BaBar Drift Chamber

Recovering from High Voltage Trips

Last updated: 2 November 2003 (images), Michael Kelsey


To ensure smooth "factory" operation of the BaBar experiment, the Drift Chamber must recover from trips routinely, or our efficiency and integrated luminosity will be impacted. This process has been partially automated, for the situations (described) below, which are clearly understood as due to machine-related backgrounds.

There are two ways the the DCH can trip during normal operations, excessive power supply currents or repeated current spikes on single wires. We have software running on the DCH IOC (DCH-MON) which will automatically implement the instructions below, and recover from trips in 10-20 seconds.

Other kinds of trips (gas system interlocks, chillers, safety systems) are more dangerous and should be dealt with as with any alarm condition: Page the DCH Operations Manager at (650) 846-0981.

The DCH can also drop voltage briefly (up to five seconds) without tripping. This will appear as a yellow alarm in the Alarm Handler and on the DCH voltage and current indicators in EPICS. The shift leader does not need to take any action, but if it happens frequently, it indicates poor beam conditions.

1. Bring up the DCH High Voltage Panel
 

Normal trip

Status indicator shows Tripped

"DCH Spike" trip

[Spike Ack.] is Red
2. Take the DCH out of Global Control if necessary Select [Spike Ack.] Panel
3. Set the chamber to [Injectable] [Acknowledge] the alarm
4. Ensure that the BACKGROUNDS indicator is green.

You cannot put the chamber back in a [Runnable] state if the background indicator is red.

5. Return the DCH to Global Control.

You must reset the trip as described above before returning the DCH to Global Control.

6. Record the trip in the logbook, along with any special conditions.

Drift Chamber HV Status

DCH Main EPICS Panel When the high voltage trips, the first thing you need to do is bring up the DCH High Voltage control panel. From the main DCH panel (left) select the [High Voltage] button.

HV Control EPICS
Panel The HV control panel (right) indicates the current state of the HV system. When it trips, the kind of trip will be shown in the box at the lower left. As noted above, there are two "routine" trip conditions from which the shift leader is authorized to recover from and continue operations: excessive power supply currents or repeated current spikes on single wires.

Over-current Trips

Our CAEN high voltage power supplies have been configured with current limits at approximately three times the level during normal running (at 3.5 x 1033 luminosity).
HV Control EPICS Panel If the current on a channel exceeds the limit for more than five seconds, the CAEN will raise an error flag and ramp the voltage down in a controlled manner. High background conditions typically will trip several superlayers in a region, occasionally the whole detector.

These trips will generally be recovered automatically, as described above. If too many trips occur during a single fill, or during a run, the automated recovery is disabled. After contacting the DCH Operations Manager you should reset this trip from the DCH HV Control panel.

  1. Take the chamber out of "Global Control" with the blue Global Control [Yes] button.
  2. Set the chamber state [Injectable]. This resets the trip condition on the CAEN power supply, and ramps all the channels to the same voltage. NOTE: Turning the voltage [Off] does not reset the power supply.
  3. Ensure that the background levels are tolerable. The BACKGROUNDS indicator must be green.
  4. Return the chamber to "Global Control". If the backgrounds are too high, this step will fail.
  5. Record the trip and any special conditions in the logbook.

You cannot put the chamber back in a [Runnable] state if the background indicator is red. This indicator summarizes the SVT "soft abort" diode readings, which dump the beams if they remain high for ten minutes.

DCH Backgrounds The DCH backgrounds summary panel (left) shows the values for each of the four SVT PIN diodes (Forward East, Forward West, Backward East, and Backward West) and the limits set for the DCH. The values typically fluctuate around 20 during normal running.

The background limits ("DCH Pain Threshold" shown in blue) can be changed either by a DCH HV expert or by the BaBar shift leader. Under certain conditions, the SVT PIN diodes, especially Backward West, can experience a pedestal drift, so that their "normal" reading ends up significantly higher than our "Pain Threshold."

If this happens, and prevents running when the beam conditions are clean, the shift leader should raise the threshold for the problem diode. For example, Backward West could be raised to 100 or 1000, effectively removing it from the decision process. Once the SVT PIN diodes are recalibrated, the DCH pain threshold should be restored.

Spike Monitor Trips

HV Control EPICS
Panel We have special software running in EPICS which watches for excess currents on individual channels, above the normal level but below the power supply trip threshold. If the same channel has two or more spikes in a short time, and no other channels show any problems, it is possible that one of the wires in the chamber is breaking down. The software will ramp the whole chamber OFF and will automatically page the DCH Operations Manager.

DCH Single Channel
Spike Panel When this happens, the [Spike Ack.] button (left) will be outlined in red, and the chamber will automatically be taken out of Global Control. You must use the Spike Acknowledgement panel to reset the chamber for normal operation.

  1. Bring up the [Spike Ack.] panel.
  2. [Acknowledge] the alarm condition. You may have to [Page DCH Expert] as well, but sometimes not.
  3. [Close] the panel when you're finished.
  4. Return the chamber to "Global Control".
  5. Record the trip and any special conditions in the logbook.


For comments, suggestions, or corrections, please contact Michael Kelsey <kelsey@slac.stanford.edu>