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 provided by Max; these had the respective mean of x, x', y, y', and t subtracted to simplify integration of different guns into the model without having to adjust RF phases and correctors.
  • Beam comparisons between R28-2 and R30-3 guns
• 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

• Plan A is the 16°/30° design. We are going to start the design/fabrication process to quickly get a practice part and go through all the steps.
• Alicia's GPT simulations of the injector with Max's distributions indicate all models from 15 to 18 degrees should be manageable. Higher Pierce angles (shorter focal lengths) give a large beam size in the first lens and give high transmission loss at apertures, but the latter is because everything has so far been simulated with the same lens strengths. Max and Alicia will continue this work to re-match the optics for each case to see if the whole system of gun + 1 or 2 lenses actually differs. These studies may identify a suitable Plan B geometry and tell us how much deviation from the nominal geometry can be tolerated.
• Meeting with Keith on Friday to get the ME design started.
• Presentation at the upcoming B-Team meeting (2/13/24) about the strategy followed to solve the R30-3 focusing problem. Carlos will make some brief introductory remarks; Max and Alicia will decide to divide the rest of the work as it makes sense.

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