Basic Theory of Gas Discharge in 222nm UV Lamp
1. Townsend Electron Avalanche Theory
The phenomenon of any electric current passing through a gas is called gas conduction or gas discharge. Different operating conditions will produce different gas discharge phenomena and have different discharge properties. When studying gas discharge phenomena, discharges are usually divided into two categories: non-self-sustained discharge and self-sustained discharge. The phenomenon of discharge transitioning from non-self-sustained to self-sustained is called gas breakdown.Non-self-sustained discharge refers to a phenomenon where discharge can only be maintained under the condition of an external ionization source. Self-sustained discharge refers to a phenomenon where discharge can still be maintained when the external ionization source is removed. Under the action of an external ionization source, when the voltage across the 222nm uv lamp discharge tube increases to a certain sufficient value, the current inside the tube suddenly increases. At this time, if the ionization source is removed, the discharge current is still large enough, that is, the formation of the discharge at this moment has nothing to do with the presence or absence of the external ionization source, and this state is called self-sustained discharge. The process of discharge transitioning from non-self-sustained discharge to self-sustained discharge is called the gas breakdown process or ignition process. This discharge phenomenon and theory were first studied and established by scientist Townsend in the early 20th century, so it is called Townsend discharge. After analyzing a large number of experimental situations, Townsend believed that electrons move in a uniform electric field, continuously gaining energy from the electric field, and electrons collide with gas atoms to cause ionization and lose energy. In the equilibrium state, these two parts of energy should be equal. The newly ionized charged particles are accelerated by the electric field and can also cause ionization. When an initial electron runs from the cathode to the anode and causes collision ionization, the 1st, 2nd, 3rd, 4th, ... collision ionizations occur in sequence, and the newly generated electrons also move to the anode and produce collision ionization. The number of electrons moving to the anode increases to 2, 4, 8, 16, ... The number of electrons increases like an avalanche, so it is called an electron avalanche. The electron avalanche theory is applicable to the discharge region where the directed motion of electrons dominates under the action of the electric field, and the irregular thermal motion of electrons is in a disadvantageous position, which is also reflected in the working process of the 222nm uv lamp.The volt-ampere characteristic curve of Townsend's electron avalanche theory is shown in Figure 2-1, which can be divided into three parts: T0, T1, and T2. In the T0 region, the voltage between the electrodes is very low, and the current flowing through the gas is also very small. As shown in Figure 2-1, it rises from zero and then tends to saturation. This is the result of the directed migration of charged particles under the action of the electric field under residual ionization. Due to the radiation of cosmic rays and radioactive elements in the earth's crust, there is a certain amount of electrons and ions in any gas at any time and place. This phenomenon is called residual ionization. In the absence of an external field, these charged particles move randomly in space like gas molecules. When a low voltage is applied across the 222nm uv lamp discharge tube, electrons and ions move directionally under the action of the field, so the current gradually increases from zero. When the voltage between the electrodes is large enough, all charged particles can reach the electrodes, and at this time, the current reaches a certain maximum value. The density of charged particles generated by residual ionization is generally very weak, so the saturation current value in the T0 region is still very small (on the order of 10^-12 A).Based on the above basic discharge process, Townsend's discharge theory introduces coefficients α, β, and η to describe the mechanism of gas ionization, which is crucial for understanding the discharge mechanism of 222nm uv lamp.Townsend's first ionization coefficient α: It represents the number of free electrons generated by collision ionization between an electron and gas atoms when an electron moves from the cathode to the anode direction over a unit distance. This ionization process is also called the α process.Townsend's second ionization coefficient β (volume ionization coefficient of positive ions): It represents the number of collisions between a positive ion and gas atoms when a positive ion moves from the anode to the cathode direction over a unit distance. This process is also called the β process.Attachment coefficient η: It represents the attachment probability of an electron per unit length along the electric field direction. Attachment reduces the number of free electrons and, in a sense, competes with ionization, which affects the sustainability of discharge in the gas environment of 222nm uv lamp.
2. Streamer Theory and Leader Theory
Townsend's theory derives the self-sustained discharge criterion. Its basic model of collision ionization gain and the idea of converting from a primary electron avalanche to a secondary electron avalanche as a self-sustained discharge condition have always been the basis of gas discharge theory. However, Townsend's theory does not consider the distortion of the externally applied electric field by space charges, nor does it fully consider the source of initial electrons in secondary electron avalanches. It cannot explain the formation time delay of the breakdown process, the branching of discharge channels during large partial discharges, and the phenomenon that the discharge process is not affected by cathode materials. In 1940, Week and Leob established the streamer theory (also known as streamer theory), which complements Townsend's theory. Peterson suggested in 1970 that streamer theory is more applicable to inhomogeneous electric fields under actual conditions, which is of great significance for optimizing the electrode structure of 222nm uv lamp. According to streamer theory, when a single electron avalanche reaches a critical size, a very fast breakdown process will occur. High-speed electrons form a spherical head of the electron avalanche, moving toward the anode. Compared with electrons, positive and negative ions move very slowly and can be regarded as immobile, remaining where they are generated to form a long positively charged avalanche tail. At atmospheric pressure, the electron diffusion speed is small, and the charge of the electron avalanche is relatively concentrated, which obviously disturbs the original electric field and changes the electric field at the head of the electron avalanche. Due to the strengthened electric field in front of the electron avalanche, the accelerated movement of electrons leads to further ionization, and many electron collisions form excited particles that release photons in all directions when returning, which are easily absorbed by the gas to produce photoionization, generating secondary electrons. Thus, secondary electron avalanches are generated by collision ionization in the electric field strengthened by space charges and converge with the head of the initial avalanche to form a streamer channel, further developing to gap breakdown. Streamer theory believes that the discharge from initiation to breakdown is not a quantitative change process of collision ionization, but a qualitative change result of space photoionization when the initial electron avalanche develops to a certain extent and the number of charges generated by the sub-electron avalanche exceeds a certain critical value, and the space charge significantly distorts the external electric field. The secondary electron avalanche caused by photoionization converges with the main electron avalanche to form a streamer, the discharge turns into self-sustained discharge, and the development of discharge is maintained by space photoionization. The space photoionization process of streamer theory can well explain the discharge time delay, discharge morphology, and the influence of motor materials and other physical processes that cannot be explained by Townsend's theory, which needs to be considered emphatically when studying the discharge stability of 222nm uv lamp. Processes such as lightning with long gap discharges need to be explained by the thermal ionization of leader theory. Both streamer theory and leader theory only provide qualitative physical models, and there is no relatively complete quantitative mathematical model. Subsequent research work is mainly carried out by experimental methods.According to the characteristics of the discharge process in the corona layer, corona can be divided into two forms: electron avalanche form and streamer form. When the curvature of the corona electrode is large, the corona layer is very thin and relatively uniform, and the discharge current is relatively stable. The self-sustained discharge adopts Townsend discharge form, that is, the corona when an electron avalanche appears. As the voltage increases, the corona layer continues to expand, and individual electron avalanches form streamers, resulting in a pulse form of discharge, and then transition to streamer form corona discharge. If the radius of curvature of the electrode is increased, the corona is very intense at the beginning and forms a streamer form as soon as it appears. When the voltage is further increased, individual streamers develop strongly, resulting in brush discharge, and the pulse phenomenon of discharge becomes more intense, eventually penetrating the gap and causing complete breakdown of the gap, which is related to the service life and performance of 222nm uv lamp.It is generally believed that the electron avalanche form is suitable for uniform electric fields, and streamer theory is more suitable for inhomogeneous electric fields. Based on Townsend discharge combined with streamer theory, the breakdown criterion for inhomogeneous electric fields can be derived, which has guiding significance for the design and optimization of 222nm uv lamp.
