Marx Generator

The Febetron 2020 pulser uses a Marx type high voltage generator, very similar to the circuit originally developed by E. Marx in 1924. The circuit in the Febetron system consists of 80 modules each of which contains two energy storage "stages". In addition to a high voltage capacitor, each stage also consists of two inductors and a spark gap, set at approximately 0.100".

The inductors in the module column prevent the shorting of capacitors at the beginning of the pulser discharge, as they function as open circuits to a quickly changing current. However, on the tail end of the discharge, they act as a pulse shaping network. This is a useful output pulse shape, as natural high energy discharge phenomena such as lightning, (which is often desireable to simulate for electrical surge testing), typically have a fast pulse rise time and relatively slower pulse abatement.

As can be seen in the above schematic, the energy storage element utilized by each stage is a capacitor. All of the capacitors in the module column are charged in parallel by a 0-35 kV, 10mA power supply. When the capacitors have been charged to the desired voltage level, the first stage is triggered by an external circuit. This trigger pulse causes the dielectric gas in the first stage to breakdown and become ionized (and thereby conducting). This in turn causes the potential across the second stage gap to be twice the charging voltage, which must exceed the breakdown strength (as low as 3 kV/mm) of the Marx gas in use (typically Nitrogen) for a full discharge pulse to be produced. As each subsequent stage breaks down, the potential across each respective gap is the sum of the charge voltage on the capacitors of all of the preceding stages. As much as 5.6 MV can be developed across the output of the final stage when a 35 kV charging voltage is used. Typically, the load in use has an impedence matched to the internal 380 ohm impedence of the pulser network, making the maximum attainable load voltage approximately 2.8 MV.

The main advantage of the Marx circuit configuration over a more direct approach to charging is that it overcomes the need for (expensive and bulky) very high voltage capacitors (2.5+ MV), while at the same time building in the function of a pulse forming network.