[en] KfK is continuing the development of 140 GHz high power gyrotrons for electron cyclotron wave (ECW) applications. Experiments with a TE_10,4 mode gyrotron have been performed using an axial rf output coupling. The achieved maximum output power of this version amounted to 0.7 MW in the design mode with a mode purity of about 99% and an electronic efficiency of 26%. Since these experiments have been performed using the superconducting magnet system designed for the previous TE_0,3 mode tube the pulse length was limited to 5 ms. With a more appropriate magnet system with increased bore hole diameter which allows the use of a built-in quasioptical converter a pulse length of up to 300 ms with 0.5 MW rf power is expected. In this paper the authors present some experimental results with respect to frequency step-tuning of the TE_10,4 mode gyrotron and describe the radial output coupling system