Norimitsu Takamura

Ozone generation using positive- and negative-nano-seconds pulsed discharges

Pulsed discharges in atmospheric pressure gases have been studied for many years since it is one of the promising technologies for the removal of the hazardous environmental pollutants and ozone generation. In the literatures, especially, some researchers reported that an effect of pulse polarity on pollutants treatment. These experimental results indicate that the negative pulsed discharge gives an improvement of an energy efficiency of pollutant gas treatment. It is described that the polarity effect of pulsed discharge caused by difference of streamer propagation mechanism between both pulse polarities. However, no comparison results of the polarity effect in nanoseconds range pulsed discharge has been reported so far. In this paper, an investigation of the ozone generation using positive and negative nano-seconds pulsed discharges was conducted. In the experiments, ns pulse generator which has 5 ns of pulse duration in output pulsed voltage is employed. The results showed negative voltage polarity has higher ozone production yield than positive one at high voltage application.

Growth Control of Leaf Lettuce Using Pulsed Electric Field

Agriculture in Japan is approaching a crisis for two reasons: a declining self-sufficiency rate and a decreasing number of agriculture workers. To solve these problems, plant factories have been attracting attention recently. Plant factories are facilities that aid the steady production of high-quality vegetables year round by artificially controlling the cultivation environment, allowing growers to drastically decrease production time. Despite the many advantages of plant factories, a main impediment is the reduction of initial and running costs. This paper utilizes leaf lettuce (Early impulse), which is a typical item cultivated in plant factories, as a target to improve productive efficiency of plant factories due to its relatively high price. Various pulsed electric fields (PEFs) were applied to roots of the lettuce to increase lettuce growth rate. The experimental results show PEF intensity from 0.2 to 1.0 kV/cm is positive for growth stimulation. Conversely, those over 1.0 kV/cm resulted in growth inhibition. Furthermore, roots of the samples which exhibited increased leaf weight grew more robustly than those of decreased leaf weight. In addition, analysis results showed that there was no significant difference when liquid fertilizer was applied prior to or after application of PEF. The results suggest that PEF does not affect the composition change of liquid fertilizer but directly influences the growth of leaf lettuce.

Growth control of leaf lettuce using pulsed electric field

In order to improve productive efficiency of plant factory because the vegetable price of plant factory is high, in this study, leaf lettuce (Early impulse) was employed as specimen in plant factory and root of the lettuce was applied various PEFs for increasing lettuce growth rate. The experimental results showed PEF intensity from 0.2 to 1.0 kV/cm was occurred to growth stimulation. On the contrary, over 1.0 kV/cm was occurred to growth inhibition. Furthermore, the roots of increased leaves weight were lead to more growth than the roots of decreased one. It was suggested that PEF application have a positive effect on the root cells. On the other hand, analysis results of liquid fertilizer showed no significant difference between prior to and after application with PEF. The result suggests that PEF does not affect the composition of liquid fertilizer but directly influence the growth of lettuce in this study.

Influence of test method and gas pressure on negative lightning impulse breakdown voltage in ultra-pure water

Insulating oils have been used as dielectric materials in transformers, and also in capacitors and cables. The main advantages of insulating oils are low cost and their superior insulating properties. However, once oils leaks from the equipment, it adversely affects the environment. Therefore, we focused on ultra-pure water (UPW) as one of environmental friendly liquid insulating materials. In this study, we have investigated the fundamental insulation characteristics of UPW using negative lightning impulse (L. Imp.) voltage (1.0/40 μs) and a half-inch (12.7 mm) diameter sphere electrodes at atmospheric pressure (0.1 MPa). The gap length of electrodes, the insulation resistivity, and the temperature of UPW were set in the range of 1.00–2.50 mm, 15.6–18.2 MΩ·cm, and 17–24°C, respectively. In our recent studies, we investigated and compared the L. Imp. breakdown voltages (BDVs) in UPW using Progressive Stress Test (PST) and Up-and-Down Test (UDT) methods. In both PST and UDT methods, the average L. Imp. BDV tends to increase with increasing gap length, and there was no significant difference. In contrast, the average L. Imp. breakdown electric field tends to decrease with increasing gap length in both methods, in common with insulating oils. As a result, there was no significant difference in breakdown characteristics of UPW in two different test methods. On the other hand, it was also found that the average L. Imp. BDV of UPW tends to increase with increasing gap length for both w/wo pressurization, and the BDV of UPW increased approximately 1.3 times by pressurizing UPW in the range of 0.26–0.28 MPa, compared with non-pressurizing (0.1 MPa). Furthermore, the average standard deviation of L. Imp. BDV decreased to approximately 30% by pressurizing UPW, in common with insulating oils. These results indicate that pressurization might prevent air bubble generating in UPW, which decreased the number of weak points in UPW for breakdown.

Matching a Nanosecond Pulse Source to a Streamer Corona Plasma Reactor With a DC Bias

Matching a pulse source to a plasma load is one of the main challenges to overcome to maximize the full potential of pulsed discharges for air purification applications. In this paper, we propose experiments that investigate the matching of a nanosecond pulse source to a corona plasma reactor that is aided by a high voltage on a tertiary electrode. The corona plasma reactor is a wire-cylinder reactor, as is commonly used in pulsed power plasmas. A tertiary electrode is situated on a dielectric layer against the cylinder of the reactor. On this tertiary electrode, we can apply a pulsed RF or dc voltage to provide an additional plasma and bias voltage in the corona plasma reactor. The pulse source that energizes the main corona plasma is the nanosecond pulse source of Kumamoto University, Japan. In this paper, we show the results of experiments with a dc voltage on the tertiary electrode. We varied the amplitude of the pulsed voltage as well as the dc voltage and investigated their effect on the corona plasma with energy and ozone measurements. The results show that the matching to the reactor increases with increasing dc voltages. However, the matching effect decreases with higher repetition rates. Furthermore, ozone measurements confirmed that a better matching also results in a higher ozone production, but in a lower ozone production energy yield.