Artist's conception of a single Region 2 Sector. A few example wires are shown strung between the two curved endplates.

Charged particles are tracked by drift chambers which are arranged in three regions: Region 1 close to the target, Region 2 between the coils, and Region 3 outside of the coils. The Region 2 drift chambers consists of six separate sectors, one for each of the six sectors of the CLAS. Each sector contains one axial superlayer with approximately 1200 sense wires in six layers and one stereo superlayer with 1200 sense wires in six layers at an angle of 6 degrees with respect to the axial wires. The wires are arranged into a hexagonal pattern, with up to 192 sense wire in each layer. Each superlayer is surrounded with a row of guard wires to minimize edge effects. The logical layout of the different wire types within a superlayer is shown below:

Logical wire layout of a superlayer in Region 2

There are approximately 6000 guard and field wires per sector for a total of 50,000 wires in the Region 2 drift chambers. The sense wires are made of 20 micrometer thick Tungsten wires (gold-plated) strung with 19 g tension and held at +1750 Volts. The field wires are 150 micrometer thick gold-plated Aluminum wires strung with 150 g tension. Each sense wire has 6 field wires at -960 Volts around it, for a total potential difference of 2710 Volts. The nominal cell "radius" is about 1.2 cm. The guard wires are similar to the sense wires, but are held at a potential of +600 Volts.

The wires are strung between curved endplates that are approximately 5 meters long by 50 cm wide. Each endplate has been placed as close to the coil cryostat as possible so that the magnet and not the drift chamber limits the active area of the CLAS. The following figure shows a schematic drawing of one drift chamber sector and the superconducting coil, projected onto the midplane between coils.

Cross sectional view of a Region 2 drift chamber

The constructing of these drift chambers has posed some interesting design challenges. The drift chamber endplates, dead region near the endplates, feedthrus, sense wire connections, high voltage connections, amplifiers, gas supply, and supports had to fit into the 5 cm shadow region next to the cryostat. The endplates have been designed so that they do not deflect significantly under their own weight plus the tension of all of the wires; they also must not deflect if the CLAS coils quench causing a rapidly changing magnetic field. The wires hat to be positioned to 0.1 mm RMS or better. The chambers were designed and built on a rather short time scale (less than 4 years from the first conceptual design ideas to installation). The Region 2 drift chambers were the first to be installed into the CLAS. For that reason, these drift chambers were on the critical path for the entire CLAS detector.

To address some of these design concerns, we have chosen to use Stesalit, a nonconducting, composite material, for the endplates in order to meet the space requirements and to be stable in case of a CLAS coil quench. With the help of CEBAF engineers and designers, we have modeled the mechanical properties and produced a detailed design of a Stesalit drift chamber. All endplates were fabricated by Stesalit in early 1994, and the approximately 100,000 precision holes were drilled by Remmele Engineering, Inc. Several prototypes were constructed to test the properties of Stesalit as a wire chamber material. The final chambers were strung at ODU (5 sectors, including full scale prototype) and JLab (remaining 2 sectors). The chambers are now all completed, installed, and tested. After a series of commissioning runs (through October 1997), Physics research with CLAS begins in earnest in November 1997.