E907 TPC Anode Testing

Contents

Introduction
1.  Capacitors
2.  Control Code
3.  Test Each Channel
4.  Initial Anode Ramp
5.  Normal Operation

Introduction

The E907 TPC anodes are powered by a LeCroy 1440 High Voltage System. using 1444 High Voltage Cards. These cards have been modified to increase the current sensitivity.

This note describes the checks we should perform on the hardware and software, the initial ramp/training for the anodes, and the normal ramp and operating points.

1.  Check for Aged Capacitors

The 1441 and 1443 cards have electrolytic capacitors which age, causing significant voltage errors. See the Fermilab PREP 1440 System Manual for details.

We need to check with PREP (Mike Behnke) that either these are not an issue with the 1444, or that our cards have had the ECO done (replace with different spec cap.)

2.  Control Code

1. There are documented issues using the v1.7 firmware (Fermilab standard) with the 1444 card. These have to be addressed by our control software or we have to get a crate controller with version 2 firmware.


2. These cards have a low current modification which affects the sensitivity of the current read back, and possibly the demand voltage. At least one of the cards has the LeCroy "MOD nnn" sticker. I've asked Mike Behnke for help, and he has referred this to Hank Connor.

   In the BNL E895 TPC Logbook (the TPC's last experiment), page 46 I ran across this:

   

   I read this as

   Note on currents in 1440 HV:

   Monitor & trip current levels are normally read out in µA.

   However, we are now using modified 1440P cards which monitor currents with 100 times normal sensitivity, and trip with 50 times normal sensitivity.

   Modified cards can be recognized by either MOD 300 or MOD 100 on the top or bottom of the panel where the SHV connectors are located.

3.  Test Each Channel

4.  Initial Anode Ramp

When the TPC was in Industrial Center, we tested the anodes. We used 100 V steps, and waited between each step until the indicated current had dropped well below 500 nA.

Since the 1440 read back update time is so slow (0.75 s), we need to set up a volt meter to monitor the current in real time, using the IMON connection on the 1444 front panel:

For each of the 16 anode channels, we should perform a similar procedure:

1. Set the voltage to 100 V. Record the time and initial current.
2. When the current drops below 200 nA record the time and the final current.
   
   
   • If the current is noisy, use the peak readings, not the baseline value.
   
   
3. Increase the voltage by 100 V, again recording the time and initial current.
4. Repeat the previous two steps until we reach 1300 V.
   
   
   • If the current on a channel hangs well above 200 nA, we should stop raising the voltage on that channel.
   
   
5. Record the time and current periodically until the current drops below 100 nA. My best guess is that will happen over roughly an hour for most channels, so recording the current every 5-10 minutes should suffice.

5.  Normal Operation

The BNL E895 TPC Logbook pages show that normal operating currents were generally around a few nA, with a few channels up as high as 25 nA, in nitrogen. It doesn't quote normal currents in P10.

The operating voltage was 1180 V, 1250 V, or 1300 V, at various times.

The log doesn't quote the normal ramp rate, though makes reference to "pauses during run-up."

I suggest we plan for a programmed ramp in 100 V steps to 1250 V (obviously, the last step will be 50 V). We will have to set the time delay after seeing how the anodes behave once burned in.