Performance simulations of the TRIAC IH-linac

An ISOL-based radioactive nuclear beam facility: Tokai Radioactive Ion Accelerator Complex (TRIAC) is operating in collaboration between High Energy Accelerator Research Organization (KEK) and Japan Atomic Energy Agency (JAEA). The TRIAC located in JAEA tandem accelerator facility comprises a 25.96-MHz split-coaxial RFQ (SCRFQ) and a 51.92-MHz interdigital-H (IH) linac for accelerating radioactive ions with a charge-to-mass ratio (q/A) greater than 1/9 from 2.1 keV/u to 1.09 MeV/u. The IH linac is a separated function drift-tube linac consisting of four tanks and three quadrupole triplets placed between tanks. In order to increase the beam energy up to 5-8 MeV/u, we have made a plan to connect the IH linac to the 129.8-MHz super-conducting (SC) linac existing at the tandem facility. Although the plan is not realized yet, the frequencies of the SCRFQ and IH linacs have been already changed from 25.5 to 25.96 MHz and 51 to 51.92 MHz, respectively, so as that their frequencies become equal to 1/5 and 2/5 of the SC-linac frequency. In the present paper, the performance of the 51.92-MHz IH linac is found by the beam simulation using TRACEP code. After the beam simulation for normal operation based on the design, our attention is paid on the special operation to bring the output beam energy to the maximum restricted by the accelerating voltage limit. As a result of the simulation for normal operation, the relation between output beam energy and synchronous phase of the first accelerating cell is obtained for each tank. This relation is efficiently used as a standard for adjusting the accelerating voltage and phase in the real operation of the linac. Most importantly, by a subsequent simulation for the special operation, we show that the IH linac could accelerate ions with a q/A=1/4 up to 1.4 MeV/u higher than designed energy of 1.09 MeV/u. (author)