Cockcroft-Walton generator

The Cockcroft-Walton (CW) generator, or multiplier, was named after the two men who in 1932 used this circuit design to power their particle accelerator, performing the first artificial nuclear disintegration in history. John Douglas Cockcroft and Ernest Thomas Sinton Walton used this voltage multiplier cascade for most of their research, which in 1951 won them the Nobel Prize in Physics for "Transmutation of atomic nuclei by artificially accelerated atomic particles". Less known is the fact that the circuit was first discovered much earlier, in 1919, by Heinrich Greinacher, a Swiss physicist. For this reason, this doubler cascade is sometimes also referred to as the Greinacher multiplier.

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Design

The CW is basically a voltage multiplier that converts AC or pulsing DC electrical power from a low voltage level to a higher DC voltage level. It is made up of a voltage multiplier ladder network of capacitors and diodes to generate high voltages. Unlike transformers, this method eliminates the requirement for the heavy core and the bulk of insulation/potting required. Using only capacitors and diodes, these voltage multipliers can step up relatively low voltages to extremely high values, while at the same time being far lighter and cheaper than transformers. The biggest advantage of such circuits is that the voltage across each stage of the cascade is equal to only twice the peak input voltage, so it has the advantage of requiring relatively low cost components and being easy to insulate. One can also tap the output from any stage, like a multitapped transformer.

Operational characteristics

In practice, the CW has some drawbacks. As more and more stages are added, the voltages of the higher stages will 'sag', primarily due to the AC impedance of the capacitors. The ripple also increases, which may or may not be important to the application at hand. A transformer needs the diodes anyway in the rectifier. The choice between a transformer and a capacitor depends on the voltage to current ratio (the resistance) of the power supply one is to build. The border lies at about 1 kΩ. A common configuration uses a transformer for the low voltage stages and a CW for the higher voltage stages.

CW multipliers can be used to generate bias voltages of a few volts or tens of volts or millions of volts for purposes such as high-energy physics experiments and lightning safety testing.

Usage

CW multipliers are also found, with a higher number of stages, in laser systems, high voltage power supplies, X-ray systems, LCD backlighting, traveling wave tube amplifiers, ion pumps, electrostatic systems, air ionisers, particle accelerators, copy machines, scientific instrumentation, oscilloscopes, TV sets,CRT sets, bug zappers and many other applications that use high voltage DC.

See also

A similar circuit is the Marx generator, which has the same "ladder" structure, but consists of resistors, capacitors and spark-gaps. The Marx generator produces short pulses, whereas the CW generator produces a constant DC.

Further reading

• Cockcroft, J.D. and Walton, E.T.S. (1932). Proc. R. Soc. London A137:229.