Sector 20 has a dead anode region (20-5) over padrows 14-20, and partially over padrow 21. It should have electric field lines (magenta) which are unusual on the last "live" anode wire above padrow 21. These field lines come from a smaller radius and are not counter-balanced by field lines at larger radius. If nothing else, this will lead to a bias in where the electrons fall along the anode wire due to the Lorentz effect (these electrons will not be traveling parallel to the magnetic field) which will be directionally dependent on the sign of the magnetic field.

I think this different arrangement of field lines also implies that this anode wire may appear to have more gain than other anode wires as it has more field lines terminating at it.

Even if the anode section won't hold full voltage, it appears to me that keeping the anode section at some significant fraction of full voltage (but still too low for any notable gain) would better stabilize the field lines in this region to reduce these effects.

-Gene

[adapted from the TPC hardware documentation]