The selective ablation behaviour of nanocomposite thin films under femtosecond laser irradiation was systematically investigated. The experimental sample, a Nb/Ti multilayer nanocomposite consisting of alternating metallic bilayers deposited on a Si substrate, was fabricated by ion sputtering. The fluence range for selective photomechanical ablation, e.g. the selective removal of nano-layers, was determined and the spatial periodicity of laser-induced periodic surface structures was estimated for the given experimental conditions. Single-pulse laser irradiation was carried out in air using a focused, linearly polarised Gaussian beam with a pulse duration of 170 fs and a wavelength of 513 nm. To induce selective removal of one or two individual nano-layers within the multilayer thin film, the laser fluence varied from 0.05 to 1.0 J/cm². Detailed morphological analysis of the irradiated spots revealed well-defined selective ablation of the top one or two layers, depending on the applied fluence. Furthermore, multi-pulse irradiation with up to 20 overlapping pulses resulted in the formation of periodic surface structures on the Nb/Ti nano-layer thin film, which can be attributed to interference between the incident laser field and surface-scattered waves on roughness formed after the first laser pulse.
The three enduring paradoxes of tokamak physics are: 1. Good neoclassical conductivity contradicts the anomalously high transport of particles. 2. Particle density has maximum at the core, while the main particle source is near the edge. 3. Improvement of confinement by plasma current ramp-down and thus weakening of the poloidal magnetic field near the edge. While resolving these paradoxes is imperative for establishing a predictive theory of tokamaks, limited discussions on these complexities exist in the literature. The authors suggest that resolution of the first paradox is a coexistence of regular and chaotic trajectories of particles; resolution of the second paradox is turbulent equipartition of particles and a decrease of the specific poloidal volume to the center; resolution of the third paradox is an increase of the specific poloidal volume near the edge, resulting in the expansion of plasma, lowered density and temperature, consequently reducing the heat flux.
Particle accelerators require energetic positron sources to satisfy next generation requirements. Positrons are currently sourced using solid targets. Here, we consider the collision of an energetic electron beam (≥5 GeV) with an intense laser pulse (2×1022 Wcm−2) as a potential positron beam source. Prolific pair production occurs due to strong field quantum electrodynamic effects in the interaction resulting in a high positron yield. Through the decoupling of initial and final positron energy, positrons converge towards the same average energy regardless of the initial energy they are produced at. Through 1D particle-in-cell simulations we demonstrate that positron beams with 200MeV average energy may be generated through non-linear Breit-Wheeler pair production. By reducing the electron beam energy, the energy spread of resulting positron beam decreases by up to 38% while the final positron energy remains constant. Comparison between the simulations and the analytical model show that the simulated positrons agree with the model within 4%. The low energy spread and low shot-to-shot variation in energy make these beams ideal for injection into particle accelerators.
Research on atmospheric-pressure plasma jets (APPJs) generated using argon (Ar), helium (He), and their oxygen (O₂) mixtures has been widely reported; however, direct comparisons of optical emission characteristics under identical operating conditions remain scarce, particularly for sinusoidal AC excitation. Most previous studies have focused on radio-frequency or pulsed discharges, which do not fully represent excitation modes relevant to stable AC-driven jets. In this work, the optical emission characteristics of APPJs operated in Ar, He, and their O₂ mixtures are systematically investigated using a dielectric barrier discharge configuration driven by a high-frequency sinusoidal power source. The relative intensities of emissive species, including Ar I, He I, O I, OH, and N₂(SPS)/N₂⁺(FNS), were analyzed in the ambient region downstream of the plasma plume using optical emission spectroscopy (OES). Electrical measurements were simultaneously performed to support the interpretation of the optical emission trends. Notable intensity variations were identified for Ar I (650–850 nm), He I (725 nm), O I (777 nm), OH (307 nm, A²Σ⁺→X²Π), and N₂(SPS)/N₂⁺(FNS) bands (335–405 nm), indicating that O₂ addition significantly modifies excited-species populations and plasma chemistry. The addition of O₂ to Ar or He favors the excitation of the N₂(SPS) band. In the Ar plasma, a small amount of O₂ slightly increases the Ar I emission, whereas higher O₂ concentrations progressively suppress Ar I, OH(A–X), and O I emissions. In the He plasma, a small O₂ addition slightly decreases OH emission while enhancing atomic oxygen excitation, resulting in a moderate increase in the O I (777 nm) line at low O₂ flow rates, whereas He I emission remains relatively stable. The results represent time-integrated optical emission from the plasma effluent under ambient conditions and are interpreted in a comparative manner under identical operating parameters.
Disinfection and sterilization are essential processes in healthcare determined by the level of contact between medical devices and the human body. While disinfection eliminates most pathogenic microorganisms, sterilization ensures the complete destruction of all microbial life, including spores. Conventional sterilization techniques such as steam autoclaving and ethylene oxide (EtO) gas sterilization are widely used; however, they present limitations when applied to temperature-sensitive medical devices such as catheters and fiber-optic instruments. Additionally, EtO sterilization involves long cycle times and raises environmental and safety concerns.Low-temperature plasma sterilization has emerged as a promising alternative, but existing systems typically rely on radio-frequency (RF) power sources, which are costly, complex, and often imported, making maintenance challenging. To address these limitations, this study presents the design and development of an indigenously built pulsed plasma sterilizer. The system incorporates a variable high-voltage pulsed power supply (PHVPS) integrated with a controlled vacuum environment to generate effective plasma for sterilization.The performance of the developed system was evaluated using chemical indicators in accordance with the ANSI/AAMI/ISO 11140-1:2014 standard. Both simulation and experimental analyses were conducted to validate system functionality and sterilization efficacy.The results demonstrate stable plasma generation at 200–400 Pa and 300–400 V using a pulsed high-voltage power supply operating at 10–30 kHz. A base pressure of 3.6 Pa was achieved within 4–5 min, and chemical indicator testing confirmed repeatable sterilization performance. The developed system satisfies the functional requirements for low-temperature sterilization while providing a cost-effective and scalable alternative to RF-based plasma sterilizers. The key novelty lies in replacing conventional RF plasma generation and impedance matching networks with a simplified indigenous pulsed architecture.
