Marx generator

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A Marx generator is a type of electrical circuit first described by Erwin Marx in 1924 whose purpose is to generate a high voltage pulse. It is extensively used for simulating the effects of lightning during high voltage and aviation equipment testing. A bank of 36 Marx generators is used by Sandia National Laboratories to generate X-rays in their Z Machine. It can also be used as an ignition switch for thermonuclear devices.

Marx generator diagrams; Although the left capacitor has the greatest charge rate, the generator is typically allowed to charge for a long period of time, and all capacitors eventually reach the same charge voltage.
Marx generator diagrams; Although the left capacitor has the greatest charge rate, the generator is typically allowed to charge for a long period of time, and all capacitors eventually reach the same charge voltage.
To deliver 5 ns rise time pulses the Marx generator is often built into a coaxial wave guide. The spark gaps are placed as close as possible together for maximum UV light exchange for minimum jitter. DC HV coming from bottom, pulsed HV leaving at top into the coaxial line. The double line of spheres in the middle are the spark gaps, all other spheres are to avoid corona discharge. Blue=water capacitor. Grey=solid metal. Black= thin wire. The outer conductor also functions as a vessel, so that the gas and the pressure can be optimized.
To deliver 5 ns rise time pulses the Marx generator is often built into a coaxial wave guide. The spark gaps are placed as close as possible together for maximum UV light exchange for minimum jitter. DC HV coming from bottom, pulsed HV leaving at top into the coaxial line. The double line of spheres in the middle are the spark gaps, all other spheres are to avoid corona discharge. Blue=water capacitor. Grey=solid metal. Black= thin wire. The outer conductor also functions as a vessel, so that the gas and the pressure can be optimized.

A number of capacitors are charged in parallel to a given voltage, V, and then connected in series by spark gap switches, ideally producing a voltage of V multiplied by the number, n, of capacitors (or stages). Due to various practical constraints, the output voltage is usually somewhat less than n*V. Proper performance depends upon selection of capacitor and the timing of the discharge. Switching times can be improved by doping of the electrodes with radioactive isotopes caesium 137 or nickel 63, and by orienting the spark gaps so that ultraviolet light from a firing spark gap switch illuminates the remaining open spark gaps. Insulation of the high voltages produced is often accomplished by immersing the Marx generator in transformer oil or a high pressure electronegative gas such as sulfur hexafluoride (SF6).

Note that the closer a capacitor is to the charging power supply, the faster it will charge. If the generator is allowed to charge long enough, all capacitors will attain the same voltage.

In the ideal case, the closing of the switch closest to the charging power supply applies a voltage 2*V to the second switch. This switch will then close, applying a voltage 3*V to the third switch. This switch will then close, resulting in a cascade down the generator (referred to as erection) that produces n*V at the generator output (again, only in the ideal case).

The first switch may be allowed to spontaneously break down (sometimes called a self break) during charging if the absolute timing of the output pulse is unimportant. However, it is usually intentionally triggered by mechanical means (reducing the gap distance), triggered electrically, triggered via a pulsed laser, or by reducing the air pressure within the gap after all the capacitors in Marx bank have reached full charge.

The charging resistors, Rc, need to be properly sized for both charging and discharging. These resistors are sometimes replaced with inductors for improved efficiency and faster charging. In many generators the resistors are made from plastic or glass tubing filled with dilute copper sulphate solution. These liquid resistors overcome many of the problems experienced by the more normal solid resistive materials which have a tendency to lower their resistance over time under high voltage conditions.

Contents

[edit] Variants

Avalanche diodes can replace the spark gap for voltages around 500 volt. The charge carriers easily leave the electrodes, so no extra ionization is needed and jitter is low, also the diodes live longer. The Marx generator is not only useful for its high voltage, but also for short pules, at 5 V for signal processing tunnel diodes may be useful.

[edit] Applications

[edit] Further reading

  • M. Obara, "Strip-Line Multichannel-Surface-Spark-Gap-Type Marx Generator for Fast Discharge Lasers", IEEE Conference Record of the 1980 Fourteenth Pulse Power Modulator Symposium, USA, Jun. 3-5, 1980, pp. 201-208.
  • G. Bauer, "A low-impedance high-voltage nanosecond pulser", Journal of Scientific Instruments, London, GB, Jun. 1, 1968, vol. 1, pp. 688-689.
  • Graham et al., "Compact 400 KV Marx Generator With Common Switch Housing", Pulsed Power Conference, 11th Annual Digest of Technical Papers 1997, vol. 2, pp. 1519-1523.
  • S.M. Turnbull, "Development of a High Voltage, High PRF PFN Marx Generator", Conference Record of the 1998 23rd Int'l Power Modulation Symposium, pp. 213-16.
  • CNess, et al. "Compact, Megavolt, Rep-Rated Marx Generators", IEEE Transactions on Electron Devices, vol. 38, No. 4, 1991, pp. 803-809.
  • Shkaruba et al, "Arkad'ev-Mark Generator with Capacitive Coupling", Instrum Exp Tech May-Jun. 1985, vol. 28, No. 3 part 2, May 1985, pp. 625-628, XP002080293.
  • I. C. Sumerville, "A Simple Compact 1 MV, 4 kJ Marx", Proceedings of the Pulsed Power Conference, Monterey, California, Jun. 11-24, 1989, No. conf. 7, Jun. 11, 1989, pp. 744-746, XP000138799.

[edit] Patents

[edit] External articles

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