GEM Foil Leakage Current

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Special thanks to Jeremie Merlin for writing the indepth guide for this experiment, available LINK GOES HERE.

Introduction

Discussion from Jeremie's document to go here.

Theory

Discussion & images from Jeremie's document go here.

Measurement & Procedure

If you have ordered a 10cm-by-10cm GEM chamber kit from CERN there will be three GEM foils and one drift foil inside of a protective case. This case should only be opened inside a clean room of class <= 10,000. It is important to know the difference between the GEM and the drift foils.

The GEM foils are slightly transparent and have two electrodes, one each for the top and bottom of the foil, external to the frame. The GEM foils are also coated with copper on both sides giving a "mirror" finish to the foil as shown below:

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on the other hand, the drift foil is coated with copper on only one side; and has only one electrode external to the frame:

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Do not unseal the protective case until you are inside a clean room.

Initial Setup

The leakage current measurements should only be performed in a clean room of class <= 10,000. The temperature, ambient pressure, and humidity of the room should be monitored as these can have an impact on the results of the leakage current test. The procedure below illustrates the steps to test one foil at a time; parallization of this process is implied.

1. Remove the GEM foil from their protective case.
2. Place the GEM foil on its support structure:
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3. Place the GEM foil and support structure inside the plexiglass box. Affix connections to GEM foil electrodes.
4. Check the connectivity to the foil from outside the plexiglas box by measuring the capacitance of the foils. For a good quality (read "clean enough to continue") 10cm-by-10cm foil this value should be ~5.6nF as shown below:
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5. Flush the plexiglass box with nitrogen gas (flow rate > 50 mL/hr). Monitor the temperature, pressure, and humidity inside the box. The humidity inside the plexiglas box should be well below the humidity outside the box.
6. Connect the bananna cable end of the SHV cable to the GEM foil. Connect the other end of the SHV cable to channel A, located on the back of the module, of the HV power supply on the right of the image below.
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You are now ready for the "One Minute Test."

One Minute Leagakge Current Test

This test should only be performed on GEM foils which have a capacitance of ~5.6nF.

Please click through the 14 steps below before starting the test. GEM foils can be easily damaged by high currents or excessive localized discharges. If a high sustained current (> 1 nA) is observed for longer than 60 seconds or if consecutive discharges (sparks) occur in the same position immediately remove the applied voltage from the foil.

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A foil that passes the "One Minute" test can move to the Automatic Measurement described in the next subsection.

Automatic Measurement of the Leagakge Current Test

This test should only be performed on GEM foils which have passed the "One Minute" test.

This test consists of measuring the leakage current by applying a series of alternating voltages to the GEM foil and monitoring its stability throughout this procedure. An automated test protocol has been developed using LabVIEW.

1. Connect the bananna cable end of hte SHV cable to the GEM foil if it is not already connected. Connect the other end of SHV cable to channel 0, located on the back of the module, of the programmable power supply on the left of the image below:
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2. Enable the HV for channel 0 of the CAEN N1471A HV programmable power supply by moving the kill switch up (shown below) if it is not already enabled.
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3. Connect both the USB cables from the Keithley multimeter and the CAEN N1471A HV programmable power cable to the computer if they are not already connected.
4. Open "Device Manager" to check for proper communication. If the computer recognizes both devices a "National Instruments GPIB Interfaces" field will appear in the main list and a new USB entry will appear under "Ports (COM & LPT), as shown below:
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5. Note the port of the CAEN N1471A HV programmable power supply. In the example above it is "COM10," although it may be assigned a different number in your setup.
6. Open the "LeakageTest-N1471A-KILL.vi" shortcut on the desktop if it is not already open.
7. Navigate to the "Configuration" tab. Change the "Set Data Folder" location to a pre-existing filepath. Then start the LabVIEW virtual interface (VI). These steps are shown graphically:
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8. Fill out the "User Name" and "Foil ID" fields in the pop up menu then press "OK" as shown below. Note the "Foil ID" must be unique . A subdirectory inside the "Set Data Folder" filepath will be created with the name of the "Foil ID" field.
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9. Ensure the "VISA resource name in" drop down menu has the CAEN N1471A Module's com port (identified above) selected then press the "Start Test" button. Graphical reference shown below:
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10. The program will switch to the "Leakage Current Test" tab and display a pop-up window. Click "OK" without changing any values. Graphical reference shown below:
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11. Navigate to the "Vmon and Imon Monitoring" tabs as shown in the image below. If you do not observe the voltage to begin ramping up immediately in the highlighted Vmon graphs double check that the HV on the CAEN N1471A HV programmable power supply is enabled. If this is enabled restart the LabVIEW program and repeat steps X - Y until you see the voltage ramping.
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12. Once you observe the voltage begin to ramp you may observe some discharges on the foils. An example of a discharge is shown in the image below:
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13. The procedure will terminate when either a) the maximum number of allowed discharges has been reached, or b) the test procedure has reached the final step. An example of what the monitoring graphs should look at the end of the automated procedure is shown below:
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Repeat this procedure until three GEM foils have been identified with leakage currents below 1nA and a sufficiently low number of discharges occurring. Once this occurs you are ready to move onto detector assembly.

-- BrianDorney - 26 May 2014