Bias Voltage on the Wire Planes --------------------------------- Initial Rev. 29-OCT-2005 Current Rev. 14-NOV-2005 In order to design the field shaping electrodes one needs to know how the wire planes will be biased. The cryostat must be defined as being at 0 Volts. The Voltage of the wire planes and of the field shaping electrodes is measured with respect to the 0 Volt cryostat. In principle one could bias the wire planes in any arrangement that gives the proper filed lines. Three basic arrangements of wire plane Bias Voltage are: Wire Plane Setup #1 Setup #2 Setup #3 ------------------- ---------- ---------- ---------- Cathode -HV -HV -HV 1st Induction plane - Bias - 1/2 Bias 0V 2nd Induction plane - 1/2 Bias 0V + 1/2 Bias Collection plane 0V + 1/2 Bias + Bias For normal operation the ICARUS 600T detector appears to prefer a bias arrangement that is closest to the "balanced + and - Bias" shown in basic setup #2 above. The setup for normal ICARUS operation is: -220 V on the Induction 1 plane; 0 V on the Induction 2 plane; +280 V on the Collection plane. They say that they picked this arrangement of Bias Voltages because it, "optimize the transparency and the signal to noise ratio on all the wire planes". See page 94 of, "Design, construction and tests of the ICARUS T600 detector", Preprint submitted to Elsevier Science 5 January 2004 I expect that this rather balanced setup, with the center wire plane being at 0 Volts, gives the best results because it minimise the Voltage across the bias network capacitors and resistors. It would be typical for the "excess noise" produced by these components in the bias network to increase as the Voltage across them increases. I assume that the bias on the Collection plane is a little higher than the bias between the other two planes in order to make the second induction plane transparent and to guarantee that the Collector is in a deep enough potential well to trap all electrons that come into it. I.E. electrons can not come through the middle plane with enough kinetic energy to escape the Collection plane. Also on page 94 of this document they talk about initially running and debugging the system with a different arrangement of Bias Voltages specifically: 0 V on the Induction 1 plane; +180 V on the Induction 2 plane; +420 V on the Collection plane. I have not understood why this different arrangement was used during the initial running and testing. Do these values make sense ? ---------------------------- Do the actual values of their Bias Voltage (-220V and +280V) make sense ? The ICARUS plane spacing is 3 mm. Thus these Bias Voltages provide a 733 V/cm field between the two transparent planes and a 933 V/cm filed between the middle plane and the collection plane. This compares with the 500 Volt/cm field in the bulk of the ICARUS drift volume. In another publication form the ICARUS experiment they indicate that a field of 700 V/cm is enough to insure, "complete transparency (of the wire planes) to drifting electrons". See Page 38 of, "First observation of 140-cm drift ionizing tracks in the ICARUS liquid-argon TPC", Nuclear Instruments and Methods in Physics Research, A 449 (2000) 36-41 They say, "Second wire plane (collection in this experiment) set to virtual ground through the pre-ampliers. Voltage on the first wire plane (induction) set to -280V; this corresponds to a drift field in the gap between induction and collection of 700 V/cm, enough to ensure complete transparency to drifting electrons." Sketch of how to pick the wire plane Bias Voltage for FLARE ----------------------------------------------------------- Pick the center plane to be at 0V. This makes sense because you could even skip putting the DC blocking capacitors on these wires and thus cut down on the number of parts that can fail. You also can eliminate the Bias Voltage power supply for the center wire plane. This should make the center wire plane the electrically quietest plane in the detector and thus let you verify whether or not the other planes have excess noise from their Bias Voltage components. How much Bias Voltage is needed to make an induction plane transparent ? - My current understanding is that for the plane to be transparent you just need to satisfy: Ea/Eb > (1+x) / (1-x) Ea is the field after the grid Eb is the field before the grid where x = 2pi r/d r is the wire radius d is the distance between wire centers - I would hope that in the FLARE Detector that the required Bias Voltage is not over 200 or 300 Volts. If the required Bias Voltage becomes too high then it becomes more difficult to have nice quiet (no excess noise) DC blocking capacitors and distribution resistors in the Bias Voltage network. I assume that the Bias Voltage on the Collector plane would be positive and higher in magnitude by 60 Volts than the bias on the first induction plane.