In this article, Otway provides a solution to the closed Dirichlet problem which is a mixed eliptic-hyperbolic equation. This type of equations are encountered in electromagnetic wave propagation in cold plasmas.
Continue reading “Review: UNIQUE SOLUTIONS TO BOUNDARY VALUE PROBLEMS IN THE COLD PLASMA MODEL”
BURFIT-80, a prototype radio-frequency ion thruster, is designed, built and tested
at the Bogazici University Space Technologies Laboratory. This paper presents the design parameters and numerous design improvements of this thruster. Three different versions of the thruster, with the same discharge chamber inner diameter of 80 mm, have been built and tested. The latest version of this prototype thruster presents significant improvements of the DC electrical connections to the grids and RF electrical connections to the RF antenna. For the second version of the thruster, plume ion energy distribution measurements are conducted using an indigenously developed retarding potential analyzer, and some of the measurement results are presented.
Continue reading “Design Improvements and Experimental Measurements of BURFIT-80 RF Ion Thruster”
This article describes plasma sources, the possible movements of particles created inside the plasma, the origins of these particles, the definitions of thermal/non-thermal plasmas, surface and volume interactions, the processes of electrical breakdown, plasma discharge with boundary relations, 0-D model for density, temperature and electric field, and 1-D model for different pressure levels.
The article presents introductory materials for the processes inside atoms and between atoms based on energy levels. Firstly, the particles which have a probability to interact with the other species are described. Among the interacting species, photon and electron are elementary particles. The energy of a photon is calculated depending on its frequency. Electron has only translational kinetic energy calculated by translational speed. For atoms and molecules, their total energy is a summation of translational, vibrational and rotational energies. There are many energy levels representing excited levels. If we consider an atom, the minimum energy above the ground level causes excitation and constitutes a free electron. When energy transmitted to atom exceed the ionization energy (series limit), a free ion is created. After ionization, particles can attain any energy level so they create a continuum. All energy levels correspond to a configuration of possible energy states, which is called as degeneracy. It can be defined as the number of different quantum states with the same energy. For electron, there are only two possible states resulting from electron spin. Atoms can have larger degeneracy values depending on their quantum states.
In this book chapter, particle collisions are examined. Electrons and ions can experience elastic collisions by preserving total momentum and energy. Otherwise, they lose their energy in the form of ionization or excitation and this type is named inelastic collision. Electrons and fully stripped ions have only kinetic energy but excited and ionized atoms possess internal energy, analogous to potential energy. While their internal energy is constant, kinetic energy is redistributed between the colliding particles in elastic collisions. For the super elastic collisions, an excited atom can be de-excited and the total kinetic energy becomes larger.
In this article, different electron energy distribution functions (EEDF) for plasma conditions in xenon dielectric barrier discharge (DBD) are explored in plasma modelling. At the beginning, ionization and excitation rates resulting from electron-neutral collisions are discussed. Generally, local-field approximation (LFA) is used for those collisions by assuming electrons gain energy in nanoseconds and reaches equilibrium. For the LFA models, electron energy distribution function is governed by the Boltzmann equation for the primary elastic and inelastic collisions. Although LFA is useful to calculate the primary ionization and excitation rates, diffusion and mobility coefficients, the secondary processes are omitted. These processes can be superelastic collisions, stepwise ionization, electron-ion recombination etc. The models using LFA neglect the secondary processes to reduce the computational cost.
Continue reading “Summary: EEDF FOR MODELLING THE PLASMA KINETICS IN DIELECTRIC BARRIER DISCHARGES”
Sanders investigated the Townsend coefficient (α) in the regions with different pressure and distance levels in his two papers. For the former one, the author criticized the early measurements of Townsend, Bradbury and Paavola to give an explanation to the current values for higher X/p (the ratio of electric field strength [V/cm] to the pressure [mm]). Townsend explained the energetic ions cause more ions and introduced value which represents new pairs of ions per centimeter. However, his experiments were conducted in a low pressure, small distance, high electric field environment and the results failed to explain lower X/p values in high pressure conditions. The researchers proposed that at higher pressure level, the number of the created ions increases due to electron impact ionization in the dark current just before the breakdown. The distorted fields with space charge satisfy the Townsend equation with new values.
Continue reading “Summary: MEASUREMENT OF TOWNSEND COEFFICIENT”
In this chapter, the types of electron emissions from the surface of a metal, which are thermionic emission, field-enhanced thermionic emission, field emission and thermo-field emission, are introduced. While cathode is emitting electrons, anode collects electrons passively and this process simply determines the roles of anode and cathode in gas discharges. The electric current is mostly carried by electrons because of their lower mass and higher energy compared to ions. Also, current density is critical to evaluate power density distribution and energy balance which are responsible for all the processes such as electron emission, phase transition and plasma production.
In this part, surface adsorption-desorption reactions and secondary electron emission process are explained in detail. Surface processes have a key role on surface etching, for example. The gas phase on top of the surface interacts with the surface layer. Adsorption-desorption reactions affect gas-phase species concentration. Also, positive ion neutralization and secondary electron emission have effects on gas discharge. Therefore, gas and surface reactions are coupled.
In the article, low energy ion induced electron emission (IIEE) is investigated on Si and Ge semiconductors. Plasma interactions on semiconductors are recently used for etching and deposition processes. According to the previous studies, the IIEE strongly depends on the surface process but this research focuses on the sub-surfaces processes such as doping type, Fermi level of the material and cleanliness level. The studies from the literature show that the IIEE measurements are affected by the electron density in the conduction band of the semiconductor. They claim that more electrons near the vacuum potential result in larger IIEE. However, the IIEE theory predicts less dependence on electron density by assuming all the emitted electrons from the valance band. Thus, there should be no direct relation between the IIEE and doping density and type for semiconductors, on the contrary to metals.
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