The assembly and operation of a polarized target inside the CLAS
spectrometer presents an array of technical problems not encountered at SLAC.
Advantage must be taken of the near 
geometry of CLAS so that a minimum of
solid angle is occluded by the structure and supports of the target magnet and
refrigerator. It is likely that, once installed, the target system will stay
in place for a lengthy period so that as many experiments as possible can be
completed. This means that some flexibility must be built into the target
configuration to allow, for example, different targets to be used. To further
these objectives it has been decided to design a new magnet customized to the
parameters of CLAS. For instance the supports of the coils of this magnet can
be positioned to be in the shadow of the CLAS coils. Some of the experiments
require beam intensities at a level to which a 
He refrigerator is best suited
and can be designed and built as a package with the magnet. In the past six
months various discussions have taken place with Oxford Instruments, the
builder of the CLAS magnet, and previous polarized target magnets and
refrigerators. They have done design studies to find the best coil
configuration for the target parameters and this cooperation will continue.
After discussions with interested parties at TJNAF (CEBAF), a preliminary design is being drawn up. Because of height constraints in CLAS the refrigerator has to be fitted into the polarizing magnet at approximately 30 degrees from the horizontal. However it does appear that the various targets necessary for the different experiments can be mounted vertically and controlled to move into the beam on demand.
The GDH sum rule experiment and any other using low intensity photon beams will be greatly enhanced by using a dilution refrigerator, operating in frozen spin mode, and with material which contains no polarizable nuclei, such as butanol, rather than ammonia. It would be advantageous to have a dilution refrigerator in our UVA laboratory to carry out some of the material investigations. Such a refrigerator would be a simple, quick-load type and will be built at Virginia.
The design, assembly and testing of these different pieces of cryogenic equipment would take place over the three year lifetime of the proposal as would the development and testing of target materials. Components of the system will be available for installation at TJNAF (CEBAF) in late 1997.