Characteristics of Large Beam Hole Biperiodic Accelerator Structures

Large beam hole biperiodic rf structures operating at π/2 mode with a/R of 0.31 and 0.51 have been studied. A larger beam hole reduces the amount of higher order mode excitations for modes with frequencies below the beam pipe cutoff frequency. Further reductions can he made by reducing the accelerating cell lengths. Endcell effects can be corrected by detuning the endcell; a residual field imbalance in the endcell remains because of the reduced coupling coefficient caused by field leakage into the beam pipe. The dependence of the coupling coefficient and cell frequency of a detuned cell has to be taken into account when a structure with a large tuning range is analyzed using a RLC equivalent circuit model. The reductions in the first neighbour coupling coefficient in the two structures with aperture radius 8 cm and 13 cm are respectively 0.023% and 0.056% per MHz increase in cell frequency. A comparison with a singly periodic structure operating in the π mode shows that: 1. The additional couplers in a biperiodic structure increase the energy loss slightly (9% in this case). 2. Although the field flatness of a π mode structure is worse than that of a π/2 mode structure close to the design frequency, a singly periodic structure, which has no lossy couplers, may be preferred if a large tuning range is required. 3. An 'idle' biperiodic structure can be detuned to reduce the Q and therefore cause less disturbance than a singly periodic structure.