Skip to content

Minutes of #3 Remote E-Beam Meeting (draft)

The meeting was held on Vidyo on 08/04/2020 - See indico

Actions

  • D. Gamba: check with G.Tranquille about available public documentation on the ELENA cooler modeling in COMSOL. Also check if a "compiled" version of the COMSOL model can be made available.

E- Beam orbit optimization in ELENA e-cooler (B. Galante)

Bruno showed the experimental results on e- beam trajectory optimization in ELENA e-cooler performed during last a few months of 2019.

ELENA features e-cooling during two plateaus, at 350 eV and 50-55 eV. Two BPMs are installed in the drift solenoids in the ring straight section, potentially seeing both antiprotons and electrons. The BPM signal was generated by coupling a sinusoidal signal on the electron gun grid electrode voltage, as shown in previous presentation (E-BEAM #8).

The main electrostatic and magnetic components that can be adjusted to steer and shape the e- trajectories are shown in the following figure: ELENA e-cooler main parameters. Courtesy A. Frassier ECV[12][PN] are clearance electrodes normally designed to prevent backscattered e^- to go back toward the gun, but were grounded during 2018 operation.

The electron beam could be run in DC mode (i.e. independently from the actual magnetic cycle of ELENA) or in pulsed mode, i.e. following ELENA's cycle and running only during plateaus.

Several important observations have been reported:

  • At the low-energy plateau, it was found easy to steer the beam through the center of the BPMs simply acting on the electron gun drift orbit correctors (LNR4.ECGH and LNR4.ECGV), therefore most studies where then carried on/shown for the ELENA mid-energy plateau (350 eV).
  • with 90 A expansion solenoid (i.e. small expansion), beam was expected to be well smaller than beam pipe (good trajectory seen on BPMs), but losses were evident looking at the vacuum.
    • suggested that losses could be at the collector.
  • with 50 A expansion solenoid (i.e. without beam expansion, therefore expecting a beam size of about 8 mm radius equal to the cathode radius =to be checked!=), beam was touching at least the first H BPM, but no strong vacuum activity was seen.
    • acting on collector/repeller voltages, it was possible to better control the beam (no saturation of BPMs, better vacuum)
  • some differences were observed between pulsed and DC mode of e^- beam
    • final touch: setting all clearance electrodes to -20V (instead of grounded), and some minor corrections on ECGV and ECT1V, it was possible to almost zero the losses (both on BPMs and vacuum) for both plateaus.

The final settings, were very close to the settings used in operation in 2018 regarding beam steering, but very different on collector and repeller voltages as well as on clearance electrodes. With those settings, nominal e^- beam current could be produced with almost nominal expansion (90 A expansion solenoid) and with no evident losses visible on the vacuum on both plateaus.

Bruno concluded saying that:

  • more tests could be useful to see if results are reproducible now that we have a better picture of the different effects
  • BPM saturation should be further investigated, especially understand why BPMs still saturates when vacuum doesn't show any sign of losses.

Discussion

  • C. Carli asked if e^- beam orbit measurement will be made available during normal operation.
    • Bruno and G. Tranquille confirmed that this has been already agreed and this task is progressing for all CERN coolers.
  • D. Gamba asked if for all cooler we will be using LLRF as basic modulation frequency for the e^- beam excitation.
    • G. Tranquille replied this is at the moment the baseline, but details still to be looked at. In LEIR, in the past, it was done with an independent local oscillator.
  • L. Ponce asked when the orbit system was last available/used/seen in AD.
    • D. Gamba mentioned that the local head amplifiers for the BPMs were not in place during 2018 run, but BI already agreed to install them during LS2. The question if the BPMs themselves are still operational is not known.

ELENA ecool settings 2018/19 (G. Tranquille)

Gerard reported the different settings for magnets (expansion, toroid, drift, squeeze), as well as as all gun and collector voltages used at the end of 2018 run and during the investigation of e^- orbit. He has then chosen a few representative settings to evaluate the theoretical beam transport using his COMSOL model of the cooler. Note that this model assumes an in-line cooler made of a gun, drift solenoid and collector, i.e. without the difficult-to-model toroids. Since the low-energy plateau was experimentally seen to be less problematic, the simulations presented assume the ELENA mid-plateau energy (350 eV e^- energy).

The simulations showed that at end of 2018 run the e^- where hitting the collector voltage at about 300 eV, i.e. they were almost not decelerated due the very low setting of the collector voltage (7 V). According to the literature, secondary electron emission is expected to be maximum around 200-300 eV for the incident beam, therefore this could explain the observed on activity on the vacuum while pulsing the e-cooler.

In simulations, the two settings used for the expansion solenoid (50 and 90 A) don't show any particular difference on the beam dumping in the collector side, except clearly the different size of the incoming beam.

The final configuration of the collector (repeller at -450 V, collector at -233 V) was also analyzed. Despite the high repeller voltage (well above the electron beam energy of about 350 eV), it turns out that the voltage on the beam axis is indeed just below the beam energy, allowing for ideal dumping of the electron beam.

Gerard also investigated the effect of a possible misalignment (5 mm displacement and 1 mrad angular deflection) of the incoming beam at the entrance of the collector. Simulation shows that the setup used in 2018 was more prone to beam losses right at the entrance of the collector than the new setup found in 2019. The latter also have much stronger squeeze solenoid (-23 A instead of -5.6 A used in 2018), which allows for shrinking the beam size before entering the collector.

Overall, the set-up found in 2019 is compatible with simulations and it seems to be pretty robust against beam imperfections.

Discussion

  • A. Rossi asked why the beam looked bigger in the 2018 case shown in the last slide (left) than in 2019.
    • Gerard replied that the difference is due to the different settings of the squeeze coil. The different colors of the beam traces is then linked to the different settings of collector and repeller voltage
  • D. Gamba asked if Gerard consider the COMSOL model a good representation of ELENA cooler.
    • Gerard replied that in reality the model he has used is missing the toroids, which are a difficult object to model. The overall beam dynamics is then not possible to simulate, but one could use a compiled version of this COMSOL model as a guideline tool.
    • Offline discussion will be organized to see if such a compiled model can be produced and distributed.
  • D. Gamba asked if the previous IPAC papers document the actual magnetic system of the ELENA cooler and related COMSOL modeling.
    • Gerard said that the IPAC papers might be still based on a earlier version of the cooler. This can be crosschecked offline.
  • D. Gamba asked if COMSOL could or is being used for the electron lenses simulations.
    • Gerard said that in principle this is possible, with the caveat of the toroids being still the difficult component to model.
  • C. Carli asked if there are plans to go to higher values of the expansion solenoid.
    • Gerard replied that the 90 A used in 2018 is almost the nominal value, but indeed the plan is to go up to nominal as soon as possible.
    • C. Carli stressed that it would be interesting to study the impact of different expansion factor on cooling time and equilibrium emittance. Gerard confirmed that this is the plan.