Florescence (BIF) generated by the interaction of the e^- and ion beam with an injected gas. The gas is injected as a ~1 mm thin curtain at 45 degrees with respect to the beam, effectively generating the equivalent of a beam screen. The light generated by BIF is collected by a standard optical system and a CCD camera.

  • depending on gas used, one obtains different spectra and different photon yield
  • different gas species have been considered. Neon seems to be most promising one, allowing to get a profile for electrons and protons using about 0.5 seconds integration time
    • cross section of e^- with different gasses is being investigated at Cockcroft Institute
    • measurements are consistent with expectation within 10-20%
  • parasitic measurement in LHC with an old system are promising for ions (good profile measurement of with sigma of 2.2mm sigma beams), but not so great for protons
    • plans to install a new chamber in March 2020 to be used for more systematic measurements during LHC Run 3
    • several studies are ongoing to optimize the design of the instrument for HL-LHC, also profiting of present experience in LHC
      • plans to install one prototype in HEL e- test stand in mid 2020
  • cross section of protons (14 MeV) with Neon is being measured at GSI
    • even though proton energy is much smaller than HL-LHC, expectation from Bethe-Born scaling is just to loose about one order of magnitude
      • R. Veness stressed that nobody is expecting a very different scaling than Bethe-Born. Any small error could be compensated by adjusting the integration time
    • no older data seems to be available with Neon
      • S. Mazzoni commented that in the SPS measurements where only performed with Nitrogen

Status of e- beam orbit measurement in ELENA e-cooler (D. Gamba)

Two methods for inducing an e^- beam intensity modulation has been tested in ELENA:

  • switching the grid voltage between two given setting at a given frequency (up to 20 kHz)
    • gives a strong sawtooth-like signal on the BPMs, but at very low frequency
  • coupling the grid voltage lead cable with a transformer (Pearson), inducing a (undetermined) voltage oscillation (up to ~10 MHz)
    • it gives a nice sinusoidal signal on the BPMs
      • a few uA modulation over the ~mA DC beam.
    • amplitude of the signal depends on the excitation frequency (0dBm -- -20dBm)
      • M. Wendt commented this could be estimated knowing the capacitance of the grid electrode/circuit, which may limit the response of the instrument
    • it can get be easily converted to \muA/mm provided the good calibration factor are used (or at least the same factor as for the ion beam)

The sinusoidal excitation has been used for a first optimization of the e^- orbit in the e-cooler for both 355 eV and 55 eV plateaus. The signal could also be used to measure the time of flight between 2 pickups (note that cable length was assumed to be << 200 ns). The optimization was limited by charging up effect, which still need to be investigated. Additional observations:

  • No independent measurement of each electrodes available, only sum and difference.
  • Changing modulation frequency does not change the saturation behavior, that seems to be due to losses of beam.

In coolers outside CERN it seems useful to have the grid electrode being divided in 4 sectors, such to modulated only one corner of the beam, see for example A. Denisov. This option should be considered for future e-cooler guns.

The voltage step implementation seems to be more interesting for cooling force measurement studies, but in this case one should use it to "switch" in a single step the voltage of the cathode. Also this implementation should be considered for future e-cooling studies.

Discussion

  • Should we sum 4 instead of 2 plates to get orbit?
    • apparently not according to L. Soby.
  • Possible to include BPM of e-cooler in the orbit measurement system?
    • L. Soby: Electron beam not exited at the same revolution frequency as the circulated beam, so not possible at the moment, but could be done.
  • M. Wendt: you want lock-in technique to make the system more sensitive.

BPMs for E-Coolers and E-Lenses at CERN (L. Soby)

An overview of the BPMs in the different e-coolers with parameters has been presented (see slides):

  • All coolers use two electrostatic BPMs
    • charging up effect mush be investigated
  • ELENA is presently the only one "operational" with intensity (or energy in case of AD) modulation
  • plan to make all coolers fully "operational", with digitalization from both Oasis and standard BI orbit system has been agreed and on the way
    • some concern about the status of the AD BPMs, but preparation of the analogue acquisition system is being completed.

For the Hollow Electron Lenses it has been decided to use strip-line BPM design, to profit from the counter rotating beams and directivity of such a device.

  • First 3D design done and being analyzed
    • already found some modification needed to improve the RF performance
  • Present design based on 0.4 m long, 23 deg width strip lines, which should give about 0.2 V and 88 V signal for electrons and protons, respectively
  • A design for the acquisition system has also been made
    • it can use a single "direction" for measuring both beams (not at the same time!)
    • the forward (wrt proton beam) direction signal can be used additionally as (quasi-) independent measurement of protons.
      • in any case, protons must not be there to measure the electron beam
    • the use of a "standard" DOROS acquisition system is also being considered

A spare LHCBPLX BPM, which is very similar to the HL-LHC HEL BPM design, is going to be installed in March in the HEL test stand.

Discussion

  • A. Rossi asked what is the expected resolution for all kind of BPMs, but this has not been computed, yet.
  • M. Wendt suggest to have a look more in detail for a "lock-in" acquisition system for the e-cooler BPMs: this could provide very high resolution measurement.

AoB: Betacool for UNIX (D. Gamba)

  • The source code of Betacool has been retrieved and temporarily stored on CERN Gitlab (Betacool) and modified to be compiled for Linux/Mac systems
    • presently only accessible to E-BEAM e-group members
  • Minimalistic Python-Betacool interface is also being developed and available on Gitlab: pybetacool