The high-energy beam line (4) which delivers the proton beam from the main stage cyclotron to the target area is split into two parts: 1) Beam line 3 (BL3) which is between the main stage cyclotron and the bend, and 2) Beam line 5 (BL5) from the bend to the beam swinger area. Both BL3 and BL5 include a precession solenoid and a beam line polarimeter.
Of necessity, the beam must pass through the cyclotron with a polarization normal to the plane defined by BL3 and BL5, which is parallel to the fields in the cyclotron. The combination of the precession solenoids and the bend make it possible to precess the beam polarization to any of the three states: normal, sideways, and longitudinal (N, S, and L). The precession solenoids are capable rotating the spin of the proton in the plane transverse to the beam line. The bend will precess any polarization component in the plane defined by BL3 and BL5 according to the Thomas precession formula:
where is the proton mass, and is the proton magnetic moment and is equal to ( nuclear magneton ). At 200 MeV the precession can be given, to a very good approximation, as . So, any in-plane polarization component will be rotated by .