18" Photogun ala GTS
This photo-gun was intended for the UITF to operate at 200 kV. It is based on the GTS 18" diameter vacuum chamber, doped R30 insulator and spherical electrode. The vacuum chamber is NEG coated, and the anode is the CEBAF style, with 6 cm anode-cathode gap. The GTS 18" photo-gun does not have a NEG coated chamber, and the anode is larger than the CEBAF type, with holes for laser in/out.
It was first high voltage conditioned in the UITF [1], but processing was very slow with unprecedented gas load with voltage increase. On July 28 2017 the insulator arced over at 227 kV: [2].
The photo-gun was refurbished in August-September 2018 but this time with a pure alumina (white) R30 insulator, as there were eno more doped R30s [3]. The gun was installed, baked but the cable was not changed. Although a visible black (carbon likely) track was on the cable plug, it cleaned off easily. On October 20 1017, the insulator arced over at only 164 kV [4], likely due to the cable plug being damaged from the first breakdown. Not changing the cable was a mistake on our part. The low voltage at which the insulator suffered electrical breakdown is surprising, considering that this type of pure alumina insulator with a shed demonstrated 375 kV in the GTS using a dummy ball (2016, see our IEEE paper).
Upon the second insulator failure, the 18" photo-gun was removed from the UITF beam line, parked in our TL1137 lab, and replaced with the 14" photo-gun and R28 insulator.
The spherical electrode was disassembled and stored in the TL1137 clean room cabinet. The shed has been polished and clean. The electrode is ready for assembly. A new pure alumina insulator has been welded to a 10" CF that has holes for S&A fixtures, and that has been vacuum-degassed.
The anode was removed from the vacuum chamber and is now in the clean room at TL11237. The anode has scratches and some of the ceramic rings and washers are broken. This anode features the capability fo independently biasing the anode or the anode drift tube.
The WP-1250 cartridges are still installed in the vacuum chamber, but the woven wire shielding screen has been removed. The NEG cartridges have an electrical short and would not be activated running current through them.
The vacuum chamber is dusty, and has pieces of nitrile glove, bits and pieces of metal like washers, wire, and screws. There is no longer an extractor gauge attached to the vacuum chamber.