High intensity bunch length instabilities in the SLC damping rings

The SLC damping rings function to provide low emittance, short bunches for injection into the linac. As the beam intensity in the rings was raised, as part of the SLC luminosity improvement program, we observed a threshold above which beam jitter would occur in the linac. Frequent machine trios, caused by beam loss on some bunches, were associated with the jitter. These erratic “flier” pulses were traced to extraction phase errors from the damping rings. This paper covers the diagnosis of this condition and the observations of the transient bunch length instability phenomena. The evidence of this turbulent (microwave) bunch length instability is seen in both time domain and in frequency domain measurements. The instability has been dubbed the “sawtooth” because of the characteristic time dependent nature of the phase and bunch length signals. It is present in both the SLC electron and positron damping rings at about the same threshold of 3×10¹⁰ particles per bunch. There is also some evidence in numerical simulations of transient bunch lengthening phenomena, which is discussed later in this paper