1. It is the best to set HV on -1910. There is no need to vary when beam current or target changes. The efficiency is no worse than 2005 condition. With the current limit setting used now, 40uA on C target may not be a problem since there is still a significant margen.
2. For threshold, 5.5V is in agreement with that used in 2005. However, we can certainly also make a simple enough type of variation rule for an optimization for different beam-target condition during different data taking run periods:
(A) DC1-F current under 50uA, set threshold to 2.5V (B) DC1-F current above 50 but under 100uA, set them to 4.0V (C) DC1-F above 100uA, use 5.5V (D) DC1-F above 150uA, 6.0-6.5V must be used to quiet down Fastbus.
Once the physics data taking starts, this rule does not make frequent
changes. Higher threshold under high beam-target condition is mainly
reducing the multiple-hits per plane which affects the efficiency just
like a typical beam line MWPC. The last two scenaniors happen only when
we run higher than 30uA on C or high current on Cr and Si targets.