A Comparison of pi/2-mode standing wave structures for Linac4

Cell coupled structures at twice the basic frequency provide a higher shunt impedance for proton energies above 90 MeV for Linac4 when compared to drift tube based geometries. For this reason the nominal accelerating structure for the energy range of 90-160 MeV in Linac4 was chosen to be a Side Coupled Structure at 704.4 MHz. High power klystrons will feed this structure, with the consequence that a large number of cells are coupled together in one module. To provide field stability in a long chain of resonators, a pi/2-mode structure will be used. In this report, the three best known and most widely used ones - the Annular Ring Coupled Structure (ACS), the On Axis Coupled Structure (OCS) and the Side Coupled Structure (SCS) - are compared in terms of the electrical parameters Q-value and shunt impedance as well as structure dimensions.