The Alternating Gradient Synchrotron (AGS) is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, United States.

The AGS became the world's premiere accelerator when it reached its design energy of 33 billion electron volts (GeV) on July 29, 1960.

Until 1968, the AGS was the highest energy accelerator in the world, slightly higher than its 28 GeV sister machine, the Proton Synchrotron at CERN, the European laboratory for high-energy physics.

It became increasingly clear that if further progress was to be made in high energy nuclear physics by experiments using artificially accelerated particles some new principle must be found that would cheapen the cost per GeV.

A cheaper machine could be built if the amplitudes of the free and forced oscillations of the accelerating particles could be decreased in some way so that the vacuum chamber size and the cross-section of the magnet ring could be reduced.