Extended lifetime railgap switch

An improved railgap switch for use with pulse discharge gas lasers. The blade of the prior art railgap switches is replaced by an electrode having an "T" shaped cross section which provides two edges along which arcs are generated. The thickness of the "T" cross section near the edges at which arcs are formed is relatively uniform and oriented at a constant distance from the second electrode so that the thickness and distance remains unchanged despite ablation of the edges of the electrode. As a consequence the electrical properties of the switch are not altered significantly by ablation caused by repetitive operation of the switch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to switching devices for connecting high voltage, 
high-energy sources to gas discharge lasers so as to discharge high 
currents in short times through the gas laser medium. More particularly, 
this invention pertains to railgap switches for use as switching devices 
in pulse discharge gas laser systems. 
2. Description of the Prior Art 
FIG. 1 depicts a prior art, railgap switch (1). The switch consists of an 
elongated metal blade (2) that is located adjacent to elongated bar (3) 
and which blade is separated from the bar along the elongated length of 
blade (2) and bar (3) by a small distance or gap (4). Bar (3) typically is 
attached to plate (5). Blade (2), bar (3) and plate (5) all are made of 
conducting materials. A housing (6) made of an insulating material 
maintains the position of blade (2) relative to bar (3), and encloses gas 
(7), which gas fills the volume enclosed thereby, including the gap 
between blade (2) and bar (3). A gas mixture such as one composed of 10 
percent argon and 90 percent nitrogen molecules (N.sub.2) typically is 
used in the prior art devices. Typically, blade (2) has a thickness of the 
order of 0.5 millimeters, bar (3) has a diameter of 2 centimeters, the gap 
(4) is from 0.5 to 2 centimeters in width, and the blade (2) and bar (3) 
are from 20 centimeters to 1 meter in length. 
FIG. 2 shows in cross section, the shape of blade (2) and bar (3) and their 
relative positions. In operation, a sufficiently high voltage is applied 
to blade (2), relative to bar (3), to cause the gas in gap (4) to 
breakdown and form a series of conducting arcs between the blade and bar 
along their elongated lengths. The initial formation of the arcs may be 
encouraged by partially pre-ionizing the gas with ultra-voilet or other 
radiation. Once the arcs are formed, the arcs act, in effect, to connect 
blade (2) electrically to bar (3), thus, forming a relatively low 
resistance and low inductance path for the high currents required for 
exciting pulse discharge gas lasers. 
The relatively thin shape of the edge (8) of blade (2), acts, in the 
initial stage of the formation of arcs between the blade and bar, to 
concentrate and intensify the electric fields near edge (8), which in 
turn, reduces the voltage required to initiate arc formation and also 
causes the arcs to form along edge (8). Repeated operation of the switch 
at high current densities, however, causes edge (8) of blade (2) to be 
ablated, thus, effectively increasing the size of the gap between blade 
(2) and bar (3), as a consequence, changing the electrical characteristics 
of the switch. Eventually, the ablation of blade (2) is sufficient to 
require replacement of the blade. 
SUMMARY OF INVENTION 
In the present invention, blade (2) is replaced by an elongated plate 
having two edges located at a substantially constant distance from a 
planar second electrode. The planar electrode has two edges, along which 
arcs form, thus, for a given application, reducing the current density at 
the edges in half, which reduction, by itself, reduces the ablation rate 
by much more than a factor of two. In addition, ablation of the edges of 
the plate, although it reduces the width of the plate, does not alter 
significantly the distance between the edges of the plate and the second 
electrode. As a consequence, the electrical properties of the switch 
remain relatively constant, despite ablation of the planar electrode. By 
making the thickness of the plate relatively uniform near its edges, the 
thickness, and hence, the concentration of the electrical fields near such 
edges also remains relatively constant despite ablation of the edges, 
which constancy contributes to stable and consistent electrical 
performance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a cross sectional view of a preferred embodiment of the 
invention. In the preferred embodiment, the blade electrode of the prior 
art is replaced by an electrode (9) having a "T" shaped cross section, and 
the bar shaped electrode of the prior art is replaced by a flat conducting 
plate (10). The "T" shape of electrode (9) provides two portions (11) of 
the blade located near the edges (12) thereof, which are relatively thin 
and relatively uniform in thickness. As a consequence, the electric field 
concentration near each of edges (12) of electrode (9) is similar to that 
near edge (8) of blade (2) of the prior art depicted in FIG. 2. 
Referring again to FIG. 3, because the portions 11 of electrode (9) are 
located approximately parallel to, and at a substantially constant 
distance from, surface (13) of plate (10), the spacing from electrode (9) 
to plate (10), as measured from edges (12), at which the arcs form to the 
nearest portions of surface (13), remains relatively constant as edges 
(12) of electrode (9) ablate with repeated use of the switch. 
Because the electrode (9) of the invention has two edges (12), along which 
arcs form to plate (10), the currents in the arcs are reduced by a factor 
of two for a particular application and given length of device. The 
halving of the current density reduces the rate of ablation by much more 
than two. Furthermore, even with ablation, the distance from edges (12) to 
plate (10), remains relatively constant with ablation. As a consequence of 
these two effects, the electrical characterists of the switch remain 
almost constant with repeated use despite the eventual ablation of 
significant portions of electrode (9). As a consequence, the present 
invention exhibits a significantly longer lifetime than those devices of 
the prior art. 
As in the prior art, an insulated housing encloses a gas (15). In the 
preferred embodiment, the gas is composed of a mixture of 10 percent 
Argon, and 90 percent nitrogen molecules at a pressure of one atmosphere. 
In the preferred embodiment, the electode (9) is approximately 20 
centimeters long, the thickness of the portions (11) is approximately 
0.375 millimeters, and the width of the gap between the electrodes is 
approximately 5 millimeters. These dimensions, the gas mixture, and the 
gas pressure, of course, can be varied to appropriately suit particular 
applications. 
The two important elements of the invention are the doubling of the number 
of edges at which arcs are formed, thus, reducing the ablation rate, and 
the reorientation of such edges so that ablation of the edges does not 
signicantly alter the distance of the edges from the second electrode. 
Accordingly, it is apparent that the invention may have other embodiments, 
one of which is depicted in FIG. 4. 
In FIG. 4, electrode (17) has two edges (18), along which arcs originate 
between electrode (17) and bar (20). The electrode (17) has relatively 
uniform and thin portions (19) near the edges (18), such that as the edges 
(18) ablate, the thickness near the edges remains relatively constant so 
that the concentration of electrical fields near the edges remains 
substantially unchanged with ablation. As depicted in FIG. 4, the shape of 
electrode (17) in the portions (19) thereof, is curved so as to maintain 
the portions (19) thereof at a relatively constant distance from the 
nearest surface (21) of the second electrode, the bar (20). It is apparent 
from the geometry of the electrodes depicted in FIG. 4 that as edges (18) 
ablate, the distance of the edges (18) from surface (21) of bar (20) 
remains substantially constant. Thus, in the embodiment depicted in FIG. 
4, the electrical properties of the switch also remain relatively constant 
with use. 
FIG. 5 is a cross-sectional view of another embodiment of the invention 
wherein the housing (6) depicted in a FIG. 1 has been replaced by a 
toroidal shaped housing (22) depicted in FIG. 5 through which the gas 
contained within the housing can circulate past electrode (23) and plate 
(24). A further advantage of the "T" shaped electrode (23) depicted in 
FIG. 5 as compared to the shape of electrode (2) depicted in FIG. 1 is 
that the "T" shape creates much less turbulance than the prior art shape 
when gas is circulated passed the electrode in an embodiment such as that 
depicted in FIG. 5. In an application where the switch is used at a high 
repetition rate, a high velocity of gas flow is necessary in order to 
clear the conducting channels between current pulses. In such 
circumstances, the reduction in turbulance obtained by the use of the "T" 
cross section is important.