Beam line and condition

A Jlab standard beam line will be used for this experiment. Table 21 outlines the beam requirement and the CEBAF general specifications are also given as a reference (table 22). A beam energy of 1.8 GeV is the preferred and optimized energy in terms of yield, background, and resolution, although the experiment can use energies in the specified range. Certain kinematic mismatch will take place when using an energy other than 1.8 GeV since the Splitter field MUST be changed to ensure the beam transport from the target through the dump line to the beam dump. This causes a kinematic mismatch between two arms in central momenta.

In order to achieve the overall energy resolution of 300-400 keV (FWHM), the total energy spread must be below 10 $^{-4}$ (FWHM). Both the energy jitter and the bunch spread are required to be $\sigma = 3 \times 10^{-5}$ level or better. Such accuracy was demonstrated and achieved during the experiment E89-009 (HNSS), when a fast energy lock system installed on the Hall-C line was activated. For this high precision experiment, it is essential to use such a lock system on Hall C for ACC to control energy stability.

The slow lock system on the ACC arc is also necessary. The fast lock system occasionally becomes unstable (tripping off sometime during data taking). The slow lock can maintain the energy stability within $(1-2) \times 10^{-4}$ (FWHM). If such a period is less than 10% of overall beamtime, the effect to the resolution might be not significant.

There is a plan to install a special device at the location of middle arc in the Hall C beam line where the momentum dispersion is the largest. This device will measure the beam position at that location and the result can be put in monitoring display and in data stream along with readouts from BPMs at the entrance and exit of the arc. Energy shift correction can be done in off-line analysis when it becomes necessary. This device has no material in the beam, thus producing no additional beam halo for the experiment. It replaces the OTR that has been used in the HNSS experiment for the same purpose. A small halo was, however, found with the OTR from its thin carbon foil.

Beam position fast lock and a tune for small beam spot size are also required as listed in the table 21. This is important to stabilize and optimize the spectrometer optics. The requirement is the same as achieved during the HNSS experiment. Readout of the BPM (beam position monitor) closest to the target will also be fed into the data stream for off-line correction as done in HNSS.

There are other additional precautions taken during the HNSS experiment, such as monitoring arc dipole field stability with the 9th dipole chained in the power supply and arc dipole temperature variation. It was found that power stability was an order of magnitude better than spectrometer magnets' power supplies and the temperature changed less than half a degree. Therefore, they made negligible effects on the arc optics. Such beam line and items should be maintained in E01-011 again.

The existing BCM (beam current monitor) will be used. Its accuracy is sufficient for this experiment as its contribution to the systematic error in calculating the cross section is much less than the statistical error.

表 21: Beam requirements for the E01-011 experiment (Hall C)

Beam Parameters				Targets
	Range	Stability	Max	C	CH	CH	CH	Si
Beam Energy (GeV)	1.62.0		2.0	1.8	1.8	1.8	0.9	1.8
Beam Energy Jitter ( $\sigma$ )		$3\times10^{-5}$	$1 \times 10^{-4}$
Bunch Momentum Spread ( $\sigma$ )		$3\times10^{-5}$	$1 \times 10^{-4}$
Total Beam Energy Spread (FWHM)		$1 \times 10^{-4}$
Energy Lock Required				$\surd$	$\surd$	$\surd$	$\surd$	$\surd$
Beam Current ( $\mu$ A)			50	30	5	30	5	30
Beam spot size $\sigma_x$ (rms, $\mu$ m)
$\sigma_y$ (rms, $\mu$ m)
Beam Angular Spread $\sigma_x, \sigma_y$ (rms, mrad)		0.05	1, 1 $^{(1)}$
Beam Position (rms, $\mu$ m)
Fast Beam Position Lock Required				$\surd$	$\surd$	$\surd$	$\surd$	$\surd$
Beam Halo at 5 mm $^{(2)}$		$1 \times 10^{-4}$

(1) The beam divergence is not tightly controlled, i.e. the beam is focused to the desired spot size.
(2) Ratio of integrated beam currents at specified beam radius. While direct measurements are not available (apart from Hall B), it has been demonstrated not to be a problem up to 100 $\mu$ A.

**表 22:** CEBAF Accelerator Parameters $^{(1)}$
Beam Parameters	Nominal
	Value/Range
Beam Energy (GeV)
Beam Current ( $\mu$ A/Hall)
Repetition Rate (MHz/Hall)	499
Charge per Bunch (pC)
Extracted Beam Energy Spread (rms)	$<1 \times 10^{-4} ^{(2)}$
Bunch Length ( $\mu$ m)
Normalized Emittance (mm $\cdot \mu$ rad)	1
Transverse Beam Size (rms, $\mu$ m)
Beam Angular Spread (rms, rad)	$< 0.1 / \gamma$
Beam Position ( $\mu$ m)
Beam Halo (Hall B)	$<2 \times 10^{-6}$
	6mm, 5nA $^{(3)}$
Beam Scraping ( $\mu$ A)

(1)G.A. Krafft et al., Measuring and Controlling Energy Spread in CEBAF, JLab-ACC-00-13
(2) $5 \times 10^{-5}$ for one Hall.
(3)Preliminary estimate.