2025/12/02 by Saptarshi Biswas, Biswas, Saptarshi, Sven G. Bilén +1
Engineering · Physics and Astronomy · #FOS: Physical sciences #Gyrotron and Vacuum Electronics Research #Particle Accelerators and Free-Electron Lasers #Particle accelerators and beam dynamics #Plasma Physics (physics.plasm-ph)
paper · pdf · doi:10.48550/arxiv.2512.02415
openalex publication_date 2025/12/02 · openalex created_date 2025/12/04 · openalex updated_date 2026/07/28
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→ 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 ≈ 0.55~mm and backshort distance ≈ 0.80~mm, the return loss reaches |S11| ≈ -30~dB near 17.8--18.1~GHz while sustaining peak electric fields of ∼ 1.8 × 105~V/m at the nozzle (normalized to 1~W). The measured through loss of the launch assembly is |S21| ≈ 0.7--0.8~dB at resonance. A parametric study shows that backshort lengths L_\rm bs ≥ 0.5 λ\rm g excite a parasitic stub resonance, introducing a second S11 minimum and localizing energy behind the doorknob; keeping L_\rm bs ≤ 0.4 λg avoids this. In helium plasma discharges at P_\rm in = 10~W, in-situ retuning of the short and probe maintained a favorable match as the plasma impedance evolved, increasing absorbed power from ∼ 43% to ∼ 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.