The AGS partial snake experiment

The AGS partial snake experiment E880 is intended to study the possibility of accelerating polarized beams from 1.5 GeV to 25 GeV kinetic energy in the Brookhaven AGS with a 5 percent partial snake. [see Huang, et al. Phys. Rev. Lett. 73, 2982 (1994)]. A 4.7 Tm conventional solenoid was installed in the AGS in 1993. Two experimental runs with polarized beams were carried out to test the possiblity of polarized beam acceleration. Figure 1 shows the measured polarization as a function of the G*gamma value for a 5 percent snake.

The polarized beams were then accelerated to G*gamma=21 without using the tune jump quadrupoles. We find that the partial snake can indeed provide imperfection resonance correction for the AGS. Figure 2 shows the final polarization as the polarized beams were accelerated to 12 GeV. We note here that the depolarization arised from intrinsic resonances, which will be corrected by tune jump quadrupoles.

In this experiment, we have also found that the linear coupling can cause beam depolarization even in the presence of partial snakes. Figure 3 shows the polarization as a function of the vertical betatron tune. We note that the beam depolarization occurred when the horizontal and vertical tunes are equal.

Unsolved Puzzle

Why the vertical polarization, shown on Fig.~3 above, decreases when the vertical tune approaches 8.5?
What is the equivalent Froissart-Stora formula, when the horizontal and the vertical betatron tunes are equal with linear coupling? This is important to understand the depolarization shown in Fig.~3 when Q_x=Q_z.