  This paper reports the first spin tune measurement at high energies (24 GeV
and 255 GeV) with a driven coherent spin motion. To maintain polarization in a
polarized proton collider, it is important to know the spin tune of the
polarized proton beam, which is defined as the number of full spin precessions
per revolution. A nine-magnet spin flipper has demonstrated high spin-flip
efficiency in the presence of two Siberian snakes [1]. The spin flipper drives
a spin resonance with a given frequency (or tune) and strength. When the drive
tune is close to the spin tune, the proton spin direction is not vertical
anymore, but precesses around the vertical direction. By measuring the
precession frequency of the horizontal component the spin tune can be precisely
measured. A driven coherent spin motion and fast turn-by-turn polarization
measurement are keys to the measurement. The vertical spin direction is
restored after turning the spin flipper off and the polarization value is not
affected by the measurement. The fact that this manipulation preserves the
polarization makes it possible to measure the spin tune during operation of a
high energy accelerator.

Accelerator Physics