Why we need one

   The minitorus was originally proposed by Mark Pitt in order to "sweep away" the Moller electron background which would have been as issue for the middle chambers. The elastic ep rate at the middle chambers is approximately 4 kHz/nA, and the Moller rate at that location is approximately 1200 kHz/nA (See Figure 1 ). This wouldn't have been a problem, except that it would be desireable to run at 10 nA, and the total rate would then have been more than the middle chambers could handle. Another solution to this problem was to place the minitorus in the main magnet field, but the cost of doing this would have been equal to or more than the cost of building the minitorus. In addition, it is desirable to have a tracking point from the chambers before the elastics eps enter the main magnet field.

Will it work

   The first step in investigating the possibility of the minitorus was to see if it would, in fact, be able to bend the Mollers into the secondary collimator. This could be accomplished by bending them up, away from the beam pipe, or down toward the beam pipe (See Figure 2). The issue of bending them down is that the minitorus also bends the eps, and it bends them enough that the tolerance for a support bar of the main magnet is now unacceptably small (See Figure 12, middle plot ). Bending up, however, is not possible within real constraints on size and operating current of the minitorus itself - because it does not reduce the Moller rate enough. Bending up would require the minitorus to bend the highest energy Mollers the entire opening of the secondary collimator (See Figures 3, 4 and 5). This dilemma was solved by the suggestion of changing the mean angle of the experiment from 8 degrees to 8.4 degrees. Then, even with bending the eps toward the beampipe, the tolerance for the main magnet support bar in question is actually better than it was without the minitorus (See Figure 12, rightmost plot)!

Problems

   Once it was shown that the minitorus could, in fact, be used to bend the Mollers into the secondary collimator, it was necessary to investigate the effect on the eps. It is believed that the eps still clear the infrastructure, especially if the mean angle of the experiment is increased to 8.4 degrees (See Table 1), but how does this change and the presence of the minitorus itself affect the position, shape and focus of the eps at the cerenkov bar location (See Figures 6, 7, 10 and 11)? Related to this is the question of whether the position/shape/focus at the cerenkov bar location would change too drastically to run with the minitorus on and off. Some other potential problems are helicity-correlated and helicity non-correlated effects related to the stability of the minitorus current, and possible background issues due to the Mollers hitting the secondary collimator.

Specifications

   The minitorus consists of 8 coils in a toroidal configuration. Each coil consists of 2 double pancakes of .7 cm square water cooled copper conductor, 25 turns per pancake. The inner radius of a coil is 8.75 cm and the outer radius is 26.25 cm, giving a cross sectional area of 49 cm². The maximum current density that we want is about 470 A/cm² (See Figures 8 and 9) . Taking into account the .39 cm diameter hole for the water cooling, that gives us a total minitorus current of about 175 A. The minitorus will be placed just after the primary collimator, with the outer radius about 10 cm away from the outer radius of the beamline. In this configuration, the maximum B field halfway between the coils at the center of the coils is at a radius of about 25 cm (See Figures 13 and 14). The value of the B field here is about .9 kGauss.