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Sunday, 5 June 2011

The Electric Drift Field for the STAR TPC

 The Electric Drift Field for the STAR TPC   When a charged particle traverses the TPC volume, it ionizes gas atoms every few tenths of a millimeter along its path and leaves behind a cluster of electrons.  The electron clusters then drift to the anode plane under the influence of an externally applied electric field where their time of arrival and location is recorded.
An animated simulation of the drift is available here.

In the STAR TPC, the electric field is provided by the outer field cage (OFC), the inner field cage (IFC), and the high voltage central membrane (CM).  The purpose of the OFC and IFC is to provide a nearly perfect electric field in which to drift the electrons to the anode plane since any distortions in the field will result in a distortion of the recorded tracks.  The OFC and the IFC also serve to define the active gas volume and were designed to contain the TPC gas and prevent it from being contaminated with outside air.  The central membrane is located in the middle of the TPC and is held at high voltage.  The anode and pad planes are organized into sectors on each end of the TPC and the pads are held at ground potential.   The OFC and IFC include a series of gradient rings that divide the space between the central membrane and the anode planes.  The total distance from the CM to either anode plane is slightly greater than 2 meters. There is approximately one ring per centimeter and the rings are biased by a chain of resistors that connect to the CM, the anode plane ground, and each of the gradient rings inbetween.  The rings are separated by two-megohm resistors and there are 182 rings and 183 resistors in each chain. 

The last two resistors are adjustable and are housed in a rack which is external to the TPC.  Note that the outer field cage has a "ground shield" attached to ring 182.  It marks the end of the TPC drift volume and is used to better define the shape of the field at the terminus.   The inner field cage does not have a "ground shield".   The spark gaps are safety devices for the protection of personnel and equipment in case the external resistors are disconnected while the field cages are biased.
There are four resistor chains.  One for each end of the two field cages.  Thus, IFC East, IFC West, OFC East, and OFC West.

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