Cockcroft-Walton Voltage Multiplier - STEM2

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cles to study the atomic nucleus. They used an early particle acceleratorwhich generated high voltage electric fields using a voltage multiplier, a cascadeof capacitors and rectifiers. These high voltage generators are calledCockcroft-Walton voltage multipliers and are used today to generate highvoltage electrostatic fields for such applications as laser printers and copiers.2 The Cockcroft-Walton AcceleratorFrom a document from the American Institute of Physics:http://www.aip.org/history/lawrence/epa.htmThe Cockcroft-Walton AcceleratorJohn D. Cockcroft and Ernest Walton at the Cavendish Laboratory inCambridge, England, sought a way into the nucleus through a prediction ofquantum mechanics. George Gamow had suggested that a particle with toolittle energy to overcome the electrical repulsion of the nucleus through thebarrier. (The trick was that the energy of the particle was not actually welldefined,according to Heisenberg’s Uncertainty Principle). In 1930 Cockcroftand Walton used a 200-kilovolt transformer to accelerate protons down astraight discharge tube, but they concluded that Gamow’s tunnelling did notwork and decided to seek higher energies.To penetrate the nucleus, Cockcroft and Walton built a voltage multiplierthat used an intricate stack of capacitors connected by rectifying diodes asswitches. By opening and closing switches in proper sequence they could buildup a potential of 800 kilovolts from a transformer of 200 kilovolts. They usedthe potential to accelerate protons down an evacuated tube eight feet long. In1932 they put a lithium target at the end of the tube and found that protonsdisintegrated a lithium nucleus into two alpha particles. A Soviet team inKharkov found the same result several months later.The original Cockcroft-Walton voltage multiplier used mechanical switchesin place of rectifiers, which I thing is what we see in the machine on displayat the London Museum of Science.3 The Villard Doubler CircuitThe input voltage to this circuit is an alternating voltage of amplitude V .This could be a sinusoidal AC voltage, or a square wave voltage. The capac-2
A B C D EV2V 4V 6V 8VFigure 1: Voltage Multiplier Circuits. Upper left: The Villard DoublerCircuit. Upper right: The Greinacher Doubler Circuit. Lower: TheCockcroft-Walton Voltage Multiplier Circuit. The Cockroft-Walton Multiplieris constructed by repeatedly adding 3 the circuit elements shown in thecenter. The voltages at points A, B, C, D and E are stepped voltages. Theoutput voltages at the bottom nodes are DC voltages. The ideal voltage atthe extreme lower right node is a DC voltage of 8V , where V is the amplitudeof the input AC voltage. The minimum of the stepped voltage at E is 6V ,the maximum 8V .
Page 1: Cockcroft-Walton Voltage Multiplier
Page 5 and 6: itor in this circuit will charge to
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Cockcroft-Walton Voltage Multiplier - STEM2

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