Marx generator

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Marx generator under construction in a high-voltage laboratory

A Marx generator is a pulse generator for generating short high-voltage pulses. It is named after the engineer Erwin Otto Marx , who first developed such generators in 1923.

The high voltage pulses of the Marx generator are coupled with spark or gas discharges and have significantly higher energies than artificial electrostatic discharges .

Marx generators are required in the high-voltage laboratory for test purposes and experiments, as well as to prove interference immunity in terms of electromagnetic compatibility . Marx generators are also used to feed gas lasers , for example nitrogen lasers .

functionality

Marx generators are based on the idea of charging a large number of capacitors in parallel with direct voltage to the so-called step voltage and then suddenly connecting these capacitors in series. When the capacitors connected in parallel are charged, the individual charging currents add up; when the capacitors are then connected in series, the voltages of the individual capacitors add up. This high-voltage "trick" makes it possible to dimension the charging voltage and the associated equipment (charging transformer, rectifier) ​​for significantly lower voltages than the desired pulse voltage. While the charging process can take a relatively long period of time (several seconds to about 1 minute), the capacitors are connected in series and discharged across the DUT in an extremely short time (microseconds). The Marx generator therefore collects charge over a long time at low voltage, and releases the charge again in a short time and at high voltage.

Single-stage surge generator (basic circuit)

A capacitor is charged by means of a DC voltage source (charging voltage) via a charging resistor (current limitation). This charging process usually takes a relatively long time (a few tens of seconds). The voltage at the surge capacitor follows a function and then reaches a quasi-stationary end value depending on the charging resistance and capacity. A spark gap connects the capacitor with the test item or the load. It is dimensioned in such a way that it does not break down with the charging voltage - it is caused to flash over at a freely defined point in time with an ignition electrode. At this moment the plasma of the spark forms a low-resistance connection and practically the full charging voltage is applied to the test object.

For impulse voltage tests, the time curve of the impulse voltage is defined by standards within a tolerance band. In order to maintain a standard-compliant surge voltage curve, calculations and preliminary tests with a reduced surge level may be necessary in advance; this is due to the different electrical behavior of the individual test items. The waveforms (rise time and fall time of the pulse) are achieved through series and parallel resistances as well as through a capacitance connected in parallel to the test load.

The disadvantage of the single-stage surge circuit is that the DUT cannot achieve a voltage higher than the charging voltage. For this reason, Marx developed multi-stage surge circuits, which are known today as the Marx generator.

Multi-stage circuit

Basic sketch of the Marx generator: the current-carrying connections and components are highlighted

To generate pulses of higher voltage, a multi-level arrangement according to Marx is used, as shown in the adjacent figure. Such a Marx generator forms a series connection of several of the surge circuits described above when ignited.

All surge capacitors are charged simultaneously via the DC charging voltage . The charging resistors limit the charging current and although they charge more and more slowly due to their series connection to the right, they are all connected in series during ignition and therefore only have to be dimensioned for the voltage of one stage. In order to shorten the charging process, chokes are used for high pulse repetition frequencies instead of resistors.

The gap widths of the spark gaps are chosen so that the lines do not break through when the maximum charging voltage is reached.

If all surge capacitors are charged to their quasi-stationary final value of the voltage, an ignition spark gap (trigger spark gap, see below) is used to ignite the lowest section at a freely selectable point in time, which then breaks down. At the next spark gap , double the charging voltage is already available, so that it will definitely ignite. Within an extremely short time, all of the generator's spark gaps ignite and the individual step voltages add up to the total voltage, which is then applied to the test object as test voltage.


Practical execution, ignition and operation

Triggerable switching spark gap

In principle, it is possible to determine the time to ignition and thus the point in time at which the surge voltage begins by selecting the striking widths of the individual spark gaps. In practice, however, the influences of air humidity, the cleanliness of the spherical surfaces and metal dust from the discharges play a major role, so that the time at which the spark gaps will ignite cannot be precisely foreseen in this way. Such self-triggering Marx generators are therefore only used if the ignition times or the repetition frequency of the pulses are not decisive.

For testing and experimentation purposes, however, one would like to set the ignition point exactly. For this purpose, all spark gaps in the generator are dimensioned so that they do not ignite by themselves when the quasi-stationary charging voltage is reached. The lowest spark gap is designed as a trigger or ignition spark gap:

One electrode (1, see picture) of this trigger section is equipped with an ignition electrode (2), which is insulated from the main electrode. It is held in place by means of a ceramic sleeve (3). At the moment of ignition, an auxiliary generator H supplies a high voltage pulse of a few kV to the ignition electrode, whereupon a flashover forms between this and the main electrode, which ionizes the air gap between the two balls. The ionization leads to the breakdown of the ignition spark gap within a short time (10 to a few 100 ns), which results in all other spark gaps of the Marx generator being ignited.

An industrially used, self-triggering Marx generator for continuous operation has, for example, the following features:

  • Charging of the capacitors via chokes
  • Oxygen-free inert gas with gas exchange, pressure control, cooling and filtering
  • Switching spark electrodes made of copper / tungsten sintered metal,
  • Output voltage approx. 360 kV
  • Pulse current approx. 8 kA
  • Pulse repetition frequency 20 ... 30 Hz

The EMP test device ATLAS-I contains two opposing polarity, synchronously ignited Marx generators with 50 stages each and a charging voltage of 100 kV. This means that ± 5 MV can be generated which, using transmission lines and antennas, generate an electromagnetic pulse with a peak power of 200 GW. According to, the energy is 200 kJ, which results in a pulse duration of approx. 1 µs.

Applications

Marx generators are able to generate electrical impulses of very high power (several megavolts, currents in the double-digit kiloampere range). This makes them the only man-made devices that can approximately simulate the parameters of lightning .

Testing purposes

Energy network

Right in the picture Marx generator with spherical spark gaps, disc-shaped capacitors and rod-shaped charging resistors (far right at the edge of the picture)
Ten-stage Marx generator; The main discharge can be seen in the middle of the picture, on the right the 9 switching spark gaps and next to it the disk-shaped capacitors

High-voltage equipment must withstand the overvoltages that occur in practice. A distinction is made between overvoltages that can occur as a result of direct or indirect lightning strikes in the energy network (lightning impulse voltage or external overvoltage) and those that occur as a result of switching operations in the high-voltage network (internal overvoltages). If surge arresters ignite while the transient overvoltage is present, the high-frequency components in the voltage curve are subject to a particular load for the equipment; this is referred to as a cut-off surge voltage.

In order to test the equipment with regard to its behavior in the event of such transient network overvoltages, it is exposed to standardized high -voltage pulses that are generated with Marx generators.

EMC

The electromagnetic compatibility of devices (aircraft, defense technology) with regard to lightning strikes and EMP weapons is tested with Marx generators or with pulse generators for electromagnetic radiation fed by them.

science and technology

Marx generators can be used for the following purposes:

Web links

Commons : Marx Generators  - Collection of Images, Videos and Audio Files

Individual evidence

  1. ↑ Kurt Jäger (Ed.): Lexicon of electrical engineers. VDE Verlag, Berlin, 1996, ISBN 3-8007-2120-1
  2. ↑ https://patents.google.com/patent/DE102004001782A1 Martin Kern: Bipolar Marx generator with switch tower, direct triggering and gas conditioning for continuous industrial operation ; Patent DE200410001782
  3. ↑ https://patents.google.com/patent/DE102004001782A1 Martin Kern: Bipolar Marx generator with switch tower, direct triggering and gas conditioning for continuous industrial operation , patent DE102004001782A1
  4. ↑ http://alibi.com/news/35291/Empire-My-Prince.html Charles Reuben: In Memoriam Empire My Prince Carl Baum, trestle-maker
  5. ↑ http://ece-research.unm.edu/summa/notes/trestle.html Charles Reuben: The Atlas-I Trestle at Kirtland Air Force Base
  6. ↑ Stefan Töpfl, Claudia Siemer, Guillermo Saldaña-Navarro, Volker Heinz: Overview of pulsed electric fields processing for food . In: Da-Wen Sun (Ed.): Emerging technologies for food processing . 2nd Edition. Academic Press, London 2014, ISBN 978-0-12-411479-1 , pp. 93-114 .