In physics, a well-defined plasma is described as the fourth state of matter. It is made up of charge particles, ions, and electrons, as well as other excited gas species and highly reactive charge carriers. The microwave plasma discharge will occur over a wide range of gas pressures, beginning at mbar vacuum and ending at atmospheric pressure. Plasmas are classified as either equilibrium or non-equilibrium. In the equilibrium section, the temperatures of electrons and ions are roughly close to each other, whereas in non-equilibrium plasmas, the temperatures of electrons and ions differ greatly. Microwave plasmas are non-equilibrium plasmas because the electron temperature is around 11000K and the massive species temperature can range between 2000 and 6000K. Because of the high collision rate of charged particles in atmospheric pressure, particularly electrons, with background molecules, the atmospheric microwave plasma jet is a thermal plasma. The collision is the source of heat transfer from electron energy to the molecular energy.

The Figure 2 depicts the microwave plasma jet schematic. The microwaves that created with magnetron device, can interact with an insulation tube, such as a quartz tube. The igniter is a metal rod that is introduced into the microwave medium and creates an arc to create a gas discharge. Because plasma is a good absorber of microwaves, it continuously absorbs microwave energy. In microwave plasma jets, two types of gas flow are typically used. The swirl flow, also known as radial flow, and the axial flow. The main flow is radial, and it serves two functions: (i) Separate the plasma from the inner surface of the tube to prevent it from melting, (ii) Increase the residence time of the gas inside the tube to interact with microwaves more effectively.

The moving plunger is a metal plate in a specific position to apply the maximum electric field of microwave in the centre of the tube.

The 915MHz plasma jet is widely used in industrial heating with high power up to 100kW. A photograph of an operating unit generating such a 915MHz plasma jet is shown in Figure 2 (right). The power in microwave plasmas is reflected by plasma with electrons. The three stub Tuner is used to reduce reflection power and maximize plasma power absorption.