Plasma is another form of substance that exists outside of solid, liquid, and gas states, known as the fourth state of matter. A collection mainly composed of charged positive and negative particles (including positive ions, negative ions, electrons, free radicals, and various active groups), where the number of positive and negative ions (electrons) is equal, therefore, it is also known as plasma.

Plasma is widely present in the universe, with over 99% of matter existing in a plasma state. Due to its unique properties (such as electromagnetic properties) that distinguish it from the solid, liquid, and gas states, plasma is referred to as the fourth state of matter. Plasmas can be classified into high-temperature plasmas and low-temperature plasmas based on the electronic energy state, gas temperature, and ion density. High temperature plasma usually refers to nuclear fusion plasma, which has a high plasma density and is difficult to generate in the laboratory; Low temperature plasma can be divided into equilibrium plasma and non-equilibrium plasma. In equilibrium plasma, the electron temperature and gas temperature are almost the same (several tens of eV), and the pressure is generally above atmospheric pressure. The system is in equilibrium at a temperature of 2000K-50000K and is therefore called equilibrium plasma; The electron temperature of non-equilibrium plasma is much higher than the gas temperature (around room temperature), and the system is in a non-equilibrium state, hence it is called non-equilibrium plasma.

Classification of low-temperature plasma

According to the different working gas pressures of low-temperature plasma, it can be divided into low-pressure plasma and atmospheric pressure plasma. Low pressure plasma is generated in a vacuum state, while atmospheric pressure plasma is generated at atmospheric pressure.

Low pressure plasma

According to the discharge method, low-voltage plasma can be classified into direct current glow discharge (DC) plasma and radio frequency glow discharge (RF) plasma. The discharge phenomenon caused by the collision and ionization of electrons and neutral particles in a low-pressure gas excited by a high voltage under the action of an electric field is called glow discharge, and the resulting plasma is called glow discharge plasma. The voltage of glow discharge plasma is high, but the current is low. The DC glow discharge plasma power supply is a DC power supply, while the RF glow discharge plasma power supply is a high-frequency AC power supply. Compared to glow discharge plasma, radio frequency glow discharge plasma has higher working pressure and lower voltage.

low pressure plasma

Atmospheric pressure plasma

Compared to low-pressure plasma, atmospheric pressure plasma has a lower molecular free path, resulting in more frequent collisions of active component molecules. The temperature of atmospheric pressure plasma is slightly higher than that of low-pressure plasma. Atmospheric pressure plasma mainly includes dielectric barrier discharge (DBD) plasma and cold plasma jet. Due to the presence of an insulating dielectric between the discharge electrodes, an AC power source is required. When the potential between the plates is large enough, gas is broken down to form plasma, and the insulating dielectric makes the plasma distribution uniform. Cold plasma jet is a process of generating plasma through high-frequency alternating current between open plates, and blowing the plasma into the working area through a high-speed flowing working gas.

atmospheric pressure plasma