Equipartitioning design method of a quasi-periodical focusing-accelerating coupled system

High-current proton linac sets a very severe requirement on the control of beam loss and emittance growth. Beam physics theory demonstrates that driven by the beam coherent instability, beam emittance grows due to the coupling of the strong space charge effect when an intense beam has different temperatures between different degrees of freedom. Therefore it is necessary to design an intense beam accelerator in line with the equipartitioning principle. However, this design becomes too tedious to reach full equipartitioning because a real proton linac is, in fact, a quasi-periodic coupling system. Taking the advantage of the code TRACE3-D, the authors modified it and added equipartitioning design function to it. It becomes a useful tool to readily realise equipartitioning design with the assistance of PARMILA code. The modification to TRACE3-D code is introduced, and then illustrate its application to the design of a DTL linac section, which shows the importance of equipartitioning design in the intense beam linac