1. NEG-coated gun: Black R30 arc at 227kV, white R30 arc at 164 after refurbishing and second bake
Carlos
Wednesday, August 10, 2018

2. The NEG-coated gun was assembled originally with a black R30 insulator
Barrel polished electrodes
July
Yan’s design shed
Old Cathode vs. New

3. History of events for gun with black R30 insulator
With a NEG coated chamber, the gun showed very good vacuum while sitting mounted to the beam line and to cathode prep chamber for about one year.
High voltage conditioning a year later (July 2017), surprisingly, we started to see voltage induced gas desorption at 120kV, while this usually happens at about 240 kV.
The gas load made high voltage conditioning almost impossible, very very long time for the vacuum to recover even with small (0.1kV) voltage increments.
An RGA in the prep chamber showed mass 20, perhaps double-ionized Argon coming off form the coating?.
The partially conductive "black" R30 inverted insulator suffered arcing at only 229kV
An identical gun in the GTS has been conditioned to 365 kV without problems and has been operating at 300kV delivering beam since November 2016.
The GTS chamber is NOT NEG coated
A smaller black R28 insulator reached 365 kV with a dummy ball electrode in the same GTS gun vacuum chamber.
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4. kV CPS A Arcing at 227kV Gas at 120kV July 28 July 12
Black R30 insulator first time HV conditioning: gas at 120kV, too much gas load before arcing at 227kV
Arcing at 227kV
Gas at 120kV kV
CPS A
Ion pump
current
July 28
July 12
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5. The gun was refurbished using a WHITE R30 insulator and SAME HV cable
This insulator was tested previously in the GTS with the dummy ball electrode to 375kV, and later in Summer delivered 1 mA beam with John's shed design and Mamun's CsK2Sb cathode.
September
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6. kV CPS A Arcing at 164 kV Gas at 130kV October 10
Refurbished gun with WHITE R30 insulator: less gas load now at 130, but arced at 164kV while using SAME cable used when the gun first arced at 227kV
Arcing at 164 kV
Gas at 130kV kV
CPS A
Ion pump
current
October 10
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7. The overall gas load is ~ 1/3 after the second bake/HV conditioning
After second bake (Oct) AND HV conditioning to 227KV
First time (July)
Gas at 130kV
Gas at 120kV kV
CPS
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8. Things to do for IF >350C bake to drive gas off the NEG coating
Weld a new black R28 insulator to a 10” custom cut flange
Get dummy electrode from GTS shelf
Barrel polish (for touchup) dummy ball electrode
Remove from gun chamber
Damaged white R30 Insulator & electrode assembly
10” anode assembly flange
Extractor gauge
Right-angle valve
Ion pump magnet
NEG feedthrough
Aluminum standoffs holding the NEG screen
Check anode spacer length, is it for 6.3 cm or for 9 cm anode-cathode gap?
Blank 10” anode flange
Install 10” flange with black R28 and dummy ball electrode assembly
Add a tube on the 4.5” flange to extend outside oven, Add a gate valve on the other end, with a turbo pump connected directly to the gate valve, and back the turbo with a turbo pump cart during HV conditioning.
Pumping during bakeout through right- angle valve on I/P? too conductance restricted? Can we pump with turbo pump on gate valve too?
Check 240 VAC power on North side of cave O

9. We are going to do this instead:
Have revised R28 10” flange made
Weld R28 insulator to flange
Barrel polish dummy ball (sitting in GTS shelf)
Mount dummy ball to R28, then to chamber
Add gate valve and turbo to front end 4-1/4” gun flange
Evacuate
Run NEG pumps to ~1/2 full activation current and measure temperature around chamber, at the HV end of the insulator (plug side) and along the insulator for a few hours.
Ensure temperature in the ceramic remains ~ 30 C
Turn off NEG current and let it cool down to room temperature
Insert HV cable plug
Start HV ramp and record voltage at which gas desorption happens first
Heat up chamber with NEGs, let it stabilize overnight
Start HV conditioning with NEGs running at ½ full current and record voltage at which gas desorption happens first
Continue HV processing to 250kV
Do HV conditioning by heating up the chamber using the NEGs at half the full activation current.
We expect to see gas desorption at a lower voltage than at room temperature, AND because the chamber is being heated by the NEGs, the ceramic should stay more or less at room temperature., but first:
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