The forward chambers provide tracking and triggering from 33 degrees down to 11 degrees in theta.
FCA consists of 2 arms on both ends of the TPC. The one in the backward direction is called side A, and the one in forward direction is called side C.
One side consists of 3 chambers, each with two staggered layers and split into half-discs with an outer radius of 103 cm. Each layer consists of 128 square drift tubes with a 100 micron anode wire in the center. The chambers are rotated with respect to each other by 120 degrees. The wire orientation of each double layer ("coordinate") corresponds to that of a group of four layers in FCB, thus providing 2 x 3 coordinates (x,u,v). Consequently each of these coordinates forms a basic unit for the forward / backward track trigger.
In addition there are 32 cathode strips on each outer side of a module, rotated by 60 degrees with respect to the corresponding anode wires, to improve local pattern recognition. Thus there are 768 anode wires and 384 cathode strips per endcap.
All readout electronics of the FCA (for both sides) is located in Control Room B1.
For each endcap 6 LTDs per coordinate are used for the readout. In order to launch the drifttime measurements at the correct time, the BCO warning from the PANDORA is delayed and distributed to the LTDs via the CTS module.
The LTDs measure the drifttime of the streamer emerging from the ionization caused by the traversing particle. At the same time, the LTDs produce a one-bit hit pattern indicating which channels were hit. 256 wire signals per coordinate (corresponding to 2 chamber layers) are processed in the LTDs (one LTD has 48 channels). There are 32 cathode strips in each detector layer, for each coordinate there are 64 cathode strips. The drifttime information from 4 cathodes are multiplexed via a MUX and the 4x16 signals are stored in the remaining 32 channels of 2 LTDs (LTD3 and LTD6).
Dedicated electronics is used to produce the local trigger results for the first and second level. FCA trigger data are correlated with the FCB ones. The third level is done by software.