The Design of a Single Cavity Harmonic Buncher for Argonne´s Low Beta Heavy Ion Linac

In order to increase the beam capture efficiency of Argonne\´s heavy ion linac to over 70%, a single cavity harmonic buncher has been designed as a replacement for the existing fundamental frequency buncher Because the beam line space between the 1.5 MeV Xe+1 preaccelerator and first accelerating cavity is at a premium, especially in the tunnel area near the preaccelerator, a single cavity design was undertaken. In addition, to further conserve access space, the cavity was designed to fit directly beneath the beam line. The cavity is designed to resonate at the fundamental linac frequency of 12.5 MHz and its first harmonic, 25 MHz. This was accomplished by nesting the 25 MHz resonant section inside the larger 12.5 MHz resonant section. Both sections are heavily capacitively loaded, folded coaxial lines with two 0.008 m accelerating gaps per section. The cavity was designed using a transmission line model taking account of the capacitances of each discontinuity and by use of the RF cavity computer program "Superfish." The transit time factor for the cavity gaps was calculated using the computer program "Poisson" and are 0.44 for the 25 MHz section and 0.70 for the 12.5 MHz section. The transit time factors are "poor" because of the large linac aperture of 0.049 m.