Note: Descriptions are shown in the official language in which they were submitted.
~6~
1 FIEI,D_OF_THE_IN_ENTION
This invention relates to high voltage switches ana in
particular those which employ the use of a spark gap.
BACKGROUND OF THE INVENTION
______._________ _________ _
High voltage switch technology is used in many areas
such as laser technology, lightening protection devices and
other areas re~uiring the switching of high voltage. Spark
gaps are often used as effective high voltage switches, the
technology o~ which is summarized in a paper by Tommy R.
Burkes et al, A Review of High Power Switching Technology,
IEE~ Transactions on Electron Devices, Volume ED-26, No. 10,
October 1979, 1405. Spark gaps commonly employ the use of a
trigger arrangement to ionize the gap between the electrodes
to cause a breakdown of the gap with a consequent discharge
arc between the electrodes. In using a trigger arrangement,
if the potential between the spark gap electrodes is at a
su~ficiently high potential for the spacing~ the ionizing o~
the gas in the gap by triggering the arrangement with a
trigger signal provides a selectively operable high voltage
~0 switch.
To improve the ~iring of the txigger arrangement, a
dielectric material may be used as spaced between the trigger
electrodes. According to Lavoie et al in their paper "Spark
Chamber Pulsing System", The Review of ScientiEic
Instruments, Volume 34, No. 41, November 196~, 1567, such use
of a dielectric material reduces the voltage requirements in
the signal to trigger a main discharge in the spark gap.
According to this paper, barium titanate having a high
dielectric constant is useful.
It is common to use a spark gap in laser circuitry to
'~
7~
1 redllce almost instantaneol}sly the potential of ~ne of the
electrodes of a laser cavity to excite the cavity region and
produce lasing action. ~asson, United States paten~
~,035,~83 discloses the use of â spark gap with a laser
cavity to control the timing in initiating or commencing the
lasing action of â laser. The important aspects in using a
spark gap for a laser is that the jitter time~ that is the
period from when the trigger signal is applied to the trigger
arrangement and the discharge arc occurs in the spark gap, is
reasonably constant. In some laser applications, a ~itter of
only three to Eive nanoseconds is desirable.
A spark gap using a high dielectric constant ~aterial
between the trigger pin and the trigger electrode, such as
the arrangement of Lavoie et al where the main discharge is
between the high voltage electrode and the trigger electrode
configuration,is particularly advantageous for use with
lasers. Only a comparatively low voltage is necessâry to
pulse the trigger pin to cause â sufficient electron and ion
clensity in the spark gap to provide breakdown and consequent
arcing in the spark gap. ~owever, it has been found that the
solid high dielectric constant material is chipped away or
fractured by the intensity of the discharye arc as it travels
to the adjacent trigger electrode. As the barium titanate is
worn away by the discharge arc, jitter of the spark gap is
increased and the arrangement becomes impractical from a
precise switching standpoint and may become inoperable.
The spark gap, according to this invention, overcomes
the above problem, has extended life and the capability of
producing relatively low jitter so that the spark gap is
particularly useful with lasers.
~ ~G7~L
1 SU~MARY_O~_THE_INVENTION_
The spark gap, according to this invention~ for
switching high voltages comprises opposite and adjacent
electrodes having opposing areas spaced-apart a predetermined
distance to establish a spark gap A main discharge arc
occurs between the opposing electrode areas when the spark
gap is triggered~ The trigger arrangement, which includes
the use oE a solid insulative material, is located rearwardly
of the adjacent electrode area to position such insulative
material in a manner to prevent a triggered main discharge
arc afEecting the structural integrity of the insulative
material. Such an arrangement provides for the clesirable use
of the various well known insulative materials in the
triggering arrangement for the spark gap, yet avoids any
destructive effects the high current discharge arc has on the
insulative material to provide a reliable, long-wearing,
low-jitter spark gap.
When the spark gap is used as a switch in controlling
the firing of the laser, the opposite electrode is
electrically connected to a laser electrode. In triggering
the spark gap, the laser electrode has its potential suddenly
lowered relative to the other electrode as is common in the
Blumlein laser circuit.
A housing may be provided for the spark gap to contain
insulating gases, which may be under pressure in the spark
gap. The position of the opposite electrode may be
ad~ustable to vary the distance between the opposite and
adjacent electrodes dependent upon the voltage on the
opposite electrode and gaseous pressure. The relative
spacing between trigger electrodes may be adjustable in the
~8~
1 housing to provide an optimum trigger discharge when the
trigger voltage is applied.
The insulative material on the trigger arrangement
preferably has a dielectric constant greater than 80. Such
material may be barium titanate, although it is appreciated
that many other types of solid insulative materials can be
used such as other ceramics of synthetic plastics.
BRIEF DESC~IPTION OF THE DRAWINGS
___________________.__________ _ ___
Preferred embodiments of the invention are shown in the
drawings wherein
Figure 1 is a schematic of a standard Blumlein circuit
for a laser;
Figure 2 is a top plan view of a laser cavity;
Figure 3 is a perspective view of the spark gap
according to this invention;
Figure 4 is a cross-section of the spark gap with
housing particularly adapted for use with lasers;
Figure 5 shows an alternative embodiment of the spark
gap employing a screen as an adjacent electrode; and
Figure 6 is another preferred embodiment for the spark
gap according to this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
_ ___.__ ____ _____ ___________________ __ _ __ _
The high voltage switch, according to this invention,
has many uses in the field oE spark gaps for rapid switching
of high voltages. To demonstrate a preferred use of this
spark gap, reference is made to controlling the timing of
firing a laser. Referring to Figure 1, a schematic of a
laser circuit is shown. The laser 10 comprises a cavity 12
with spaced-apart laser electrodes 14 and 16. A direct
current high voltage source 18 charges the electrodes 14 and
1 16 to the same voltage through storage capacitor 20 and pulse
shaping capacitor 22. The electrodes 14 and 16 are
galvanically interconnected by an inductance 28. With the
electrodes 14 and 16 charged to the same level by source 18,
no discharge in gap 12 occurs. To initiate a discharge in
gap 12, a trigger arrangement 30 is used. The trigger
arrangement comprises spaced-apart electrodes 32 and 34 with
a trigger pin 36. A trigger signal 37 is applied to the
trigger pin 36 which causes a breakdown between the gap 32
and 34. The developed arc discharge across electrodes 32, 34
drops the voltage on electrode 16 towards the level o~ ground
to set up a highly stressed electric field across the laser
cavity 12 o~ short duration to result in a population
inversion, which gives rise to a stimulated emission of
lasing radiation from the laser cavity.
Exemplary of the laser eavity of Figure 1 is that shown
in Figure 2. The eleetrode 14 is spaced apart ~rom the
electrode 16 and is interconneeted by inductance 280 A spark
gap arrangement 30 is in contact with the electrode 16 by way
o~ the eontaet at 38. The voltage from source 18 is applied
to electrode 14 through pin connection 40~ The lasing action
in cavity 12 propagates outwardly o~ the cavity through
window 42.
The spark gap device 30 is shown generally in Figure
3. The spark gap co~lprises an opposite electrode 32 and an
adjacent eleetrode 34 whieh were schematically shown in
Figure 1. The trigger arrangement 36 comprises trigger
electrodes 44 and 48. Electrode 48 is a cylindrical pin
which is surrounded by a solid insulative sleeve S0. The
sleeve is of a material, which according to this preferred
~ ~8~
1 embodiment, has a high dielectric constant Eor use in
separating the trigger pin 48 from the trigger electrode 4~.
The adjacent electrode 34 is electrically connected to the
trigger electrode 44 by way of a perforated circular plate 35
which has a plurality of apertures 52 formed therein to
permit the electrons generated by a discharge in the trigger
arrangement 36 to flow towards the positively charged
opposite eleçtrode 32 to trigger a breakdown in the spark
gap. Photons generated by the trigger discharge may also
radiate through the apertures 52 into the spark gap to ionize
the gas. According to this preferred embodiment, the central
portion of the adjacent electrode 3~ is raised at 54 to
oppose a rounded area at 56 on the opposite electrode to
provide the opposing areas between which a main discharge arc
occurs. In providing the raised portion for the adjacent
electrode, the jitter of the spark gap is slightly reduced
compared to the use of a planar adjacent electrode area. It
is thought that the raised area provides a better defined
region to which the arc can consistently travel, once the
discharge is triggered to result in a more constant period
between trigger pulse and spark gap breakdown. It is
appreciated that sharp edges on the opposite and adjacent
electrode areas are to be avoided to not create highly
s-tressed electrical fields in such areas which cou]d result
in an uncontrolled breakdown of the spark gap. Thus all
functional edges of the electrodes are smoothly rounded.
This structure, as shown in Figure 4 with a housing
generally designated 58, is particularly adapted for use with
a laser circuitry of the type of Figure 1. The spark gap
arrangement 3~ has pin 38 spring loaded in the opposite
2~
1 e].ectrode area by spring 60, for contacting electrode 16 of
the laser cavity of Figure 2. The sleeve 50 oE insulative
high dielectric constant material is useful in not only
reducing the signal voltage to in:itiate trigger discharge,
but also to facilitate physical location of the trigger pin
48 in the aperture 62 of the trigger electrode 44. The
sleeve 50 may contact the aperture wall 62 in providing the
nece.ssary spacing between pin end 49 and wall 62 without
detracting from the reliability in forming a trigger
discharge.
The housing 58 for the spark gap comprises endcaps 64
and 66 which have sandwiched therebetween circular block 68
to de~ine a space 70 housing the opposite electrode 32, the
adjacent electrode 34 and the trigger arrangement 36. The
space 70 is sealed by O-rings 72, 74 and 76, so that
pressurized insulative gases may be contained in the space
70. To complete the cavity and seal with the opposite
electrode, an additional housing block 78 is provided which
is connected within the support 80 ~or the opposite electrode
32. The opposite electrode 32 is mounted in the s~pport 80
so as to be movable inwardly and outwardly relative to the
adjacent electrode 34. This provides adjustability in the
space between the opposing areas 54 and 56 of the spark gap
electrodes in handling various magnitudes oE voltages for
switching.
The trigger electrode 44 and the supportive plate 35
~or the adjacent electrode 34 are electrically interconnected
as mounted to block 68 by spaced-apart threaded bolts 82. By
tightening bolts 84, the endcaps 64, 66 are squeezed together
to complete the enclosed space 70. Insulative gas ~or -the
1 spark gap between electrode areas 54, 56 may be supplied to
the enclosed space 70 through a duct 80, as shown in dotted
lines, with appropriate connective elementsO The gases may
be pressurized in enclosure 70 to provide for a decrease in
the spark gap spacing.
In the arrangement shown, opposite electrode 56, as
connected to electrode 16 of the laser, is at a high
potential relative to adjacent electrode 34. ~he spacing
between electrode areas 54, 5~ is such that the gap does not
break down for the high voltage applied to electrodes 32 and
34. To cause a breakcdown, flow of electrons and radiation oE
photons into the spark gap is provided for by the triyger
arrangement 36. To initiate a trigger discharge between the
end 49 o~ the trigger pin 48 and the trigger electrode
aperture wall 62, a signal in the form of a voltage pulse is
applied to pin 48. As previously mentioned, the trigger
electrode 44 may be electrically connected to the adjacent
electrode 34 so that they are at the same potential.
According to the circuitry of Figure 1, this is at ground.
The signal to pin 48 may, therefore, be a negative pulse
below ground which repels and thereby enhances the flow of
electrons into the spark gap.
It has been found that in using a solid high dielectric
constant material, such as barium titanate, as the insulative
sleeve 50, the voltage applied to achieve a trigger discharge
is considerably less than with other arrangements and may for
certain parameters be in the range of 4000 volts. The signal
generates a discharge in the trigger pin area and is
sustained by continuing the application of the signal to pin
~8. The positively charged opposite electrode 3~ attracts
1 the so generated electrons into the spark gap area through
the apertures 5~ of the plate support 35 for the adjacent
electrode 34~ As the electrons and photons, which move into
the spark gap, ionize this space~ the gap breaks down and a
discharge arc is initiated between opposing electrode areas
5fi, 56 to pull down the potential on electrode ]6 of the
laser cavity by discharging capacitor 220
Jitter is an expression used to quantify the variation
in the time from when the signal is first applied to the
lG trigger pin 4~ to the time when the main discharge arc occurs
between surfaces 54 and 56~ It has been found with this
arrangement that jitter can be maintained in the range of 1
to 3 nanosecond variation for a 40 nanosecond delay time from
the commencement of forming the trigger signal to the
Eormation of the discharge arc. It is appreciated, of
course, that the delay time after the trigger signal may be
changed as desired by altering to a new level the pressure of
the gas in the space 70, or the spacing between electrodes
5~ 5~ or the potential applied to the opposite electrode 32
It has been found that the extent of jitter is in
direct relation to the delay time between trigger signal and
spark gap breakdown. It is, therefore, important, when it is
desired to minimize jitter, to reduce as much as possible the
delay time. By using the high dielectric constant material
in the trigger arrangement, this permits the use oE a lesser
trigger voltage. A part of this delay time is the time taken
to actually produce the trigger signal. With an avalanche
transistor trigger circuit of the type disclosed in the
aforementioned Lavoie et al paper, a 4000 volt signal may be
generated in approximately ten nanoseconds, it taking another
1 approximately 30 nanoseconds to generate the trigger
discharge and cause a main discharge in the spark ~ap.
Therefore, the avalanche transistor circuit has proven most
use~ul in minimi~ing the delay time in providing a switch Eor
hi~]h voltages.
In keeping with the invention, an alternative
embodiment is shown in Figure 5. An opposite electrode 32 is
mounted in the manner of Figure 4 relative to an adjacent
electrode 34 which is in the form of a metal wire mesh or
screen having spaced-apart wire members 90. Located beneath
and spaced from the adjacent electrode 3g is the trigger
arrangement 36. The wire mesh 90 permits the electrons
generated by a trigger discharge to flow through the mesh
towards the opposite electrode 32 and thereby breakdown the
gap between the area 56 of the opposite electrode and the
wire members 90 beneath this area to provide for a discharge
arc.
An alternative embodiment is shown in Figure 6 where
the opposite electrode 32 has its area 56 opposing an
adjacent electrode 34, which has an annular raised area 92
which opposes area 56 of the opposite electrode. The trigger
arrange~ent 36 has a trigger pin 4a with insulative sleeve 50
to separate the pin 48 from annular region 94 which is
integral with the adjacent electrode 34. Annular region 94
~unctions the sa~e as the trigger electrode 44 o~ ~igure ~.
The trigger discharge occurs, as the signal is applied to the
pin 48, between the pin end 49 and region 94~ The plasma
generated by the trigger discharge travels along the bore 96
in the adjacent electrode 34 into the spark gap region
between area 56 and annular region 92. On initiating the
~6~
1 main discharge, the spark travels to annular raised area 92
and is so spaced from the insulative sheath 50 to thereby
prevent such discharge having an effect on the structural
integrity of this insulative material.
Although barium titanate is a pre~erred form of
lnsulative material, it is appreciated that other materials
having dielectric constants greater than 80 are available,
such as TiO2, LiNbO3 and KDP ~potassium
dideuterophosphate K~12PO4).
The use of the insulative material, which may have a
dielectric constant greater than 80, provides for a more
efficient triggering system for the spark gap, in terms of
requiring a lower voltage signal to generate a trigger
discharge and also to facilitate the physical location of the
trigger pin relative to an adjacent trigger electrode. The
insulative material may be in contact with both the trigger
pin and electrode to establish a minimum spacing, so that
consistency is obtained in the generation oE the trigger
discharge. In systems o~ the prior art which do not employ
the use of a high dielectric constant material in the trigger
electrode arrangement, substantially higher voltages are
needed for the trigger signal. For example, with some
arrangements 20,000 volt s:ignal is required which is
difficult, if not impossible, to generate in short periods
such as ten nanoseconds and requires far more complex
electrical equipment than that needed to generate the 4000
volt trigger signal.
The arrangement, according to this invention, provides
for the continued use of the dielectric material. It was
experienced in using the insulative material in the manner
11
67~2~
1 suggested by the previously referred to paper by Lavoie et al
that, for Erequencies of usage of approximately 10 hertz, the
barium titanate would begin to fracture and chip away after
approximately two hours use. The lifetimes reported by
Lavoie et al were not experienced. It is difficult to
speculate as to why the lifetimes of Lavoie et al were not
realized; however, it is believed this may be due to Lavoie
switching at substantially lower power loads. The advantages
in using a dielectric material, as suggested by Lavoie et al
could not be commercially realized in switching the high
power loads in the firing of a laserO The arrangement,
however according to this invention, provides a structure
which positions the triggering arrangement behind the
adjacent electrode of the spark gap~ As can be gathered from
the preferred embodiments discussed, the location of the
dielectric material is such that the main discharge arc is
prevented from having any substantive degradative eEfect on
the structural integrity of the material. Accordin~ to the
preferred embodiments of Figures 4, 5 and 6, this is
accomplished by spacing the insulative sleeve S0 a sufficient
distance from the adjacent electrode area.
Although the prererred insulative material has a high
dielectric constant, that is materials which have dielectric
constants in excess of 80, it is appreciated that any other
forms of solid insulative materials may be used which would
be subject to degradation if exposed directly to the main
discharge arc. For example, other solid insulative
materials, which may be used, are those of synthetic plastics
such as Mylar (trademark to identify a polyethylene
terephthalate sold by Dupont) and Delrin (trademark to
7~
1 identify the acetal resin sold by Dupont) which have
d;electric constants less 10. Epoxies are also usable which
have dielectric constants in the range oE 30 to 35. Other
usable forms of insulative materials may be the more common
forms of ceramics which would have dielectric constants less
than 10, such as boron nitride. It is appreciated that in
using insulative materials having the lower dielectric
constants a thinner section of insulative materi~l would be
used and that with some arrangements, it may be necessary to
use a trigyer signal of a higher voltage. This could result
in increased jitter for the spark gap; however, with some
applications this may be acceptable. In using the
arrangement, according to thi~ invention, the insulative
material, whether it be a plastic such as Mylar or other
ceramics, continues to be protected from the hazardous
eEfects of the main discharge arc. For example, with Mylar
if subjected to the main discharge arc would degrade rapidly
due to the heat generated. In locating the Mylar in the
trigger arrangement, which creates a corona discharge at
lower temperatures, it is thereby protected according to the
arrangement of this invention from the effects of the main
discharge arc. Preferably the insulative material, as it
surrounds the trigger pin of the trigger arrangement shown in
Figures 4, 5 and 6, have intimate contact therewith to
minimize air gaps between the insulative material and the
trigger pin to provide for a better corona discharge when the
trigger signal is applied to the trigger arrangement.
In terms of the longevity of the device, i-t has been
experienced, in using barium titanate as the insulative
material, that up to five million discharges can be expected
1 before it is necessary to disassemble the unit to clean
around the trigger area. No structural degredation of the
insulative sleeve was evident after the many million
discharges.
Various types of gases may be used in the spark gap
enclosure, such as nitrogen, helium, argon. Sulphur
hexafloride provides a very good form of insulative gas;
however, it i9 corrosive and expensive, so that it is not
always desirable to use unless a particular application so
requires.
It is appreciated that the polarity of the spark gap is
not crucial in that the system could work as well with the
reversal of polarities on the opposite and adjacent
electrodes. There may, however, be an experience of greater
delay b~fore main discharge arcing occurs, because the
electrons generated by the trigger discharge would be
attracted to adjacent electrode 34. However, the photons
generated by the trigger discharge are not polarity sensitive
and would radiate and flow into the spark gap region to
2~ initiate the breakdown.
Although preferred embodiments of the invention have
been described herein in detail, it will be understood by
those skilled in the art that variations may be made thereto
without departing from the spirit of the invention or the
scope of the appended claims.
14
