Target Chamber

The Hall A target chamber is a large evacuated multistaged can that contains the target struck by the CEBAF electron beam. The chamber was designed to isolate the beam line vacuum from each HRS so that each HRS could rotate around the target without vacuum coupling and without jeapordizing certain desired kinematic and acceptance specifications of both high resolution spectrometers needed for approved experiments. It was also designed to simultaneously contain a liquid or gas target and an array of water cooled thin metallic foils, both remotely controlled and also be adaptable for the waterfall target. The desired kinematic specifications that were considered included momentum and energy resolution in both arms, angular range of spectrometers, angular acceptance, and luminosity. The chamber vacuum is isolated from the HRS by using thin aluminum foils.

The target chamber is designed so that each spectrometer will have continuous coverage in the standard tune from $\theta_{min}=$12.54 $^\circ$ to $\theta_{max}=$ 165 $^\circ$.

The target chamber is supported by a 24 $in$ diameter pivot post secured in concrete, rising about 93.6 $in$ above the Hall A cement floor. The Hall A target chamber consists of an aluminum middle ring, a stainless steel base ring, each with a 41.0 $in$ inner diameter, and a stainless steel cylindrical top hat with 40 $in$ inner diameter to enclose the cryotarget and secure the cryogenic connections.

The aluminum ring with an outer diameter of 45.0 $in$ and wall thickness 2.0 $in$ is necessary for a sturdy support structure and to permit machining of the outside surface to accommodate the flanges for fixed and sliding seals mounted on opposite sides of the ring that vacuum connect the chamber to each HRS. The height of the aluminum ring shown is 36.0 $in$, which is designed to accommodate the mounting flanges. The stainless steel base ring is 11.50 $in$ in height with one pump-out 6 $in$ diameter port and with seven 4 $in$ viewing and electrical feed-through ports. The base ring will also contain support mechanisms for the solid target ladder assembly, a rotisserie for collimating slits, radiators, and magnetic fingers for removing the solid target vacuum-lock can. The total height of the top ring, middle ring, and base ring is 93.81 $in$. This length is partly determined by our desire to include with the cryogenic extended target a solid target vertical ladder secured in an inverted hat through a hole in the base of the chamber.

The base ring includes an end plate through which the inverted hat will be adapted to fit into the large vertical pipe serving as the pivot post for the Hall A spectrometers.

The stainless steel cylindrical top hat has 40.0 $in$ inner diameter, and is 0.375 $in$ thick and 46.31 $in$ high , which is necessary to permit the cryotarget to be withdrawn and to make space available to expose the solid targets to the electron beam.

The 200 $\mu$A electron beam, presumably focussed to a mm spot and rastered $\pm$5 mm horizontally or vertically on the target, enters through a oval hole in the middle ring which is 2.06 $in$ wide and exits through a 1.81 $in$ hole connected to the exit pipe.