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How does a Van de Graaff generator create a very high voltage?

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Diagram of a linear proton resonance accelerator, or linac, showing protons entering from a source, passing through a series of labeled drift tubes powered by a radio-frequency source, and exiting as a beam through a cavity. — particle accelerator The accelerator is a large-diameter tube within which an electric field oscillates at a high radio frequency. Within the accelerator tube are smaller diameter metallic drift tubes, which are carefully sized and spaced to shield the protons from decelerating oscillations of the electric field. In the spaces between the drift tubes, the electric field is oriented properly to accelerate the protons in their direction of travel.

A Van de Graaff generator creates high voltage by continuously transporting electric charge to a hollow, spherical high-voltage terminal using a rapidly moving belt of insulating material.123

The process functions as follows:
* Charging: A power supply charges a comb of sharp needles near the grounded end of the belt. The resulting intense electric field causes ionization of the surrounding gas, creating a corona discharge that deposits ions onto the belt.2
* Transport: The moving belt carries this charge into the interior of the large, spherical metal shell.13
* Accumulation: Inside the terminal, a second comb of needles removes the charge from the belt and transfers it to the outer surface of the sphere.23

The shell continues to accumulate charge and increase in potential until it reaches a limit defined by the load current or an electrical breakdown. When insulated by pressurized gas, these devices can achieve potentials of approximately 20 megavolts.32