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Dielectric Gases 367

FIGURE 9 Formative time lag versus percentage of overvoltage in (a) N 2 and (b) SF 6 for different field distributions
n ≡ E a /E max (n values of 0.84, 0.54, and 0.32 correspond, respectively, to approximately homogenous, weakly in-
homogenous, and inhomogenous fields), d = 4 mm, and P = 1 bar. In (a), for N 2 , curves 1–5 are (respectively) for
n = 0.84, polarity +; n = 0.54, polarity +; n = 0.54, polarity −; n = 0.32, polarity +; and n = 0.32, polarity −. In (b), for
SF 6 , curves 1–5 are (respectively) for n = 0.84, polarity +; n = 0.54, polarity +; n = 0.54, polarity −; n = 0.32, polarity +;
and n = 0.32, polarity −. [Data from Peiffer, W. (1984). In “Gaseous Dielectrics IV” (L. G. Christophorou and M. O.
Pace, eds.), pp. 329, 331, Pergamon Press, New York.]

2
( 150 × 10 −17 V cm ). The successful operation of such IV. USES OF DIELECTRIC GASES
switching devices depends on the availability of a gas that
is a good conductor in the conducting stage and a good in- The most abundant “traditional” dielectric gas is atmo-
sulator in the transferring stage. To optimize conduction spheric air. It naturally insulates overhead transmission
under the low-E /N conditions of the conducting stage, lines that crisscross the countryside. Overhead transmis-
the electrons produced by the external source (e-beam or sion lines up to 800 kV are presently in service, and
laser) must remain free and must have as large a drift
velocity w as possible. To optimize the insulating prop-
erties under the high-E /N conditions of the transferring
stage, the gas must effectively remove electrons by at-
tachment (have a large attachment rate constant at high
E /N). These requirements are schematically illustrated in
Fig. 10.
Gas mixtures with such desirable characteristics have
been reported by various authors. In particular, it has
been shown that binary gas mixtures of buffer gases
such as Ar and CH 4 , whose electron-scattering cross sec-
tions have a Ramsauer–Townsend minimum at low en-
ergies (∼0.5 eV), and electron-attaching gases such as
C 2 F 6 , and C 3 F 8 , which attach electrons efficiently at high
E /N and have much-reduced electron-attachment rate
constants at low E /N, are most appropriate for diffuse FIGURE 10 Schematic illustration of the desirable characteristics
discharge switching applications. Such mixtures have dis- of the w(E/N) and k a (E/N) functions of the gaseous medium in
tinct maxima in the w(E /N) at E /N values appropriate an externally (e-beam) sustained diffuse discharge switch. Indi-
for the conducting stage of the switch, w values in excess cated are rough estimates of the E/N values for the conducting
7
−1
of 10 cm sec , and breakdown strength 150 × 10 −17 and the opening stages of the switch. [From Christophorou, L. G.,
et al. (1983). In “Proceedings 4th IEEE International Pulsed Power
2
Vcm for mixtures containing 10% of the attaching 1983, Albuquerque, New Mexico” (T. H. Martin and M. F. Rose,
gas. eds.), p. 702, Texas Tech University Press, Lubbock, TX.]