Accurate estimation of start oscillation current for maximum output power in 42 GHz, 200 kW gyrotron

Design of a 42 GHz, 200 kW gyrotron operating at TE03 involves optimization of a guiding magnetic field across the tapered cavity. The start oscillation current (SOC) or starting current is one of the most important parameter to estimate the guiding magnetic field to excite the desired operating mode irrespective of large numbers of competitive modes. This paper presents an accurate method to estimate SOC, based on the linear theory, yielding the guiding magnetic field at which the maximum desired power level is also achieved through the beam-wave interaction simulation using 3D MAGIC-PlC code. The method involves interpretation of the beam wave interaction length as e-1 times of the normalized axial Gaussian electric field profile. This method is successfully applied to low order symmetric operating mode gyrotron. The minimum SOC from the linear theory for the operating mode TE03 and the maximum power from the non-linear theory/code are obtained at the same guiding magnetic field 1.607 T with an error of 0.12% only. The paper also reports the effect of noise on SOC and output power, simultaneously. (author)