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100kV marx generator

Introduction

A Marx generator is a simple high voltage pulse generator which has various scientific uses, ranging from insulation and lightning safety testing to fusion research. It is based on slow, parallel charging of a bank of capacitors, which are then quickly connected in series, usually by means of spark-gaps. Due to extraordinary power handling capabilities of spark gaps, Marx generators are capable of providing pulses of unparalleled peak powers along with multi-megavolt voltages. Marx generators have produced some of the longest man-made arcs and sparks. Probably the most famous Marx generator shot is the one from Russian SIBNIIE high voltage research facility, producing arcs reaching over 100 meters in length from just several megavolts of peak voltage. Another good example is Sandia Z-machine, a powerful Marx generator used for fusion research. The picture of it commonly circulated on the internet shows a beautiful streamer display on the surface of oil pool, produced by rapid change in electric field when the machine fires.

Marx Generator spark from SIBNIIE research facility. Courtesy of Bert Hickman’s website

“Z machine” streamer display

Project description and construction

When I originally started building Tesla coils, I made quite a bit of bad decisions – one of them was to buy considerable amount of high voltage ceramic capacitors with the intention to use them for my tank capacitor. Witnessing their poor performance, after some research I realized they are not suitable for that kind of application: These capacitors are most often used in line filter or DC bypass applications, and are not designed for repetitive high frequency stress. The ceramic dielectric used – barium titanate, is piezoelectric and ferroelectric, displaying considerable losses at high frequencies. The dielectric constant, and hence the capacitance, is also heavily temperature sensitive, making these capacitors poorly suitable for Tesla coils.

This way, I ended up with a  considerable amount of ceramic HV capacitors, and decided it may be a good idea to put them to use as a marx generator. They were 3kV, 10nF capacitors, a voltage somewhat too low for spark gaps to work reliably with it. I ended up using series of 5 capacitors for each “Marx capacitor”, which along with 10 stages allowed a maximum 150kV output. In order to allow for some margin of safety, the generator was charged only up to about 10kV per stage, resulting in ~100kV maximum output (though no measurement could be made at the time to confirm this, and hence this is just an upper estimate).

For simplicity, the whole design was built onto a printed circuit board for the sake of simplicity; this is not a best choice for a high voltage circuit and has resulted in some surface flash-overs which ultimately required board modification. The resistors chosen were standard voltage, 1 mega-ohm, 0.5W carbon film resistors. It is usually unadvised to over-volt the resistors, especially if they use thin carbon film instead of carbon composite resistive element; so far though, they have withstood this duty without flashing over or going open, as carbon film resistors sometimes do in pulsed conditions.

The spark gap design was minimalistic, consisting of pieces of carefully bent solid copper wire soldered to the backside of the PCB. The spark gap “heads” were created by simply bending the wires into a semi-circle.

Marx generator Schematic and the PCB. All unmarked resistors are 1 megaohm, 0.5W resistors. All capacitors are series strings of 5 3kV, 10nF ceramic capacitors.

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Completed Marx generator

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Front closeup

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Spark gap detail

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The bleeder resistors that were first soldered onto the PCB started a fire due to their inadequate voltage ratings. This portion of PCB had to be cut out, and a special 100 mega-ohm HV resistor was put in place

Results

The Marx generator was powered by about 8kV DC from a rectified oil burner transformer, producing loud 10cm sparks. Unfortunately, I couldn’t find any pictures that demonstrate this! But I made a video, in which one frame manages to capture the short-lasting spark event:

Conclusion

A small marx generator like this has little use other than the educational value of building one, and showing it to others. It is a device that can be scaled to almost any size, and with proper design it can produce some of most powerful electrical pulses known to scientists – with lots of hopes that one day this power might be used to initiate nuclear fusion.

Links and references

[1] Quick and dirty Marx generator http://www.electricstuff.co.uk/marxgen.htm

[2] Claimed 1MV Marx generator http://www.electricstuff.co.uk/marxthree.html

[3] Steve Ward’s Marx generators http://www.stevehv.4hv.org/Marxindex.htm

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One Comment
  1. Insightful .. I was not aware the power that is being generator have such purpose. In the dynamic world which demands more power, the engineers should harness the power in a better way…

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