Dynamic Π-type impedance matching network design for radio frequency slab CO₂ laser

Objective Radio-frequency (RF)-excited slab CO₂ laser acts as one of the indispensable core excitation sources for extreme-ultraviolet (EUV) lithography research as well as high-end precision non-metallic material processing. Benefiting from its advantages of compact resonator structure, high beam quality and scalable output power, this type of laser has drawn increasing attention within advanced manufacturing and optoelectronic research communities. Nevertheless, its plasma discharge load exhibits strong time-variant characteristics.

The load impedance continuously drifts in response to multiple physical variables, including gas-mixture composition, resonant cavity internal pressure, inter-electrode gap distance, and real-time injected RF power. Under practical operating scenarios, these factors frequently couple with each other, further broadening the scope of impedance fluctuation. Traditional fixed-parameter impedance matching networks are merely optimized for a single nominal operating point and lack adaptive capacity for wide-range impedance deviation. Once severe impedance drift occurs, considerable reflected power will be generated at the power-load interface, which directly reduces RF-to-plasma energy-coupling efficiency, gives rise to non-uniform plasma glow discharge, and may even induce device overheating or irreversible component damage in severe cases. For a 40.68 MHz, 4 kW high-power RF excitation system intended for slab CO₂ lasers, a high-performance dynamically reconfigurable matching topology is urgently required to suppress power reflection and maintain stable plasma discharge. Targeting these critical technical bottlenecks, this paper develops an improved dynamic Π-type impedance-matching network, aiming to achieve high-efficiency RF-power delivery under drastically variable discharge-load conditions.

Methods To accurately depict dynamic discharge-load behaviours, a dual-component equivalent-circuit model is established. This model comprises parallel plasma resistance-capacitance branches representing the bulk plasma region and series sheath capacitance characterizing the near-electrode boundary layer, which quantitatively describes the low-resistance capacitive nature of plasma load and reveals inherent dynamic-impedance evolution rules. On the basis of the inherent merits of the conventional Π-type topology, namely broad potential matching bandwidth and relatively low insertion loss, two high-voltage ceramic tunable capacitors are introduced to construct a dual-variable-capacitor cooperative-regulation architecture. The tunable capacitor C1, whose capacitance ranges from 150 pF to 1500 pF, is deployed on the power-source side to calibrate input impedance. Another series-connected tunable capacitor C3 with 10–100 pF adjustment scope is placed at the load branch to compensate reactive impedance and modify the overall impedance-transformation ratio. Matlab numerical simulation is performed to compute cooperative adjustment curves for these two tunable components and derive optimal parameter-adjustment trajectories corresponding to varying load resistance. ADS circuit simulation combined with Smith-chart impedance analysis is further adopted to refine network parameters and validate RF performance at the target frequency of 40.68 MHz. An experimental test platform is built integrating self-developed 4 kW RF power supply and slab CO₂ laser prototype. Fixed experimental boundary conditions are maintained throughout validation: electrode dimension of 840 mm×40 mm, adjustable electrode spacing of 1–mm, cavity pressure of 12 kPa, and gas mixing ratio of CO₂:N₂:He=1:1:6.

Results and Discussions Experimental and simulation results demonstrate that the proposed matching network can cover load-impedance variation with real-part resistance ranging from 2 Ω to 9 Ω and capacitive imaginary-part impedance varying between −60 Ω and −1 Ω. ADS simulation results exhibit S₂₁ close to 0dB and S₁₁ lower than −20 dB at 40.68 MHz, confirming near-lossless power transmission and extremely low signal reflection within the matching network. With coordinated tuning of C1 and C3, the voltage standing-wave ratio remains below 1.05 and power reflectivity is suppressed under 1% across all tested working states. When the actual injected RF input power reaches 2800W, reflected power is restricted below 28 W while the effective laser optical output achieves 265.7 W. Continuous discharge experiments obtain uniform, arc-free glow distribution over the full electrode surface, verifying reliable plasma excitation. Capacitance-adjustment trajectories acquired from Matlab simulation show great consistency with practical debugging datasets, verifying that the dual-capacitor cooperative-control strategy can effectively counteract real-time impedance drift triggered by pressure fluctuation and electrode-gap alteration. Under identical experimental set-ups, conventional fixed-parameter matching circuits fail to accomplish such wide-range impedance adaptation.

Conclusions The dual-capacitor collaborative-regulation scheme breaks the matching limitation of classic fixed Π-type networks facing time-varying plasma loads. The presented dynamic Π-type matching structure substantially improves RF power-injection efficiency and plasma-discharge stability. The completed design fulfils impedance-matching requirements for 4 kW-class 40.68 MHz RF-excited slab CO₂ lasers and provides solid technical support for popularizing high-power CO₂ lasers in EUV lithography and advanced precision-manufacturing applications. Furthermore, the load-equivalent-modelling approach and parameter-optimization workflow established in this work can be generalized to impedance-matching design for other RF-excited gas-laser systems.