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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
B) Beam Dynamics
Each IH tank is independently phased from separate rf amplifiers. With the exception of the first and last tank the tanks consist of bunching sections and accelerating sections. The accelerating sections in the IH tanks are designed for 0° synchronous phase. The beam is injected into the accelerating sections with a reduced phase spread and velocity higher than the design velocity so that the bunch drifts to more negative phases during acceleration and emerges with a reduced energy spread. Quadrupole triplets between tanks provide periodic transverse focussing. Short -60° sections at the entrance of every accelerating tank provide periodic longitudinal focusing to allow matching to the next accelerating section (see figure 3). Key to achieving an improved longitudinal acceptance is the addition of an extra long drift-tube between the bunching and accelerating sections to further reduce the phase spread entering the accelerating section. With this novel technique a beam of more than 3 p keV/u-nsec can be accelerated with minimal emittance growth. The velocity of the incoming beam (beta = 0.02) and the chosen resonant frequency (97 MHz) results in a gap length of about 1.5 cm for the first resonator. The inner diameter of the tube is fixed at 1.4 cm to minimize the penetration of field into the tubes. The length of the tanks are chosen in such a way that for the given input emittance (e = .3 p mm mr normalized), the maximum beam size inside any drift tube is less than half of the tube inner radius. A schematic of the proposed DTL along with the transverse & longitudinal beam envelopes are shown in Figure 2. Length and number of cells in each tank along with the output energy after each tank is shown in table 1.
Figure 2: A schematic of the proposed DTL along with the transverse & longitudinal beam envelopes
Figure 3: accelerating tank and longitudinal focusing
Table 1: DTL Tank Details