|Jochen's High Voltage Page|
As the name implies, it should be possible to "cascade" cascades to obtain even higher output voltage. Unfortunately, TV cascades, which are readily available compared to other high voltage components, lack the second output which could provide an AC input for the next stage (compare the basic villard circuit). However, this additional output is present within the module, and it can be made accessible by simply drilling a hole. This makes it possible to put (in the example presented here) four TV cascades in series, for 120kV output.
The circuit above (shown here are only three cascades) is nothing but a 9 stage Villard multiplier. Everything inside the dashed lines is provided by the TV cascade modules. Only between two cascades there is an additional 10kV/1nF capacitor to make up for the lacking cap (compare the TV cascade schematic). The external damping resistor, consisting e.g. of 20 33kOhm resistors, is important to protect cascade modules and input AC source from voltage spikes as a result of rapid discharges.
The tricky thing is to drill the hole. Try to have an X-ray made of the type of cascade you´re going to use. As an example, see the X-ray picture in the TV cascade section. The place where the hole has to go is indicated by a circle. Drill very slowly and cautiously from the side until you hit metal, then drill to the same point from top and make a little cavity around the metal wire you hopefully hit. Solder a piece of hv cable to the wire and fill the holes with epoxy. As you´re making a mess anyway, also drill out the "A", "D" and "~" connections, attach hv wire and fill with epoxy. If there is a 8kV output, drill it out as well and fill with epoxy without attaching a wire, as this output is not needed.
|Finished cascade module. You can still see the solder joint of the additional output through the expoxy.|
The finished modules are mounted to some insulating structure and the appropriate inputs and outputs are connected directly or through a 10kV/1nF cap, as indicated by the schematic. To prevent corona (which is a very serious problem above, say, 30kV), insulate joints and caps in wax, epoxy or some potting compound. Alternatively, you can put the whole setup under oil, of course.
Because of the large number of stages, the Super-cascade should be fed from a 8kV, 20kHz flyback supply. With my four-stage Super-cascade, I achieve approx. 120kV, which already drops to about 100kV when loaded with 1GOhm for measurement. The maximum spark length is around 20cm, the repetition rate is several discharges per second.
|Four-stage Super-cascade. Left, the green cascade modules, with caps and joints between them potted inside the black plastic containers. Right, metal warm bottle on insulating plastic tube acting as a terminal, preventing corona. Between both, near the upper edge of the photo, the damping resistor. It consists of a resistor chain inside PVC hose.|
|Remember that capacitors can hold their charge for quite some time. Discharge the cascade carefully before handling it.|
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