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.
Author: Nazli Turan
Summary: EEDF FOR MODELLING THE PLASMA KINETICS IN DIELECTRIC BARRIER DISCHARGES
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.
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Summary: MEASUREMENT OF TOWNSEND COEFFICIENT
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.
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Summary: PHYSICS OF CATHODE PROCESS
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.
Summary: SURFACE PROCESSES
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.
Summary: THE EFFECTS OF THE FERMI LEVEL ON ION INDUCED ELECTRON EMISSION FROM CHEMICALLY AND SPUTTER CLEANED SEMICONDUCTERS
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.
Summary: PARTICLE AND FLUID SIMULATIONS OF LOW-TEMPERATURE PLASMA DISCHARGES: BENCHMARKS AND KINETIC EFFECTS
The review article has evaluated three different particle and fluid simulations for low-temperature plasma discharges. The investigated methods are fluid, particle-in-cell and hybrid simulations. The plasma discharge includes complexities in itself but the disciplined models can offer research guidelines, better design alternatives and better operation conditions. The selected model is adapted to the specific plasma conditions. The weakness of the simulations results from the uncertainties of the input parameters and the assumptions. For plasma simulations, cross sections, secondary electron emission coefficients and rate constants cannot be identified strictly.
Summary: LOW-PRESSURE GAS BREAKDOWN IN UNIFORM DC ELECTRIC FIELD
In this paper, the breakdown phenomenon is explored in low pressure DC discharges by introducing a new variable (L/R). The various gases and cathode materials are tried experimentally to obtain data for the breakdown voltage. For the experiments, E/p is remained constant while pL value is changing. In literature, Paschen’s law illustrates that the breakdown voltage is a function of p (pressure) and L (the discharge gap). However, Townsend and McCallum showed that for the same pL values, the voltage is significantly higher for higher L values. This implies there should be another factor affecting the voltage. This parameter is decided as L/R, where R is the radius of the discharge vessel.
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Summary: THE MULTISCALE NATURE OF STREAMERS
This paper presents a broad research to understand the self-generated field enhancement at the head of the discharge channel, growing streamers and branching concepts. They discuss the several types of streamers observed in nature such as lightning and sprite discharges. The comprehensive photographs obtained with ICCD in nanoscale provide the propagation pattern clearly. Since the streamers carry highly energetic electrons, there are many different applications emerging in the areas of deposition, chemical processing or gas convection. In the paper, the physical mechanisms behind the streamer formation are evaluated in detail to proceed a full-scale computational model. Some arguments are included to the model. In the regions with high electric field, electrons and ions are produced. Those electrons move with drift velocity and diffuse. The applied electric field is distorted by the space charges.
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Summary: THE BOHM CRITERION AND SHEATH FORMATION
In this comprehensive review article, the regions between quasi-neutral plasma and wall are investigated by defining sheath and presheath. Bohm criterion, which is the required condition to form an electrostatic sheath vanishing in the plasma or the condition for an electrostatic potential to satisfy the boundary condition at the wall, is examined with a cold ion fluid model to illustrate the traits in between the plasma-sheath transition. To match the solutions, including singularity, on the both sides of the sheath boundary, a transition layer is inserted. The sheath thickness is defined with Debye length for the thin sheath approximation. The region in which ions are accelerated specifies presheath. The Bohm velocity states the acoustic velocity and ions should be accelerated to supersonic velocities to satisfy the Bohm criterion. This is a similar approach as we see in fluid dynamics in the concept of breaking the sound barrier. The passing from subsonic to supersonic velocities implies there should be strong space charge formation.
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