Beam-line Magnets near the Interaction Point

The final-focus quadrupole magnets (QCS) are located inside the field volume of the detector solenoid and are common to both beams. In order to facilitate the high gradient and tunability, these magnets are superconducting at the expense of a larger size. In order to minimize backgrounds from QCS-generated synchrotron radiation, their axes are aligned with the incoming $e^+$ and $e^-$ beams. This requires the radius of the backward-angle region cryostat to be larger than that of the one in the forward-angle region. The inner aperture is determined by the requirements of injection and the need to avoid direct synchrotron radiation incident on the beam pipe inside the cryostats. The $z$-positions are determined by the detector acceptance (17 $^o \leq \theta \leq$ 150$^o$).
To minimize solenoid-field-induced coupling between the $x$ and $y$ beam motions, superconducting compensation solenoid magnets are located near the interaction point (IP), occupying the same cryostat as the QCS magnets. Since the $\int B_z dz$ between IP and QCS is required to be nearly zero, these magnets run as high as 4.8 T. The cryostats for QCS and the compensation magnets are supported from a movable stage that provides a common support base for all the accelerator magnets located in the experimental hall.
The QC1 magnets are located outside the QCS cryostats and help provide the vertical focus for the high energy beam only. Although these are normal conductor magnets with an iron return yoke, a special design is necessary because of the small beam separation in this region. The one in the forward region is a half-quadrupole with the iron septum. In order to reduce the synchrotron radiation background from the incoming beam, the backward region QC1 is a special full-quadrupole [5]. The locations are chosen so as to avoid the leakage field of the detector solenoid.