Experimental investigation of the coherent longitudinal instability of relativistic particle beams

In order to verify the theoretical predictions of stability of relativistic proton beams against coherent longitudinal oscillations driven by interaction with the surrounding vacuum chamber wall, a special high-Q cavity is built with remotely controlled tuning and damping. The first experiments with the cavity are successful, and the beam is driven unstable at thresholds quite close to those calculated for simplified models. These results also confer more credibility to the claim that it is necessary to continue the costly efforts of keeping the wall impedance low. (2 refs).