Active Tailoring of the Electron Energy Distribution in an Atmospheric-Pressure Dual-Frequency Argon Jet

The electron-energy distribution function (EEDF) is the primary determinant of processes in low-temperature plasmas. In conventional glow RF discharges at atmospheric pressure, frequent electron–neutral collisions rapidly force the EEDF toward a single Maxwellian, constraining both physical and chemical controls. Here, we overcome this barrier with a dual-frequency atmospheric-pressure argon plasma jet; a sinusoidal 5 MHz power is supplied to the main pin electrode, while a low frequency power applies 50-kHz bipolar square voltage to the counter electrode. Nanosecond-resolved laser Thomson scattering captures the phase-resolved EEDF, along with electron temperature (T_e) and density (n_e). Analysis of the scattered spectra with super-Gaussian fitting and Bayesian inference provides phase-dependent deviations from a Maxwellian EEDF. The EEDFs respond dynamically to the time derivative of the voltage applied to the counter electrode; a high-energy tail emerges during a falling phase of the 50 kHz voltage, whereas low-energy electrons are preferentially populated during a rising phase, sharpening the low-energy part of distribution. These µs-scale modulations of the EEDF are accompanied by corresponding oscillations in n_e (up to 2.2×10¹⁹ m⁻³) and T_e (0.85–1.0 eV). These findings suggest that dual-frequency excitation provides a practical µs-scale lever for tailoring the EEDF and, in turn, tuning plasma chemistry in atmospheric-pressure plasma jets.