Beam line and requirements

A Jlab standard beam line will be used for this experiment. In order to achieve the overall energy resolution of 300-400 keV (FWHM), we require that 1) beam energy spread is better than $1 \times 10^{-4}$, 2) beam energy stability is better than $1 \times 10^{-4}$, 3) beam X and Y spot size at target $<= 60$ and 100 $\mu$m (r.m.s), respectively, and 4) beam position stability is better than 100 $\mu$m. Such beam accuracy was achieved during E89-009 by the energy and beam position lock systems. The energy lock system contains two independent subsystems, slow and fast locks. The slow lock uses the accelerator arc to measure the energy spread. Instability with a rate of a few 10 Hz is corrected with feedback to the last cavity. Using this slow lock technique, the $1 \sim 2\times 10^{-4}$ energy spread and stability was achieved. The residual instability was dominated by the high frequency (kHz) noise. The fast feed back energy lock uses the Hall C arc to measure the spread and instability with a frequency of few kHz. With this fast lock system, the beam energy stability and spread were better than $5 \times 10^{-5}$, which meet the requirements of the experiment. Sometimes, the problems due to electronics and communication systems stops the fast lock system work and, thus the accuracy deteriorates to $1 \sim 2\times 10^{-4}$. More than 90% of the E89-009 experiment period, the fast and slow combined system provided better than $5 \times 10^{-5}$ accuracy. The fast feedback position lock system uses a series of the Hall C arc beam position monitors (BPMs) to measure and control the corrector magnet before the arc. This system maintained the beam position drift to be less than $100 \mu$m which satisfies the requirement of the experiment. The beam spot size was tuned to achieve $\Delta X = 30$ and $\Delta Y = 75$ $\mu$m for E89-009, and it also meets our requirement.

Therefore existing resources can be used to realize our requirements for stabilities of the beam energy , spot size, and position. Both Hall A and C are equipped with such a fast lock systems and one of the energy lock systems is able to control the overall. However, it is known that the energy quality of one hall deteriorates by a factor of 2-3 when the other hall's energy lock system is used. To ensure the accuracy of the present experiment, we request that the fast energy lock of the hall which our setup is installed at controls the overall energy quality during this experiment period.