Feb 7 2024

From last time

• Carlos will incorporate into the time line a potential vent and electrode swap in case of failure to HV-condition or get the beam out (by no means anticipated, but good to know how long it would take)
• Max, Gabriel, and Alicia will complete simulations of:
  • shallow junction angle
  • tolerance bands around nominal values, especially cathode recess, to evaluate injector acceptance (see parameter table from last meeting).
  • Final checks: tilted anode, biased anode, 140 kV
• Deadline to define Pierce angle: Today. Max, Alicia.
• Deadline to provide Keith Harding with the chosen models (TWO Pierce angle options): Friday, February 9, 2024. Max, Gabriel.
• February 13, 2024. Present before the B-Team meeting the strategy followed for arriving at the optimized Pierce angle. Max and Alicia.
• Geometrical parameters downselect

Table of parameters

Parameter	Unit	Nominal value	Fabrication uncertainty	Simulation band	Comments
Hole inner radius	mm	6.4135	+/- 0.3?	+/- 0.3	to edge of actual metal, as measured with caliper
Pierce angle	°	16	?	+/- 2
Cone/sphere junction angle	°	30	?	30 -- 62	was 62°; smooth transition = 26.971°
Cathode recess	mm	0.188	0.05	0.1	from Keith's drawing; error may be larger for old pucks
Cathode tilt	°	0	+/- 0.5	1
Laser spot size (RMS)	mm	0.5	0.1	0.2

Updates

• Max, Gabriel, Alicia: Parameter variation studies. The nominal values above give good agreement with the R28 model; the smooth transition at the junction gives minimum field enhancement at the lip and has been shown not to cause problems due to the protruding edge (Sajini's design).
  • Pierce angles, junction angles: Beam envelopes // Focal length vs. spot position -- no major optical difference between junction angles, just changes focal length; 16/30 model is closest to R28.
  • Anode tilt with nominal parameters, same angle as R30-3 (2°): Beam envelopes // Focal length vs. spot position -- worth keeping, no adverse effects, partial cancellation of downward kick
  • +/- 0.2 mm hole size: Beam envelopes // Focal length vs. spot position -- suggest at most +/- 0.1 mm machining tolerance (final part after polishing etc.)
    • Important to control. Keep radius small to reduce tolerance of longitudinal distance between edge and photocathode surface.
  • +/- 0.1 mm recess: Beam envelopes // Focal length vs. spot position -- less-than-nominal recess gives unduly long focal length (note, variation is twice the limit from comparative stack-up drawing)
    • could increase focusing (e.g., 1° more Pierce angle)
    • but history indicates focal length tends to be shorter than simulated, i.e., more recess
    • unlikely to be operationally problematic
  • 140 kV instead of 200: Beam envelopes // Focal length vs. spot position -- less voltage focuses more, but not a big issue
• Alicia: What phase-space parameters are desirable? (simulations done with R30-4 candidate phase spaces)
• Carlos:
  • Sent an un-polished 25 deg Pierce cathode front end to CMM to measure outer diameter and hole size. These are expected to indicate if these dimensions changed compared to a mechanically polished piece.
  • Keith Harding is ready to produce the drawing of the new cathode Pierce angle from end. He says a sketch with dimensions will be easier to model than receiving a step file.

Path forward, conclusions

• Max, Alicia: Present at the upcoming Team meeting (2/13/24) strategy followed to solve the R30-3 focusing problem?

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