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As one of a drift tube type linac, an Interdigital H-type (IH) linac has been applied for ion acceleration in low beta range. It can realize a resonant cavity of convenient size at low frequency band and higher shunt impedance at low velocity range. These characteristics are advantageous especially for heavy ion acceleration; therefore, this structure has been applied for heavy ion cancer therapy. The RF field is resonated in TE111 mode and the electric field does not have the axial field component. The accelerating field is excited by using the electrode of Interdigital shape. In order to apply this structure to intermediate beta range, we propose a IH linac with the TE11n mode of the higher-order mode (HOM). Although the operating frequency becomes higher by using HOM, it is convenient to accelerate ion beam of intermediate energy. The design of the cavity structure and the possibility are presented.
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