The TPC field cages are made up of conducting rings separated by resistors which step the potential from the central membrane to ground at the endplanes. We have experienced a few electrical shorts between rings over the years, leading to non-uniformity of the TPC electrical field, distorting the data. Where the shorts are located has changed in different run years, and we have experienced fluctuating shorts.
Some distortion maps can be found here.
We now have code to automatically calibrate for the distortions caused by fluctuating field cage shorts in near-real time (on the order of an hour or so delay) based on measurements of the currents from the field cages. However, this code needs to be told where the shorts are and how to translate a current measurement into an amount of missing resistance. In other words, it is not meant to calibrate the position and logistics of the short itself, but rather handle automatic filling of the DB for varying corrections. If the logistics are not understood, then this automatic code should not be run.
The code is located under $STAR/StRoot/macros/calib as:
procTpcFieldCageShorts.csh
matchTpcFieldCageShorts.C
The shell script is meant to be run about once per hour and uses the ROOT macro to match information from multiple sources. This code has run as a cronjob under the starreco account.