Increasingly Accurate Stability Study of the Experimental W-Band TWT Using Code TESLA-Z Stability Analysis Framework

We present the results of our comprehensive numerical stability study of the experimental, high-power, two-stage Serpentine
${W}$
-band traveling-wave tube (TWT) with ~10% working bandwidth. This device demonstrated an output power level of ~215 W at 92 GHz at the nominal beam voltage of 20 kV and ~285 W at an increased beam voltage of 20.8 kV. The experimental device was observed to be stable in all cases when operated within the designed beam voltage range. The onset of instability near the lower band edge was observed at higher beam voltages approaching a threshold value of ~21.0 kV. We use the recently developed stability analysis framework, based on the Naval Research Laboratory (NRL) 2-D large-signal code TESLA-Z, to find the predicted threshold of zero-drive instabilities, which could develop near the lower band edge of the experimental
${W}$
-band TWT. An initial study was done by using a simplified model for geometry of the experimental electrodynamic structure of the TWT, which ignored all interface elements. TESLA-Z predictions for onset of instability, in this case, were found much higher in beam voltage than its measured value. Next, an advanced TESLA-Z-based stability study was performed using a more accurate, detailed model for geometry of the experimental electrodynamic structure of the TWT, including important elements such as couplers (transformers), lossy load (sever), and windows. This more detailed analysis gave more accurate results for the onset of instability, in good agreement with the measurements.