Low-current operation

For experiments with the polarized target, the CEBAF accelerator should be operated with average beam currents in the range 10nA to 100nA. This could represent a problem, as the instrumentation of the accelerator is designed for a minimal beam current of 1 A. In this section we show how this difficulty can be circumvented.

Low-current operation actually encounters a set of problems:

• The present gun is not capable to deliver simultaneously low currents and currents in the range of 100 A, thus restricting the flexibility in the choice of beam currents delivered to simultaneous users in different halls.
• The polarized source equipped with a stressed photocathode (delivering 80% polarization) is not suitable for high beam intensity operation (above several A) due to the small quantum efficiency.
• The accelerator is not instrumented for currents much below 1 A.

These complications all solvable, however the solutions will not be implemented at the time we plan to run our experiment.

The solution for running the G-experiment is a straightforward one:

• Running the experiment during the interval of the first 1.5 years where only Hall C is operative makes Hall C the single user of the accelerator, and the question of greatly differing beam currents for simultaneous users does not arise.
• The G experiment only requires low beam currents ( 100nA for the physics running, potentially A for additional irradiation of the ND target at 1K). The experiment therefore gets all the current needed from the presently available stressed GaAs cathodes (which are used in experiment E143 at SLAC which runs in Fall 1993)
• The monitoring of the beam in the accelerator can be carried out by adding to the low intensity CW-beam spikes of 50-100 s duration and 1 A peak intensity. This scheme again is already implemented at SLAC where the beam monitoring system has the same difficulties with the low intensities required by the polarized target. Such a spike can be produced in the polarized source by adding a further Pockels (or Kerr) cell.

The monitors at CEBAF used for the steering of the beam have time constants of less than 50 s. A spike of >50 s duration and 1 A intensity can be sampled, and used to control the beam position. The rate of these spikes is expected to be in the 10 - 100 s range, contributing an additional averaged current of 0.5 - 5 nA. This additional intensity is perfectly acceptable to the polarized target.

During these spikes of high peak intensity, we plan to veto the data acquisition (a loss of duty cycle of less than 1%). We also envisaged using the beam rastering system to steer the beam during the spike to a point 2 mm below the ND target, such that, due to the low target density at that point, the spike would add nothing at all to the overall radiation dose received by the target.