On May 9-10, 2006, a test was performed where we reversed the polarity of the Gated Grid as shown in the top of Fig.3 in the GridLeak Simulations ["inverted" mode] for the west side of the TPC. Here are the results of that test.

Z-dependence

Below are plots of the distortion gap (as measured by track residuals) as a function of Z for normal runs (71290[234]*, 57 files) and test runs with the west GG reversed (71290[56]*, 39 files). Here one can see the slight improvement caused by reversing the polarity. At this time I have no explanation for why the distortion seems to abate near the central membrane (the distorting field should abate because of the boundary conditions, but the distortion is an integral in Z over this field).

Normal:

West reversed:

West/East Ratio

Using a linear fit to the distortion as a function of |Z| as a measure of the amount of leaked ions (distortion scales linearly with Z and with leaked current), I can take the ratio of the west side to east side to see if anything changed with the test. I find that the test appears to show a visible decrease of about 2-3% in the ion leakage, as seen in the following (normalized) histograms. I had more files available when I made this plot (70 normal, 69 reversed test files), and only fit over the range 25cm<|Z|<150cm.

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Gene Van Buren
gene@bnl.gov