Field Shaping

  The usage of surfaces (wires, planes, strips) with controllable potential configuration to get a desired shape for the electric field in a given volume, typically in drift chambers. This includes, of course, the introduction of additional wires or grids, in order to optimize the field, e.g. to get a uniform drift field in the cells of a drift chamber, or a simple relation between drift time and distance, independent of track parameters. Field shaping wires are in principle all those wires in multiwire chambers which contribute to the shape of the electric field in which the electrons drift towards a sense wire. This includes, of course, the cathode wires. Sometimes, wires with different potential are used to obtain optimal conditions; resistive voltage dividers allow one to obtain the desired potential. In drift chambers one also talks about the cathode wires as field shaping wires and about the (thicker) wires that alternate with the sense wires as field wires. When executed as conductive strips on printed circuit boards, usually in narrow bends and long straight sections, the name racetracks is also in use. Separate grids are introduced in time projection chambers and time expansion chambers, to gate out ions and control the drift region.

For a fixed geometry, the solutions to the differential equations for electrostatic fields can be found in textbooks of classical electrodynamics (for a few basic formulae, see [Barnett96]). For literature on detectors, see [Blum93], [Sauli91], [Sill90a], [Sill90b], and [Beingessner80].