Gas mixtures for spark gap closing switches

Gas mixtures for use in spark gap closing switches comprised of fluorocarbons and low molecular weight, inert buffer gases. To this can be added a third gas having a low ionization potential relative to the buffer gas. The gas mixtures presented possess properties that optimized the efficiency spark gap closing switches.

A spark gap switch can be described in a most basic manner as a pair of 
electrodes with a gas between them that can sustain a voltage across the 
electrodes that is near that of the breakdown voltage of the gas. If a gas 
has good electron attachment capability, it can sustain a high voltage 
making it a good insulator when the switch is open. The same gas, to be 
efficient in a spark gap closing switch, must free up electrons when the 
switch is closed making it a good conductor in the closed phase. 
Therefore, there is a need for gas mixtures that are both good insulators 
when the spark gap closing switch is open and good conductors when closed. 
SUMMARY OF THE INVENTION 
In view of the above need it is an object of this invention to provide gas 
mixtures that improve the efficiency of spark gap closing switches. 
Another object of this invention is to provide gas mixtures that are good 
insulators when spark gap switches are open. 
A third object of this invention is to provide gas mixtures that are good 
conductors when spark gap closing switches are closed. 
It is also an object of this invention to provide gas mixtures that have 
good electron attachment characteristics at ambient temperatures. 
Another object of this invention is to provide a gas mixture that frees 
attached electrons at high temperatures. 
A final object of this invention is to provide a spark gap closing switch 
having improved efficiency, repetition rate and recovery characteristics. 
Other objects and advantages will become apparent to persons skilled in 
the art upon study of the specifications and appended claims. 
To achieve the foregoing and other objects in accordance with the purpose 
of the present invention, the gas mixture of this invention may comprise a 
gas component that strongly attaches electrons at low energies, said 
attachment being exclusively nondissociative, and detaches from electrons 
as energy increases. Many fluorocarbons have these electron attachment and 
detachment characteristics and a number of them such as C.sub.6 F.sub.6, 
1-C.sub.3 F.sub.6, n-C.sub.4 F.sub.10, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8, 
c-C.sub.4 F.sub.6, or c-C.sub.5 F.sub.10 have proven to be effective. If 
fluorocarbons comprise the gas component, it is necessary to dilute it 
with a second component because the spark will cause decomposition of the 
gas and carbon can deposit in the switch. Another reason to add the second 
component is to increase the electron drift velocity in the system which 
thereby increases the conductivity of the gas mixture. A suitable second 
component is one that has low molecular weight and is nonreacting, such as 
an inert gas or a diatomic gas. 
The invention is also a ternary gas mixture comprising a fluorocarbon, a 
second gas that is nonreactive and of low molecular weight and a third gas 
that has a low ionization potential relative to the second gas component. 
The invention is also a spark gap closing switch that has a gas mixture 
between the switch electrodes that strongly attaches electrons at low 
energies, said attachment being exclusively nondissociative, and detaches 
from electrons as energy increases. 
The gas mixtures described by the specifications of this application can go 
from a good insulator to a good conductor rapidly at breakdown voltage. 
This property is found in some gases that attach electrons to form 
negatively charged gas molecules instead of dissociating into positive 
fragments and electron pairs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
When a spark gap closing switch is in the open phase, there is a high 
sustained voltage across the electrodes approaching the breakdown voltage 
of the gas as shown in FIG. 1. V.sub.o represents the sustained voltage 
and V.sub.s represents the breakdown voltage. To maximize the speed of 
closing, thereby maximizing the efficiency of the switch, it is necessary 
to approach V.sub.c, the voltage during the conducting phase, as rapidly 
as possible. The gas must transform from one that is a good insulator to 
one that is a good conductor in a minimum of time. It is also desirable 
for V.sub.o to be very near the breakdown voltage while V.sub.c is as low 
as possible. 
In the open phase, when the gas must insulate, electron attachment is an 
important characteristic; therefore the gas mixture must be able to tie up 
the electrons that are present in a system that has a high electric field. 
A suitable type gas would be one that forms negatively charged aolecules, 
i.e., AX.sup.-. 
The switch is closed by introducing energy using a laser trigger or other 
triggering device that will induce voltage breakdown. When this occurs at 
time, t.sub.o, the gas must release electrons when the voltage, V(t), 
begins to drop. Such a gas must have an electron attachment rate that 
decreases with increasing temperature since the temperature will increase 
at breakdown when the current, i(t), begins to flow. It must also not 
dissociate into positively charged molecular fragments and electron pairs. 
There are few gases that possess all these characteristics and applicants 
have identified the following that meet the criterion of the invention: 
C.sub.6 F.sub.6, 1-C.sub.3 F.sub.6, n-C.sub.4 F.sub.10, C.sub.3 F.sub.8, 
c-C.sub.4 F.sub.6, c-C.sub.4 F.sub.8, and c-C.sub.5 F.sub.10. When diluted 
by the addition of a nonreactive gas having low molecular weight, the 
electron drift velocity increases and conductivity is improved, resulting 
in a more efficient switch having better repetition rate and recovery 
characteristics. 
It is very important to remember that electron attachment must go down with 
an increase of energy (temperature) in the system. Without this 
characteristic, the conductivity would suffer and the switch would be less 
efficient. Examples of gases that have good electron attachment properties 
at low energy are known, but their behavior at high temperatures is 
unpredictable. 
It is believed that the efficiency of the switch could be further improved 
by addition of a small amount of a gas having a low ionization potential 
resulting in an increase in the number of free electrons in the switching 
mechanism during the conducting phase. This phenomenon, which is briefly 
explained here, is more fully discussed in applicants' patent application 
Ternary Gas Mixtures for Diffuse Discharge Switch S.N. 884,857 filed on 
July 14, 1986. When the system experiences breakdown, the released energy 
can elevate gas atoms to higher energy states when electrons are excited 
to higher electron shells but not fully released. Excited electrons 
continuously return to the groundstate and emit photons which may be 
resonantly reabsorbed by other atoms; therefore, the gas is in a constant 
state of absorbing and emitting photons when the switch is closed. The 
energy in the system incidental to this continuous photon emission does 
not contribute to the efficiency of the system and is wasted. However, it 
has been found under similar circumstances that a gas having a low 
ionization potential can capture this energy and become ionized to release 
electrons and significantly increase the electron density in the switch. 
EXAMPLE 
Various mixtures of gases having good nondissociative electron attaching 
properties were tested to compare their attachment rate with electron 
energy. Although actual switch measurements were not taken, the 
relationship of attachment rate and electron energy is indicative of 
suitable gas mixtures for use in spark gap closing switches, see FIGS. 2 
through 6. 
FIG. 2 shows a maximum attachment rate for n-C.sub.4 F.sub.10 in Ar at 
about 300.degree. C. which drops as the temperature increases to 
500.degree. K. A similar behavior is shown in FIG. 3 for C.sub.3 F.sub.8 
in Ar. It was found that above 500.degree. K. the attachment rate of these 
two gas mixtures increased, therefore, for these mixures it is necessary 
that the temperature be maintained at 500.degree. K. or less when the 
switch is closed. 
For the other gas mixtures shown in FIGS. 4 through 6, no temperature 
limitation was demonstrated and attachment rate continued to decrease to 
the maximum temperature that was measured in each instance. 
The binary gas mixtures found suitable comprise from about 2 percent to 
about 20 percent fluorocarbon in a nonreacting buffer gas of helium, 
argon, hydrogen or nitrogen. The ternary gas mixtures comprise from about 
2 percent to 20 percent fluorocarbon, 0.5 percent to 2 percent low 
ionization potential additive and the remainder is buffer gas. The amount 
of low ionization potential additive is a projection based on previous 
findings as described in the patent application Ser. No. 884,857 filed by 
inventors on July 14, 1986. Although the gas mixtures tested comprised 
only one gas from each catagory of fluorocarbon, buffer, or low ionization 
additive, the gas mixtures could also comprise combinations of gases in 
any one catagory and still be functional, although no particular advantage 
is forseen in such combinations. 
Therefore, based on the above data and considerations, the following 
gaseous media possess the most favorable properties for use in closing 
switches. 
GAS MIXTURES FOR CLOSING SWITCHES 
Binary Gas Mixtures 
I. 2-20% Fluorocarbon 
c-C.sub.4 F.sub.6 
c-C.sub.4 F.sub.8 
C.sub.3 F.sub.8 
C.sub.6 F.sub.6 
1-C.sub.3 F.sub.6 
n-C.sub.4 F.sub.10 
c-C.sub.5 F.sub.10 
II. Balance Buffer Gas 
Argon 
Helium 
Hydrogen 
Nitrogen 
Ternary Gas Mixtures 
I. 2-20% Fluorocarbon 
C.sub.3 F.sub.8 
n-C.sub.4 F.sub.10 
c-C.sub.4 F.sub.8 
1-C.sub.3 F.sub.6 
c-C.sub.5 F.sub.10 
c-C.sub.4 F.sub.6 
C.sub.6 F.sub.6 
II. 0.5-2% Low Ionization Additive 
C.sub.2 H.sub.2 
20C.sub.4 H.sub.8 
III. Balance Buffer Gas 
Argon 
Helium 
Hydrogen 
Nitrogen