Observed laser distortions
First look
The first listed attached file was my initial look at distortions to laser tracks in the TPC (not that it is upside down from subsequent plots as I accidentally took the non-distorted minus the distorted here). These are radial tracks, and the plots are off the difference between run 7068057 (with excess current) and 7061100 (without excess current). Also, run 7068057 was taken with collisions ongoing, so there is some SpaceCharge effect as well (7061100 was taken without beam). I believe this explains the rotation of the tracks at low positive z (west side).
Second look: dedicated runs
On March 17, we took a couple laser runs without and with a compensating resistor to get the IFC east current at least approximately correct. I plotted 1/p of laser tracks and took the profile. Straight tracks give very low curvature = low 1/p. The distortion brings up the curvature, as can be seen in the IFC east without resistor. The same plot also shows large error bars for the negatively charged laser tracks because there aren't many: the curvature tends to bring them positive. The IFC west shows the appropriately low level without any distortion, but the timing on the west lasers was wrong, so they are not reconstructed where they are supposed to be in Z. I am uncertain whether this bears any relation to the odd behavior of the first laser on the west side (showing up here at Z of about +67cm). The IFC East plot for the no-compensating-resistor run also shows that 1/p begins to drop somewhere around Z of about -100cm. The short would be located where the largest slope occurs in this plot (because the distortion to tracks is an integral of the distortion in the field, and the short is where the field distortions are largest), but the data isn't strong enough to pin this down very well. The negative tracks indicate a short between the lasers at -145cm and -115cm. But the seemingly better quality positive tracks are less definitive on a location as the slope appears to get stronger at more negative Z, implying a short which is at Z beyond -170cm.
No compensating resistor (run 7076029):
With 1 MOhm compensating resistor (run 7076032), which brings IFC east current to correct value within ~20 nAmps, according to 10:27am 2006-03-17 entry in Electronic ShiftLog:
Again, I looked at the distortion as seen be comparing TPC hits from the distorted runs to those from an undistorted run as I did at the beginning of this page (but this time taking distorted minus undistorted). For the undistored, I again have only run 7061100 to work with as a reference. The plots for each Z are in the second listed attached file below, where the top 6 plots are for the no-compensating-resistor run (7076029), and the bottom 6 are the same ones with the resistor in. I also put on the plots the value of the difference from a simple fit (meant only to extract an approximate magnitude) at a radius of 60cm (approximately the first TPC pad row). Those values are also presented in the following plot as a function of Z, confirming the improvement of the distortion with the resistor in place.
These plots seem to point at a short which is occurring somewhere between the lasers at Z = -145 and -115cm.
Gene Van Buren
gene@bnl.gov