One can clearly see that there are luminosity dependent drops in the current, as has been observed and discussed previously. However, the magnitude of the drops was generally much larger this year (it was as much as 30 nAmps in Run 11, but was often between 50 and 100 nAmps in Run 13), and for portions of the data, immensely larger, as shown in the below plot of the IFC east current vs. time [hours, arbitrary zero] from fill 17533, for which the nominal current is ~76.46 μAmps, but 74.8 μAmps is seen, a drop of 1660 nAmps (2.17%)!!!
This huge drop was seen in all four field cages, as visibile in the above plot on day 141, along with other notably large drops in the the day 141-154 range. This first immense drop was reported at the TPC meeting on May 23, 2013 by Alexei.
And along with the large drops, this range of days also seems to have a higher nominal (zero-luminosity) current (visible as a distinct red band of non-run data at the top of the first set of plots), rising from ~76.425 μAmps up to ~76.46 μAmps, and again happening for all four field cages. Here are some specific examples of IFC east current vs. day for fills (17562 and 17569) demonstrating the varying nominal current by plotting not only the data from during the fill, but from just before it as well, with the cathode presumably on, but no colliding beams. In both cases, the current drops by ~80 nAmps when the beam turns on, but it is clear that before fill 17569, the nominal current is very low (by perhaps 200 nAmps) and rising.
These incidents of large current drops do not appear to be tied to particularly high luminosity occasions, as demonstrated by this plot of ZDC coincidence rate vs. day for runs with TPC:
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A more relevant metric of what could lead to possible distortions in the TPC is the difference between the outer and inner field cage currents, which can lead to radial components in the E field in the volume of the TPC. Here are the OFC - IFC (west and east) current differences vs. day for Run 13, and vs. time [hours, arbitrary zero] for fill 17533:
NB: West has a larger difference than east primarily because of the short in the OFC west, which increases its current.
Some observations from these difference plots::
- Current differences in Run 13 are up to ~twice as large in Run 13 as they were in Run 11.
- The trends in current differences appear to follow the luminosity (ZDC coincidence rate) rather well.
- It appears that the immense current drops for fill 17533 do not translate into abnormally large current differences between the outer and inner field cages. Perhaps the immense current drop was due to a cathode voltage drop instead of the hypothesized current leakage due to ionization in the air outside the TPC??? A 2% drop in cathode voltage would be roughly half a kV!
- There are mild drops in the current differences every ~40 minutes. I believe these are due to some activity of C-AD that occurs every ~30-40 minutes, as some blips were also seen in RICH scalers.
- There is a second band in the west difference which is perhaps 8 nAmps lower than the main band. It appears to be from the OFC west (it's difficult to see unless a reference is subtracted to remove the primary time variations due to luminosity), but I see no obvious time pattern. Rather, it seems that a random ~10% of the measurements are in this second band. I have no theories other than the possibility that the short in the OFC west is no constant?!?
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Following up on the possibility of a cathode voltage drop, one might expect to see drift velocities drop by a perhaps 0.003 cm/us for a 2% drop in cathode voltage [Garfield calculations, which Yuri says are off by a scale factor in drift velocity (not E field) due to methane content]. Here are the drift velocities for Run 13, along with a zoom in near May 21, when fill 17533 occurred (from ~19:00 GMT until ~00:30 GMT just after midnight). The drift velocity should hypothetically slowly rise with a slowly rising cathode voltage, but there's no evidence of this during this period of time.
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Following up on the luminosity dependence of the drops in current...
I fit a first order polynomial to each field cage current as a function of zdcx, giving intercepts and slopes. The slopes are the dependence of the field cage current on luminosity as measured by the ZDCs. Here are the slopes [μAmps/Hz] for IFC east, IFC west, OFC east, and OFC west vs. day, binned in half days:
Now taking the differences of the slopes using (east - west) for inner and outer field cages, and ratio of the slopes (east / west), we see that something clearly happened around day 129 to alter the inner field cage currents.
Additional observations regarding huge current drops
I'll start by showing the early and late IFC west current (Iiw) vs. day:
So the immediate question is, are these dips seen in the voltages. There are several voltages recorded, including points labeled "0" and "1" for each field cage, described here., and cathode setting and readback voltages. Here are the voltages at 0 and 1 for IFC west for the late period:
Clearly the huge changes are seen, and the voltages seem to track the currents. We can come back to this in a little bit, but first let's also look at the cathode voltages. The readback voltage is in [kV], but I do not understand the units of the setting voltage. These voltages are recorded with precision which is on the scale of their variance, so one needs to take mean values (profiles) to understand systematic behavior. Here are the setting and readback voltages for the late period:
While there are some features in the data, nothing stands out as being related to the huge current dips. How do these cathode setting and readback voltages appear for the early period?
The features look similar, and point to no evidence of the huge current drops observable therein. To make the point even clearer, here are the cathode and setting voltages versus IFCW current, with the late period in dark blue, and the early period in red:
The early period shows that the cathode setting voltage is correlated with the drops in current generally accepted to be due to luminosity (and ionization outside the TPC volume). The late period shows a similar correlation for the higher currents (though it is curious that the correlation is not quite the same), but the correlation generally vanishes for lower currents, which are associated with the huge current drops. The readback voltage, on the other hand, shows no correlations with current at all, but also shows a curiously different relationship for early and late periods.
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Let's come back to the field cage voltages for the late period and take a closer look. If we divide the voltage by the current, we should get a fixed effective resistance. Interstingly, this seems not to be constant with time. Here are the -Res0 = Viw0/Iiw, and -Res1 = Viw1/Iiw vs. day, using colors to correspond to day:
And here again is the Iiw vs. day plot using colors to help correlate:
The first huge current drop occurs before the discontinuous step in the Res0/1 values. How is it possible for these effective resistances to change?!? And does the east field cage show anything similar? Here now are some plots correlating the effective resistances, using the same color scheme:
While Res1 east and west seem correlated (both have similar values regardless of time), the discrete Res0 values from east and west do not appear well correlated with each other, nor with their same-chain Res1 values. If one treats the resistor box as seeing infinite impedence "up" the resistor chain, both Res0 and Res1 measure effective resistances using exactly the same resistors, just at different points. So they should be very correlated. It is mysterious!
-Gene