Development of an APS Cavity for Tristan Main Ring

Normal conducting 508 MHz cavities of alternating periodic structure (APS) with nine accelerating cells will be installed in the TRISTAN main ring (MR). Rationale for this choice was discussed in ref. 1. The structure is biperiodically loaded with straight disks. The cells are coupled only electrically through the beam apperture of a 100 mm diameter and the coupling constant is about one percent. The length of the coupling cell is 15 mm. The adopted dimensions are shown in Fig. 1. The parameters of the cavity were calculated by SUPERFISH and listed in Table 1. It was found that each accelerating cell frequency should be tuned well to obtain a high Q value. Thus, tuning plungers were installed to all of the accelerating cells. The shunt impedance was estimated to be 23 M¿/m and this value was 85 percent of the calculated one. It was also found from model calculations that the frequency of the coupling cell fc should be higher than or equal to that of the accelerating cell fa to stabilize the power distribution among the accelerating cells. The cavity designed under this constraint was excited up to 220 KW (CW) which was 150 percent of the designed power dissipation. The measured shifts of fa and fc due to the thermal distortion arising from the power dissipation were in agreement with those estimated from a computer analysis of thermal distortion. The transverse instability caused by the deflecting mode of the cavity was observed in the TRISTAN accumulation ring (AR).