BEAM DYNAMICS MEASUREMENTS IN THE VICINITY OF A HALF- INTEGER RESONANCE

The operating point of the betatron tune set near to a half-integer is a crucial parameter to produce high luminosity in electron/positron ring colliders. Dynamic beam-beam effects change the optics parameters, depending on the betatron tune and the beam-beam parameter. On the other hand, the existence of the half-integer stopband affects the beam stability. Therefore, the beam behavior near a half-integer might provide interesting issues from the viewpoint of beam dynamics. We measured the frequency response of the beam across a half-integer for measuring the betatron tune at KEKB. A sharp spike just at a half-integer was observed in the tune spectrum. We believe the spectrum to be a nonlinear resonance caused by some off-momentum particles in a bunch. The horizontal beam size measured using a synchrotron radiation monitor indicated a slight increase when the tune approached a half-integer. The variations of the size due to dynamic beam-beam effects should be evaluated using an H-mode tune not an L-mode.