Marx generator

Marx generators are used in high-energy physics experiments, as well as to simulate the effects of lightning on power-line gear and aviation equipment.

The circuit generates a high-voltage pulse by charging a number of capacitors in parallel, then suddenly connecting them in series.

This process of the spark gaps connecting the capacitors in series to create the high voltage is called erection.

At some point, the spark gaps stop conducting, and the low-voltage supply begins charging the capacitors again.

In many generators the resistors are made from plastic or glass tubing filled with dilute copper sulfate solution.

Shortness is relative, as the switching time of even high-speed versions is not less than 1 ns, and thus many low-power electronic devices are faster.

In electrodynamic terms, when the first stage breaks down it creates a spherical electromagnetic wave whose electric field vector is opposed to the static high voltage.

This moving electromagnetic field has the wrong orientation to trigger the next stage, and may even reach the load; such noise in front of the edge is undesirable in many switching applications.

If the generator is inside a tube of (say) 1 m diameter, it requires around 10 wave reflections for the field to settle to static conditions, which restricts pulse leading edge width to 30 ns or more.

The charge carriers easily leave the electrodes, so no extra ionisation is needed and jitter is low.

[citation needed] A speedy switching device is an NPN avalanche transistor fitted with a coil between base and emitter.

Once the Marx generator has completely fired, voltages everywhere drop, each switch avalanche stops, its matched coil puts its base-emitter junction into reverse bias, and the low static field allows remaining charge carriers to drain out of its collector-base junction.

In food industry Marx generators are used for Pulsed Electric Fields processing to induce cutting improvement or drying acceleration for potato and other fruits and vegetables.

A small demonstration Marx generator (tower on the right) . It is a ten stage generator. The main discharge is on the left. The nine smaller sparks that can be seen in the image are the spark gaps that connect the charged capacitors in series.
Marx generator diagrams; Although the left capacitor has the greatest charge rate, the generator is typically allowed to charge for a long period of time, and all capacitors eventually reach the same charge voltage.
To deliver 5 ns rise time pulses, the Marx generator is often built into a coaxial wave guide . The spark gaps are placed as close as possible together for maximum UV light exchange for minimum jitter. DC HV comes from underneath, pulsed HV leaves at the top into the coaxial line. The double line of spheres in the middle are the spark gaps, all other spheres are to avoid corona discharge . Blue= water capacitor . Grey=solid metal. Black= thin wire. The outer conductor also functions as a vessel, so that the gas and the pressure can be optimized.
Marx generator. Air is a poor conductor of electricity, but with sufficient voltage (30 kV/cm), air transforms into a conductor, resulting in a spark discharge.