5 Things to know about APELC Marx Generators
Since our founding in 1998, the core of our technology at APELC has been a suite of Marx generators designed for specific customer applications. In this blog we will look at what a Marx generator is, and what makes ours so special.
The Marx Generator
The Marx Generator is a circuit created by Erwin Marx in 1924. The idea behind the Marx is simple: Charge up a bank of capacitors in parallel and then rapidly connect them in series, discharging the energy. Each capacitor (a.k.a. “stage”) in the circuit is isolated on both the high and low sides by a charging element (either resistor or inductor) that serves the dual role of defining the charge rate of the capacitor, and isolating each stage so that when switched into a series combination, the RC time constant of the parallel circuit is much greater (>10X) than the time constant of the series discharge circuit. In this way, the voltage on each stage does not have time to fall off before the series circuit is fully formed and the pulse is delivered onto a load. An example of a Marx Generator circuit is shown in Figure 1.
Figure 1 Simple 3-Stage Marx Circuit
The switching elements can either be spark gaps or solid state switches. Solid state switches are suitable for relatively low charge voltages (i.e. up to a few kV). Spark gaps become the switch of choice as the charge voltage increases to as much as 100 kV. In either case, once all of the switches have closed, connecting all of the capacitors into the series configuration, the stacked voltage (also called the erected voltage) is simply the number of capacitors multiplied by the charge voltage, or N x Vch. Typically, most of the spark gap switches are simple, two-electrode devices switched by an overvoltage. However, the first few switches are most often actively switched using three-electrode geometries (trigatrons or field distortion). By triggering the first few spark gaps, higher differential voltages are seen on the subsequent gaps, causing them to be overvoltaged, and thereby closing. Once all of the spark gaps have closed, the Marx generator has been erected.
Once the Marx generator has erected, and is connected to the load, a “double exponential” pulse of energy is delivered to the load. The rise time of the pulse is mostly related to the series inductance of the Marx generator (with all spark gaps conducting) divided by the load resistance (or Lmarx / Rload). The fall time of the pulse is typically an RC decay, where “C” is the erected capacitance, or the stage capacitance divided by the number of stages (Cstage / N). The amplitude of the pulse can simply be seen as a voltage divider, between the impedance of the Marx generator and the load impedance, or Vpulse = Verect x (Zload/Zload + Zmarx), where Zmarx = sqrt(Lmarx/Cmarx). It is easy to see that as the number of stages increase to achieve higher voltages, the Marx impedance also increases. This is because as we add stages, the erected capacitance decreases and the series inductance increases, either of which increases the Marx generator’s impedance and decreases the voltage efficiency on the load.
It may at first seem that because the Marx generator was created in the 1920’s that in the 21st century there would no longer be a need for this kind of gaseous switch and that all Marxes would now use solid-state components, as most electronic circuits now do. However, because of the fundamental physical limitations of existing solid state devices, there presently does not exist a solid state switch (e.g. BJT, IGBT, MOSFET, or SCR/Thyristor) that is capable of both high voltage (>5kV) and fast closure time (<10ns). While transformers exist for multiplying AC and Cockroft-Walton circuits exist for multiplying DC to high-voltage, the Marx generator is specifically designed to deliver a high-voltage pulse, and therefore these fast switching times may be critical depending on the application. More about this in the next section.
APELC Marx Generators- what makes them special?
1) Tailored source impedance
If you do a search for Marx generator online, you are likely to see a photograph of a Marx generator firing in open air with all of the spark gaps and capacitors visible. Due to the lack of a closely-coupled ground plane these Marxes are inherently high-impedance, due to the large amount of inductance. In applications such as high-power microwave/RF, EMP simulators, flash x-ray and pulse charging, the load is typically <100 Ohms requiring the use of a low source-impedance Marx generator to see the desired voltage on the load. Too often, however, Marx generators are too high in impedance, bringing a low voltage efficiency. There are two typical mitigation strategies, neither of which are very appealing: 1. The stage capacitance can be increased, in the attempt to reduce the source impedance. However, as we add more capacitance, we typically add more inductance. 2. Often the Marx generator is used to pulse-charge a Pulse Forming Line (or PFL). In this scenario, we use the Marx generator for voltage multiplication and energy delivery to pulse charge a lower impedance source. The drawback here is added volume and complexity.
A better practice is to design the Marx generator for a low impedance. This can be done by bringing the ground plane in closer proximity to the Marx circuit-especially with the spark gap switches, which have tiny conductive channels.
APELC Marx generators, such as the MG15-3C-940PF shown in Figure 2, utilize an aluminum housing and solid insulation that place the ground plane in close proximity to the internal components of the Marx, thereby creating a low-impedance coaxial geometry. In the case of the MG15-3C-940PF, the source impedance is ~50 Ohms, making it well suited for driving many standard high-voltage coaxial cables. As a result, the MG15 and many other APELC Marx generators are able to efficiently deliver the erected energy of the Marx onto a remote load, such as an antenna, spark gap (in the case of triggering larger pulsed power systems), or flash X-ray head. The conductive housing also acts as a pressure vessel, providing a gaseous insulation/switching medium that can be adjusted to achieve a high dynamic range. APELC Marx generators typically achieve a 1:5 dynamic range (8kV to 40kV charge), although ranges as high as 1:9 have been realized.
Figure 2 APELC MG15-3C-940PF Marx generator (input and output views)
2) Wave-erection
APELC Marxes are designed using a principle known as “wave-erection”. The “wave erection” concept was seemingly first introduced by John Francis, at Texas Tech University, and later evolved by David Platts, at Los Alamos National Laboratory. In brief, this means that the spark gaps close in a very sequential manner – first to last – by customizing the stray-to-ground capacitance of each stage of the Marx circuit. By increasing the stray-to-ground capacitance, we provide each stage with a strong ground reference. But more importantly, we force higher overvoltages on each subsequent spark gap, realizing that each spark gap represents a capacitive voltage divider, between the spark gap capacitance and the stray-to-ground capacitance. Additionally, components are placed in such a way that provides coupling of both optical (spark gaps) and electrical energy to create repeatable erection of the generator with a relatively simple trigger. In the case of the spark gaps, the optical energy from one stage pre-ionizes the next, whereas each stage is able to “see” the adjacent stage through capacitive coupling- providing an additional means for overvoltaging successive stages.
A major result of the “wave erection” method is that we can achieve very fast rise time. In prior work, APELC has demonstrated and reported on pulse rise times as fast as 200 ps, with generators that are relatively high in series inductance. So how is this done? When the last spark gap closes, the first energy out is driven by the stray-to-ground capacitance of the last spark gap, through the inductance of the last spark gap and the output inductance, which can be substantially lower than the total series inductance of the Marx generator.
3) Ease of use/maintenance
APELC Marx generators utilize a modular stage design and are engineered to be easily removed from their housing. Moreover, all of our generators employ our coaxial quick-disconnect connectors on the input (charge and trigger) and output. These features make our generators extremely easy to use and extremely easy to maintain.
Whereas many Marxes use sulfur hexafluoride- an extremely expensive, EPA-restricted gas, our generators use compressed dry air as the switching medium. Not only is this extremely inexpensive, it can be easily sourced from a gas supplier, SCUBA store, or even made on-site with a dry-air compressor.
4) Long lifetime and high repetition rates
APELC has performed lifetime studies on our generators to understand both shot life and failure mechanisms. One advantage of the air insulation switching medium mentioned previously is the ability to flow the gas and clear-out/cool the switching channel. Our studies have created a set of metrics for flow-rate vs. repetition rate vs. duty cycle. With this knowledge incorporated into the design, the newest version of our MG15-3C-940PF Marx generator is able to achieve repetition rates as high as 500 Hz.
5) Low-jitter, rise-time and repeatability
Due to the mechanisms mentioned in the Wave-erection section above, along with the knowledge gained from the afore-mentioned lifetime studies, APELC Marx generators are able to produce pulses that are repeatable both in amplitude and in time. As shown in Figure 3, 10 consecutive pulses from the MG15-3C-940PF are overlaid and nearly indistinguishable from one another. Moreover, the jitter timing for the Marx is ~2ns, meaning the pulses can be predictable triggered within nanoseconds. This is extremely useful when multiple generators need to be timed in unison (e.g. phased arrays) or when a single generator needs to predictably trigger a larger pulse power system.
To learn more about our Marx Generators, click here.
