Conceptual design of a drift tube linac for proton therapy

The conceptual design of an Alvarez-type Drift Tube Linac (DTL) for a proton therapy facility is described in this paper. The DTL will accelerate 16 mA proton beams from 3 MeV to 7 MeV. The main feature of the design is its low total RF peak power, which is only 201 kW. The error study of the Drift Tube Linac is also given in this paper. INTRODUCTION Proton therapy is blooming around the world in recent years according to the Particle Therapy Co-Operative Group (PTCOG) [1]. Under the support of the National Key Research and Development Program of China, a 7 MeV linac injector is designed for a synchrotron-based proton therapy facility. The linac mainly consists of an ECR source, a Low Energy Beam Transport line (LEBT), a 3 MeV Radio Frequency Quadrupole (RFQ) and a 7 MeV Drift Tube Linac (DTL). PARAMETERS OF DTL The DTL will accelerate 16 mA proton beams from 3 MeV to 7 MeV. The permanent magnet quadrupoles (PMQ) are used to focus the beam in the DTL. The PMQs are mounted in the drift tubes with an FD lattice. Compared with the electromagnetic quadrupoles, the PMQs do not need coils and water cooling. The PMQs have a simple structure which can effectively reduce the size of the drift tube and improve the acceleration efficiency. The design of the DTL cavity aims at low total RF peak power. The cost can be saved if only one amplifier is used for the RFQ and the DTL. The tetrode-based RF power amplifier has been tested successfully to produce the peak power up to 500 kW at 325 MHz with the pulse width of 150 μs and repetition rate of 1 Hz [2]. 500 kW can be regarded as the maximum peak power for the RFQ and the DTL from one amplifier. The parameters of the DTL cells have been optimized with respect to high effective shunt impedance per unit length ZTT to achieve a low peak power. The constant accelerating field per cell is adopted, and the average accelerating field E0 is also optimized to reduce both length and power loss. The total peak power of the RFQ is 177 kW according to the design result [3].The peak power, including the beam power and 25% margin for the cavity power, is 201 kW, which is low enough for the amplifier. The total length of the DTL is 3.41 m. The diameter of the cavity is 61.87 cm while the diameter of the drift tube is 8.4 cm and the bore radius is 0.7 cm [3]. The model of the DTL is presented in Fig. 1. Figure 1: Model of the DTL cut along the beam axis. The design parameters of the DTL are listed in Table 1. Table 1: DTL Parameters Parameter Value Ion type Proton Input beam energy 3 MeV Output beam energy 7 MeV Peak current 16 mA RF frequency 325 MHz Pulse length 40~100 μs Pulse repetition rate 0.5~10 Hz Cell number 36 Average accelerating field 1.6 MV/m Maximum surface field 0.55 Kilp Total RF peak power for the DTL (25% margin) 201 kW