Increasing reliability and efficiency of pulsed power sources used in plasma

Master's thesis in the field of power electronics, power electronics

Abstract:

The use of pulsed power sources in various plasma processes is increasing due to the connection established between them. According to the research done in this case, the design of pulsed power sources with the aim of reducing losses and increasing efficiency can have important effects on plasma applications. The basis of the pulsed power system technology is based on storing a lot of energy in a relatively long time and releasing it very quickly. The goal of the energy release process is to increase its instantaneous power. One of the prominent features of pulsed power sources to increase efficiency and reliability is its complexity and detail. Improvement of efficiency and reliability in pulsed power sources due to its application in plasma is fundamentally related to the characteristics of pulsed power systems. This thesis proposes a new topology based on a modified (positive) buck-boost converter at the input of the plasma pulsed power source circuit. Based on this topology, in a certain range in the plasma pulsed power source, a set of switch-diode-capacitor is connected sequentially, which is used to generate high voltage and dv/dt. The key components of the proposed topology to increase reliability and efficiency are: a new topology structure based on a DC-DC converter, using a suitable control method (voltage source) and determining the amount of energy stored in the main elements of the circuit (sleeve and capacitor). Therefore, the presented topology can be easily adjusted, upgraded and developed with a wide scope in various applications of pulsed power sources. Considering the effect of its key components, the proposed topology has been accurately obtained from the simulation in the MATLAB/SIMULINK software environment, which confirms the efficiency and applicability of this topology to achieve the desired goals, which is to produce high power pulses with high voltage and improve the efficiency and reliability of plasma pulsed power sources.

Key words:

Topology, plasma, reliability and efficiency , pulsed power source, buck-boost positive converter.

1 Introduction

The basis of the technology of the pulsed power system is based on storing a lot of energy in a relatively long time and releasing it very quickly, and the goal of the energy release process is to increase its instantaneous power. One of the key characteristics of pulsed power sources is the voltage level and the duration of its increase, which is determined based on the characteristics of the required load [1]. Adaptation methods of pulsed power sources with different loads by existing technology is one of the key discussions of pulsed power system technology used in plasma. The use of advanced knowledge and recent approaches in power electronics and semiconductors is due to the level of industrial and scientific requirements, which has caused the rapid development of pulsed power sources in the last decade. One of the prominent features of pulsed power sources to increase its efficiency and reliability is its complexity and minuteness [2]. Optimal control of the power generation process in pulsed power generation sources is an important and vital method to increase efficiency. On the other hand, the use of high-voltage pulsed power sources requires high-power switches whose breakdown voltage and switching time are limited. 2.1 Getting to know plasma The word "plasma" was first used in 1927 by Irwin Langmuir [1] for a neutral mass of charged particles [3]. Plasma can be created by creating a potential difference between two electrodes in a gaseous environment. The electric field created between two electrodes, anode and cathode, causes ionization of neutral gas particles and creates a conductive path. Figure (1-1) shows an example of electrodes. The simplest case, the electric field lines between the anode and the cathode, where the electric field is almost uniform, depends on the size and shape of the electrodes (two flat electrodes with a small gap between them)[4].

Gas discharge curve of voltage - plasma current

Figure (1-2) shows the gas discharge curve of voltage - current of the electrodes in dc mode] 5[. This curve has several areas, the names of the areas are summarized in table (1-1).  The dark plasma discharge region, where the discharge starts. However, this discharge does not excite the particles sufficiently to cause failure.. This discharge is called dark because in this discharge there is no energy transfer to the electrons to lead to the emission of visible light. In a dark discharge with ionization, the ions and electrons alone produce cosmic rays and other forms of it (such as natural ionizing radiation) that are accompanied by voltage increases. In the saturation state with ionization, all charged particles are removed and electrons do not have enough energy due to ionization. In the Townsend mode, with the start of ionization, an electric field is created and the current and voltage increase exponentially [6]. Between the Townsend state and breakdown in the plasma, corona discharge may occur, resulting in the electric field being concentrated on the sharp edges of the electrode. Corona discharge can be visible or dark depending on the amount of current passing through it. The radiation discharge zone starts with the breakdown mode and ends with the formation of an electric arc. Mainly, the processes that lead to the formation of the fracture state and radiation discharge can be divided into two main groups: (a) gas plasma processes, in which ionization takes place from the collision of electrons and ions. (b) Cathodic plasma processes, in which electrons are released from the cathode. This process is also called secondary process due to the creation of electrons in it [7].    By studying the published articles on this matter, it can be seen that the type of cathode has a great effect on the failure mode. Through the secondary process, it is possible to emit different types of radiant energy in the form of photoelectricity, in which light energy causes the release of electrons. In this case, we can also refer to the ionic heat state in plasma, where thermal energy creates electrons and leads to the generation of an electric field. The sparks caused by the discharge in this case are very intense and have a high brightness and current density. Arcs caused by discharge can be considered equivalent to high current density in kiloamperes per square centimeter. However, the natural intensity of the arc can be the cause of faster erosion of the electrodes [9,8].

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Regions

Dark discharge

Gradiant discharge

Spark mode

Ionization mode

Saturation mode

Corona mode

Townsand mode

Failure mode

Radiative mode

Number

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Areas

Abnormal radiant mode

Transitional mode from radiant to spark

Thermal mode

Thermal mode with spark

3.1 Applied aspects of pulsed power sources in plasma

The first application of pulsed power sources in the 1960s in nuclear power plants and nuclear weapons was to produce pulses with megavolt voltage and terawatt powers (1 terawatt is 1000 gigawatts) and pulse widths of several tens of nanoseconds to several hundred nanoseconds to stimulate plasma electron accelerators [10]. The limitation of energy storage elements and the lack of power pulse switching technology prevented its expansion in more general areas. But now, with the development of these resources and the improvement of the technology of making capacitors, inductances and switches, many problems in the production of power pulses, with high energy and reasonable price, have been solved. Recently, one of the main and key goals to increase the efficiency and reliability of pulsed power systems is the frequent use of pulsed power generators with maximum power in industries such as: food industry, medical treatment, water and sewage (water treatment, etc.), ozone gas production, concrete recycling, steam engine combustion system and ion implantation in plasma [11]. The most common uses of pulse power sources can be mentioned: Marx generator, electromagnetic pulse compressors, insulation, transmission lines and pulse shaping. Although pulsed power generators are also widely used with maximum power in military applications and nuclear fusion.  Also, pulsed electric fields[2] have many direct and indirect applications in the industry, and recently the application of these fields in sterilizing food has received much attention [12]. A summary of the specifications of pulsed power sources required for different applications is described in table (1-2).

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