Argon Discharge in the GEC Reference Cell
The GEC cell was introduced by NIST in order to provide a standardized platform for experimental and modeling studies of discharges in different laboratories (Ref. 1). The plasma is sustained via inductive heating. The Reference Cell operates as an inductively-coupled plasma in this model.
Due to the complex physics involved in inductively coupled plasmas, it is advisable to begin modeling with a simplified chemical mechanism. Argon serves as an ideal starting point at low pressures because of its relative simplicity. In this approach, the electronically excited states are grouped into a single species, resulting in a chemical mechanism comprising only three species and seven reactions:
list of volumetric plasma chemical reactions considered
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
-
Stepwise ionization (reaction 5) can play an important role in sustaining low pressure argon discharges. Excited argon atoms are consumed via superelastic collisions with electrons, quenching with neutral argon atoms, and ionization or Penning ionization where two metastable argon atoms react to form a neutral argon atom, an argon ion and an electron. In addition to volumetric reactions, the following surface reactions are implemented:
list of surface reactions considered
reaction
formula
Sticking Coefficient
1
Ars=>Ar
1
2
Ar+=>Ar
1
When a metastable argon atom or ion makes contact with the wall, it will revert to the ground state argon atom with some probability (the sticking coefficient).
From an electrical perspective, the GEC reactor functions like a transformer. A current applied to the driving coil (the primary) induces a current in the plasma (the secondary). In turn, the plasma generates an opposing current in the coil, which increases the coil’s effective resistance. The current flowing through the plasma depends both on the coil current and the plasma’s reaction kinetics. The total plasma current can range from zero (when the plasma is not sustained) to matching the primary coil current, which represents perfect coupling between the coil and plasma. In this example, the coil is powered at a fixed input of 1500 W.