EP0979548B1 - Method for triggering a gas insulated switching spark gap and device using said method - Google Patents

Description

The invention relates to a method for triggering a switching spark gap and a switching spark gap which is operated according to the method and is used as a DC voltage switch or as a dynamically stressed switch.

The goal is for high voltage discharges in the most diverse Processes to have a switch available to them a predetermined time reliably in the conductive state is transferable. For this purpose, a solution is known in which the insulating gas in the spark gap chamber is easily photoionizable, gaseous additives (fluorene scene) are added, which is then more suitable by irradiation with a light source Wavelength via photoionization the starting electrons for the Clear the formation of the ignition channel (see Dougal, R. A. et al .: "Fundamental Processes in Laser-Triggered Electrical Breakdown of Gases ", J. Appl. Phys., Vol. 17 (1984), p. 903 - 918, printed in Great Britain).

As light sources, the spark gap in the conductive state offset, so-called trigger light sources, have become incoherent Light sources such as UV lamps or coherent light sources proven like lasers. The latter is under the term laser triggering known.

JP 1-81 185 A describes a method for triggering a Switching spark gap and a corresponding device are known. A spark arises between the electrodes 6 and 1 by partially illuminating the space between the electrodes with a light source 14 of predetermined wavelength ionized metal vapor generated.

From US 4,604,554 is a device with light triggered Switching spark gap known. An auxiliary spark gap G5 causes ultraviolet radiation that ignites a trigger spark gap G4, which in turn leads to the ignition of the switching spark gap G1.

With laser triggering, the temporally targeted resolution electrical breakdown in a spark gap the irradiation of the electrode gap or the electrode surface achieved with laser light. The one for triggering the Spark gap required laser energy depends on the used Mechanism of photoelectric charge generation and on the operating mode of the spark gap.

A basic distinction is made between dynamically stressed switches and DC voltage switches. The voltage to be isolated is constantly present at DC voltage switches before the spark gap is triggered. An electrical breakdown in a gas-insulated homogeneous field arrangement can only take place if the effective impact ionization coefficient α _eff in the gas is greater than zero and consequently an avalanche-like increase in freely movable charge carriers as a result of the impact ionization can take place. For field strengths less than the static breakdown field strength, α _eff ≤ 0 applies. Since the DC switch should isolate reliably before the trigger event, its operating voltage must be below its static breakdown voltage U _DC .

The ionization energy of gas atoms is above W _i = 12 eV and is therefore significantly larger than the photon energy of the laser radiation, which is less than W _ph = 4 ... 5 eV depending on the wavelength of the laser. The mobile charge carrier of the plasma can thus only by a simultaneous absorption of multiple photons, by the so-called M ulti p hotonen i onisation (MPI) can be generated (see Gray Morgan, C .: "Laser-Induced Breakdown of Gases", Rep. Prog. Phys., Vol. 38, 1975, p. 621-665).

Multiphoton ionization is a strongly non-linear effect that only appears at high irradiance levels. To generate a conductive plasma which has a charge carrier density of n> 10 ¹⁶ cm ^-3 sufficient for triggering a DC voltage switch (Dougal, RA et al .: "Fundamental Processes in the Laser-Triggered Electrical Breakdown of Gases", J. Appl. Phys ., Vol. 60, No.12, 1986, p. 4240 - 4247), the required irradiance is I = 1 GWcm ^-2 . It can only be achieved by focusing the laser beam in the space between the electrodes. The required laser energies are W> 100 mJ.

The laser beam is focused on the electrode surface the trigger energies are about an order of magnitude less than with a focus in the gas volume between the electrodes. The required laser energy is W = 10 mJ (see Dougal, R. A. et al .: "Fundamental Processes in Laser Triggered Electrical Breakdown of Gases ", J. Appl. Phys., Vol.17 (1984), p. 903-918). Evaporated metal from electrode material increases here the conductivity of the plasma. In addition, they are freely movable Electrons from photoemission from the electrode surface triggered.

In pulsed power technology and in almost all short-term physics Applications are the switching spark gaps with pulse-shaped Voltages u (t) acted upon and thus dynamic claimed. For triggering dynamically stressed switches the energy consumption is lower. The voltage at the switch exceeds the static breakdown voltage very quickly. The spark gap breaks at comparatively high field strengths of even if, in a natural way, d. H. by radioactive radiation or by radiation from the outside free starting electron was formed.

The targeted transfer of a dynamic switch into the conductive state takes place before the appearance of a natural Start electronics created in this way. Before the spark gap are breached by themselves at the trigger time using Laser light generates starting electrons. During the pre-discharge period the avalanche builds up and the streamer spreads between the electrodes. After the pre-discharge period breaks the voltage between the electrodes and the spark gap is in the conductive state.

In contrast to DC voltage switches, dynamically stressed Switches the prerequisite for training a Discharge channel a field strength value over the static Breakthrough field strength due to short-term exceedance the static breakdown voltage already met. It is sufficient therefore a comparatively low charge carrier density, in Ideally, a single starting electron around the spark gap trigger specifically. In addition there is a lower irradiance required to generate a highly conductive plasma with high charge carrier density.

The trigger laser energies to be used are in the range of 1 mJ and the irradiance levels are a few MW / cm ² . It is not necessary to focus the laser beam. When the electrode surface is illuminated, electrons are provided by photoemission from the metal surface in addition to the charge carriers formed in the gas volume. The trigger laser energy to be used is then, similar to DC voltage switches, lower than when the interelectrode space is only illuminated.

If the trigger laser beam is not focused and parallel to the electrode surfaces, there is the possibility trigger several discharge channels simultaneously. To be as possible Generating many discharge channels is therefore elongated and rail-shaped electrode geometries in particular suitable. Such multi-channel switches are called Railgap spark gaps. You have an extreme low switch impedance and because of the comparatively large the electrode surface to be stressed has a long service life.

Railgap switches with an electrode length of 50 cm were developed by Taylor et. al. at the National Research Council of Canada. A KrF laser (λ = 248 nm) and a nitrogen laser (λ = 337 nm) served as trigger lasers. With Ar / SF ₆ and N ₂ / SF ₆ switching gas mixtures and without optimizing additives, the spark gap was triggered with laser energies in the range of W = 20 mJ.

A reduction in the trigger laser energy required was achieved by adding easily photoionizable gas additives such as fluorobenzene when using the KrF laser and tri-n-propylamine when using the nitrogen laser. With 1 mJ KrF laser radiation, 70 - 100 discharge channels per meter electrode length could be achieved. The minimum trigger energy was W = 100 µJ, the lowest irradiance was I = 300 kW / cm ² . When triggered by means of an N ₂ laser, the energy expenditure was W = 60 μJ (see Taylor, RS et al .: "UV Radiation Triggered Rail-Gap Switches", Rev. of Scient. Instrum., Vol. 55, No. 2, 1984, pp. 52-63). However, the irradiance here was approx. I = 4 MW / cm ² and thus significantly higher than when triggered with KrF radiation.

W. Frey and A. J. Schwab reported at Ninth International Symposium on High Votage Engineering in Graz, Austria, Aug 28 - Sep 1 1995 on laser-triggered rail gap spark gaps with starting electron generation by photoemission of metal aerosol particles. For this a laser beam is used suitable wavelength and low laser energy directed through the gap between the electrodes of the spark gap. The Interior of the spark gap is with a gas, e.g. Bar, filled, in which metal aerosol particles are distributed. The Laser light sets in on the aerosol particles through photoemission Starting electrons free, under suitable conditions, such as Switching gas-tight interior of the spark gap and more initial Potential difference between the electrodes, the short circuit initiate between the electrodes. It is essential that the starting electron generating laser beam through the Electrode gap goes.

The need for light or laser energy for error-free triggering the spark gap is high. This goes hand in hand with the need on trigger light sources with higher energy, which in particular reflected in the cost of the trigger light system.

ein Verfahren für eine Schaltfunkenstrecke bereitzustellen, mit dem die Funkenstrecke zeitlich exakt mit möglichst geringer Laserenergie vom sperrenden in den leitenden Zustand übergeführt werden kann.

Weiter soll eine Schaltfunkenstrecke bereitgestellt werden, mit der sich das Verfahren zuverlässig durchführen läßt. Die Triggerlichtquelle soll energiearm sein.

Die Schaltfunkenstrecke soll als Schalter in einer Hochspannungsimpuls-Erzeugungsanlage eingesetzt werden können.

This results in the object on which the invention is based, namely:

to provide a method for a switching spark gap, with which the spark gap can be transferred from the blocking to the conducting state with exactly the least possible laser energy.

Furthermore, a switching spark gap is to be provided with which the method can be carried out reliably. The trigger light source should be low in energy.

The switching spark gap should be able to be used as a switch in a high-voltage pulse generation system.

The object is achieved by a method according to claim 1 and with a switching spark gap according to claim 6. The switching spark gap is used according to claim 13 as a DC voltage switch or as a dynamically stressed switch.

Advantageous method steps are in subclaims 2 to 5 characterized. Characterize subclaims 7 to 12 construction measures advantageous for the execution,

The light required to trigger the switching spark gap or laser energy is very compared to the prior art low. The method does not require a beam-focusing Means such as lenses and the necessary fine adjustment devices. The process is for the optimization of existing laser switching systems applicable without significant design change. Especially The optimization of the switching behavior is advantageous of spark gaps with similar, rail-shaped electrodes, i.e. multi-channel switches, the so-called Railgap spark gaps.

Figur 1 den prinzipiellen Aufbau der Schaltfunkenstrecke,

Figur 2 den Aerosolgenerator im Prinzip,

Figur 3 die Zündverzugszeit der Railgap-Funkenstrecke,

Figur 4 die Standardabweichung der Zündverzugszeit(Jitter) der Railgap-Funkenstrecke,

Figur 5 die Selbstdurchbruchspannung der Funkenstrecke in Abhängigkeit der Aerosol-Partikel-Konzentration bei 2% SF₆ in Ar,

Figur 6 die Selbstdurchbruchspannung der Funkenstrecke in Abhängigkeit der Aerosol-Partikel-Konzentration bei 10% SF₆ in Ar.

The method and the switching spark gap and the drawing are explained in more detail below.
It shows:

FIG. 1 shows the basic structure of the switching spark gap,

FIG. 2 shows the aerosol generator in principle,

FIG. 3 shows the ignition delay time of the rail gap spark gap,

4 shows the standard deviation of the ignition delay time (jitter) of the rail gap spark gap,

FIG. 5 shows the self-breakdown voltage of the spark gap as a function of the aerosol particle concentration at 2% SF ₆ in Ar,

6 shows the self-breakdown voltage of the spark gap as a function of the aerosol particle concentration at 10% SF ₆ in Ar.

The switching spark gap 4 is a rail gap spark gap that perpendicular to the axis of the electric field lines and parallel illuminated to the two electrodes with a nitrogen trigger laser 9 becomes. The aerosol is a magnesium aerosol, accordingly is at least one of the two sacrificial electrodes of the aerosol generator 1 made of magnesium.

Crucial for the application of the metal aerosol trigger method to optimize the switching behavior of existing systems, that no premature self-breakthrough due to the particle admixture the laser switch before triggering the trigger laser 9 occurs.

Measurements of the self-breakdown voltage of the rail gap spark gap 4 as a function of the particle concentration n _p , which is proportional to the spark fruity f _{F of} the aerosol generator 1, show that the self-breakdown behavior of the spark gap 4 is not influenced by the particle admixture (FIGS. 5 and 6). The trigger voltage interval is not restricted by the use of metal aerosol switching gases.

Error-free triggering occurs with Mg particles in the switching gas the spark gap 4 with laser energies of W = 200 nJ. At this trigger energy, the switching control is less than without Particle admixture and a laser energy factor 1000 times higher, Figure 3, right.

The irradiance at the lowest investigated trigger laser energy is I = 300 Wcm ^-2 and is therefore 4 orders of magnitude lower than with previous approaches to reduce the required trigger laser energy. The required laser energy itself is 3 orders of magnitude lower.

The method of operation does not depend on a specific electrode geometry tied to the spark gap. A targeted release of a dynamically stressed laser switch depends first Line depends on whether starting electrons at a certain laser energy can be generated. The used one plays Start charge carrier process the decisive role and not that Electrode geometry.

The physical basis of the metal aerosol trigger method is the high quantum yield of the photoemission of electrons from small spherical metal particles in a gas atmosphere. It is of the order of Y> 10 ^-4 . When the particles are irradiated with light, N _e = 10 ⁴ photons are sufficient to generate a freely mobile electron.

The reason for the high quantum yield is the negligible one Backscattering of electrons on gas particles in the direction the particle surface with subsequent absorption of the electron viewed. An electron emission in the direction of the surface normal has the highest probability of leaving.

The metal particles 2 are created using the aerosol generator 1 generated, which works on the spark erosion principle. For technical Implementation of the trigger method is the gas supply line 3 Switching spark gap 4 separated and the spark erosion generator 1 interposed, Fig. 1. This type of aerosol generation and admixture is for continuous operation of the laser switch suitable with constant switching characteristics. With other methods of aerosol generation, such as the Wire explosion method, long-term stability of the Switching properties during repeated operation of the spark gap cannot be reached.

The spherical metal particles 2 arise in the spark erosion generator 1 as a result of the spark discharges between the two sacrificial electrodes 7, FIG. 2. The discharge is fed from the capacitance C _S and burns repeatedly with the spark frequency f _F. At the base of the arc, electrode material is melted and flung in liquid form into the gas space, where it solidifies in a spherical shape and is transported by the gas stream 8 into the switching spark gap 4.

The sacrificial electrodes 7 consist of the specified metal. The work function of the particle material W _A must be smaller than the photon energy of the trigger laser radiation W _ph . To avoid sedimentation of the particles in the spark gap, the particle diameter must be less than Dp = 500 nm. The required particle concentration is of the order of np = 10 ⁴ cm ^-3 . This is achieved with a gas flow of> 1 l / min and with Mg electrodes. The discharge circuit of the aerosol generator is designed so that the storage capacity C _S = 20 nF, the charging voltage is 1 kV and the repetition frequency is at least 5 Hz.

The trigger method is used on the Railgap spark gap 4 and is investigated with the addition of magnesium particles 2. The work function of magnesium is W _A = 3.66 eV. The photon energy of the N ₂ trigger laser 9 used (λ = 337 nm) is slightly higher at W _ph = hv = 3.68 eV. The mean magnesium particle diameter is D _p = 100 nm and the particle concentration in the switching gas n _p > 10 ⁴ cm ^-3 .

First, for experimental reasons, a basic gas mixture of argon and SF _{6 was used} (FIGS. 5 and 6). In principle, however, the use of a mixed gas is not necessary for the trigger method to function. A one-component or higher-component switching gas can also be used to operate the switching spark gap 4.

The low energy requirement for triggering the spark gap 4 with aerosol switching gas becomes particularly clear when measuring the ignition delay time of the spark gap 4, the time from the beginning of the laser pulse to the beginning of the voltage breakdown across the spark gap 4, depending on the trigger laser energy, FIG. 3 A basic gas mixture of 10% SF ₆ in argon, a gas pressure of p = 2 bar _absolute and without the addition of Mg particles is only occasionally used for laser triggering at a laser energy of W = 20 µJ. In more than 50% of all trigger tests, the spark gap 4 breaks through automatically at higher voltage values considerably after the time of laser irradiation. The ignition delay time (FIG. 3) and the switching spread of the spark gap 4 (jitter) (FIG. 4) are correspondingly high at 145 ns and 167 ns, respectively.

Reference list

1

Spark erosion generator, aerosol generator

2nd

Metal particles

3rd

Gas supply line, connecting line

4th

Switching spark gap, Railgap spark gap, spark gap

5

Gas supply line, supply line

6

Switching gas supply

7

Electrodes, sacrificial electrodes

8th

Gas flow

9

Trigger light source, trigger laser, N ₂ trigger laser

Claims (13)

1. Method of triggering a gas-insulated switching spark gap, which is subjected to a prescribed insulating gas pressure, by means of a light source, said method comprising the following steps:

   a spark-erosion generator (1) is incorporated in the supply line (3) for supplying an insulating gas component to the switching spark gap (4) and is subjected to a prescribed pressure,

   in the spark-erosion generator (1), in which there are spark discharges between two electrodes (7) - the disposable electrodes - and which generator is operated at a prescribable repeating frequency, electrode material is fused at the base of the spark arc, which is produced by the respective discharge, and centrifuged in liquid form into the intermediate space between the disposable electrodes (7), where said material solidifies to form small spherical, suspendable particles - called metal aerosol - which do not sink in the traversing insulating gas component, said material is entrained by the flow of gas and transported to the switching spark gap (4),

   the intermediate space between the electrodes of the spark gap is at least partially illuminated by a light source (9) of a predetermined wavelength - the triggering light source - for the purpose of ignition, whereby the starting electrons for forming at least one discharge channel between the electrodes of the switching spark gap (4) are released from the metal aerosol particles, present in the insulating gas, at the prescribed time via photoemission, and

   the axis of the triggering light beam (9) extend centrally through the space between the electrodes of the switching spark gap (4).
2. Method according to claim 1, characterised in that a one-component insulating gas, such as SF₆ or N₂, or an insulating gas which has at least two components, such as an N₂/Ar mixture or air in the simplest case, is used in the switching spark gap (4).
3. Method according to claim 2, characterised in that a mixture of 98 - 86 % Ar and, complementary thereto, SF₆ is used as the insulating or switching gas, and the gas component Ar, which is not electronegative, flows through the aerosol generator (1).
4. Method according to claim 3, characterised in that an incoherent light source, which is suitable for the photoemission of electrons from aerosol particles, is used as the triggering light source (9).
5. Method according to claim 3, characterised in that a laser is used as the triggering light source (9), which laser releases electrons from particles of the insulating gas aerosol by photoemission.
6. Apparatus for accomplishing the method according to the method claims 1 to 5, comprising a light-triggered switching spark gap and having the following features:

   during transverse triggering, the axis of the light beam of the triggering light source (9) extends through a light-permeable window, such as quartz glass, in the wall of the switching spark gap (4) and through the centre of the intermediate space between the electrodes or,

   during longitudinal triggering, said axis extends through such a window in one of the two electrodes,

   characterised in that

   a spark-erosion generator (1) communicates directly with a switching gas supply means (6) provided with pressure regulating arrangements and, moreover, is connected to the chamber of the switching spark gap (4) via a gas pressure line (3),

   there is at least one additional supply line (5) for an additional insulating gas component in the connection line (3) between the spark-erosion generator (1) and the spark chamber of the switching spark gap (4), so that an insulating gas, which has at least one component, can be supplied to the switching spark gap (4), and

   at least one of the two electrodes of the spark-erosion generator (1) is configured as a disposable electrode, which is the source for the metal aerosol, and it is formed from an easily ablatable, metallic material or is coated therewith.
7. Apparatus according to claim 6, characterised in that the electrodes of the switching spark gap (4) are designed in such a manner that, in the connected position, there is at least one arc channel between the two electrodes.
8. Apparatus according to claim 7, characterised in that the two electrodes of the switching spark gap (4) are identical, rail-like, and lie parallel (rail gap) to each other.
9. Apparatus according to claim 8, characterised in that the triggering light source (9) is an incoherently radiating light source, such as a UV light source for example, the wavelength of which source is smaller than the long-wave limit for the photoemission of electrons from the aerosol particles, and it radiates in a prescribed intensity.
10. Apparatus according to claim 9, characterised in that the triggering light source (9) is a laser, such as a nitrogen laser for example.
11. Apparatus according to claim 10, characterised in that the source for the metal particles of the aerosol is from magnesium or copper or a metal which otherwise easily dispenses metal particles.
12. Apparatus according to claim 11, characterised in that the gas supply line (3) at the spark-erosion generator (1) terminates directly at the intermediate space between the two disposable electrodes (7).
13. Use of the switching spark gap which is operated according to the method claims 1 to 5 and formed according to the apparatus claims 6 to 12, characterised in that the switching spark gap (4) is used as a direct-voltage switch or as a dynamically loaded switch.

Images