A direct current (DC) powered low-temperature atmospheric pressure plasma (LTAPP) jet device was built and used to sterilize Escherichia coli (E. coli) bacteria. The plasma jet’s general properties, such as length and temperature, were first tested and found to be strongly related to the plasma jet’s operational flow mode (laminar or turbulent flow). The optical emission spectra of various gas mixtures were measured to confirm the presence of active radicals, which is critical for sterilization success. Pure helium gas or a combination of helium with a small percentage of oxygen (6.25%) was found to have the highest intensities of bactericidal species such as atomic oxygen (O) and hydroxide (OH). These mixtures were then used to treat E. coli bacteria previously grown in a Petri dish. Sterilization was accomplished by repeatedly treating the bacteria for 10 s for 5–10 rounds for short periods. The best results were obtained when the bacteria had enough time to rest between rounds.

A hybrid DC–RF inductively coupled plasma (ICP) driven by a single-turn internal antenna was experimentally investigated to quantify magnetic confinement effects in low-pressure argon discharges. Superposition of a dc current on the RF antenna generated an azimuthal magnetic field that modified electron transport and reduced cross-field diffusion in the near-antenna region. Spatially resolved measurements of plasma density, electron temperature, plasma potential, and magnetic-field components were obtained using Langmuir, emissive, and B-dot probes. Increasing the dc current enhanced electron confinement and increased the plasma density by up to an order of magnitude at low RF power, together with improved radial and axial uniformity. A semi-empirical diffusion model incorporating electron-temperature-dependent ambipolar transport reproduced the measured ion-density distributions, ni(R,Z), within ±15%. The results support the interpretation that the discharge behaviour is governed by the coupled effects of localized magnetic confinement and inductive power deposition, and show that hybrid DC–RF excitation provides an effective route to denser and more spatially extended plasmas under low-pressure conditions.

A spatially resolved investigation of bacterial inactivation using a radiofrequency (13.56 MHz) capacitively coupled plasma (RF CCP) discharge operating in ambient air at 4.0 mbar is presented. The plasma was generated in a parallel-plate reactor without external gas precursors and characterized using Langmuir probe diagnostics and optical emission spectroscopy (OES). Electron densities on the order of 109 cm³ were measured near the powered electrode, exhibiting pronounced axial and radial gradients across the discharge volume. OES revealed strong excitation of oxygen- and nitrogen-containing emitters, including O I (777 nm), N₂ s positive system (337–380 nm), and N₂⁺ first negative system features, with emission intensities increasing monotonically with applied RF power. The bactericidal performance was evaluated using Escherichia coli American Type Culture Collection (ATCC) 11775 exposed at different axial and radial positions within the reactor. At a fixed exposure time of 60 s, the log₁₀ reduction increased nonlinearly with RF power, rising from 0.29 at 20 W to 0.81 at 40 W, followed by a sharp transition to the assay reporting ceiling (≥2.95-log₁₀ under the adopted half-count correction) at 50 W and above. Time-resolved measurements at 50 W demonstrated rapid inactivation kinetics, with measurable reductions occurring within 5–10 s and reaching the reporting ceiling within 60 s. In contrast, samples positioned at the chamber periphery or approximately 20 cm from the discharge center exhibited negligible inactivation, confirming strong spatial localization of the biocidal effect. These results identify a threshold-like operating regime in which increased discharge intensity produces rapid inactivation in the plasma core while remaining strongly position dependent. The findings establish medium pressure, air-based RF CCP as an efficient, gas-free, and spatially controllable platform for localized surface decontamination under non-thermal conditions.

Antenna geometry strongly influences electromagnetic-field distribution, mode transition, and plasma-density formation in low-pressure inductively coupled plasma (ICP) sources. This study compares three external antenna configurations in the same 13.56 MHz, 3.0 mTorr argon plasma system: a helical-type antenna, a planar spiral antenna, and a combined spiral–helical antenna. The comparison is based on spatially resolved measurements of plasma density, RF plasma-potential oscillation, and RF magnetic-field components. The helical-type antenna produced a gradual density increase and a source-localized plasma, whereas the planar spiral antenna showed a clearer abrupt transition from a capacitively influenced low-density state to a higher-density inductive state. The combined spiral–helical antenna gave the strongest overall experimental response, producing the highest measured plasma-density values, the best density uniformity among the three configurations, the broadest useful high-density region, and strong suppression of RF plasma-potential oscillation in the high-density regime. The experimental magnetic-field measurements showed that the combined antenna produced a hybrid electromagnetic structure, with a hill-shaped axial RF magnetic-field profile and an off-axis radial-field structure. To support the interpretation, a verified three-dimensional vacuum-field Biot–Savart benchmark was developed under equal-current conditions. The benchmark is used only as a geometry-controlled field comparison and does not include plasma loading, dielectric boundaries, chamber currents, antenna-current variation, capacitive coupling, or self-consistent particle balance. The combined experimental and numerical results show that RF magnetic-field structure and plasma-density distribution are related but not identical, because density formation also depends on induced electric field, plasma-current density, ionization, transport, and wall losses. The work provides guidance for optimizing antenna geometry in controllable, energy-efficient, low-pressure cold plasma sources. This study should be read as a geometry-comparison case study: energy-efficiency gains are inferred from higher plasma density at equal applied RF generator power, not from direct absorbed-power measurements, and the combined-antenna geometry and 3.0 mTorr operating pressure investigated here represent a single tested case rather than a generally optimized design.