In Part 1, we talked about fires inside the Marx generator — the kind that melt plastics and fill the housing with soot. Part 2 covers a different kind of expensive: 100’s of kV launched back into your power supply and trigger unit when the generator self-breaks.
Part 2: Damaging the supporting equipment
Typically, using a Marx generator requires a high voltage power supply to charge the generator, a source of pressurized gas to further insulate the spark gaps, and a pulsed high voltage source to trigger the generator. It’s a fairly easy setup. But problems crop up, because high voltage has a way of doing the unexpected.
There are essentially three scenarios resulting from triggering a Marx generator, including (1) a no shot, (2) a triggered shot, and (3) an untriggered shot, commonly referred to as a “self-break.”
The basic circuit: how the components connect
First, let’s consider the following simple circuit, describing the simple components to operate a Marx generator. Both the high voltage power supply and the high voltage pulse generator are connected to the Marx generator via coaxial cables (typically 50 Ω), and these sources are generally high in their output impedance.
How the trigger switch works: the trigatron method
Next consider the geometry of the trigger switch, which is the first switch in the Marx generator. A typical method is referred to as the “trigatron” method, which buries an insulated pin inside the cathode of the first gap, as shown. Applying the trigger pulse creates a plasma from the trigger pin to the ground electrode, thus effectively tying the two together, electrically, while also splashing UV into the primary gap. This ionization leads to the breakdown of the primary gap.
What a self-break looks like — and why the order matters
The order of breakdown is very important. We now consider the “self-breakdown” process. In general, the Marx generator can (and will) self-trigger at the worst times, while creating the worst of scenarios. A terrible case is when one of the later spark gaps self-breaks (or closes). For this discussion, let’s assume that we have a 10-stage Marx generator, and switch 5 decides to close.
The backward-moving wave: how electronics get destroyed
When the “self-break” happens, we momentarily see two voltage waves moving away from this closed switch. The “self-break” of switch 6, results in a forward-moving wave toward the load. The “self-break” also results in the sequential closure of switches 5, 4, 3, 2, and finally 1, and is described as a backward moving wave toward the input of the generator. As noted in the drawing, the “point of interest” is our problem, since there is a finite amount of time (i.e. a few nanoseconds) before switch 1 closes, finding ground. It is at this moment that the “point of interest” may realize a voltage potential of 100’s of kV.
In general, the trigatron gap can be represented by several capacitances (without detail). And when the “point of interest” is subjected to the voltage potential resulting from the backward-moving wave, that voltage potential charges these stray capacitances, also to very high voltage. The capacitance between the trigger pin and ground is loaded by the coaxial cable connecting the high voltage trigger to the Marx generator. So, in effect, we now have very high voltage pulses launched onto both the high voltage supply connection and the trigger connection.
With backward wave voltages in the 100’s of kV, you can see how a self-break can send that energy straight back into the power supply and trigger unit — potentially killing both.
Most engineers don’t think about the backward wave until they’ve lost a power supply to one. Once you understand how it propagates and what it finds on the way back, you start designing your system differently.
If you’re designing or operating a Marx generator system and want to talk through safe triggering practices and system protection, APELC is glad to help. These are lessons we’ve learned the hard way over decades of building and testing these systems. 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.
