Introduction

The control and measurement equipment along the Hall A beamline consists of various elements necessary to transport beam with the required specifications onto the reaction target and the dump and to simultaneously measure the properties of the beam relevant to the successful implementation of the physics program in Hall A. The resolution and accuracy requirements in Hall A are such that special attention is paid to the following:

1. Determination of the incident beam energy

2. Control of the beam position, direction, emittance and stability

3. Determination of the beam current, total charge and polarization.

Table 2.1 gives a listing of all the various elements along the Hall A beamline from the switch yard to the dump. Figures 2.1, 2.2, 2.3 show a schematic of the Hall A line starting at the shield wall.

Figure 2.1: Schematic of the Hall A beamline starting at the shield wall to end of alcove.

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Figure 2.2: Schematic of the Hall A beamline from the end of the alcove to the target chamber.

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Figure 2.3: Schematic of the Hall A beamline from the target chamber to the dump diffuser.

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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 vacuum conductance except through magnetic and beam diagnostic elements where it is usually 1 inch. Sections are isolated by vacuum valves and these are listed in Table 2.1. 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 interface their equipment. Their individual systems are tested leak tight (to $\le 10^{-9}$ Atm cm$^3$/sec).

The Beam Optics Channel

These consist of dipoles, quadrupoles, sextupoles, and 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.

The beamline optics elements are designed 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 2.5. For the basic beamline we are able to deliver beam onto the hall A target in the achromatic mode.

Beam Diagnostic Elements

These consist of beam position monitors (BPMs), beam current monitors, wire scanners (superharps) and beam loss monitors. 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 are 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 are also used to absolutely calibrate the two associated beam position monitors located in front of the target.

Beam Exit Channel

After the target vacuum chamber, which was built by the University of Virginia, there is an exit beam pipe which transfers the scattered 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 cross-sectional area of 3'' are welded to its side. A vacuum of 10$^{-5}$ Torr is maintained with a turbomolecular pump. The exit face of this pipe has a 12'' port and is connected to the diffuser with a Beryllium window.