Integrated Sliding-Short/Probe Tuner with Doorknob Transition for High-Q Cavities

Abstract: We present an integrated three-knob tuner that internalizes impedance matching inside the launch adapter of a waveguide-fed, high-$Q$ cavity. The tuner combines a waveguide sliding short, a doorknob transition, and a micrometer-driven adjustable coaxial probe. A transmission-line/ABCD model is derived that maps the three mechanical degrees of freedom to the electrical objectives $Γ\rightarrow 0$, $β$, and $Q_{\rm L}$, explicitly including the fused-silica feedthrough capacitance. The model yields closed-form matching conditions and predicts the critical-coupling set. Full-wave FEM simulations and bench measurements validate the approach: with $h \approx 0.55$~mm and backshort distance $\approx 0.80$~mm, the return loss reaches $|S_{11}| \approx -30$~dB near 17.8--18.1~GHz while sustaining peak electric fields of $\sim 1.8 \times 10^5$~V/m at the nozzle (normalized to 1~W). The measured through loss of the launch assembly is $|S_{21}| \approx 0.7$--$0.8$~dB at resonance. A parametric study shows that backshort lengths $L_{\rm {bs}} \geq 0.5 λ{\rm g}$ excite a parasitic stub resonance, introducing a second $S{11}$ minimum and localizing energy behind the doorknob; keeping $L_{\rm {bs}} \leq 0.4 λg$ avoids this. In helium plasma discharges at $P{\rm {in}} = 10$~W, \textit{in-situ} retuning of the short and probe maintained a favorable match as the plasma impedance evolved, increasing absorbed power from $\sim 43\%$ to $\sim 76\%$ while increasing helium propellant flow rate from 25 to 351~sccm. The compact tuner eliminates external stub boxes and generalizes to other waveguide-coupled resonators and plasma sources.