I have calculated the bias necessary on the STAR TPC field cage electrodes and wire planes in order to achieve 100% transmission through the wires without distortions. A postscript file of an example (MathCad) calculation for the Outer sector with P10 is available. This example is useful because it shows the basic calculation clearly. However, the calculation of V2max is not quite right. It has been corrected in the java code.

I have also written a java applet that does the same calculations. Click here to run the applet, or click here to look at the calculational part of the java code. Unfortunately it does not yet work across the net due to security violations but you can see an example calculation for P10 here and an example calculation for He/C2H6 here. These examples use voltages that result in drift fields of 400 V/cm and 130 V/cm for He/C2H6 and P10 respectively along with estimated anode voltages for each gas. In each case the requirement on the gated grid voltage for 100% transmission has been satisfied and the effective gated grid voltages (Veff) on the outer and inner sectors have been matched. The applet works as follows:

Inputs:

Anode voltages (determined by gas gain requirements)
Membrane voltage (determined by desired drift field)
Gated grid voltages (to be determined by using the program)

Outputs:

Appropriate bias for the last 2 stripes on each field cage.
Appropriate bias for the shield electrodes.
Gated grid bias condition for 100% transparency.
Effective voltages on all 3 wire planes.
Magnitude of the drift field.
z coordinates of objects from survey data. (incomplete)

The idea is for the user to adjust the voltages until the 100% transparency condition on the gated grid is satisfied and the inner and outer gated grid effective potentials are matched.