An improved topology of Extended interaction oscillators (EIO) is presented with enhanced efficiency at 0.35 THz. A bi-periodic ladder-type structure is used for optimizing the performance of the TM31-2π mode operation. The proposed topology is based on bi-periodic interaction gaps composed by staggered short and long slots. The bi-periodic interaction mechanism permits to operate the circuit with a standing-wave field between the π and 2π modes, thus providing the potential to combine the advantages of both π and 2π modes in terms both of coupling and output performance. The resulting transverse TM31 operating mechanism exhibits a good agreement with bi-periodic structure in terms of coupling performance, with the short slot length positively correlated with the value of effective characteristic impedance M 2 R/Q. The optimization of M 2 R/Q and Qe, the circuit demonstrates attractive coupling and output characteristics. A state-of-the-art value M 2 R/Q of 59.69 Ω and a moderate Qe of 2523.52 are achieved for the proposed ladder-type EIO. A RF power of 540 W is obtained with a 42.5 kV, 0.6A sheet beam, with 2.1% electronic efficiency at 0.35 THz.