Plasma is a state of matter that is completely or partially ionized, or any state of matter that contains a sufficient number of positively and negatively charged particles with nearly equal charges is called plasma. During the discharge process of plasma, there are generally six basic particles: photons, electrons, ground state atoms (or molecules), excited state atoms (or molecules), as well as positively charged cations and negatively charged anions. Plasmas can be classified into high-temperature plasmas and low-temperature plasmas based on their different temperatures. High temperature plasma is a plasma in thermodynamic equilibrium, which typically exists inside stars and other high-temperature environments. Low temperature plasma is an incompletely ionized plasma, in which the temperature of electrons is much higher than that of heavy particles. Low temperature plasma can be further divided into hot plasma and cold plasma. Hot plasma is close to or in thermodynamic equilibrium, with electron temperature and ion temperature roughly the same. Common examples include arc plasma discharge and magnetohydrodynamic discharge. Cold plasma is in a non thermodynamic equilibrium state, with electron temperature significantly higher than ion temperature. Common examples include glow discharge and corona discharge. Cold plasma has a wide range of applications in materials processing, semiconductor industry, biomedical and other fields.

Common atmospheric pressure cold plasma generation devices

The device for generating atmospheric plasma jets generally consists of a gas source, a power source, and a plasma generator. The commonly used gas sources mainly include argon, organic compounds, other inert gases, etc. Power supplies are divided into intermediate frequency AC power supplies, DC power supplies, RF power supplies, etc. according to their different properties and characteristics. Due to different discharge forms and principles, common atmospheric pressure cold plasma generation devices can be divided into dielectric barrier discharge devices and bare electrode discharge devices.

(1) Dielectric barrier discharge device

Dielectric barrier discharge (DBD) is a non-equilibrium gas discharge in which an insulating medium exists in the discharge space. Due to the presence of at least one insulating dielectric barrier, DBD is similar to a capacitor. The insulating material (medium) is placed between the high and low voltage electrodes of the device, and alternating current is applied between the two electrodes. The potential difference gradually increases, and eventually the insulating medium can be broken down, generating a plasma jet, as shown in Figure 1. This type of device has strong adaptability and can work normally both high and low voltage. Dielectric barrier discharge devices can be divided into various types according to the performance characteristics of the electrodes and the different placement positions of the insulating medium, such as needle plate electrode structure and needle ring electrode structure. Structure, dual ring electrode structure, etc., but the discharge principle is similar.

Figure 1 Dielectric Barrier Discharge Generator

(2) Bare electrode discharge device

Dielectric barrier discharge devices require a certain voltage and current to penetrate the insulating medium between the electrodes; The electrode position for bare electrode discharge is different, and such devices often place the electrode in the discharge space. After discharge, the current intensity gradually increases until the working gas is broken down, thereby generating a plasma jet. The schematic diagram of the bare electrode discharge device is shown in Figure 2. There is no medium inserted between the high and low voltage electrodes, and tungsten needles are often used as high voltage, while the conductive pads on the periphery have low voltage. In this way, a low voltage cold plasma jet can be generated.