Cockcroft–Walton generator

The Cockcroft-Walton generator emerged because physicists needed to accelerate particles to high energies without building impossibly large transformers. Heinrich Greinacher's 1919 voltage multiplier circuit solved this elegantly—and when Cockcroft and Walton used it in 1932, they achieved the first artificial nuclear reaction in history.

The physics was clear by the late 1920s: bombarding atomic nuclei with high-energy particles might reveal their structure. But the voltages required were enormous. Ernest Rutherford at Cambridge's Cavendish Laboratory had calculated that protons accelerated to several hundred thousand electron volts might penetrate the lithium nucleus. The problem was generating such voltages.

Traditional transformers would have been impractically large. Greinacher, a Swiss physicist, had developed an alternative in 1919: a cascade circuit that multiplied AC voltage through a ladder of capacitors and rectifiers. Each stage doubled the voltage of the previous one. The circuit could generate hundreds of thousands of volts from relatively modest components.

John Cockcroft and Ernest Walton, working under Rutherford at the Cavendish Laboratory, saw the potential. They built a four-stage voltage multiplier using large rectifiers and high-voltage capacitors. By 1932, their apparatus could accelerate protons to approximately 700,000 electron volts—enough, Rutherford believed, to split atoms.

On April 14, 1932, Cockcroft and Walton directed their accelerated protons at a lithium target. The lithium nuclei split into pairs of helium nuclei—alpha particles—that could be detected by their characteristic flashes in a zinc sulfide screen. For the first time in history, humans had artificially transmuted one element into another without using radioactive sources.

The achievement vindicated Einstein's mass-energy equivalence. The alpha particles' combined kinetic energy exceeded what the protons had carried in—the extra energy came from the mass lost in the reaction, exactly as E=mc² predicted. Nuclear physics had entered the experimental age.

Cockcroft and Walton received the 1951 Nobel Prize in Physics 'for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles.' The generator that bears their names (sometimes called the Greinacher multiplier, acknowledging its inventor) remains in use today. Modern applications include particle accelerators, X-ray machines, and air ionizers. The simple principle of cascading voltage multiplication still powers devices that reach into realms where transformers cannot go.