Description

The polarized $^3$He target system has four target positions: the actual polarized target cell (which can be replaced by a water cell for NMR calibration (see Chapter ), a solid BeO target inline with a seven-foil Carbon target for alignment and optics calibration, an empty target position that contains no target and allows the beam to pass undisturbed through the apparatus and a reference cell position (see Chapter ). These targets are mounted on a target ladder as shown in Figure .

Figure: Schematic diagram of the target ladder and target positions.

The target ladder can be positioned vertically using a stepper-motor-driven motion control system. The controller can drive the ladder to five different fixed positions, three of which correspond to the three targets, the fourth to the empty (no target) configuration, and the fifth to the position used for NMR measurements where the target cell is surrounded by the pick-up coils. The total required vertical motion range is approximately 15cm.

The motion controller employed was developed at JLab and will be first used in experiment 99-117 (Precision Measurement of the Neutron Asymmetry A$_1^n$ at Large x using CEBAF at 6 GeV). Vertical target motion is provided by a screw-driven cylinder (Industrial Devices Corporation model N2P22V1205A12MF1MT1L). The cylinder incorporates a 200 step/rev stepper motor with a 1:12 gear, resulting in a position repeatability of about $\pm 13 \mu$m at a maximum load of 600 lbs (272kg). The maximum range of motion is 31.5cm. Mechanical position sensors are mounted on the target support frame to indicate motion limits. These switches are wired normally closed and fail open. A magnetic position sensor (Reed Sensor, PSR-1Q) mounted on the motor cylinder serves as the home switch. This switch is wired normally open, using only two of its three wires (12 V is not connected for this type of switch). Additionally, a linear potentiometer provides independent readback of the cylinder position.