Note: This discussion is ultimately a discussion about the charging methods of Marx generators. Let’s start with the bad.
Every pulsed power engineer eventually learns the hard way that Marx generator safety isn’t just about the discharge event. Fires inside the housing. Dead power supplies. Destroyed oscilloscopes. Damaged control electronics. These aren’t theoretical risks. They’re things APELC engineers have witnessed firsthand, and in some cases, things every new hire seems destined to experience at least once.
This is Part 1 of a two-part series on safely operating a Marx generator, written from the operator’s perspective. Part 1 covers the most dramatic failure mode: internal fires caused by how the generator is charged. Part 2 covers the destruction of supporting electronics. Both are expensive lessons. The goal is to help you avoid learning them the hard way.
How a constant voltage supply sets the stage for a fire
Since our beginning, APELC has mostly charged Marx generators using constant voltage high voltage power supplies. These supplies are simple to use and we get to testing sooner. However, this approach can be very dangerous, not only to personnel, but also to the hardware.
It’s straightforward — just connect the supply to the Marx charge line. The tendency with most, it seems, is to use smaller and smaller resistive charge elements to speed up the charge time. I’ve always liked using 1–3 MΩ. But most engineers prefer pushing down toward 100–300 kΩ, and sometimes less. What happens? 30 kV / 300 kΩ = 100 mA — plenty of current to keep the plasma excited and conductive. Likely, the power supply is current-limited to a few mA’s. But even a few mA’s can sustain a plasma.
Why the spark gaps don’t recover — and what happens next
Here’s where things can go wrong — the Marx generator is triggered, closing all the spark gaps and releasing all of the stored energy within a few 10’s of nanoseconds. However, the gaps typically remain ionized for a few milliseconds, meaning that the resistance of the spark gaps remains low and therefore conductive, and thus, “short-circuiting” the generator.
Once the Marx generator has discharged, the constant voltage high voltage power supply immediately begins pushing energy back into the generator. And since the gaps are still conductive, we’re simply driving current through the very conductive gap, keeping the plasma sustained — potentially for seconds.
What the fire actually looks like inside the housing
Plasmas are very hot, measuring in the 1,000’s of degrees, and primarily dissipating heat into the metallic electrodes, which are supported by plastic supporting structures.
As one might imagine, the electrodes super-heat and begin melting the plastics. And left long enough (i.e. seconds), the plastics will begin to burn, producing soot, damaging everything inside the generator’s housing.
The bottom line
A constant voltage supply and low charge resistance is a combination that can turn a routine test into an expensive afternoon. Design your charging circuit accordingly.
Coming up in Part 2
In our next blog, we will discuss the potential damage to supporting control systems.
If you’re commissioning a Marx generator system or reviewing your operating procedures, APELC is glad to talk through safe charging practices and system design. These are lessons we’ve learned building and operating these systems for decades. Reach out directly, or explore our Marx generator product line to see how we approach safety and performance from the ground up.
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About the Author
Jon Mayes is the Founder and President of APELC. With over 25 years of experience in pulsed power system design and a Ph.D. in Electrical Engineering, Jon has led the development of industry-leading Marx generators, EMP simulators, and high-voltage test systems. His work has supported the Department of Defense, Department of Energy, and major research institutions across the U.S.
