GTS meeting 7 25 16

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  • 1 mA semi-stable running for nearly 7 hours: [1]
    • At 300kV, with anode bias at +1500 V, and
    • Better beam focusing and centering in the viewers, rather than on the solenoids.
    • We think is the latter that made the greatest contribution to high current beam
    • Even so, we still see sudden "photocathode-anode arcing" that reduces the QE on the illuminated area, but only slightly
    • Slide09..jpg
  • The cathode is amazingly robust!
    • Slide10.jpg
  • By moving a hand-held magnet around the gun chamber, "something changed" and now there is field emission starting at around 230kV.
    • This induced several gun HV trips (on current) while attempting to operate at 300kV.
    • We tried Kr processing at no avail. We are then operating at 225kV.
  • The QE scanner is operational!
    • The key to make it work was to bias the gun at -1kV, using the Glassman HVPS, and the anode grounded through the Keithley pico-ammeter that reads in EPICS. YEs, for this we need to lock the GTS enclosure in Run mode.
    • The following scan was taken after doing Kr processing. This is why the QE in the un-damaged area is smaller than 2.5%.
    • Comparison between QE map and damaged cathode picture.jpg
  • The masked cathode was installed:
    • Picture of masked photocathode.png
    • We got beam, with a ring!
    • QE is very low.
    • This cathode is being heat cleaned and will be activated again. Mamun has a better scheme for masking the alkalies.
    • Masked cathode compilation picture-QE-beam.png