DC Glow Discharge

DC glow discharges in the low-pressure regime have long been used for gas lasers and fluorescent lamps. DC discharges are attractive to study because the solution is time independent. This model shows how to use the interface to set up an analysis of a positive column. The discharge is sustained by emission of secondary electrons at the cathode.

Model Definition

The DC discharge consists of two electrodes, one powered (the anode) and one grounded (the cathode). The positive column is coupled to an external circuit:

Figure 1: Schematic of the DC discharge and external circuit.

domain equations

The electron density and mean electron energy are computed by solving a pair of drift-diffusion equations for the electron density and mean electron energy. Convection of electrons due to fluid motion is neglected. For detailed information on electron transport see Theory for the Drift Diffusion Interface in the Plasma Module User’s Guide.

where:

The electron source Re and the energy loss due to inelastic collisions Rε are defined later. The electron diffusivity, energy mobility and energy diffusivity are computed from the electron mobility using:

The source coefficients in the above equations are determined by the plasma chemistry using rate coefficients. Suppose that there are M reactions which contribute to the growth or decay of electron density and P inelastic electron-neutral collisions. In general P >> M. In the case of rate coefficients, the electron source term is given by:

where xj is the mole fraction of the target species for reaction j, kj is the rate coefficient for reaction j (m3/s), and Nn is the total neutral number density (1/m3). For DC discharges it is better practice to use Townsend coefficients instead of rate coefficients to define reaction rates. Townsend coefficients provide a better description of what happens in the cathode fall region Ref 1. When Townsend coefficients are used, the electron source term is given by:

where αj is the Townsend coefficient for reaction j (m2) and Γe is the electron flux as defined above (1/(m2·s)). Townsend coefficients can increase the stability of the numerical scheme when the electron flux is field driven as is the case with DC discharges. The electron energy loss is obtained by summing the collisional energy loss over all reactions:

where Δεj is the energy loss from reaction j (V). The rate coefficients may be computed from cross section data by the following integral:

where γ = (2q/me)1/2 (C1/2/kg1/2), me is the electron mass (kg), ε is energy (V), σk is the collision cross section (m2) and f is the electron energy distribution function. In this case a Maxwellian EEDF is assumed. When Townsend coefficients are used, the electron energy loss is taken as:

For non-electron species, the following equation is solved for the mass fraction of each species. For detailed information on the transport of the non-electron species see Theory for the Heavy Species Transport Interface in the Plasma Module User’s Guide.

The electrostatic field is computed using the following equation:

The space charge density ρ is automatically computed based on the plasma chemistry specified in the model using the formula:

For detailed information about electrostatics see Theory for the Electrostatics Interface in the Plasma Module User’s Guide.

boundary conditions

Unlike RF discharges, the mechanism for sustaining the discharge is emission of secondary electrons from the cathode. An electron is emitted from the cathode surface with a specified probability when struck by an ion. These electrons are then accelerated by the strong electric field close to the cathode where they acquire enough energy to initiate ionization. The net result is a rapid increase in the electron density close to the cathode in a region often known as the cathode fall or Crookes dark space.

Electrons are lost to the wall due to random motion within a few mean free paths of the wall and gained due to secondary emission effects, resulting in the following boundary condition for the electron flux:

(1)

and the electron energy flux:

(2)

The second term on the right-hand side of Equation 1 is the gain of electrons due to secondary emission effects, γp being the secondary emission coefficient. The second term in Equation 2 is the secondary emission energy flux, εp being the mean energy of the secondary electrons. For the heavy species, ions are lost to the wall due to surface reactions and the fact that the electric field is directed toward the wall:

plasma chemistry

Argon is one of the simplest mechanisms to implement at low pressures. The electronically excited states can be lumped into a single species, which results in a chemical mechanism consisting of only 3 species and 7 reactions (electron impact cross-sections are obtained form Ref. 2):

Table 1: table of collisions and reactions modeled.
reaction	formula	type
1	e+Ar=>e+Ar	Elastic	0
2	e+Ar=>e+Ars	Excitation	11.5
3	e+Ars=>e+Ar	Superelastic	-11.5
4	e+Ar=>2e+Ar+	Ionization	15.8
5	e+Ars=>2e+Ar+	Ionization	4.24
6	Ars+Ars=>e+Ar+Ar+	Penning ionization	-
7	Ars+Ar=>Ar+Ar	Metastable quenching	-

In this discharge, the electron density and density of excited species is relatively low so stepwise ionization is not as important as in high density discharges. In addition to volumetric reactions, the following surface reactions are implemented:

Table 2: table of surface reactions.
reaction	formula	Sticking Coefficient
1	Ars=>Ar	1
2	Ar+=>Ar	1

When a metastable argon atom makes contact with the wall, it reverts to the ground state argon atom with some probability (the sticking coefficient).

Results and Discussion

The electric potential, electron density and mean electron energy are all quantities of interest. Most of the variation in each of these quantities occurs along the axial length of the column. Figure 2 plots the electron density in the column. The electron density peaks in the region between the cathode fall and positive column. This region is sometimes referred to as Faraday dark space. The electron density also decreases rapidly in the radial direction. The is caused by diffusive loss of electrons to the outer walls of the column where they accumulate a surface charge. The buildup of negative charge leads to a positive potential in the center of the column with respect to the walls.

Figure 2: Surface plot of electron density inside the column.

In Figure 4 the electric potential is plotted along the axial length of the column. Notice that the potential profile is markedly different from the linear drop in potential, which results in the absence of the plasma. The strong electric field in the cathode region can lead to high energy ion bombardment of the cathode. Heating of the cathode surface occurs, which may in turn lead to thermal electron emission where additional electrons are emitted from the cathode surface.