Tune-split effects at the ALS storage ring

This paper is concerned with operational characteristics of the Advanced Light Source (ALS) storage ring, a synchrotron light source of the third generation that is capable of operating between 1.0 and 1.9 GeV beam energy. Even though the magnetic properties of its lattice magnets appeared to be very well understood an anomaly was observed with the measured betatron tunes when the working point of any of the three quadrupole families or the bend magnets was switched from the upper to the lower hysteresis branch. In no case was it then possible to recover the standard horizontal and vertical tune values simultaneously at any given excitation current; either one was considerably off normal when the other one was set to the proper value. The nature of this so-called “tune-split effect” was investigated, and the solution to the problem is presented here, together with an outlook on consequences for operational scenarios resulting from this effect