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.
Month: July 2018
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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Summary: OPTICAL AND ELECTRICAL DIAGNOSTICS OF A NON-EQUILIBRIUM AIR PLASMA
The paper investigates a non-equilibrium atmospheric air plasma which is created with an AC discharge between a metal cathode and water anode with the gap distance of 1.3 cm. The current density in liquid electrode AC (60 Hz) glow discharge is tried to be obtained. OH emission intensity distribution is observed with high speed CCD camera. A band-pass filter is placed in front of the camera. The rotational and vibrational temperatures of N2 are found to calculate the composition of the plasma. Electron density in plasma is related with electrical conductivity and it is shown that electron density follows Gaussian radial distribution.
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Create your own stars!
Sometimes I’m taking photos and combining them in a video to convey a story. Here, I prepared cards with the drawings of James Jean. My LEGO characters are the stars!
Again and again and again…
Alice in Chains… Again!
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