TPC High Voltage Tests

During the period 25 July — 3 August 2000, after we installed the cathode plane, we applied high voltage to the cathode plane and anode wires, and measured the current. This note documents the high voltage test procedure and results.

While we were installing the cathode plane (documented elsewhere), Fermilab technicians established a flow of dry nitrogen through the TPC. Flow was initiated at ~4 PM on 1 August 2000. The flow path is illustrated below.

Dry nitrogen gas from a bottle flowed at ~1 psig through a flow meter into the TPC. The vent from the TPC ran to a mineral oil bubbler. The TPC operates at ambient pressure, that is to say, the pressure inside the cleanroom. To accommodate the cleanroom overpressure with respect to the outside air, the bubbler was filled (initially) with ~1.2 cm of oil above the exhaust tube.

The connections to the TPC can be seen in the following picture. The gas inlet tube is the copper one which is being connected with the wrenches. The exhaust is the black plastic tube, which is connected to the two exhaust ports, one of which is visible in the upper corner of the TPC.

Initially, the regulator was set so the flow meter indicated 1.5 cfh (cubic feet per hour) = 0.04 m³/h. The bubbler tube extended ~12 mm into the mineral oil. Despite the continuous flow, we saw no bubbles in the bubbler, suggesting that the gas was venting upstream of the bubbler. The liquid level in the entry tube was below the free liquid level, indicating a slight over pressure, as expected, but evidently not enough to bubble. We also heard periodic "pings" from the TPC, though we could see no indications of dynamic activity, by watching for flex in the side panels, for example.

At this point, because of a shortage of screws of the correct length, only ~1/3 of the lid screws were installed. The lid is approximately 1 x 2 m². With as little as 1 psig pressure difference, the force is ~3200 pounds. Guessing that this could be enough to open the O-ring seal, we filled in the remaining screw holes in the lid with long screws with washers. This caused the liquid level in the bubbler tube to drop, but still not enough to bubble.

We left the TPC and gas system in this state for several hours while we went to dinner. When we returned ~10 PM, we decreased the quantity of mineral oil in the bubbler. When the level reached ~8 mm, we observed bubbling at a rate of 2-3 Hz. This did not change the flow rate indicated on the flow meter. We left the system in this state over night.

By the time we turned on the high voltage on the morning of 2 August 2000, nitrogen had been flowing for 18 hours. The TPC volume is approximately 1 x 1 x 2 m³, so at 1.5 cfh, it requires ~47 h per volume change. Therefore, we had change ~1/3 of the volume, i.e., the gas was likely 14% O₂, 84% N₂, and 60% relative humidity (assuming that the ambient air was 90% RH–it was very humid that week).

We connected a 1" wide ground braid from one of the copper ground lugs on the TPC to the High Voltage Rack.

We used the Bertan "Field Cage Supply" to apply high voltage to the field cage input of the TPC.

To power the anode sections we used two dual channel (four channels total) NIM high voltage power supplies, Bertan Model 362 (0—2000V). These have SHV outputs, matching the SHV anode connections on the TPC. We fed each output through a channel of the "Anode Current Monitor," then on to the TPC.

It appeared throughout the testing that several channels of the current monitor were (or became) unresponsive, so we moved the cabling around several times, especially for the first set. The calibration of the current monitors is unknown.

The tables below show the applied voltage and indicated current for each anode section, as well as the power supply and meter channel used.