APELC’s basis for a Marx Generator Design – Part 2 Using Mica Film Capacitors
Using Mica Film Capacitors
Last summer, we talked about the design of Marx generators using the ceramic doorknob capacitor. I realized that I needed to continue this discussion with the other types of capacitors we use.
What Are Mica Film Capacitors?
Today’s discussion is based on APELC’s design of Marx generators using the mica film capacitor. One disclaimer – I am not a capacitor expert. So, the following discussion is simply from a “user’s experience”.
My understanding of these capacitors is somewhat limited to the discussions with a single supplier/manufacturer. From their description, layers of mica film and an insulating material are cylindrically-wound, then pressed into a rectangular (or more formally, a rectangular prism or a cuboid). Somewhere in the process, protruding tabs bring out the connection to the foil and the entire component is encapsulated in some type of epoxy.
Why Mica Film Was Appealing
Many years ago, we worked with Texas Tech University to build a Marx generator using these capacitors. The voltage ratings were undesirably low, limited to 20 kV; however, with a capacitance of 100 nF, this capacitor became very appealing for moderate energy storage. In our case, one Marx stage could store approximately 20 J. A similar design using the ceramic doorknob capacitors would require approximately 29 capacitors (assuming a 20 kV, 5.2 nF part), which is density difference of almost 6X. That’s a big difference.
Texas Tech also did some really nice testing on the capacitors finding that they could handle a 100% voltage reversal before failing. This is also very appealing, since vacuum diode loads too often fail to a short, which can be lethal to a Marx generator.
Where It Gets Difficult: The Challenges
However, the mica capacitor also brings some undesirable nuances.
- Loop Geometry and Inductance
Being a double-ended device is challenging. Unlike the doorknob capacitor, where the geometry is relatively tight, the mica capacitor is lengthy. If we are targeting a “clean” UV path between neighboring spark gaps, we must use long buswork to connect the capacitor to the next spark gap, as shown below. As you can see, we are setting up a loop, or really, a single turn of an inductor. Now imagine 30 stages creating a 30-turn inductor. The impedance of the generator has now dramatically increased, killing the voltage efficiency.
We can split the spark gaps, giving up the optical alignment. However, we still see the giant loop.
For this reason, we work to limit the number of stages for this type of Marx generator. Our MG30-1C-100NF Marx generator perfectly illustrates the problem, where the rise time is approximately 100 ns, which is horribly painful to see,
- Low Charge Voltage
That charge voltage limitation (20 kV) is difficult. We more ideally would like to charge to 40 kV, which would necessitate a series stack of two capacitors. This brings in design challenges, wanting to avoid excessive inductance due to geometry. This problem really compounds the problem described in the first bullet. Placing these capacitors in series dramatically expands the loop, further increasing the voltage rise time. A great example is with our MG30-3C-100NF Marx generator, which used three mica capacitors in series in each stage. The rise time increased to a horrible 250 ns. However, the impedance is kept to a low 24 Ohm, since the erected capacitance large. We no longer offer this generator.
- Inductance at the Connection Points
The effective “inductance” of the capacitor is not ideal. The manufacturers like to make the connections to the capacitor with “single point” contacts. Our supplier originally used thin wire connections. With more resistance than expected, they eventually manufactured our capacitors with wide tabs. What they didn’t buy into was a “multi-contact” connection. Why is this a problem? We’re either dealing with a large inductance, or some kind of transmission line effect, essentially “waiting” to get the energy out of the capacitor.
Summary: Where Mica Film Fits Best
As with any component, context is everything. Mica film capacitors offer great energy density, but when rise time and inductance matter, other options may win out.
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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.
