Jan 24 2024

Action items from last time

• Gabriel will work w/ ME to confirm the R28 cathode insert hole size/shape, i.e. post the shop print,
• Gabriel/Max will decide on prototypical R30-4 parameters like that of the R28 cathode insert (hole, cone shape, lip shape) at CEBAF, then Gabriel will generate the corresponding CST model, and Alicia will include this in her comparison with (R28 & R30 @ z=19cm optics match-point),
• Gabriel will look at testing a cylindrically symmetric CST model against Max's POISSON model to confirm differences, intending to confirm that the differences are just 2-D vs. 3-D, nothing else,
• Alicia will continue to develop bands of acceptance parameters at Z=19cm, where parameters refers to the beam/optics parameters acceptable/typical for the CEBAF injector (implicitly meaning up to max current),
• Max and Alicia will continue to develop and experimental match-point for the purpose of comparison between measurement and model; as mentioned above a simulation match-point at Z=19cm has already been found in a drift free region after the gun,
• Max will provide some sets of optics optimized cathode parameters (hole=fixed, cone shape, lip shape) to Gabriel, so he may generate a detailed CST model to explore max E-field strengths at surfaces/triple-points,
• Max will take the lead to develop the table of parameters with uncertainties which will be used in the simulations (GPT/CST), these will be used to develop allowable tolerances in fabrication/assembly so we can with high likelihood end up within Alicia's acceptance bands,

Max and Gabriel

• Resolved discrepancy related to different definition of hole diameter: now we consistently use the dimensions from the T-cathode drawing
• Correct cathode position for field map export/import relative to particle start coordinates is important; this changes with cathode recess
• CST field maps tend to be unphysical close to cathode if export boundaries do not align with mesh cells, causing sensitivity to step size in exported file
  • The correct strategy is to start the field map exactly at the photocathode surface. Choose subvolume end point to be start point + multiple of step size to avoid confusing the exporter. In some cases, we need more decimal places than displayed by the idiotic point-picking text box; use trick to get the accurate number, e.g., start defining a cylinder from the picked point and get number from dialog box.
  • Then and only then, the step size makes no difference, 0.5 mm is enough.

• Comparison R28, 0.18 mm recess, hole ID per drawing
  • CST: 73/53 cm, Poisson: 46 cm, measured at CEBAF 180 kV: 42/37 cm, measured at UITF 200 kV: 27/25 cm
  • difference simulation/reality may be related to recess and/or hole diameter

• Comparison R30-4, 0.18 mm recess, hole ID same as R28, 15° cone, cone/sphere junction same as R30-3
  • Sanity check: symmetric CST model (NEG stuff and HV stalk + guard ring deleted) gives almost the same field map as Poisson, Ez within 0.2 % everywhere
    • CST: 82 cm, Poisson: 78 cm
    • OK, we know what we are modeling, tracking results match
  • Complete CST model
    • CST: 92/56 cm, Poisson: 78 cm
    • Too little focusing. Recommend slight increase in Pierce angle to better match R28 results; likely involves a compromise due to stronger astigmatism

Carlos

• The 25 deg Pierce cathode front end piece from the GTS spherical electrode gun (R30-1, Yan's thesis) was measured by SRF's Coordinate-Measurement Machine (CMM).
• Findings:
  • Pierce angle design: 25 deg. Measured: 24.4 deg.
  • Cathode hole diameter to extrapolation of conical edge: Design: 0.432 inches = 10.97 mm. Measured: 0.467 in = 11.86 mm
  • Drawing and CMM findings can be found in the SharePoint folder here

Table of parameters (tentative)

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	°	15	?	+/- 2	likely need to increase
Cone/sphere junction angle	°	62	?	30 -- 62	to be decided based on field-emission concerns; 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

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Conclusions, path forward

• Carlos will develop a high-level time line for the rest of the project.
• Now that the simulation models are working well, Max and Gabriel will optimize
  • the Pierce angle to give R28-like optics as a baseline and
  • the junction angle to allow for a larger lip radius, less field enhancement. Carlos does not insist on the steep angle of R30-3, it can be a smooth transition.
• We will make two electrodes to have a spare; one can consider making two different shapes as a contingency plan. Carlos will ask Keith to join the meetings.
• Max will consolidate the set of field maps for apples/apples comparison in Alicia's injector model. Alicia will make a comparison between the updated R28, R30-3, and R30-4 models and work toward defining an acceptance band for phase-space parameters at the match point.