The Hall-A electron and hadron spectrometers rotate around the central bearing to achieve the desired scattering angle with respect to the beam line. The electron spectrometer angular range is 12.5 to 165 while the hadron spectrometer has an angular range of 12.5to 130.Each spectrometer weights 1000 Tons and it can be viewed as composed of two main sub-units: the "spectrometer proper" and the carriage system.
The "spectrometer proper" is composed of the dipole and a 25 meters long steel structure (the Box Beam) rigidly attached to it. Viewed from the spectrometer side, the set resembles a "T" with a stubby central leg (dipole) and long arms (Box Beam). The remaining spectrometer magnetic elements (quadrupoles Q1, Q2 and Q3) as well as the detector package are rigidly attached to the Box Beam. Consequently, all spectrometer magnetic elements and detector package form a single rigid body.
The carriage sub-unit is composed of two parts: the cradle and the detector shield house gantry. The cradle supports the "spectrometer proper" and transports it around the central bearing to achieve the desired scattering angle. The main pourpose of detector shield house gantry, as it name indicates, is to support the detector shield house and to move it around the central bearing as the "spectrometer proper" rotates. This structure also carries all spectrometer magnet power supplies as well as all instrumentation associated with the basic spectrometer ecosystem (i.e. cryogenics, vaccum, magnet power supplies, magnet field measurement, spectrometer motion, calibration mask, etc). The detector shield house is designed so that it surrounds the detector package but it does not put any weight on the Box Beam supporting the detector package and quadrupoles.
The spectrometer carriage system moves on 40 connical wheels. The wheels are arranged in groups of 4 wheels (a bogie) for a total of 10 bogies per spectrometer. One wheel of each bogie is driven by a servomotor. All the spectrometer weight is carried by the wheels (i.e. the central bearing is used only to constraint the spectrometer motion radially). For each spectrometer, the 10 bogies are distributed into 3 groups: 2 bogies are located at the front of the cradle (closest to the target), 4 bogies at the back of the cradle and, 4 bogies at the back legs of the detector shield house gantry. The front legs of the detector shield house gantry are supported by the cradle. Each group of bogies move over steel plates imbedded in the floor. The floor plates form three concentric rings centered at the target point. The servomotors propel the spectrometers at a maximum angular speed of 3/minute.
The cradle supports the "spectrometer proper" via three vertically mounted hydraulic jacks (a three point suspension). One jack is located at the front end of the dipole (closest to the target) and below it. The other two jacks are located at the back of the dipole, one on each side. The three jacks allow to change the elevation, pitch and roll of the "spectrometer proper" to compensate for floor deformations. As the spectrometer rotates around the central bearing, the pointing of the "spectrometer proper" towards the target may change. To allow correction of the "spectrometer proper" pointing, the two vertical support hydraulic jacks located at the back of the spectrometer rest on special rubber pads. Two horizontally mounted hydraulic jacks, one at each side of the spectrometer cradle, act in a push-pull configuration to deform the rubber pads and force a rotation of the "spectrometer proper" around the front vertical support hydraulic jack.
Power for the wheel servo-motors come from servo-amplifiers located at the back of the spectrometer, under the power supply plataform (lower floor of the shield house gantry). The servo-amplifiers for the cradle front and back row of motors are located at the back of the cradle inside two gray looking cabinets placed next to each other. In case of the electron spectrometer, cabinet EMC71BI houses the servo-amplifiers for the cradle front servomotors while cabinet EMC71BM performs the same function for the cradle back servomotors. In the case of the hadron arm, these cabinets are lebeled EMC72BI and EMC72BM respectively. The servo-amplifiers for the shield house gantry wheels are located in a gray looking cabinet located about midway between the detector shield house gantry legs, right accross from the cradle front and back wheel servo-amplifiers. The electron spectrometer cabinet is lebeled EMC71BO while EMC72BO is the nomenclature used in the case of the hadron arm. These cabinets MUST remained closed and locked during operation and can only be serviced by qualified personnel (see list below). Lethal voltages and currents are present inside these cabinets.
The horizontal positioning servo-amplifiers of each spectrometer are controlled by a VME based single board computer (Motorola MVME-162) running the VxWorks operating system. In the case of the electron spectrometer, the VME crate hosting the CPU and associated intrumentation cards is lebeled ECV71A and it is located in the power supply platform at the bottom of rack IH71B05. This rack is about midway into the third row of racks from the back of the dipole. In the case of the hadron arm, the crate is labeled ECV72A and it is located in the hadron power supply platform at the bottom of rack H72B05. As in the case of the electron arm, this rack is about midway into the third row of racks from the back of the dipole.