Marx-Generator

A Marx generator, also referred to as a pulse generator, generating electric voltage pulses of very short duration and high amplitude is used. They are named after the engineer Erwin Otto Marx, who developed such generators for the first time in 1923.

The high voltage pulses are required in the high voltage laboratory for testing and trials as well as for the detection of immunity to electromagnetic compatibility. Continues to be used to Marx generators for feeding gas lasers ( pulse lasers, for example, nitrogen laser).

• 3.1 test purposes
• 3.2 Science and Technology

Operation

Marx generators based on the idea to charge a large number of capacitors in parallel with a DC voltage on the so-called step voltage and then abruptly turn these capacitors in series. When charging the capacitors in parallel add up the individual load currents, the voltages of the individual capacitors are added in the subsequent series. This high voltage technical "trick" it, the charging voltage and the associated resources ( charging transformer, rectifier ) for significantly lower voltages to dimension than the desired pulse voltage allows. May take a relatively long period of time during the charging process (in the range a few seconds to about 1 minute), is carried out, the series circuit of the capacitors and their discharge on the device under test in a very short time (of the order of microseconds ). So you can the Marx generator (also called surge generator ) colloquially regarded as a kind of reservoir, which collects charge over a long period of time at low voltage and then concentrated giving this back in a short period of time and at high voltage.

Single stage impulse generator ( basic circuit )

About a high voltage transformer and a rectifier, the DC voltage (load voltage) is provided. She invites via the charging resistor on the surge capacity. This charging process is relatively slow as a rule, in the range of several tens of seconds. The voltage at the pulse capacitor here follows a function and practically reached after their quasi-stationary final value. The spark gap is adjusted so that they just do not even breaks down when quasi-stationary final value of the voltage. About an ignition device (see below) can be made to rollover to a freely given time the spark gap. At this moment, the plasma of the electric arc in the spark gap forms a closed switch low, and virtually the full charging voltage is applied to the ohmic voltage divider, consisting of, and where.

The load capacity is already part of the specimen. The time course of, and thus resulting to the DUT impulse voltage is affected by the component values ​​of the generator as well as those of the specimen. For high voltage tests of the time course of the impulse voltage within a tolerance band is well defined by standards. To maintain a standardized shock curve for test purposes, extensive calculations leading up to the impact test and, if necessary, preliminary tests with reduced impact levels are possibly necessary; this has its origin in the different electrical behavior of the individual specimens. For instance, have transformers completely different impedances and impedances to as switchgear; both of these resources but are typically tested with impulse voltage.

The disadvantage of the single-stage shock circuit is that the test item no higher voltage can be obtained as the charging voltage. In other words, the resources charging transformer, rectifier and surge capacitor must be sized for the maximum desired impulse voltage, which in the nowadays usual high levels of the required surge voltages (greater than 6 MV ) is extremely difficult and expensive or simply impossible. For this reason, use is made of multi-stage surge circuits, which are known under the name of Marx circuits. Shock generators practice be carried out virtually exclusively as a multi-stage arrangement, the number of stages is typically on the order of up to 12 stages.

Marx circuit

For the generation of pulses of higher voltage using a multi-stage arrangement according to Marx, as shown in the adjacent figure. Such a Marx generator consists of a series circuit of the surge current circuits described above. One then speaks of a -stage Marx generator.

About the charger voltage shock all capacitors are charged at the same time. Here, the charging resistors limit the charging current. The clearances of the spark gaps are chosen so that the routes just not yet hit when the maximum charging voltage.

Are all surge capacitors on their quasi-stationary final value of the voltage charge ( order some tens of seconds), with a special spark gap (trigger spark gap, see below) to an arbitrary point, the ignition of the bottom track that breaks down then. At the next spark gap is now already at twice the charging voltage, so that light with safety. Within an extremely short time now detonate all transmission links of the generator and the individual stages voltages add up to the total voltage, which is then present at the device under test as test voltage.

To accelerate the ignition of the spark gap, it is useful when an optical line of sight between the spark gaps, because the light emitted by a radio link to UV light reduces by ionization of the air, the dielectric strength of the other radio links and is essential for the almost simultaneous firing of all the stages.

Practical embodiments, ignition and operation

In principle it would be possible to determine the choice of the clearances of the individual spark gaps, the time to ignition and thus the timing of the start of the surge voltage. In practice, however, the influences of humidity, the cleanliness of the spherical surfaces and other effects play a major role, so that the moment of ignition of the spark gaps is not exactly predictable in this way.

Since you want but specify the exact moment of firing for testing and experimental purposes, one needs a way to bring about the firing of the generator at any given time. To this end, all radio links are dimensioned in the generator so that it does not quite ignite by itself when it reaches the quasi-steady charging voltage. The lowest spark gap in a Marx generator is designed as a trigger or spark gap:

An electrode (1, see figure ) of this trigger circuit is provided with an ignition electrode (2 ) which is insulated with respect to the main electrode. It is held by means of a ceramic insert (3).

At the moment of ignition, an auxiliary generator H provides a high voltage of several kV pulse to the ignition electrode, followed by forming a rollover between this and the main electrode, which ionizes the air gap between the two spheres. The ionization leads within a short time ( from 10 to several 100 ns ) for the breakdown of the spark gap, which has the arc-through of all other spark gaps of the Marx generator result.

Applications

Test purposes

Technical high voltage equipment must overvoltages occurring in practice withstand. A distinction surges, which can occur through direct or indirect lightning strikes in the energy network ( lightning impulse voltage or external surge) and those that occur by performing switching operations in the high-voltage network (internal overvoltages). Firing during the presence of the transient overvoltage arrester, then there is due to the high frequency components of the stress history a special burden for the resource, it is called a cut-off surge voltage.

To test the equipment with respect to their behavior in such transient power surges occur, they are subjected to standardized high-voltage pulses, which are generated with Marx generators.

Science and Technology

Marx generators are used for the following purposes:

• Supply of pulse gas lasers ( TEA laser: Pulse carbon dioxide laser and nitrogen laser, transversely excited nitrogen laser )
• Supply of impulse magnetron
• Generation of hard X-ray pulses
• Generation of dense hot plasmas
• Investigation of the processes involved in lightning strikes