Acceleration efficiency of TE-mode structures for proton linacs

Tamura, Jun  ; Kondo, Yasuhiro   ; Morishita, Takatoshi  ; Naito, Fujio*; Otani, Masashi*

Various types of cavity structures are typically used in hadron linacs, depending on the energy range of the beam particle. This is especially the case in a normal-conducting linac, because the cavity's acceleration efficiency varies with the velocity of the synchronous particle. For low-energy proton acceleration, while Alvarez drift-tube linacs (DTLs) are the most prevalent, TE-mode accelerating structures, which could also be called H-mode structures, are also widely used immediately after an initial radiofrequency quadrupole linac (RFQ). At present, the representative structures of TE modes are interdigital H-mode (IH) DTL and crossbar H-mode (CH) DTL, which are based on the TE11-mode pillbox cavity and TE21-mode pillbox cavity, respectively. In this presentation, acceleration efficiency of TE-mode structures including higher-order TE-modes such as TE31 and TE41 was comparatively reviewed with Alvarez DTL. This study shows that IH-DTL and CH-DTL have a larger shunt impedance than Alvarez DTL for proton acceleration below 10 MeV, and furthermore for the TEm1-mode structures, the rotational symmetry of the electric field improves with increasing angular index m.