The regenerative multi-pass beam breakup (BBU) instability arises from the interaction of an intense recirculated beam with High Order Modes (HOM) of the accelerating structures. In a two-pass machine, the beam deflected off-axis by the HOM magnetic field on the first pass comes back to the cavity with a displacement on the second pass and induces the HOM voltage, which depends on the magnitude and direction of the beam displacement. Thus, the beam constitutes a feedback that can become unstable if the beam current reaches a threshold. In the JLab FEL Upgrade, the regenerative, multi-pass BBU was observed at a beam current of 2.5-5 mA. Because BBU imposes a potential threat to the machine operations, better understanding of the instability and development of suppression techniques are tasks of great importance. The capability of predicting the threshold and suppressing the instability is also vital for future ERL projects with beams of high intensity. In our talk we will describe results of experimental studies of the regenerative BBU at the JLab FEL Upgrade. We will present results of studies of beam behavior above and below the threshold of the instability and compare them to analytical models and simulations. Also we will describe several optical schemes and electronic damping techniques aimed at increasing the BBU threshold that were tested at the JLab FEL Upgrade.


Talk Slides: (part I), (part II)