APELC’s Basis for Marx Generator Design Part 3 — The Plastic Case Capacitor
This blog will complete our Marx Generator Design series, or at least the design from a capacitor-geometry basis. This month, we discuss the plastic case-styled capacitor, typically sourced by General Atomics, CSI technology and CDE.
Why We Use Plastic Case Capacitors
The plastic case capacitor is the industry standard for larger energy pulsed power systems. We do understand that there are larger, more energetic capacitor styles, APELC’s main experience comes with the single-ended plastic capacitor models.
APELC does offer several Marx generators based on this capacitor type, including variants of our MG12-1C-150NF, MG10-1C-150NF and MG83-1C-150NF. For moderate to higher energies, this has become our go-to choice in capacitors.
Our MG12-1C-150NF Marx generator geometry has resulted in the most interest. We’ve incorporated the design into several applications, including the UTA’s (the University of Texas at Arlington) material testbed, as a trigger generator for various DoD and DOE groups and even with our MIL-STD 188-125 E1 5 kA and 9 kA systems.
Design Benefits and Practical Trade-offs
We like and push the plastic cap geometry in moderate to higher energy systems because of the strong performance characteristics we’ve developed. Our MG12-1C-150NF in particular has shown to deliver high -performing specs, such as rise times of sometimes less than 20 ns, source impedances as low as 10 W, and pulse-to-pulse jitter values as low as 1 ns (excluding the jitter of the triggering source).
So what makes these capacitors so wonderful? Well, we have a few thoughts, without the luxury of peaking inside one of them.
- The capacitor comes with a single-side geometry. This feature helps us a lot, since we do not have to bring the bus work back to the top of the capacitator, like we would with a double-ended geometry.
- The capacitors are typically low in their series inductance (i.e. 24 nH).
- The wide connection buses on the capacitors help us keep conduction paths wide and low in inductance.
Oil Immersion: Perks and Pain Points
If we had any complaint, it might be with the very tall cap between the electrode bus work, designed to capture the air residing inside the capacitor. This feature does force the capacitor away from the spark gaps, in our geometries.
Another “nuance” with these capacitors is the requirement for them to be oil. You can also put them in pressurized vessels; but we’re not there yet. The oil requirement does bring its own set of pros and cons. On the pro-side, we always like the “self-healing” nature of transformer oil. But better, the higher (than air) permittivity increases the stray capacitance to ground, helping stabilize the Marx while it’s erecting, but also leads to faster rise times and low values of jitter. On the downside, our current geometries are limited to a single orientation, since we work to minimize trapped air voids that can lead to breakdown failures. But also, it’s a strange person that enjoys working with transformer oil.
Final Thoughts on Switching + Insulation
Ultimately, I believe that we have bridged the gap between switching and insulation, while delivering more energetic pulses.
- We insulate and switch in dry breathable air, using a singular “switch-rail” design that maintains an optical connection between neighboring switches.
- We maximize the benefits of the higher permittivity transformer oil, including the self-healing properties associated with the potential self-breakdown events.
Please let us know if you would like more information on this topic.
