GTS meeting 6 27 16

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  • This is the link to the FGTSLOG
  • While attempting to run 1 mA DC beam, there are current excursions that trip off th gun HVPS (trip limit set to 2 mA). The excursions result in localized cathode damage, while the overall cathode QE dropped from 8% to 2.5%, allowing us to continue beam ops by rotating the cathode and by slightly re-positioning the laser spot. The current excursions were present at 275kV, and also at 300kV.
  • The current continues to slowly increase over time while running at 100 micro-Amps, we did a 5-hr run. The vacuum is stable after slight steering corrections.
  • We observed that steering the beam at the dump changes the anode current, and also the radiation detected by the RadMonitors, are they picking up beam loss downstream? or are we observing ion-back bombardment effects at the gun due to beam steering further downstream?
    • Can we learn anything if we re-position the gun radMonitors along the beam line?
    • If we bias the anode positive, that will sure help with lifetime, but we do not seem to have a problem with lifetime below 1 mA. The main problem is the photocathode "arcing" when attempting to run 1 mA, vacuum is in the 1E-9 Torr in the beam line. Perhaps the localized damage associated with a current burst is triggered by increased pressure in the anode-cathode. If this is the case, will biasing the anode help?
  • Mamun and Yan re-activated the photocathode increasing the QE from 0.2% to over 8%!! until we caused damaged while attempting to run 1 mA.
  • The vacuum rise seems to be due to halo electron beam, rather than the core beam. To solve this, it would be ideal to have a cathode with a much reduced active area: i.e. use the mask to make the next cathode from the beginning.
    • We noticed the mask is held now only by one screw.