A forward-angle large-acceptance magnetic spectrometer

Abstract

A large solid angle magnetic spectrometer for high luminosity and forward scattering angles was constructed at the Thomas Jefferson National Accelerator Facility. A number of physics experiments have used this spectrometer, and a significant physics program of future experiments has already been approved. A key feature of the spectrometer concept is a horizontal slit opening that allows the beamline to pass through the yoke of the spectrometer magnet. This design enables a short distance between the target and spectrometer, resulting in a 70~msr solid angle acceptance. The residual magnetic-field on the beamline inside the slit is reduced by a two-layer magnetic shielding system, with the external layer comprising a set of iron rings. Two correcting magnets, before and after the dipole, were used to compensate for the transverse component of the fringe field outside of the dipole yoke. The mechanical stability of the tall dipole magnet in close proximity to the target was provided by means of a heavy counterweight.

Figures (9)

• Figure 1: The views of the SBS magnet. A 3D view on the left and front view on the right (a front field clamp removed for visibility of structure).
• Figure 2: The SBS magnet with removed upper portion for visibility of the beamline. Outer shielding of the beamline (rings) shown in purple for visibility.
• Figure 3: The side view of the beamline magnetic shield. In the middle picture, the iron of the correctors was removed. The upstream portion of the beam shield is shown in the bottom picture (with a bit of rotation) and zoomed with the closer half of the shield removed for visibility of the structure.
• Figure 4: The top panel shows the transverse (green line) and longitudinal (red line) field components relative to particle trajectory in the dipole midplane vs. distance from the target. The bottom panel shows the field components (red line for the transverse and green line for the longitudinal) relative to the plane orthogonal to the dipole midplane at a distance of 220 cm from the target.
• Figure 5: Shown are the fields along the beamline for the central trajectory with both dipole correctors turned ON. The transverse (Bx) and longitudinal (Bz x 0.2) are shown as green and red lines respectively. Bx along a line tilted by 0.5 degree from the beam center, as a blue line.