Experimental studies on transient beam loading effects in the presence of a superconducting third harmonic cavity

In the framework of the SUPER-3HC project, an idle superconducting third harmonic cavity is installed in the ELETTRA storage ring in order to lengthen the bunch, thus improving the Touschek Lifetime. An improvement of more than a factor three of the beam lifetime is obtained, and the induced Landau damping renders ELETTRA longitudinally stable for the first time. The paper describes the performed studies and the experimental results, showing a strong correlation between the presence of an empty gap in the filling pattern of the ring and the third harmonic cavity induced Landau damping of the longitudinal instabilities, the bunch lengthening modulation along the bunch train and the beam lifetime improvement. Finally, an experimental investigation on the bunch profile overstretching, leading to find a low overstretching threshold below which the beam lifetime continues to increase, is presented.