Sameed explained the concept of Hollow Electron Lens as collimation stage (controlled halo depletion, catting in the beam closer than the primary collimators) and mentioned the HEL soft landing program, with prototyping and testing foreseen for the HEL electron gun, BPM and collector.
He showed the HEL electron gun version 3, which was developed starting from the Fermilab concept, and adapted to a DN63 and to provide sufficient heat removal to avoid overheating of the ceramics (see D. Perini et al., Design of high-performance guns for the HL-LHC HEL).

• The gun could be tested at the Electron Beam Test Stand, using the EBTS collector (which acts as a Faraday cup). Cathode was biassed to -10kV.  The current was pulsed using a dedicated Marx Generator, acting on the anode voltage.
   

  The current produced by the gun is inferred summing the current measured onto the collector body, the repeller electrode and the insertable FC (Faraday Cup). This measurement does not take into account presence of secondary electrons. Nevertheless, the results fit to ~ 1% the current measured at the power supply.

   
• Two  different manufacturers' cathodes: BJUT (from Bejing University of Technology) and Spectramat, could be tested. Results show that
  • The BJUT cathode:
    • Failed at least 3 times due to braking of the brazing at the filament (local overheating?).
    • Reached lower temperature for the same power to the filament as compared to the one from Spectramat.
    • Maximum achieved current density @ ∆V = 10kV, T= 1140ºC –>  4.3 A/cm², and maximum current –>  6.5 A
  • The Spectramat cathode:
    • More robust design, no failure in the filament.
    • No results @ ∆V = 10kV, but following the same emission curve as the other cathode, only at lower temperature T= 1060ºC.
• Could not reproduce Perveance curve, best fitting with linear interpolation I=P•V.

Comments :

• Adriana stated that Fermilab results with same cathode-anode shape but entirely different gun seem to better follow the perveance curve (see picture from 12th HLLHC Collaboration Meeting, Uppsala (Sweden),19-22 Sept. ‘22). Gerard added that we should check if this deviation comes from the fact that we do not opereate in DC more, only pulses of  20 to 30µs duration. Similar results with e-cooler gun when pulsed.

Philip showed the results obtained with the prototype BPM, 100 mm diamenter and 400 mm stripline.

Measurements where performed with electron beam at different current, size (compressing the beam at the BPM by lowering the magnetic field at the gun) and position (moving the beam with the EBTS steerer magnets) and compared with the BTV (OTR screen downstream the BPM).

• The beam exibits plasma 3-mode behaviour (becoming almost triangular and showing non uniform distribution of electron current density) as the beam current density is increased by incresing the emission from the gun or by compressing the beam.
• As the beam is copressed, the ratio between the outer and inner radius increases (to be investigated but could come from the distribution of the magnetic field lines where the e-beam is measured).
• Good comparison, new data should be taken keeing the BPM DOROS gain constant to avoid 'jumps' in the reading.

Philip also showed a picture of the HEL collector, now installed at the EBTS, which is under commissioning. Results on its characterisation will be presented as soon as available.