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. 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.

We demonstrated for the first time the possibility of using plasma for immunotherapy of cancer using mice. When we treat a cancer tumor using plasma, anti-tumor effects are also observed for other tumors than the treated tumor, as shown in the figure, 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 also demonstrated that the plasma might suppress recurrence of resected tumor. In addition, we use the plasma in combination with immune checkpoint inhibitors (2018 Nobel Prize in Physiology or Medicine) used for cancer immunotherapy. We measure reactive species in the plasma and analyze the pathology of plasma-irradiated tumors to elucidate the mechanism of the plasma-induced immune response. This is a development of a new cancer treatment method using electrical and electronic engineering technology.

When plasma is generated on the airfoil of an aircraft, the airflow through the airfoil changes. Using this phenomenon, we study high-speed airflow control of aircraft using plasma. “Flow separation” is one of crucial phenomena that determine the performance of an aircraft. It causes sudden lift decrease and drag increase, resulting in serious accident of the aircraft. An airflow control device is installed on the airfoil of airplane to suppress this separation, but its heavy weight and slow time response are problem. In contrast, the rapid-response plasma could control the high-speed airflows. In addition, the plasma device is simple and lightweight, which are advantageous. We consider this technology not only for airplanes on Earth, but also to Mars explorers.

Oxygen production and hydrocarbon fuel production on Mars are important for future Mars exploration. We study an efficient CO2 decomposition method in the Mars environment using plasma. It is well known that plasma efficiently decompose CO2 to CO + O by stepwise excitation of the intermolecular vibrational level of CO2. We try to apply the environmental gas decomposition technology using plasma, which has been studied for many years in our laboratory, to the Mars environment.

Plasma is used for surface treatment of polymers, semiconductors, metals, and liquids. The surface reaction of reactive species is important, but its measurement is quite difficult. Tens to a hundred types of reactive species are produced in the plasma, which makes it difficult to separately measure the surface reaction of each type of reactive species. We develop a method to supply only a few types of reactive species such as OH and O to a sample surface, and measure the surface reactions of various reactive species. Based on the results, we also develop a surface reaction model of reactive species.