Tuning the Anodes, Grids, and Voltages

The gating grid, ground shield, and anodes are arrays of wires strung in a regular grid pattern.  Ideally, the voltage applied to these grids is the voltage sensed by an electron drifting towards them. However, this is not actually the case.  The grids are partially transparent and allow the electric fields on one side to leak into the other and these leaking field lines cause the effective voltages on the grids to be different than their actual potentials.  We need to be aware of these effective voltages when we tune the anode sectors.  (See Blum and Rolandi.)

Eric Hjort has put together a really cool applet to calculate Veff  on the grids.   Your job is to input a set of grid potentials in the white boxes and then punch "recompute".  The display will update with your input parameters and the effective voltages they represent.  Note that the gating grid is affected by these calculations and it is possible to set the grid so that it is less than 100% transparent to drifting electrons.  The notation that V is less than some value means that the gating grid must be set to a voltage that is more negative than this critical cutoff value and it is up to you to see that the effective voltage on the gating grid is actually lower than this cutoff.

Previously, we determined that we wanted to operate the TPC with P10 gas and a drift field of about 145 V/cm.  What voltage on the central membrane and gating grid is required to achieve this drift field and still make the grid work?  Try the applet to find out. The values listed below are typical of the setting we will use and they satisfy the constraint that the gating grid is 100% efficient.  These values are also compatible with  fixed resistor values for the last two rings of the field cage and we don't need to retune the field cage if we change gases. (ie. the central membrane, gound shield, and drift field potentials scale by a simple multiple between P10 and He-Ethane.)

 Central Membrane Gating Grid (Veff) Ground Shield Drift Field Anode Voltages P10 Gas -30,700 -127.5  (-125) -37.2 145.0 1130./1360. He - Ethane (50-50) -83,822 -350.0  (-345.0) -105.0 400.0 1330./1560.

Note that the voltage shown for ring 181 is the voltage that should be set for both the inner field cage and outer field cage resistor chains. However, ring 182 should be set to the voltage shown only for the inner field cage. Ring 182 for the outer field cage should be set to the voltage labelled "Shield" in the applet display above. See the previous page for a discussion of how to calculate the resistor values to achieve these voltage settings.

Page created by Jim Thomas, send comments to jhthomas@lbl.gov.