Plasma is generated using high-voltage electrical discharge. In the plasma, electron-impact reactions produce reactive species including radicals (O, N, OH), ions (N2+, O2-), and excited species (N2(v), N2(A), O2(a), O(D)). As a result, plasma has high chemical reactivity. There are various applications employing the high reactivity of plasma.
We have studied these application technologies of atmospheric-pressure plasmas and their reaction mechanisms. Our current, past, and planned researches are indicated in the upper figure in red. We have studied various application technologies in addition to the fundamental researches using laser spectroscopy and simulation.
1. Fundamental study of plasma reactions
(measurements and simulations of plasma)
Reactive species are very important in the plasma chemical processes. We measure the reactive species using laser spectroscopy to study the reaction processes in the plasma. The table shown below lists the reactive species we have measured. The lifetimes of reactive species are very short (1 us to 1 ms), so we need in-situ measurements. We also develop a simulation model of streamer discharge including the reactions of reactive species. Refer to the review paper for more information about our measurements of reactive species and simulations.
1.1 Streamer discharge
Streamer discharge is a typical atmospheric-pressure plasma, which is used for environmental pollution control, water treatment, plasma medicine, plasma assisted ignition and combustion, and surface treatment of materials. We measure various types of reactive species in the streamer discharge using laser spectroscopy and develop a simulation model of the streamer discharge. In addition, we cooperate to measure the density and energy distribution of electrons in the streamer discharge using laser spectroscopy.
(Left) Laser equipment. (Center) Principle of laser-induced fluorescence. (Right) OH density measurement in the postdischarge period of streamer discharge using laser-induced fluorescence.
Simulations of streamer discharge. (Left) Electric field and electron density. (Center) Streak images. (Right) Time evolution of reactive species densities after streamer discharge pulse.
1.2 Helium atmospheric-pressure plasma jet
Helium atmospheric-pressure plasma jet is used for surface treatments of materials, material synthesis, and plasma medicine. The fundamental research of the plasma jet is very important. We measure the reactive species in the plasma jet using laser spectroscopy. We also develop a reaction model of the reactive species, which can reproduce the measurement results.
Measurements of OH and O densities and air-helium mixture ratio using laser-induced fluorescence when a helium plasma jet is irradiated onto a glass surface.
2. Plasma medicine
Plasma is efficacious for cancer treatment, blood coagulation, wound healing, tissue regeneration, etc. It is believed that reactive species are effective for the medical treatment. However, the treatment mechanism is not well understood. We study the cancer treatment and wound healing (sterilization) as well as elucidate these treatment mechanisms.
2.1 Cancer treatment
We demonstrated the possibility of using plasma for immunotherapy of cancer using mice. When we treat a cancer tumor using plasma, as shown in the left figure shown below, anti-tumor effects are also observed for other tumors than the treated tumor, and the abscopal effects keep for a long period. This effect is already known for radiotherapy, but a similar effect may also appear in the plasma treatment. We study the plasma-induced cancer-specific immune response. In addition, we measure the densities of reactive species in the plasma which we use for treating cancer cells to examine the therapeutic effects of those reactive species on cancer treatment. We also perform pathological analysis of plasma-treated tumors.
(Left) Cancer-specific immune activation using plasma. (Center) Experiment using cells. (Right) Measurement of OH radicals using two-dimensional laser-induced fluorescence.
2.2 Wound healing (sterilization)
There are two important effects for wound healing using plasma: (i) regeneration of wounded cells using plasma and (ii) sterilization using plasma. We focus on the plasma sterilization, and examine the sterilization effects of various reactive species.
3. Aerospace application using plasma
Plasma produces various gas dynamic phenomena accompanied with reactions such as ionization and dissociation. We aim to apply the gas dynamics phenomena generated by plasma to the aeronautical engineering applications.
3.1 Flow control using plasma
"Flow separation" is one of the crucial phenomenon that determine the performance of the aircraft. It cause the sudden lift decrease and drag increase resulting in the serious accident of the aircraft. An airflow control device is installed on the airfoil of the airplane to suppress this separation, but it has problems for its heavy weight and time response. We aim to improve these problems by using plasma. In particular, a nanosecond-pulse discharge can generate an instantaneously high-pressure and high-temperature field in the space, whose gas dynamic property is effective for high-speed flow control. We develop nanosecond-pulsed-driven plasma actuator for high-speed flow control through the wind-tunnel experiments.
3.2 Flow control under a reduced-pressure environment
Aircraft fly in an environment at varying densities, pressures and temperatures depending on the flight altitude. On the other hand, plasma characteristics are also affected by density, pressure and temperature. Therefore, in order to put the flow control using plasma into practical use, it needs the experiments in an environment that simulates the actual atmospheric physical properties during the flight. We investigate the relationship between plasma and flow in various atmospheric physical environments, for the application of plasma flow control to airplanes flying at a high altitude, and airplanes flying in the Mars (April 22, 2021, NASA news).
3.3 Visualization of density variation occurred in a plasma disacharge
When a plasma occurs, the flow is induced and the gas is rapidly heated resulting
in decreasing gas density. This temperature increase and density decrease affect not only
the flow properties but also affect the rate of the chemical reaction, which is important
for plasma chemical applications.
We measure the density variation induced by plasma
discharge by using various density visualization method such as a Schlieren method and Mach-Zehnder interferometer.
(Right figure) Density visualization for a spark discharge using Mach-Zehnder interferometer.
3.4 Decomposition of CO2 and conversion to hydrocarbon fuels under the Mars environment
Oxygen production and hydrocarbon fuel production in the Mars environment is
important for future Mars exploration.
We have studied an efficient CO2
decomposition method in the Mars environment using a discharge plasma.
Currently, NASA is developing an oxygen generator using a Solid Oxide Electrolysis Cell (SOEC)(April 21, 2021, NASA news),
but there is a possibility that plasma can be used to generate oxygen more efficiently.
We are trying to apply the plasma gas decomposition technology,
which has been studied for many years under the earth environment, to the Mars environment.
4. Surface treatment using plasma
Plasma is widely used for surface treatments of materials and living tissues (= plasma medicine). Reactive species react on the surfaces to modify the surface chemical composition and/or to stimulate the living tissues. We work on particularly fundamental researches of the surface treatment using plasma.
4.1 Selective production of a specific type of reactive species
For the surface treatment using plasma, quantitative measurements of the effect of each type of reactive species (OH, O, etc.) on the treatments is important. However, it is difficult to isolate the treatment effect of a specific type of reactive species because the plasma supplies tens to a hundred types of reactive species simultaneously. To measure the treatment effect of a specific type of reactive species, we have developed a method to selectively produce a specific type of reactive species and supply them to the surface of materials and cells. We use vacuum ultraviolet photolysis of molecules to produce the reactive species. We also develop a simulation model, which can reproduce the measurement results.
4.2 Quantitative measurement of the effect of OH radicals on surface treatments
OH radicals are produced from water molecules in plasma. They are considered to play important roles in surface treatments. However, the effect of OH radicals on the surface treatments have not been quantitatively measured. We quantitatively measure the effect of OH radicals on surface treatments by irradiating OH radicals to the surfaces of materials and cells using the VUV method. We will also measure the effects of reactive species other than OH radicals on the surface treatments, and we have now succeeded in measuring O radicals.
5. Plasma processing of semiconductors
The plasma processing of semiconductors is one of the most successful techniques of plasma applications. Low pressure plasma is used instead of atmospheric pressure plasma. We study what related to the plasma processing of semiconductors.5.1 Measurement of reactive species in microwave water vapor plasma used for ashing process
Ashing process is to remove photoresist from substrates using plasma. A microwave water vapor plasma is developed as a new method for the ashing process. We measure the density of OH radicals, which are considered to be the most important reactive species in the microwave water vapor plasma, using laser spectroscopy. We also develop a reaction model which can reproduce the behavior of the measured OH density.
6. Past researches
Some of our previous researches on plasma applications are introduced. Our fundamental researches including laser spectroscopy and simulation are also related to these plasma applications.
6.1 Water treatment
When we generate plasma in water or on its surface, a large amount of OH radicals are produced via dissociation of H2O molecules. We had studied water treatment using plasma which utilizes the high reactivity of OH radicals.
Treatment of dye-polluted water using plasma.
6.2 Removal of gaseous environmental pollutants
We had studied decomposition of gaseous environmental pollutants including combustion exhaust gases and volatile organic compounds (e.g. benzene, trichloroethylene, and toluene) using plasma. Our measurements of reactive species and simulation are largely related to this pollution control technologies. We are doing these measurements and simulation while considering their relation with the environmental technologies.
Decomposition of environmental gaseous pollutants using dielectric barrier discharge.
6.3 Plasma assisted ignition and combustion
Plasma can improve combustion efficiency and reduce its pollutant emissions by supplying reactive species and heat. We had developed a plasma-assisted combustion technology which can reduce the pollutant emissions by stabilizing lean combustion using plasma. In addition, plasma can ignite a fuel-air mixture like the spark-ignition in an automobile engine. We had also examined the ignition of hydrogen using plasma. This research was also related to the safety of electrostatic hazard of hydrogen handling.
(Upper left) Plasma assisted combustion.
6.4 Treatment of dye-sensitized solar cells using reactive species
Dye-sensitized solar cell (DSSC) has a TiO2 nanoporous photoelectrode colored with dye. We had improved the energy conversion efficiency of DSSC by chemical modification of the TiO2 nanoporous photoelectrode with reactive species produced by plasma and UV light. The treatment with reactive species also lowers the annealing temperature for fabricating the photoelectrode from 450 C to approximately 100 C. It enables us to use low-cost and light-weight plastic substrates instead of the conventional expensive heat-resistant glass substrates.
(Left and center) Dye-sensitized solar cell. (Right) plasma treatment.
Latest update: May 06, 2021