Determination of the incident beam energy
2. Control of the beam position, direction emmittance and stability.
3. Determination of the beam current, total charge and polarization.
Table 2.2.1.1 gives a listing of all the various elements along the Hall A beamline from the switchyard to the dump at the time the beamline was commissioned on April 18, 1996. New equipment along the beamline are planned to be installed, and these are listed in Table 2 with their approximate locations.
The basic Hall A beamline consists of the following:
The Beam Entrance Channel
The beam entrance channel consists of stainless steel tubing connected with conflat type flanges. The usual opening is 2.5 inches for better vaccum conductance except through magnetic and beam diagnostic elements where it is usually 1 inch. Sections are isoltaed by vacuum valves and these are listed in Table 2.2.1.1. Each section has a roughing port and is pumped with an ion pump. The pressure is about 10-6 torr. There are several sections along the beamline where users will interface their equipment. Their individual systems must be tested leak tight (to c.c./sec).
The Beam Optics Channel
These consist of dipoles, quadrupoles, sextupoles, beam correctors with their standard girders and stands. Starting from the beam switchyard, there are eight dipoles in the arc section which (along with five other smaller beam deflectors) bend the beam 37.5 degrees into the hall. Each dipole has a quadrupole and a pair of steering magnets (correctors) associated with it. After the shield wall at the entrance to the tunnel into the hall the beam is essentially undeflected onto the target and into the dump. There are 7 quadrupoles and 6 beam correctors between the shield wall and the target.
The beamline optics elements are so designed so as to be able to deliver various optical tunes of the beam on to the physics target as well as simultaneously deliver various optical tunes at other locations along the beamline. These requirements are listed in Table 2.2.1.5. For the basic beamline we are able to deliver beam onto the hall A target in the achromatic mode. Plans in the future also include to deliver beam in the dispersive and the defocussed modes at the target location as well as have various spot sizes at the Moller and the Compton Polarimeter locations.
Beam Diagnostic Elements
These consist of beam position monitors (BPMs), beam current monitors, wire scanners (superharps) and beam loss monitors. These are desicribed in the following subsections. The wire scanners are fabricated by Saclay (French collaboration) and four have been installed along the beamline, two before the arc section and two after the arc section. They are essential for the beam energy determination by the arc method. Another two wire scanners will be installed on the bench just before the target to determine the beam position and direction of the beam at the target point with high precision and also measure the emmitance of the incident beam. They will also absolutely calibrate the two associated beam position monitors located in front of the target.
Beam Exit Channel
After the target vacuum chamber which is built by the University of Virginia and described elsewhere in this document, there is an exit beam pipe which transfers the scatterd beam onto the dump tunnel under vacuum. This exit beam pipe is made of a thin walled aluminum spiral corrugated pipe of welded construction. The largest diameter is 36 inches with a 0.164 inches wall thickness and the smallest diameter is 6 inches with a 0.042 inches wall thickness. The whole assembly is rather light (approximately 800 kg) and is supported by H shaped adjustable stands. To prevent possible linear collapse of the larger diameter sections under vacuum load, four aluminum channels of total crossectional area of 3 inches are welded to its side. A vacuum of 10-5 is maintained with a turbomolecular pump. The exit face of this pipe has a 12 inch port and is connected to the diffuser with a Beryllium window.