Observation of the tune dependence of the stability threshold current in the PSR

In the high-intensity unbunched-beam experiments carried out in the Proton Storage Ring (PSR) at Los Alamos National Laboratory, the threshold current of the vertical transverse instability showed pronounced differences when the betatron tune varied across an integer. Experimental observations are presented, and the possible relations between the threshold current and the machine impedance are discussed. The possible effects related to the distorted closed orbit are also discussed. It was found that the behavior of the threshold current for vertical tune near the integers 2 and 3 could be related to the resistive-wall-type instability of modes n=2 to 4 together with the space-charge effect. It is conjectured that the closed-orbit distortion and a nonuniformly distributed asymmetric transverse impedance may contribute to some degree to the discontinuity of the threshold current when the vertical tune crosses an integer