Spark Gaps for Industry
Over Voltage & Triggered Spark Gaps
Spark Gaps are normally two connection devices which arc over at some preset voltage. The voltage is set by choosing the gap width and the gas pressure inside the device. These are set at the time of manufacture and are not field changeable. EG&G has a website descibing their range of gaps although some of them are not allowed to be shipped out of the U.S.A. without an export license.
By adding a third electrode to a spark gap one can make triggered spark gap, turning the over voltage spark gap into a triggered switch that behaves much like a thyratron or ignitron. The triggering voltages tend to be quite high (perhaps 25% of the SV Static Breakdown Voltage) but nevertheless they make effective switches and do some jobs better than the other devices.
The best material I've found on these industrial gaps is on the EG&G site. Take a look at Miniature Triggered Gaps, Triggered Gaps, Triggered Vacuum Gaps, Standard Trigger Transformers and Series Trigger Transformers, Spark gaps have one major disadvantage over thyratrons and ignitrons in that each time they fire, some material is used up. Like in an automotive spark plug, the firing action erodes a small amount of material each time. Calculation is needed to determine the energy of each pulse for the estimated life.
The Over Voltage Spark Gap
The Over voltage spark gap is essentially just two electrodes with a gap
between. When the voltage between the two electrodes exceeds the breakdown
voltage of the gas, the device arcs over and a current is very rapidly
established. The voltage at which arcing occurs in these devices is given by the
Dynamic Breakdown Voltage, which is the voltage at which the device will
breakdown for a fast rising impulse voltage. Note that this voltage may be as
much as 1.5 times greater than the static breakdown voltage (breakdown voltage
for a slowly rising voltage.) how much greater than the static breakdown voltage
the actual breakdown voltage is will be depends almost entirely on how rapidly
the voltage rises, a shorter rise time means a higher breakdown voltage.
Commutation times for these devices are exceptionally low (sometimes less than
1 nanosecond).
Overvoltage gaps are primarily used for protection. But in combination with the
other devices mentioned here they are commonly used to sharpen the output pulses
(decrease the rise times) of very high current pulses form triggered switching
devices, e.g., Thyratrons.
The size of these devices is almost entirely dependent upon how much
current/voltage they are intended to switch, There is really no limit as to the
size of these devices they can be as small as krytrons, however they can also be
very big, and devices intended to switch MA will be just that.
Triggered
spark gaps
The triggered spark gap is a simple device, a high voltage trigger pulse applied
to a trigger electrode initiates an arc between anode and cathode. This trigger
pulse may be utilized within the device in a variety of ways to initiate the
main discharge. Different spark gaps are so designed to employ one particular
method to create the main anode to cathode discharge. The different methods
areas follows-
Triggered spark gap electrode configurations:
i) Field distortion: three electrodes; employs the point discharge
(actually sharp edge) effect in the creation a conducting path
ii) Irradiated: three electrodes; spark source creates an illuminating
plasma that excites electrons between the anode and cathode.
iii) Swinging cascade: three electrodes; trigger electrode nearer to one of
the main electrodes than the other.
iv) Mid plane: three electrodes; basic triggered spark gap with trigger
electrode centrally positioned.
v) Trigatron: trigger to one electrode current forms plasma that
spreads to encompass a path between anode and cathode.
The triggered Spark gap may be filled with a wide variety of materials, the most
common are, Air, SF6, Argon and Oxygen.
Often a mixture of the above materials is employed. However a few spark gaps
actually employ liquid or even solid media fillings. Solid filled devices are
often designed for single shot use (they are only used once- then they are
destroyed) Some solid filled devices are designed to switch powers of 10TW (10,000,000,000,000 Watts) such as are encountered in extremely powerful capacitor
bank discharges. Except (obviously) in the case of solid filled devices, the
media is usually pumped through the spark gap. Some smaller gaps do not use this
system though.
Usually Gas filled spark gasp operate in the 20-100kV / 20 to 100kA range though
much higher power devices are available. I have one spec for a Maxwell gas
filled device that can handle 3 MA - that's 3 Million Amperes! But then it is
the size of a small car!! More commonly gas filled devices have dimensions of a
few inches. Packages are often shaped like large ice pucks though biconical,
tubular and box like structures are also seen.
Sparkgaps are often designed for use in a certain external environment (e.g., they might be immersed in oil). A system for transmitting the media to the appropriate part of the device may sometimes be included. Common environments used are: Air, SF6 or Oil.
Spark gaps are damaged by repeated heavy
discharge. This is an inevitable consequence of such high discharge currents.
Electrode pitting being the most common form of damage. Between 1 and 10
thousand shots per device is usually about what is permissible before damage
begins to severely degrade performance.
EG&G makes miniature triggered spark gaps specially designed for defense
applications. these devices are physically much smaller than normal spark gaps
(few cm typical dimensions) and designed for use with exploding foil slapper
type detonators.
Laser switching of spark gaps. The fastest way to switch a triggered spark gap
is with an intense pulse of Laser light which creates a plasma between the
electrodes with extreme rapidity. There have been quite a few designs employing
this method, chiefly in the plasma research area.
Triggered spark gaps tend to have longer delay times than Thyratrons (their
chief competitor, at least at lower energies) However, once conduction has
started, it reaches a peak value exceptionally rapidly (a couple of nanoseconds
commutation).