Keiichiro Urabe

Behavior of N2+ Ions in He Microplasma Jet at Atmospheric Pressure Measured by Laser Induced Fluorescence Spectroscopy

The behavior of N2+ ions in a low-frequency driven atmospheric pressure He plasma jet effused into ambient air was analyzed from laser induced fluorescence (LIF) spectroscopy measurements. The gas temperature derived from the rotational distribution was kept near room temperature and the drift velocity of N2+ ions estimated from the line shape was almost zero as compared to the apparent speed of the plasma bunch given by the spatiotemporal intensity profile. This shows that the mechanism of moving plasma bunches can be attributed to the ionization wave propagation similar to the streamer in positive corona discharge.

Pulsed laser ablation plasmas generated in CO2 under high-pressure conditions up to supercritical fluid

Pulsed laser ablation of solids in supercritical media has a large potential for nanomaterials fabrication. We investigated plasmas generated by pulsed laser ablation of Ni targets in CO2 at pressures ranging from 0.1 to 16 MPa at 304.5 K. Plasma species were characterized by optical emission spectroscopy, and the evolution of cavitation bubbles and shockwaves were observed by time-resolved shadowgraph imaging. Ni and O atomic emissions decreased with increasing gas pressure; however, near the critical point the intensities reached local maxima, probably due to the enhancement of the plasma excitation and effective quenching resulting from the large density fluctuation.

Absorption spectroscopy using interference between optical frequency comb and single-wavelength laser

Optical frequency combs have the potential to be applied not only to frequency metrology but also to various spectroscopic measurements replacing incoherent wide-frequency band light sources and single-wavelength laser sources. In this study, we propose a system of absorption spectroscopy for single electronic transitions using the frequency-comb laser source. This spectroscopic method utilizes the interference between the probing frequency comb and an additional single-wavelength laser beam and detects power spectra of beat signals in the interfered laser beam. This method enables us to measure single-transition absorption spectra neither using a large-scale spectrometer nor scanning the laser-beam frequency.

Pure air–plasma bullets propagating inside microcapillaries and in ambient air

This paper reports on the characterization of air–plasma bullets in microcapillary tubes and in ambient air, obtained without the use of inert or noble gases. The bullets were produced by nanosecond repetitively pulsed discharges, applied in a dielectric barrier discharge configuration. The anode was a tungsten wire with a diameter of 50 µm, centered in the microcapillary, while the cathode was a silver ring, fixed on the outer surface of the fused silica tube. The effects of the applied voltage and the inner diameter of the microcapillary tube on the plasma behavior were investigated. Inside the tubes, while the topology of the bullets seems to be strongly dependent on the diameter, their velocity is only a function of the amplitude of the applied voltage. In ambient air, the propagation of air bullets with a velocity of about 1.25 × 105 m s−1 is observed.

Functional composites of plasmas and metamaterials: Flexible waveguides, and variable attenuators with controllable phase shifta)

Numerical predictions and experimental results in this study verify that plasmas with negative permittivity work as functional media for electromagnetic waves and that their composites with metallic metamaterials show further extraordinary properties. Chain structures of individual plasmas with negative permittivity, forming a straight line and a L-shaped bent line, serve flexible waveguides via coupling structures of localized surface waves standing around each plasma. Further progresses as wave controllers are achievable in an array of the composites of plasmas and micro metallic resonators; functions of phase shifters and attenuators are individually controlled, with rotation of working points on the complex refractive index plane by varying gas conditions and permeability modulation. Such proposed sets of flexible combination will lead to advanced scientific products with novel functions.

Dynamics of pulsed laser ablation in high-density carbon dioxide including supercritical fluid state

To gain a better understanding of pulsed laser ablation (PLA) processes in high-density fluids, including gases, liquids, and supercritical fluids (SCFs), we have investigated the PLA dynamics in high-density carbon dioxide (CO2) using a time-resolved shadowgraph (SG) observation method. The SG images revealed that the PLA dynamics can be categorized into two domains that are separated by the gas-liquid coexistence curve and the Widom line, which forms a border between the gaslike and liquidlike domains of an SCF. Furthermore, a cavitation bubble observed in liquid CO2 near the critical point exhibited a particular characteristic: the formation of an inner bubble and an outer shell structure. The results indicate that the thermophysical properties of the reaction field generated by PLA can be dynamically tuned by controlling the solvent temperature and pressure, particularly near the critical point.

Effect of Series Capacitance and Accumulated Charge on a Substrate in a Deposition Process with an Atmospheric-Pressure Plasma Jet

In order to investigate the effect of accumulated charge on a substrate surface in the deposition of a SiO2 film with an atmospheric-pressure plasma jet, we have measured the discharge current flowing into a copper substrate, which was placed 20 mm from the exit of the plasma jet and connected to a variable capacitor in series. We found that the discharge current decreased markedly when the capacitance of the substrate was below 100 pF, and the deposition rate of SiO2 traced the variation of the capacitance. To analyze the behavior of the plasma jet, we considered an equivalent circuit of our system and verified the validity of our supposition that the accumulated charge restricts the deposition rate. Thereby, we found that the discharge current and deposition rate were determined by the capacitance of both the glass tube wall and the substrate.

Breakdown Characteristics of Electrical Discharges in High-Density Helium Near the Critical Point

We present an investigation of the breakdown behavior of micrometer gap direct-current discharges in gaseous, supercritical, and liquid helium, which shows a critical anomaly of the breakdown voltage near the critical point. A discharge model that combines gas- and liquid-like breakdown mechanisms and takes into account the local fluid structure in a fluctuating fluid with the concept of a modified electron mean free path, allows us to reproduce the breakdown behavior. The result of the analysis suggests that the critical breakdown anomaly is caused by long acceleration paths inside low-density domains resulting from the density fluctuation.

Review of electric discharge microplasmas generated in highly fluctuating fluids: Characteristics and application to nanomaterials synthesisa)

Plasma-based fabrication of novel nanomaterials and nanostructures is indispensible for the development of next-generation electronic devices and for green energy applications. In particular, controlling the interactions between plasmas and materials interfaces, and the plasma fluctuations, is crucial for further development of plasma-based processes and bottom-up growth of nanomaterials. Electric discharge microplasmas generated in supercritical fluids represent a special class of high-pressure plasmas, where fluctuations on the molecular scale influence the discharge properties and the possible bottom-up growth of nanomaterials. This review discusses an anomaly observed for direct current microplasmas generated near the critical point, a local decrease in the breakdown voltage. This anomalous behavior is suggested to be caused by the concomitant decrease of the ionization potential due to the formation of clusters near the critical point, and the formation of extended electron mean free paths caused by the high-density fluctuation near the critical point. It is also shown that in the case of dielectric barrier microdischarges generated close to the critical point, the high-density fluctuation of the supercritical fluid persists. The final part of the review discusses the application of discharges generated in supercritical fluids to synthesis of nanomaterials, in particular, molecular diamond—so-called diamondoids—by microplasmas generated inside conventional batch-type and continuous flow microreactors.

Discharge-Mode Transition in Jet-Type Dielectric Barrier Discharge Using Argon/Acetone Gas Flow Ignited by Small Helium Plasma Jet

A discharge-mode transition in a jet-type dielectric barrier discharge (DBD) was triggered by a small fraction of acetone vapor added to an argon (Ar) gas flow at atmospheric pressure. In order to trigger a stable discharge in the Ar/acetone gas flow with a relatively small applied voltage, we used an additional small plasma jet using a He gas flow on the side of the main flow. The transition from filamentary to glow like discharge modes took place upon increasing the acetone-vapor ratio, with the transition occurring at an acetone content of approximately 0.3 vol %. We compared discharge currents, optical emission spectra, and deposited materials on the substrate in each discharge mode to characterize the discharge phenomena. The experimental results clearly indicate that the characteristics of the jet-type DBD show nonlinear dependence on the acetone-vapor ratio, especially around the transition to the discharge mode. It was also found by microscopic observations that the surface morphologies of the deposited materials were completely different in the filamentary and glow like modes.