Efficient Heavy Ion Acceleration with IH-Type Cavities for High Current Machines in the Energy Range up to 11.4 MeV/u

We propose an efficient design for heavy ion acceleration from 1.4 to 11.4 MeV/u with a design current of 15 emA for a U28+ beam as a possible injector for FAIR. The proposed linac is based on IH-DTL cavities and quadrupole triplet focusing. The KONUS beam dynamics concept [1] is used to achieve high acceleration efficiency. By optimization of the transverse focusing scheme and the longitudinal bunch center motion, low emittance growth for the entire linac is achieved. Beam dynamics simulations were performed along with 3D rf simulations of all cavities. The cavities are designed for 108.408 MHz, reaching an effective shunt impedance of 100-200 MΩ/m. The overall length of the linac is just 22.6 m which is almost a third of an alternative Alvarez layout. A mechanical realization concept employing a modular tank design is presented. The proposed design is a viable option for the GSI UNILAC poststripper linac replacement, leaving free space in the UNILAC tunnel for future energy upgrades. LAYOUT AND COMPONENTS The whole linac structure was developed with LORASR and CST Microwave Studio. The design comprises five 108 MHz IH-DTL cavities and seven quadrupole triplet lenses [2]. The linac is divided into three mechanically rigid sections (see Fig. 1). Figure 1: Layout of the IH-DTL linac. Additionally, the cavities are divided into short modules (as shown in Fig. 2) to allow copper plating and easy alignment of the modules with drift tubes and lenses. The layout features phase probes for all lenses except L1 and L2 and beam steerers between L4 and L5 to ensure optimal beam transport. The power requirements of the cavities were estimated using CST simulations of all sections. The overall consumed power per cavity is 0.82 MW at the beginning of the linac and reaches 1.22 MW towards the end of the linac (see Table 1). At a duty factor of 0.2 % the thermal losses are in the order of 2 kW and could be managed with simple cooling techniques. Significantly higher duty factors are also possible, but would require extensive cooling of the structures. Figure 2: Mechanical design of the modular sections for the first IH-DTL cavity (courtesy of D. Bänsch At 108.408 MHz the IH-cavities are 0.7-0.8 m in diameter. The design limits were chosen to be state of the art values to provide a reliable and durable machine. On axis electric field in the IH-cavities is at maximum just above 11 MV/m which is a value considered safe for an IH-structure at this frequency. The quadrupole magnets are now limited by < 1.1 T at the pole tip. This provides some operational margin, since tip fields of 1.3 T are possible using current magnet technology. The average accelerating gradient of the whole linac is 3.76 MV/m. Table 1: Cavity Properties Cav. [m] [ ] [MW] 1 4.9 19.6 0.8 2 2.8 17.2 1.0 3 3.6 18.7 1.2 4 3.7 18.7 1.2 5 3.9 16.8 1.2