| Magnet Unit(s) | 7031 8028 |
|---|---|
| Typical BDES | 3.023 kGm |
| Typical Current | 280 A |
| Typical PS Voltage | 75 V |
| Nominal Beta X | 50 m |
| Nominal Eta X | +/- 0.03 m |
| Nominal Beta Y | 20 m |
| Nominal Eta Y | -0.08 m |
HER B3,B3M is part of the collision straight bending scheme.
HER straight 2 is not straight, but it is flat. Aside from the direction of the bending, the layout is symetric in z about the IP.
The "S" shaped bending scheme of the HER straight 2 arises from the need to separate the beams at the IP using bending magnets.
Four permanent magnets inside BaBar achieve this separation. B1 dipoles (at z = +/- ~0.3m) bend the two beams differently (due to their different energies) to achieve this separation. Further, QD1 quadrupoles (at z = +/- 1.8m centered on the HER trajectory) further deflect the LER beam due to the LER's ~3cm off axis trajectory at the QD1 magnets.
Matching to this near IP HER x trajectory is done with an S-Bend geometry.
Beginning inside HER incoming arc 1, this S-bend geometry is achieved using the following magnet scheme:
Two otherwise ordinary arc 1 dipoles (the last dipole and another 180 degrees or five dipoles upstream) have "backleg" windings (BL,BR Bleg) in addition to the normal bend string windings. These two backlegs are powered together (in series also together with their corresponding magnets on the arc 3 side). On the arc 1 side of IR2, these backlegs both reduce the ordinary dipole's field. This leaves the HER trajectory at the exit of the last arc 1 dipole with an x angle outboard from the center of the ring. On the arc 3 side, these backlegs boost the ordinary dipole's field.
With this outboard angle exiting the arc (z = -61m), the HER beamline moves outboard horizontally as it moves towards the IP. At z = -40.5 m the B3 magnet bends the beam back inboard again so afterwards the trajectory is nearly parallel to the line between the two arc endpoints. At z = -26m the B2 magnet bends the beam back inboard again towards the IP, but not completely. Immediately thereafter a second weaker magnet (BLF at z = - 23.2 m) completes the bending necessary to get the HER pointed back towards the IP. The purpose of this two magnet scheme is to keep most of the synchrotron radiation associated with bending the beam back towards the IP away from BaBar (to prevent backgrounds) and onto water cooled masks.
Once inside BaBar, the HER beam is bent back (nearly straight) again by the inward B1 dipole and overlaps the trajectory of the LER. The outward B1 then deflects the HER and LER towards the inboard side (towards the center) of the ring and a symetric scheme is used to complete the "S" bend geometry into arc 3.