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Magnetron Sputtering Deposition of Chemically Modified Hydroxyapatite
Published in Sam Zhang, Hydroxyapatite Coatings for Biomedical Applications, 2013
As mentioned, glow discharge normally occurs when there is a current running through the gas at low pressure.35 Hence, it is crucial for a discharge tube to be connected to a DC voltage at both ends of A and C in order to maintain the discharge (Figure 1.1). The Townsend discharge normally occurs during this process. Townsend discharge can be interpreted as a gas ionization process where a small initial amount of free electrons, accelerated by strong electric field, will increase the electrical conduction through a gas by avalanche multiplication (a form of current multiplication). As such, the discharge itself is self-sustainable with the right amount of electrons being generated to produce enough ions to regenerate the same number of initial electrons by positive ion bombardment of the cathode. As a result, a luminous glow is produced as the voltage drops followed by a sharp rise in the current. The increase in power signifies higher voltage and current density levels, indicating that it has reached the abnormal glow region, which is suitable for sputtering. At this stage, the ion bombardment over the entire substrate’s surface would increase until the current density is almost uniform.
Fundamentals of Vacuum and Plasma Technology
Published in Andrew Sarangan, Nanofabrication, 2016
Once breakdown occurs, current flow proceeds as follows: positive ions will be attracted to the cathode and electrons will be repelled toward the anode, where the anode is usually connected to ground along with the chamber walls. The positive ions have to bombard the cathode with sufficient energy to cause electrons to be released. The released electrons will then be accelerated away from the cathode. This acceleration has to be sufficient for the electrons to collide with neutral gas atoms and cause ionization, releasing further ions and electrons in order to maintain a steady-state condition in the plasma. This is the onset of plasma discharge and is known as the Townsend discharge.
Non-Equilibrium Cold Atmospheric Pressure Discharges
Published in Alexander Fridman, Lawrence A. Kennedy, Plasma Physics and Engineering, 2021
Alexander Fridman, Lawrence A. Kennedy
Electronic stabilization of APG has been demonstrated, in particular, by Van De Sanden and his team. Uniform plasma has been generated in argon DBD during the first cycles of voltage oscillations with relatively low amplitude (i.e., αd of about 3). The existence of the Townsend discharge at such low voltage requires an unusually high secondary electron emission coefficient (above 0.1). The so high electron emission and the breakdown during the first low-voltage oscillations can be explained, probably, taking into account the low surface conductivity of most polymers applied as barriers in the system have very low surface conductivities. Surface charges occurring due to cosmic rays can be then easily detached by the applied electric field. Long induction time of the dark discharge is not required in this case in contrast to the OAUGDP with glass electrodes. Assuming that the major cause of the DBD filamentation is instability leading to the glow-to-arc transition, it has been suggested to stabilize the glow mode using an electronic feedback to fast current variations. The filaments are characterized by higher current densities and smaller RC constant. Therefore the difference in RC constant can be used to “filter” the filaments because they react differently to a drop of the displacement current (displacement current pulse) of different frequency and amplitude. A Simple LC circuit, in which during the pulse generation the inductance is saturated, has been used to generate the displacement current pulses. The method of the electronic uniformity stabilization has been used for relatively high power densities (in the range of 100 W/cm3) and in a large variety of gases including Ar, N2, O2, and air.
Modeling of Discharge Lamp Characteristics by Using Floating Memristor Circuit Emulator with Tunable Threshold
Published in Electric Power Components and Systems, 2020
Cengiz Polat Uzunoğlu, Yunus Babacan, Fırat Kaçar, Mukden Ug˘ur
In a Townsend discharge (A-C) the current increases as a function of the applied voltage. This regime is called dark discharge since discharge remains invisible with some exceptions [36]. After the transition from dark discharge to glow discharge (C-D), the gas medium emits luminous glow light since the exerted energy is capable of producing excited gas atoms by collisions. At the point E the temperature of the electrodes increases dramatically and the cathode displays thermionic emission characteristics. In the arc regime the applied voltage decreases as the current increases, where over currents are observed.