Proton bunches: they coast inside the tubes and get pushed only in the gapsPositive tubeNegative tube: the two bars swap sign every half periodDrift tubes, cut open along the beam, wired alternately to the two RF barsRuler in cm; the bold ticks mark the ten gapsA Wideröe linac for protons, slowed about 24 million times. The ion source hands a bunch to the first gap every RF period at 50 keV. Each gap holds up to 120 kV, and the two bars swap sign every half period, so a bunch that crosses each gap exactly half a period after the last always meets a push. Inside a tube there is no field, so the bunch coasts while the sign flips; as it speeds up it covers more ground in that half period, and that is why every tube is longer than the one before (5.9 cm to 20.9 cm). Ten gaps take a proton from 0.05 to 1.25 MeV. Turn the dial off 30 MHz and the bunches reach the gaps at the wrong moment: above it they slip onto the falling side of the wave and leave with far less; below it the slowed bunches drift back toward the crest, so they lose less.