Polarized Cathode Change
On Tuesday, 6 May 2003, the Sources Group (Jym Clendenin, Jim Turner,
Axel Brachmann, Dah-An Luh and Tom Galetto) and Takashi Maruyama along with
visiting scientist from St. Petersburg State Technical University, Alexander
Rochansky, began the installation and activation of a new cathode for the
SLAC Polarized Electron Source PES) in preparation for the next E-158 run
scheduled to start in mid-June. The work was done with breaks for filling
PEP (with the thermionic gun) whenever possible.
During the first day, the SF6 vessel was removed and the transporter set
up. Then the cathode tray was installed and pumped down, baked, and finally
the vacuum in the tray left to recover overnight. This was also a planned
Repair Opportunity Day (ROD) when work in PEP and BaBar continued until late
in the evening.
Second day, May 7th, PEP collisions were just getting established at 11:00,
so decided to wait until next day to continue PES work.
click on photos to see enlarged version
The third day, May 8th, the cathode was moved into the preparation chamber,
activated (1-hour at 600 C, cool down, then apply Cs and NF3), then positioned
in the gun. After a PEP fill, the transporter was removed, the SF6 vessel
replaced, and purging with SF6 started while PEP resumed normal operations.
The fourth and final day, May 9th, the thermionic gun was vacuum isolated,
then the beamline valves for the laser gun opened, the cathode bias (-120
kV) was turned on and various tests of the cathode made using a variety of
lasers. Finally the transmission of the beam was checked in the first few
meters using the installed PMTs, the timing checked, and accelerator operations
resumed but now with the polarized beam. Unfortunately the cathode showed
a severe surface charge limit (SCL).
Postscript: On the morning of Friday, May 16th, PEP started a 3-shift downtime
to repair a vacuum leak in the LER. The Sources Group (same people as above)
was given permission to change the cathode at CID. This time a backup cathode
that was put in the cathode tray last week was sucessfully activated and
the gun closed up by about 6 pm. The SF6 can was purged, and early in the
moring of the 17th the HV brought up. This time the SCL was much less severe,
so that it appears this cathode will be ok for the E-158 run!!
Last updated
17 May 2003