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The cavity was designed in CST Microwave studio. The specified frequency is 97 MHz. The prototype is fabricated using SS304 material. Flanges and all ports are welded in and the vacuum test was carried out successfully. The cavity has an inner diameter of 85 cm and length of 38 cm after final fabrication. The ridges which hold the stems of the drift tubes are made from aluminium, and the stems and drift tubes are made from copper as well as aluminium. The 11 gap IH structure has 10 drift tubes, each supported alternately from top and bottom. The machining of the ridges, stems and drift tubes has been done using the in house CNC vertical milling machine. Provision for water cooling has been made in each of the stems as well as the end walls of the cavity. The final cavity will be copper plated so that the power dissipation is within acceptable limits. Low power RF tests were conducted on the prototype cavity. The figure 1 shows the prototype during assembly.
Figure 1: Photograph of the prototype cavity
For determining the various parameters, bead pull tests and network analyzer based measurements were carried out. The measured resonant frequency of the prototype was near 98 MHz and it was brought to the design value of 97 MHz by using a tuner plate on one side. A bead pull test was done to measure the electric field profile. A sapphire bead was pulled along the beam axis of the cavity and the resonant frequency measured. The shift of the resonant frequency caused by the presence of the bead is proportional to the electric field at the position of the bead. The figure 2 shows the electric field profile along the beam axis. It can be seen that even though the uniformity of the electric fields in the central gaps has been achieved to a large extent, the end gaps have a smaller field than expected. This discrepancy can be explained by the end gaps that were larger than specified due to manufacturing issues, and will be corrected in the final tank.
Figure 2: Beadpull Measurement Data
Figure 3: Electric Field Profile from CST Microwave Studio Output