Plasma represents a unique state of matter, which is different from the three most common composition states of matter in nature - solid, liquid, and gas - and is the fourth state of existence of matter. The content of plasma in the universe is close to 100%, for example, it exists in the interiors of stars, nebulae, and the atmospheres of planets. But in our living environment, it is an exception as there is almost no natural plasma except for lightning, auroras, and so on. This is because the generation and maintenance of plasma require high energy, just as the transition of water from solid to liquid and then to gas requires the absorption of thermal energy.

Depending on the pressure at which the plasma is located, there are low-pressure plasma, sub atmospheric pressure plasma, and atmospheric pressure plasma.

Atmospheric pressure plasma jet can directly generate high-density and low-temperature plasma in open space, which has attracted increasing attention. This unique property enables atmospheric pressure plasma jet to be better applied in various fields such as material surface modification, thin film deposition, biomedical, aerospace, and environmental science than low-pressure plasma. Similar to low-pressure plasma, atmospheric pressure plasma jets can be generated through various methods, such as direct current, low-frequency alternating current, radio frequency, and microwave. Differently, due to the absence of a low-pressure discharge vacuum system, the overall portability of the equipment is greatly improved, while the cost is greatly reduced. The second advantage is that due to the phenomenon of plasma afterglow, there is a process of plasma decay, and it can still exist for a short period of time after losing the energy to maintain the plasma. Due to the high-speed flow of the working gas, the plasma inside the device is pushed out of the strong electric field region by subsequent gases to form a jet, which allows the plasma to be maintained in an open free space. Therefore, the application of plasma jet has flexibility and can adapt to different sizes and shapes of the processed material during the processing, without being restricted by these characteristics. Compared with other types of plasma sources, it has a wider application prospect.

According to the different excitation sources of plasma, it can be divided into DC plasma, low-frequency AC plasma, RF plasma, and microwave plasma. There are many advantages to using microwave excitation to generate plasma, such as no electrode contamination, stronger controllability, and sufficient experimental evidence to prove that the active species density of microwave plasma is higher.

Generation of Microwave Plasma Jet

The discharge forms of plasma can be divided according to the operating frequency of the excitation source, including low-frequency plasma, radio frequency plasma, and microwave plasma, where the frequency of microwave is between 300MHz-300GHz. The common microwave sources used for generating plasma are 915MHz and 2.45GHz frequencies.

Plasma is an electrically neutral substance formed by the breakdown and ionization of gas molecules, mainly composed of electrons, ions, active free radicals, photons, and other components. According to the basic theory of microwave electromagnetic field, the generation of microwave plasma mainly relies on the superposition and amplification of electromagnetic field by standing waves generated by microwave resonant cavity, which generates high-intensity electric field and breaks down the working gas to produce plasma.

When exciting microwave plasma jet, it is necessary to first introduce the working gas into the resonant cavity. The microwave power source is transmitted into the resonant cavity through the coaxial line and forms a standing wave at the open position of the inner electrode tip. At this time, the high-intensity electric field formed will first break down the working gas and spray it to the nozzle position, thus forming a stable plasma in the resonant cavity.