Ryo Ono

Formation and structure of primary and secondary streamers in positive pulsed corona discharge—effect of oxygen concentration and applied voltage

Primary and secondary streamers of positive pulsed corona discharge are observed in a point-to-plane gap using a short-gated intensified CCD camera in an air-like environment. The influences of oxygen concentration and applied voltage on the properties of both streamers are presented. It is shown that the propagation velocity, the diameter, and the shape of the streamers are strongly influenced by the oxygen concentration. In pure nitrogen, the primary streamer shows branching with a diameter of about 0.2–0.4 mm, while in air, the branching disappears almost completely and the shape of the primary streamer becomes quite smooth with a diameter of more than 1 mm. After the arrival of the primary streamer at the cathode, a secondary streamer develops from the anode toward the cathode as far as the middle of the gap. The propagation length of the secondary streamer increases approximately linearly with the applied voltage. It is shown that the ratio of the energy consumed by the secondary streamer to the whole energy consumed by the discharge increases with the applied voltage.

Dynamics of ozone and OH radicals generated by pulsed corona discharge in humid-air flow reactor measured by laser spectroscopy

The dynamics of ozone and OH radicals are studied in pulsed corona discharge plasma in a humid-air environment. Ozone density is measured by the laser absorption method, and OH density is measured by the laser-induced fluorescence (LIF) method. A 100-ns pulsed corona discharge occurs between a series of 25 needle electrodes and a plate electrode. After the pulsed discharge, the time evolutions of ozone and OH densities are measured in humid air or a humid nitrogen-oxygen mixture. Results show that the addition of 2.4% water vapor to dry air reduces ozone production by a factor of about 6, and shortens the ozone formation time constant from 30 to 6 μs. Water vapor may reduce atomic oxygen levels leading to the decreased production of ozone by O+O2 reaction. The LIF measurement for OH radicals shows that OH density is approximately constant for 10 μs after the pulsed discharge, then decays by recombination reaction and reactions with the discharge products of oxygen, such as ozone or atomic oxygen. Absolute...

Measurement of gas temperature and OH density in the afterglow of pulsed positive corona discharge

The gas temperature and OH density in the afterglow of pulsed positive corona discharge are measured using the laser-induced predissociation fluorescence (LIPF) of OH radicals. Discharge occurs in a 13?mm point-to-plane gap in an atmospheric-pressure H2O(2.8%)/O2(2.0%)/N2 mixture. The temperature measurement shows that (i) the temperature increases after discharge and (ii) the temperature near the anode tip (within 1?mm from the anode tip) is much higher than that of the rest of the discharge volume. Near the anode tip, the temperature increases from 500?K (t = 0??s) to 1100?K (t = 20??s), where t is the postdischarge time, while it increases from 400?K (t = 0??s) to 700?K (t = 100??s) in the rest of the discharge volume away from the anode tip. This temperature difference between the two volumes (near and far from the anode tip) causes a difference in the decay rate of OH density: OH density near the anode tip decays approximately 10 times slower than that far from the tip. The spatial distribution of OH density shows good agreement with that of the secondary streamer luminous intensity. This shows that OH radicals are mainly produced in the secondary streamer, not in the primary one.

Dynamics and density estimation of hydroxyl radicals in a pulsed corona discharge

Hydroxyl radicals generated by a pulsed corona discharge are measured by laser-induced fluorescence (LIF) with a tunable KrF excimer laser. The discharge with 35?kV voltage and 100?ns pulse current occurs between needle and plate electrodes in H2O/O2/N2 mixture at atmospheric pressure. The density and decay profile of OH radicals are studied. OH radicals decay with time after the discharge with a time constant of about 30-60??s. The OH density is estimated to be about 7?1014?cm-3 in H2O(2.4%)/N2 mixture 10??s after the discharge. The OH density is approximately proportional to the energy dissipated in the discharge. The O2 content influences the OH production. When the O2 content is varied in H2O(2.4%)/O2/N2 mixture, the OH density is maximum at an O2 content of 2%. The spatial distribution of OH density shows that OH radicals are produced in the streamers under positive?discharge.

Ozone production process in pulsed positive dielectric barrier discharge

The ozone production process in a pulsed positive dielectric barrier discharge (DBD) is studied by measuring the spatial distribution of ozone density using a two-dimensional laser absorption method. DBD occurs in a 6 mm point-to-plane gap with a 1 mm-thick glass plate placed on the plane electrode. First, the propagation of DBD is observed using a short-gated ICCD camera. It is shown that DBD develops in three phases: primary streamer, secondary streamer and surface discharge phases. Next, the spatial distribution of ozone density is measured. It is shown that ozone is mostly produced in the secondary streamer and surface discharge, while only a small amount of ozone is produced in the primary streamer. The rate coefficient of the ozone production reaction, O + O2 + M → O3 + M, is estimated to be 2.5 × 10−34 cm6 s−1.

Spatial distribution of ozone density in pulsed corona discharges observed by two-dimensional laser absorption method

The spatial distribution of ozone density is measured in pulsed corona discharges with a 40 µm spatial resolution using a two-dimensional laser absorption method. Discharge occurs in a 13 mm point-to-plane gap in dry air with a pulse duration of 100 ns. The result shows that the ozone density increases for about 100 µs after the discharge pulse. The rate coefficient of the ozone-producing reaction, O + O2 + M → O3 + M, is estimated to be 3.5 × 10−34 cm6 s−1. It is observed that ozone is mostly distributed in the secondary-streamer channel. This suggests that most of the ozone is produced by the secondary streamer, not the primary streamer. After the discharge pulse, ozone diffuses into the background from the secondary-streamer channel. The diffusion coefficient of ozone is estimated to be approximately 0.1 to 0.2 cm2 s−1.

OH radical measurement in a pulsed arc discharge plasma observed by a LIF method

Hydroxyl radicals generated by a pulsed arc discharge were measured by laser-induced fluorescence (LIF) with a tunable KrF excimer laser. It was shown that not only OH but excited O/sub 2/ (O/sub 2//sup */) and excited NO (NO/sup */) had absorption lines within a tunable range of the KrF laser, so that LIF signals of O/sub 2//sup */ and NO/sup */ were observed as well as OH signals. It was demonstrated that OH could be detected without disturbance by LIF signals of O/sub 2//sup */ or NO/sup */ through the A-X(3,0):P/sub 2/(8) excitation; OH density was measured under various conditions in the post-discharge region. The influences of humidity, discharge current, and O/sub 2/ concentration on OH density and OH decay rate were studied in H/sub 2/O/O/sub 2//N/sub 2/ mixture. The results provided some possible channels of OH production and reaction with other molecules in humid air. The absolute density of OH was estimated.

Density and temperature measurement of OH radicals in atmospheric-pressure pulsed corona discharge in humid air

Plasma application for environmental improvement is desirable, and it is worthwhile to clarify the behavior of OH radicals in nonthermal plasma. Under atmospheric-pressure humid air, the time evolutions and spatial distribution of relative density and rotational temperature of OH radicals are measured in pulsed positive corona discharge using laser-induced fluorescence with a tunable optical parametric oscillator laser. The density of OH radicals generated by discharge when 28 kV is applied is estimated to be about 1×1015cm-3 at 3 μs after discharge. The OH density increases with humidity. The rotational temperature rises after discharge. The rate of temperature rise increases with humidity. This phenomenon arises from fast vibration-to-translation energy relaxation of H2O. The spatial distributions of OH rotational temperature indicate that the temperature rises in the secondary streamer channel.

Measurement of hydroxyl radicals in pulsed corona discharge

Abstract Hydroxyl radical generated by a pulsed corona discharge was measured by laser-induced predissociative fluorescence (LIPF) with a tunable KrF excimer laser. The LIPF with a KrF laser is a suitable method for OH observation at atmospheric pressure. It can drastically reduce the influence of undesirable quenching in the excited state. It was demonstrated that the OH–LIPF could be applied to pulsed corona discharge at atmospheric pressure. OH was observed with the LIPF in H 2 O/O 2 /N 2 mixture. The dependence of OH yield on ambient gas composition and discharge voltage was studied. In addition, the reaction of OH with NO, a pollutant removed by non-thermal plasma, was monitored by the LIPF. Our research showed that the LIPF with a KrF laser is a useful tool for characterizing OH radicals in pulsed corona discharge.

Behaviour of atomic oxygen in a pulsed dielectric barrier discharge measured by laser-induced fluorescence

Atomic oxygen is measured in a pulsed dielectric barrier discharge (DBD) using two-photon absorption laser-induced fluorescence (TALIF). The ground-level atomic oxygen is excited to the 3p 3P state by two-photon absorption at 226 nm. Negative (−40 kV) or positive (+30 kV) pulsed DBD occurs in an O2–N2 mixture at atmospheric pressure. The pulse width of the DBD current is approximately 50 ns. The TALIF experiment shows that the decay rate of atomic oxygen increases linearly with O2 concentration. This result proves that atomic oxygen decays mainly by the third-body reaction, O + O2 + M → O3 + M. The rate coefficient of the third-body reaction is estimated to be 2.2 × 10−34 cm6 s−1 in the negative DBD and 0.89 × 10−34 cm6 s−1 in the positive DBD. It is shown that the decay rate of atomic oxygen increases linearly with humidity. This can explain the well-known fact that ozone production in DBD is suppressed by increasing humidity.