Design of APhF-IH Linac for a Compact Medical Accelerator

The design of a small injection linac for a compact medical synchrotron is discussed. The linac design is based on interdigital H-type (IH) drift-tube structure with alternative phase focusing (APhF). A high acceleration rate and an absence of magnetic lenses inside drift-tubes reduce the cost and length of APhF-IH linac in comparison with HIMAC linac based on Alvarez structure with magnet quadrupoles inside drift-tubes. To reduce effects of emittance growth, the RFQ structure is used in front of the APhF linac. In such linac layout, the current transmission of a carbon beam can reach up to 90-100%. In this report, the basic parameters of whole linac are presented, while the design of APhF structure is considered in details. Two reference designs of 4 MeV/u 200 MHz APhF linacs with different voltage distributions along the whole tank have been generated and analyzed numerically. For the first design, a constant voltage distribution along the tank is assumed. The total length of the structure is about 4.2 m. For the second design, a gradient type of voltage distribution is assumed. The total length of the second design is twice shorter (about 2.1 m). Both designs keep the same value of the maximum electric field on the drift-tube surfaces along the whole tanks. It is about 1.6 of the Kilpatrick limit. The second design with a gradient-type of voltage distribution looks to be more attractive. A required voltage distribution can be realized in IH-tank. An example of gradient-voltage distribution calculated with Microwave Studio code is presented.