Minor bimodal feature in TPC OFCW current

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.

-Gene


Year OFCW - OFCE [μAmps] OFCW [μAmps]
2013
2012
2011
2010
2009  
2008  
2007  
2006  
2005  
2004