Ippei Yagi

Two-dimensional LIF measurements of humidity and OH density resulting from evaporated water from a wet surface in plasma for medical use

In plasma medicine, plasma is applied to a wet surface and is often accompanied by dry-gas flow. The dry-gas flow affects water evaporation from the wet surface and influences production of reactive species derived from water vapor, such as OH radicals. In this study, the effect of the dry-gas flow on two-dimensional distributions of humidity and OH radical density are examined by measuring them using laser-induced fluorescence (LIF). First, humidity is measured when nitrogen flows from a quartz tube of 4 mm inner diameter onto distilled water and agar media from 5 mm distance. NO gas is added to the nitrogen as a tracer and humidity is obtained from the quenching rate of NO molecules measured using LIF. This measurement has a spatial resolution of 0.2 mm3 and a temporal resolution of less than 220 ns. The two-dimensional humidity distribution shows that the dry-gas flow pushes away water vapor evaporating from the wet surface. As a result, a low-humidity region is formed near the quartz tube nozzle and a high-humidity region is formed near the wet surface. The thickness of the low-humidity region reduces with increasing gas flow rate. It is 0.1–0.5 mm for the flow rate of higher than 0.3 l min−1. Next, the OH density is measured when a nanosecond pulsed streamer discharge is applied to a distilled water surface with dry-air flow. The OH density decreases with increasing gas flow rate due to decreased humidity. When the flow rate is lower than 0.1 l min−1, the OH distribution is approximately uniform in the plasma region, while the humidity distribution shows a large gradient. The importance of the thin high-humidity region on the flux of reactive species onto the wet surface is discussed.

Influence of circuit parameter on ozone synthesis using inductive energy storage system pulsed power generator

An inductive energy storage system pulsed power generator using semiconductor opening switch (SOS) diodes was employed to drive a co-axial cylinder plasma reactor for ozone synthesis with high yield. Fast recovery diodes were used as SOS diodes in the inductive energy storage system pulsed power generator to produce short-pulse high-voltage with high-repetition rate. The high-voltage pulse was applied to a 1 mm diameter center wire electrode in the reactor. The copper cylinder of 19 mm inner diameter was used as outer electrode and was connected to a ground. Oxygen and dry air were used as source gases and were injected into the plasma reactor with 2 L/min. gas flow rate. The synthesized ozone concentration increased in proportion to pulsed voltages repetition rate under oxygen gas condition. The ozone yield of about 230 g/kWh was obtained in oxygen gas and was independent of circuit parameter. The ozone yield decreased from 230 to 100 g/kWh by changing the source gas from oxygen to dry air. The ozone yield depended on circuit parameters such as capacitance of the primary energy storage capacitor and inductance of the secondary energy storage inductor under dry air condition.

Measurement of OH, O, and NO densities and their correlations with mouse melanoma cell death rate treated by a nanosecond pulsed streamer discharge

Mouse melanoma cells in a culture medium are treated using a nanosecond pulsed streamer discharge plasma and the correlations between the rate of cell death and the densities of reactive species (OH, O, and NO) in the plasma are measured. The plasma is irradiated onto the culture medium surface with a vertical gas flow of an O2/N2 mixture from a glass tube at various gas flow rates and O2 concentrations. The densities of the reactive species are measured very close to the culture medium surface, where the reactive species interact with the culture medium, using laser-induced fluorescence. In the case of the N2 discharge (O2 = 0%), an increase in gas flow rate decreases OH density because it lowers the water vapor concentration by diluting the vapor, which is required for OH production. The increase in gas flow rate also leads to a decreased cell death rate. In the case of the O2/N2 discharge, on the other hand, an increase in O2 concentration at a fixed flow rate does not affect the rate of cell death, although it considerably changes the O and NO densities. These findings indicate that some reactive species derived from water vapor such as OH are responsible for the melanoma cell death, whereas those from O2, such as O and NO, are less likely responsible. They also indicate the importance of water evaporation from the culture medium surface in cell treatment.

Streamer Propagation of Nanosecond Pulse Discharge With Various Rise Times

Streamer corona discharge was generated by applying nanosecond-pulsed voltage with many different pulse rise times. Pulsed voltage having pulse rise time of 7-40 ns was applied to a coaxial chamber consisting of a center wire, 1 mm in diameter, and an outer electrode, 20 mm in diameter, in atmospheric pressure air. Streamer propagation in the discharge was observed using a streak camera. The propagation velocity increased with a shortening rise time of the pulsed voltage.

Development of Pulsed Discharge Inside Bubble in Water

The development of pulsed discharges inside a stationary air bubble in water was observed using an intensified charge-coupled device camera with high-speed gate. The discharges were ignited from a tungsten wire inserted into a glass tube, which was immersed in the purified water. The pulsed high voltage was applied to the wire by an inductive energy storage system pulsed-power generator. The streamer discharge was propagated into the bubble from the wire tip with a propagation velocity of 0.5 mm/ns. A back discharge was observed due to a charge accumulation on the water surface and the glass tube.

Improvement of deoxidization efficiency of nitric monoxide by shortening pulse width of semiconductor opening switch pulse power generator

The deoxidization efficiency of nitric monoxide (NO) was improved by shortening the pulse width of the voltage applied to a corona reactor. The deoxidization efficiency of NO was evaluated as the NO removal efficiency in nitrogen (N2) gas containing 200 ppm NO. The corona reactor had a coaxial geometry and consisted of center high-voltage wire and outer grounded cylinder electrodes. A nanosecond high-voltage pulse was generated using an inductive energy storage pulse power circuit with a semiconductor opening switch and was applied to the center wire electrode in the corona reactor. Fast recovery diodes were utilized as a semiconductor opening switch. The pulse width of the applied voltage was reduced from 21 to 14 ns with the arrester connected in parallel to the reactor. The energy efficiency for NO removal was improved from 8.2 to 35.7 g kW−1 h−1 with the arrester connected. The pulse width was also reduced to 8 ns by optimizing the circuit parameters. It was confirmed from observation with an intensified charge-coupled device (ICCD) camera that the streamer corona discharge transited to a glowlike discharge after the streamer propagated from the center wire electrode to the outer cylinder electrode. The duration of the glowlike phase was reduced with the arrester connected. The energy consumed in the glowlike phase was also reduced from 15.7 to 4.6 mJ with the arrester connected.

Ozone synthesis using streamer discharge produced by nanoseconds pulse voltage under atmospheric pressure

A ns pulse voltage was used to drive a coaxial geometry corona reactor to synthesis ozone with high energy yield. The ns pulse voltage was produced using an inductive energy storage system pulsed power generator using semiconductor opening switch (SOS) diodes. First recovery diodes were used as SOS diodes in the inductive energy storage system pulsed power generator to produce short-pulse high voltage with high-repetition rate. The pulse voltage of 12 ns width and 17 kV peak voltage was produced at charging voltage of −5 kV and was applied to a 1 mm diameter center wire electrode in the coaxial geometry reactor. The copper cylinder of 19 mm inner diameter was used as outer electrode and was connected to a ground. The ozone yield of 230 g/kWh was obtained using ns narrow pulse voltage. This value is almost 20% higher than 190 g/kWh obtained by 60 ns width pulse.

Improvement of ozone yield using double loop type inductive energy storage circuit

A ns pulse voltage was used to drive a coaxial geometry corona reactor to synthesis ozone with high energy yield. The ns pulse voltage was produced using an inductive energy storage system pulsed power generator using semiconductor opening switch (SOS) diodes. First recovery diodes were used as SOS diodes in the inductive energy storage system pulsed power generator to produce short-pulse high voltage with high-repetition rate. The pulse voltage of 12 ns width and 17 kV peak voltage was produced at charging voltage of −5 kV and was applied to a 1 mm diameter center wire electrode in the coaxial geometry reactor. The copper cylinder of 19 mm inner diameter was used as outer electrode and was connected to a ground. The ozone yield of 230 g/kWh was obtained using ns narrow pulse voltage. This value is almost 20% higher than 190 g/kWh obtained by 60 ns width pulse.