genevb's blog

This is a historical look over the past 10 years at what appeared to be a minor bimodal distribution in the TPC OFCW current, noticed for the first time during Run 13:

Luminosity dependence of the FC currents makes looking for a minor bimodality difficult as the peaks become indistinct when examined over a large time window. Taking the difference with OFCE helps to provide a reference which varies similarly with luminosity (or time), easing the sampling of higher statistics.

All OFCW-OFCE plots below span the same range of 30 nAmps.

Observations:

The two modes appear to be separated by ~5-7 nAmps.
- It is not clear that this separation has remained constant over the years.
The fraction of time spent in each of the two modes has certainly not remained constant over the years.
There may have been an additional third mode in 2013?
The OFCW short (between the last two rings) developed between 2009 and 2010, thus the jump in the axes of several hundred nAmps.
However, the reverse large jump was seen between 2004 and 2005.
- This coincides with the intermittent partial short in the OFCW which we associated with rings 80 & 81, which is on the scale of ~75 nAmps.
- But I looked more carefully at the individual channels, and it appears that OFCW current dropped by a few hundred nAmps between 2004 and 2005 regardless of the intermittent short, whereas a short should raise the current!
There seems to be an additional jump pf perhaps ~5 nAmps between 2010 and 2011, and then another of ~2 nAmps between 2012 and 2013, which I speculate may be due to a minute difference in luminosity dependence between OFCW and OFCE.
Before 2010, there was also a great deal more noise in the single current measurements, such that the distribution was too broad to see sub-10-nAmp features.