Key Fusion Energy Subsystems: Marx Generators
If you have been around fusion energy for a while, you have probably heard the term “Marx Generator” before. Like me, you probably wondered what a Marx Generator is, how it is constructed, and what it is used for? In this article, we will explore these questions, and how they relate to the generation of fusion energy.
What is a Marx Generator, and What Is Their History?
In simple terms, a Marx Generator is an electrical device that generates high-voltage pulses from low-voltage DC power. These devices have been around for over 100 years, having been invented in 1924 by Erwin Otto Marx. While the Marx Generator has a similar purpose to the Tesla Coil (which also produces high voltage pulses, and which preceded the Marx Generator), the approach taken in a Marx Generator is quite different from a Tesla Coil. For one thing, the Tesla Coil relied on resonant transformers, capacitors, and a spark gap to function. In contrast, Marx Generators utilize a simpler circuit that charges a number of large capacitors in parallel, and then “switches” the circuit so that the capacitors are now in series, and discharge into a load. Each of these capacitor-resistor-spark gap assemblies is known as a “stage”; and the number of stages generally indicates how much voltage multiplication a given Marx Generator can achieve.
In simple terms, the output voltage is equal to the input voltage times the number of stages; for instance, a 10-stage Marx Generator with an input voltage of 10,000 volts (DC) can generate pulses of 100,000 volts (100KV). In practice, Marx Generators have been able to achieve voltage pulses of over 2 million volts (2MV).
Uses for Marx Generators
Marx Generators have been used in a number of applications where high-voltage pulses were needed. One of the most obvious uses is in the testing of components for high-voltage transmission systems, such as insulators in large high-voltage transformers, as well as the insulators that hold up power lines.
Other uses include the simulation of lightning strikes on aircraft or lightning suppression systems. In both cases, these tend to be the types of applications that utilize Marx Generators that can generate electrical pulses in excess of a million volts.
For us that are involved in fusion energy, Marx Generators are very important, especially for inertial confinement fusion (ICF), where they are used extensively in an ICF approach known as Z-pinch, the most famous of these being the Z-Machine at Sandia National Labs.
In addition to national security and science uses, the Z-Machine is used to study electrically-based ICF. Whereas ICF has historically used lasers to implode fuel pellets and cause fusion, Z-pinch technologies concentrate high-voltage pulses on the target, forcing it to implode. Another approach, known as magnetized linear inertial fusion (MagLIF), uses magnetic fields generated by the high-voltage pulses to crush a metal cylinder holding deuterium and tritium, imploding it to create very high pressures and temperatures, (hopefully) leading to fusion of the fuel. This is the approach being taken by Pacific Fusion, who The Fusion Report featured in an article last month.
Approaches to Extending the Life of Marx Generators
One of the issues that can plague Marx Generators is a limited lifetime. As you can imagine, generating large pulses of power can wear out both capacitors and switches, primarily due to temperature and medium breakdown. To put this in perspective, the Marx Generator used in the Sandia Z Machine (prior to it’s “ZR” upgrade) had an expected capacitor lifetime of between 10,000 and 17,000 “shots” (pulses) per capacitor. While this is “a lot of shots”, at a rate of 1 shot per second (1 Hz) which is probably the minimum for a commercial electrical powerplant, the capacitors would “burn out” in three to five hours (3,600 shots per hour at 1 Hz). Obviously, a commercial fusion machine would have to run at least a year before needing to replace capacitors (that would equal almost 32 million shots at 1 Hz), so a “better” Marx Generator solution is required to hit a target like this. This is where the impedance-matched Marx Generator (IMG) comes in.
An IMG combines two capacitors and a low-impedance spark gap into what is known as a “brick” (see the illustration below, which shows 2 bricks). These bricks are typically oil-filled, and are organized in a ring around a central cathode known as a “stage”, similar to the stages in a classical Marx Generator. Like in a classical Marx generator, multiple stages can be used to further multiply the voltage level achieved.
The bottom half of the illustration shows the conceptual design of a 1.05 trillion watt (terawatt, or TW) IMG that uses 20 bricks per stage, with a total of 10 stages. This IMG can deliver this energy to its target in an 80 nanosecond (ns) pulse. This is essentially how each Pacific Fusion “pulsar” module is designed (though each stage contains ten bricks, and there are a total of 32 stages (rings).
Summary
Z-Pinch and MagLIF are only two of the uses of Marx Generators for fusion energy. Other uses include providing the pulses to drive high-power lasers that are used in laser-based ICF approaches. Beyond fusion, Marx Generators are also used to accelerate the drying of fruits and vegetables such as potatoes, which I guess answers the question of whether a “Mister Fusion” appliance really belongs in your kitchen (everyone remembers “Back to the Future”, right?). Perhaps in the future, fusion will be used for more than just generating electricity 😊!
