Magnet

The solenoid magnet produces the axial field required for momentum determination from the central-drift-chamber tracks, and serves as the principal supporting structure for all detector components. To avoid inert material which would interfere with photon detection, and range and energy measurements, the magnet is made large enough to contain all the active components within the yoke and coils. The inner volume is 2.22-m long and 2.96-m in diameter.

The steel structure of the magnet is made from eleven circular plates 305-mm thick and 5.0-m in diameter. The magnet is divided into three sections (see Fig. 2c) supported independently on sets of rollers on tracks so that the sections can be moved apart axially for access to the interior volume. The center section carries the water-cooled copper coils made up of 18 sections of 22 turns and 2 cooling circuits each. It also carries the stainless-steel support frames for the barrel-photon-veto and range-stack assemblies through four 25.4-mm-thick aluminum plates spaced between the coil sections. The steel yoke is about 560-mm thick and the coil consumes 1.1 MW at 4400 A to produce the 1.0-T central field. The coil temperature rise is about 11C at full field, and the detector interior temperature is maintained at C. The interior volume is continuously flushed with dry N.

Each of the two end sections (endplates) is made of two 305-mm-thick steel plates from the Space Radiation Effects Laboratory (SREL) cyclotron separated by a 25.4-mm gap to minimize the fringe field at the upstream and downstream faces where the photomultiplier tubes (PMT) are mounted. Each endplate is penetrated by 192 holes for lightguides from the barrel photon veto, and has a 24-spoke structure in the radial region of the range stack also to give access for light collection. The amount of steel removed from the endplate is a compromise between the desired low fringe field and high light collection efficiency. The endplate retains about half its azimuthal steel for flux return. The center part of each endplate is a steel endplug to which the endcap sections of the photon veto are mounted. The upstream endplug also supports the beam instrumentation, and the downstream endplug supports the target. The central drift chamber is supported on a steel cylinder fitted between the downstream spoke structure and the endplug. The endplug with the endcap attached can be removed independently to give access to the ends of the drift chamber using a special fixture mounted on the same tracks as the magnet.

Before the detector elements were installed, the magnetic field was mapped using an array of Hall probes referenced to an NMR probe to establish the magnitude and uniformity in the central volume to be occupied by the drift chamber. The measurement showed the field to be uniform to 1% overall, and 0.6% excluding the regions not occupied by tracks of interest, leading to an estimated 0.2% contribution to the momentum resolution. The fringe fields at the PMTs are typically T at full field and adequate shielding is provided by -metal shields.

The magnet is powered by two 720-KW, 4800-A power supplies connected in series with an LC network to filter the output. The field is monitored with a calibrated Hall probe mounted in the magnet and recorded during running by the data acquisition system.