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.
Thermal velocities of the particles are introduced first, and the force exerted on the moving charges is identified as the Lorentz Force, which includes the electric and magnetic field terms. The types of collisions are described and the mean free path, the cross-sectional area, and the number density terms are explained by relating them to pressure and energy equations. One of the surprising part is to learn Ramsauer effect (so-called Townsend effect), which states that electrons with lower energy can show scattering by an atom even though they do not have the necessary energy to ionize the atom. This interaction is defined as inelastic collisions due to ionization or excitation of the atom. This behavior is explained by the wave nature of particles, or simply by quantum mechanics.
Plasmas can be categorized in terms of the charge density and the thermal energy of the electrons. Laboratory plasmas are mostly non-thermal plasmas with higher electron temperature than ion’s. These plasmas are formed with an applied electric current which supplies energy to free electrons, creating more electrons via ionization. The next step is to go deeper into the sources of electrons, which can be thermionic emission based on the material work function, the interactions at the plasma boundaries resulting in secondary electron emission, and the strong electric field at the sharp edges. The ion production in volume processes is categorized for the regions with high-pressure, photo-ionization with the incident UV radiation, the Penning effect, and highly energetic electrons. Then, ionization is discussed by introducing electron-impact cross section which can imply the probability of ionizing collisions. The cross section depends on the energy of the electron interacting with the atoms. The energy is represented by the electron temperature which has a Maxwellian distribution.
In the following section, electrical breakdown, the condition defining a transition from an insulator state to a conductor state, is discussed first for DC sources, then for RF sources. The important characteristic for both plasmas is to observe the similar Paschen curves, namely pressure and distance dependence of the breakdown voltage. Discharge structure is introduced with the glow regions and the dark spaces. Plasma sheaths – the voltage gradients near surfaces – are defined. Debye length, which is the thickness of the sheaths and Bohm velocity, which is the velocity at the sheath boundary are explained with Poisson’s equation and the potential relations. 0-D model is used for the steady state discharges to investigate density, temperature and electric field. Finally, 1-D discharge structure is presented with the effects of diffusion and electric field on the particle fluxes. Energy flow inside plasma is summarized with a figure.
The article can be considered as a brief introduction to gas discharges. Basic concepts are explained without giving much detail but also those are helpful to get an idea about discharges.
Reference: Braithwaite, N. St J. “Introduction to gas discharges.” Plasma sources science and technology 9, no. 4 (2000): 517.
Course: AME 60637