Jan 10 2024

Updates from last meeting action items

• Gabriel:
• Alicia:
• Carlos:
  • The new electrode front end can be fabricated by the machine shop in 3 days
  • The machine shop can also do the polishing, it will take ~ 1 week.
  • Final polishing with diamond paste still needs to be done by CIS. Carlos needs to re-learn how to do this, and train others.
  • Matt gave Carlos several pieces of high quality 316L SS (crossed-forged, vacuum arc re-melt)
  • Matt also explained that the shape of the R30-3 electron front end was designed by John Hansknecht to lower the gradient at the junction to the spherical electrode shell. This was done without studying the effect on gun optics. Matt does not recall why the electron from end piece was designed with a small hole diameter than the R28 Tee electrode guns.
• Max:
  • Assume insert-to-sphere junction angle does not affect optics much (CST+GPT work can start to ascertain cone angle) but consider modeling the effect of the mechanical uncertainties of the junction in Poisson. This is the most important aspect of the mechanical design at this point. May be computationally problematic, but we don't intuitively understand the problem well enough. Worst case, we'll choose the same junction geometry as R30-3.
  • Measurement of R28 aberrations underway at UITF, large/homogeneous cathode activated and running, just need to iron out kinks in beam instrumentation

Items for discussion

Max:

1. R28-2 beam dynamics: CST agrees now POISSON on beam waist and focal length.
2. R28-2 beam-based measurements in progress at UITF to compare gun optics against models.
3. R30-3: There is still a discrepancy in focal length. In beam waist size between CST and POISSON for the R30 gun.

Alicia:

1. Looking at a set of simulations done preparing for the Fall 2023 run with the R30-3 and R28-2 guns at 200 kV to show what the beam characteristics are at a field-free region between the gun and the first solenoid (roughly 20 cm). The simulations all use the same initial beam characteristics at the cathode, so essentially the only change between the simulations is the gun itself.
2. The location of the center of the first solenoid from the cathode: MFX2I01 z = 0.780796 meters.

Gabriel:

1. Worked with Keith on the two puck models: the 625 and the 350. Both agree on the gap between the puck crown and the GaAs plane: 0.18 mm.
2. Benchmarked R28-2 CST-GPT with POISSON-GPT looking at beam envelope. This includes using the same Pierce angle, hole diameter, and position of GaAs plane.
3. Performed beam envelope GTP modeling for various Pierce angles with the R30-4 gun, and compared to the R28-2 gun.
4. The best angle for R30-4 seems to be 18 degrees from Max’s POISSON model, but Gabriel’s CST model says is 13 degrees.
   

Action items for next meeting

Max:

1. Finish R28-2 beam-based measurements and benchmark models
2. Resolve the discrepancy between CTS and POISSON for R30-3 gun optics
3. Evaluate the effect of hole size uncertainties (~0.2 mm radius) on
4. Evaluate the effect on beam optics between a 30 deg and 60 degree blending angle from the spherical electrode to the Pierce cathode front piece
   

Alicia:

1. Confirm the simulations done so far are for different bunch charges, and list those
2. In the longer run, we need to determine interface points that correspond with locations amenable to measurement. There was also the suggestion to do simulations starting at the first solenoid varying the beam size and divergence to find a range that is good for the injector. Likely these two will not done for next week's meeting.
   

Gabriel:

1. Using the R30-3 "as-is" manufacturing drawing, check with Max to ensure his POISSOM model agrees with CST AND both with the "as-is" manufacturing drawing. This should help resolve the discrepancy.
2. Evaluate the gradient at the junction between the Pierce front end piece and the spherical cathode for the angles indicated

Carlos:

1. Make the schedule ending with beam hand-off to ops. Carlos needs input from Injector group for this.
2. Take R30-1 (GTS gun) Pierce cathode piece to SRF metrology

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