Design and simulations of the HEBT spiral buncher cavities for the high current injector at IUAC

The high energy beam transport (HEBT) section of the high current injector (HCI) accelerator requires two spiral buncher (SB) cavities to match the input Twiss parameters at the entrance of the superconducting super buncher (SSB) cavity by providing the longitudinal phase matching between drift tube linac (DTL) and SSB. The spiral type open-ended quarter-wave (lambda/4) resonators were chosen for their high shunt impedance, mechanical, and vibrational stability. The 48.5 MHz frequency of cavities was chosen for its broad acceptance of time width. TRACE 3D codes were simulated to determine the bunching voltage for the spiral buncher cavities. The locations of HEBT spiral bunchers have been fixed in such a way that the ion beam can be transported from DTL to SSB with negligible growth in longitudinal beam emittance within the framework of first-order linear beam optics. The cavity parameters were optimized to get the significantly high shunt impedance and quality factor to achieve the desired electric field level at the minimum input power. The inner and outer radii of the drift tubes were honed to get the uniform electric field profile along the beam direction while beta lambda/2 was kept constant during the refinement. The HEBT SB cavities require-2 kW of input power to produce similar to 160 kV across two RF gaps. The simulated quality factor and shunt impedance for two identical HEBT SB cavities are similar to 8300 and-13.5 M Omega, respectively. The cylindrical type chamber will be fabricated of copper-plated mild-steel (MS) while other components including end plates, spiral, stem, and flanges were fabricated of pure OFHC copper due to excellent electrical as well as thermal conductivity. The cavity frequency can be easily coarse tuned by varying the length of the spiral. The distinguishing feature of the design is to tailor the cavity frequency by +/- 250 kHz by varying the diameter of the drift tubes and stem, even after fabrication. A cylindrical type frequency tuner of diameter 90 mm and thickness 10 mm has been designed which is capable of providing the additional frequency correction of +/- 250 kHz in the travel distance of similar to 200 mm. The details of longitudinal beam optics, ion-optical and electrical design, simulations, and mechanical design are discussed in this article.