1. Strathclyde gun bead-pull results

2. Cathode construction

3. Cathode frequencies
Cathode 0 is the old cathode that was used for the most recent VELA run
Cathode 1 is the first of the new cathodes
All frequencies are converted to operating conditions

4. Cathode frequencies (2)
I measured all of the new cathodes’ frequencies.
The frequency of the gun with each cathode varies.
All measurements were taken under the same conditions and with the same torque.
This is consistent with reports of frequency varying when the cathode was changed before.

5. Bead-pull results with cathode 1
The field flatness is 0.59 with a standard deviation of 0.027
Simulation shows the hole in the cathode gives a negligible change in frequency and field flatness
This would give an beam momentum of ~5 MeV/c at 7.5 MW compared to a flat cavity giving ~5.7 MeV/c.
If this were the flatness of the cavity it would explain the missing momentum
However…

6. Flatness error caused by frequency variation
I modelled the frequency change from cathode 0 to cathode 1 as due to the full cathode face penetrating less into the cavity.
The frequency change of 220 kHz can be caused by 77 μm movement of the cathode face – very sensitive!
This corresponded to a field flatness of 0.62

7. Flatness error caused by frequency variation (2)
This could explain almost the full field flatness deviation to within the error on the measurement.
The question is now what is causing the frequency difference between the cathodes and can it be modelled as I have done here?
Top choices so far are
Thicker cathodes bow out more due to flange gap – Mechanical modelling to estimate magnitude
Cathode flatness is poor – metrology to determine

8. Metrology- step height
Cathode
Nominal
Manufacturer
On site with touch probe
7.3 - 1
7.8
7.704?
7.644 2
7.802? 3
7.746
7.724
Based on the frequencies would expect 1 to be longest and 3 to be shortest – not the case.

9. Metrology- surface flatness
Cathode 0

10. Metrology- surface flatness
Cathode 1
Mark was unable to do interferometer measurement as there appears to be a high slope bump in the middle third – due to drilling?

11. Metrology- surface flatness
Cathode 2

12. Metrology- surface flatness
Cathode 3

13. Design with larger gap and 4.5mm thick copper
VELA Cathode FEA
Design with larger gap and 4.5mm thick copper
2500N applied to each screw location
Current design
16.7µm in Z
Deformation of copper
15.1µm in Z
Deformation of copper
Outer 3mm of copper fixed to represent face to face contact with gun.

14. Further work
CMM/ stylus probe measurements at RAL to confirm measurements and answer concave/convex question
Consider options for hole-less bead-pull?
Design apparatus to deform cathode known amounts and re-measure frequency.