Superconducting Third Harmonic Cavity

The main challenge of the third generation synchrotron light sources is to increase the beam lifetime. In the lowmedium energy machine. Before super 3HC ELETTRA, when operated at 2.0 GeV, applied an amplitude stable longitudinal coupled-bunch mode, obtained by a detuning of the RF cavities temperature. The longitudinal bunch dilution produced by that mode increased by 30% the beam lifetime, providing in the meantime a Landau damping of all the transversal instabilities.
An  interesting consisting of using a higher harmonic RF cavity to lengthen the bunches. In fact, by manipulating the harmonic voltage and the phase in order to attend the total voltage sampled by the beam, the bunches are lengthened, therefore improving  the  lifetime, thanks to the reduced longitudinal bunch charge, without affecting the transverse emittance and thus the radiation brightness. Moreover, the nonlinearity of the rf voltage experienced by the bunches also provides a Landau damping of the longitudinal couple bunch mode (CBM) instabilities, allowing ELETTRA to even operate with longitudinal stability.
During the summer of 2002, an idle superconducting third harmonic cavity (3HC) was installed in the ELETTRA storage ring . Several theoretical studies carried out in the past decade  underlined the advantages obtained by using a superconducting (SC) cavity rather than a normal conducting one.
The SUPER-3HC represents the first superconducting application of a high harmonic RF system in a storage ring, providing not only a relevant improvement in terms of beam lifetime and longitudinal stability but also offering the big opportunity to characterize the beam in a more exhaustive way. Indeed, the very high quality factor (Q) of the SC cavity and the associated narrow bandwidth allows tuning of the 3HC cavity very near to the third harmonic of the beam, without exciting longitudinal instabilities, leading to investigate beam dynamics phenomena still not explored to date.
Using a passive harmonic cavity is a simple solution because an external RF power generator is not required to provide the voltage, which is generated by the beam itself.

The installation of the superconducting third harmonic cavity in the ELETTRA storage ring has improved the beam lifetime by more than a factor of 3 with respect to the nominal beam lifetime of ELETTRA. An in-depth investigation of the filling pattern influence on the benefit provided by the 3HC activation has been carried out. The phase shift along  the bunch train, induced by the empty gap, significantly reduces the 3HC effectiveness, especially concerning the beam lifetime improvement. Measuring the beam lifetime at several 3HC detunings shows a lifetime increase even beyond the optimum detuning, corresponding to the optimum harmonic voltage necessary to completely flatten the RF potential. This means that the bunches must be slightly overstretched and the fact that, in the uniform filling case, a low overstretching  threshold exists, for which the lifetime increase is maximum, has been demonstrated. When exceeding  this threshold, the lifetime abruptly drops. This threshold has been measured not only for the uniform filling case, finding a very good correspondence with the calculation, but also for fractional fillings. Moreover, the performance of 3HC has been compared, in terms of beam lifetime, for several fractional filling patterns, showing that best conditions are met in the case of a 96% filling. This fact suggests that, even if the maximum bunch lengthening is reached with uniform filling, ELETTRA requires a small empty gap to optimize the beam lifetime, in order to clean the residual trapped ions.
The superconducting third harmonic cavity has been in routine operation at ELETTRA since September 2003, and the new standard Users operation envisages a filling pattern of 96%. Before the TOP-UP operations,  the great improvement in beam lifetime allows for refilling of the ring every 48 h, instead of every 24, which is of greatest importance for reliable and stable operation for the Users.