Peter Barnes, LLNL
August 29, 2000

TPC High Voltage Tests

Abstract

While the cathode plane was installed at Fermilab during 25 July — 3 August 2000 we ramped the cathode and anode voltages to their operating points. This note documents the test procedure and results.

Contents

Introduction
Gas Flow
Ground Connections
Cathode Plane High Voltage Tests
Anode Wires High Voltage Tests
Conclusion


Contents

 

Introduction

Gas Flow

Introduction

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.


Contents

Introduction

Gas Flow

Cathode Plane

Gas Flow

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).


Contents

Gas Flow

Ground Connection

Cathode Tests

Ground Connection

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


Contents

Ground Connection

Cathode Tests

Anode Tests

Cathode Plane High Voltage Tests

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

Both the supply and the TPC connector mate to a Reynolds 167-3516 bayonet connector. We used a single RG 213/U cable. We set the power supply high voltage limit to 10,100 V and the current limit to 80 mA.

Using the direct control mode, Gulshan started ramping the voltage. He used the control knob to increase the voltage, while monitoring the current on the meter. We observed a steady increase in the current with voltage. During puases in the ramp we saw no evidence of current noise or drift. At ~5000 V the current limit was exceeded and the supply tripped off. He tried again and reached ~8000 V. Because the Bertan dial is rate sensitive, it is easy to twitch it while turning and introduce a large voltage step, leading to a momentary large current pulse as the TPC capacitance charges up. We believed this was the cause of the over-current trips, though we did not see large current excursions on the display. We increased the current limit to 120 mA, but this did not seem to help. Finally, by very carefully turning the knob for 10 minutes, Raja was able to reach 10,008 V.

The following plot shows the current and resistance as a function of voltage for all measurements we recorded during all the ramps. Ignoring the first point, the average of all the resistance measurements is 124.2 0.3 MW.


Contents

Cathode Tests

Anode Tests

Conclusion

Anode Wires High Voltage Tests

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.

We connected four anode sections at a time, ramping the voltage to 1300 V, but not exceeding ~500 mA indicated on any section. Most anode sections showed a slow decay of the current when we left the voltage at a fixed setting. Since the time constant appeared to be many minutes, we did not wait for the currents to reach steady state values. Because of some initial confusion about where to read the current and what the maximum voltage should be, anode sections 0—3 may have been energized to as high as 2000 V for a few minutes.

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

 

Anode Section

0

1

2

3

HV Channel

0

1

2

3

Current Monitor

(5)* 10

8

2

3

Voltage (V)

Current (nA)

100

(1)*

2

<< 1

<< 1

200

(2000)*

8

<< 1

<< 1

300

100

10

20

20

400

200

12

10

10

500

120

14

80

20

600

120

14

90

20

700

130

14

90

20

800

200

20

100

80

900

500

40

200

80

1000

 

50

100

80

1100

 

80

100

100

1200

 

90

100

100

1300

 

100

100

130

*The meter response on monitor channel 5 seemed unreliable, so we switched to monitor 10.

 

Anode Section

4

5

6

7

HV Channel

0

1

3

2

Current Monitor

10

8

3

2

Voltage (V)

Current (nA)

100

80

20

20

80

200

100

80

80

100

300

100

90

80

100

400

120

80

90

100

500

200

90

100

110

600

200

90

100

100

700

150

90

90

100

800

200

90

90

100

900

150

80

90

100

1000

120

80

80

110

1100

150

90

90

120

1200

120

90

120

120

1300

200

80

200

150

Anode Section

8

9

10

11

HV Channel

0

1

3

2

Current Monitor

10

8

3

2

Voltage (V)

Current (nA)

100

90

30

12

90

200

100

90

60

100

300

120

90

90

110

400

130

100

90

120

500

120

100

90

110

600

110

100

90

110

700

110

100

90

110

800

120

90

90

120

900

120

100

90

130

1000

120

100

90

130

1100

110

100

80

200

1200

120

100

80

400

1300

110

110

80

500

Anode Section

12

13

14

15

HV Channel

0

1

3

2

Current Monitor

10

12

3

2

Voltage (V)

Current (nA)

100

60

100

8

10

200

100

120

80

100

300

110

130

80

120

400

120

200

80

120

500

120

400

90

120

600

110

500

90

140

700

110

400

80

300

800

100

300

90

300

900

180

500

100

300

1000

200

500

100

800

1100

200

500

100

 

1200

200

800

120

 

1300

200

 

120

 

 


Contents

Anode Tests

Conclusion

 

Conclusion

We successfully brought the cathode plane to 10 kV and almost all the anode sections to 1300 V, the nominal operating points. There did not appear to be any dead shorts; the minimum effective anode resistance was 2 GW. Four of the anode sections exceeded 200 nA at or below 1300 V. This was likely due to errors in the current measurement or the dirty gas composition.


Contents

Conclusion