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How did CERN's Intersecting Storage Rings improve the energy available for proton collisions?

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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.

CERN's Intersecting Storage Rings (ISR), which began operation in 1971, improved the energy available for proton collisions by utilizing a colliding-beam technique rather than firing a beam at a stationary target.1

When a particle beam strikes a stationary target, a significant portion of the beam's kinetic energy is wasted on the forward motion of the resulting particles after the collision. By using two interlaced rings to store protons moving in opposite directions, the ISR allowed the beams to collide head-on. In this configuration, the total kinetic energy of both beams is available for the interaction at the moment of collision.21

Specifically, the ISR stored two beams of protons at 31 GeV each, resulting in a total collision energy of 62 GeV. This provided an interaction energy equivalent to what would be required if a stationary target were struck by a beam of 2 TeV, demonstrating the massive efficiency gains achieved by colliding two moving beams.1