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358 Dielectric Gases

As mentioned earlier, a dielectric gas is a relatively poor produce or deplete free electrons. While a multiplicity of
conductor or a nonconductor of electricity to high applied physical processes and species, both neutral and charged,
electrical strength (i.e., a gas with a high breakdown volt- play a role in determining the dielectric properties of a gas,
age). As such, it is used to insulate electrically various it seems that the electron is the key particle, and its inter-
types of high-voltage equipment (see Section IV). As we actions with the gas molecules are the critical processes.
shall see in Section III, the magnitude of the breakdown Knowledge of these processes often allows a prediction
voltage depends not only on the nature, number density, of the dielectric properties of the gas and a choice of the
and temperature of the gas, but also on many other fac- appropriate gaseous medium for specific uses.
tors such as the type of applied voltage and the geome-
try, material, and surface condition of the electrodes. The II. BASIC PHYSICAL PROCESSES
breakdown voltage varies considerably from one gaseous AND PROPERTIES
medium to another, and it can be—for certain electroneg-
ative gases, for example—over six times larger than the
A. Basic Physical Processes
breakdown voltage of atmospheric air, which is the “tra-
ditional” gas dielectric (see Section III). The basic physical processes that determine the properties
Depending on the form of the applied voltage and the ofdielectricgasesinvolveexcitedandunexcitedatomsand
nature and density of the gas, the transition of a gaseous molecules, electrons, positive and negative ions, and pho-
medium from an insulator to a conductor occurs in times ton interactions with the gas and the electrodes. We shall
ranging from nanoseconds to milliseconds. The transition focus on those physical processes that are associated with
is critically determined by the behavior of electrons, ions, the gas itself (not with the electrodes), and in Table I we
and photons in the gas, especially by those processes that list the principal ones. Basically, all these processes affect

TABLE I Principal Physical Processes in Electrically Stressed Gas Dielectrics
Process number Process representation Process description
Group A. Electron–molecule interactions
1 e + AB → AB + e Elastic electron scattering (direct)
2 e + AB → AB + e Inelastic electron scattering (dirct)
∗
3a e + AB ⇒ A + B + e Dissociation by electron impact
∗
3b ⇒ A + B + e Dissociative excitation by electron impact
+
4a e + AB → AB + 2e Ionization by electron impact
+
4b e + AB ⇒ A + B + 2e Dissociative ionization by electron impact
5a e + AB → AB −∗ → AB − Parent negative-ion formation
5b ⇒ A + B − Dissociative attachment
5c → AB (AB ) + e Elastic (inelastic) electron scattering (indirect)
∗
+
6 e + AB → A + B + e Ion-pair formation
−
Group B. Photon–molecule interactions
7 hν + AB → AB ∗ Photonabsorption
8a hν + AB → AB + e Photoionization
+
8b ⇒ A + B + e Dissociative photoionization
+
9 hν + AB → A + B Photodissociation
10 hν + AB (B ) → AB (B) + e Photodetachment
−
−
11a AB + C → AB + C + e Penning ionization
+
∗
11b B + C → B + C + e Penning ionization involving highly excited
+
∗
atoms (e.g., Rydberg states)
Group C. “Secondary” interactions
∗
12 e + AB → AB (AB ) Electron–positive ion recombination
∗
13 B + A → AB (AB ) Positive ion–negative ion recombination
∗
−
+
14 AB + C → AB + C + e Collisional detachment
−
−
15 AB + C → ABC + e Associative detachment
−
16 AB + C → AB + C − Electron transfer
−
−
17 AB + nC → AB C n n ≥ 1 Cluster formation