Pulsed Power and its Applications


Pulsed power is a technology that consists in accumulating energy over some period of time, then releasing it very quickly. Since power equals energy (or work) divided by time, the idea is to emit a constant amount of energy in as short a time as possible. It will only last for a fraction of a second though, but that instantaneous power has very interesting applications. With this technology, power levels of more than 300 terawatts have been obtained. Is this technology for unlimited budgets, or is this in reach of the common hacker?

Consider for example discharging a capacitor. A large 450 V, 3300 uF electrolytic capacitor discharges in about 0.1 seconds (varies a lot depending on capacitor design). Since the energy stored in it is given by 1/2 CV², which gives 334 Joules of energy, the power delivered will be 3340 watts. In fact a popular hacker project is to build large capacitor banks. Once you have the bank, and a way to charge it, you can use it to power very interesting devices such as:

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A portable, 1.25 kJ coilgun by [Jason Murray]

Railguns in particular are subject to serious research. You may have read about the navy railgun, capable of reaching a muzzle speed of more than 4,600 mph (around Mach 6), more than any other explosive-powered gun. Power is provided by a 9-megajoule capacitor bank. The capacitors discharge on two conducting rails, generating an electromagnetic field that fires the projectile along the rails. The rail wear due to the tremendous pressures and currents, in the millions of amperes range, is still a problem to be solved. lgmx

Another device that uses capacitors for high power pulses is the Marx generator. It is a very simple circuit that allows you to charge a number of capacitors in parallel and then suddenly discharge them in series using spark gaps. Very large Marx generators have been built, for high voltage component testing and other purposes, but it’s also very easy to make a small lightning simulator in under an hour if you have some high voltage capacitors and resistors. Marx generators are in use in the Z machine, a Sandia National Labs project for fusion research, that is capable of shooting 26 million amperes in 95 nanoseconds. Temperatures of 3.7 billion kelvins have been obtained.

The Marx generator is a particular case of a pulse forming network, or PFN. Capacitors, inductors and transmission lines, or a combination of them are used for energy storage in various topologies. Then, the network is discharged into the load via a high voltage switch

Transmission line PFN. By Chetvorno, via Wikimedia commons.
Transmission line PFN. By Chetvorno, via Wikimedia commons.

such as a spark gap or a thyratron. The transmission line PFN is interesting because the capacitance of the conductors in the line is used for both transmission and energy storage. When the power supply is connected it slowly charges up the capacitance of the line through RS. When the switch is closed, a voltage equal to V/2 is applied to the load, the charge stored in the line begins to discharge through the load a current of V/2Z0 and a voltage step travels up the line toward the source.

Compulsators (a portmanteau for compensated pulsed alternator) are another way of delivering high current pulses. They convert rotational energy from a flywheel directly into electrical energy. The compulsator works in a similar way as a normal alternator, but is designed with minimal inductance windings to deliver extremely high currents in very short time periods. There is little information on compulsator design and, as far as we know, no hobbyist has ever made one. You have your homework assignment.

Alternator vs compulsator designs. From Weldon et al.
Alternator vs compulsator simplified designs. From Weldon et al.

The explosively pumped flux compression generator, or EPFCG for short, is a device that generates a high power electromagnetic pulse using a high explosive to compress the magnetic flux. Million of amperes and tens of terawatts of power are produced by the EPFCG in a single pulse, since the device is destroyed in operation.

Croquant, via Wikimedia commons
Steps in flux compression. By Croquant, via Wikimedia commons.

The three basic steps in flux compression are shown above.

  1. An external magnetic field threads a closed ring conductor.
  2. The ring’s diameter is reduced by the explosive. The variation of the magnetic flux induces a current in the ring, which in turn creates a new magnetic field, so that the total flux in the interior of the ring is maintained.
  3. The external and induced magnetic fields add up so that the total magnetic flux remains constant, and a current is created in the ring.

The compression process allows the chemical energy of the explosives to be (partially) transformed into the energy of an intense magnetic field surrounded by a correspondingly large electric current. There are several designs of EPFCG’s. The figure shows the hollow tube type.

flux_compression_generator_1

Pulsed power is also used in particle accelerators and high power lasers and the technology is rapidly evolving.

If you’re starting out, you may want to experiment with capacitor banks which are a relatively simple way of obtaining pulsed power. But if you do, take all necessary precautions. The power levels can be extremely dangerous.



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