RF Cavity Bells

The cells in section along the beam, with the accelerating passband's field in each, slowed down to a turn a second: in the π mode it points the other way in every next cell, so a bunch crossing a cell in half a period is pushed the same way in all of them. A bunch out of step, or a second bunch, leaves the cells unevenly lit; a quench turns the walls red.

The accelerating passband: each mode's frequency against its phase advance per cell, on the coupled cells' curve ω = ω_c/√(1 + k cos φ). A dot's size is how strongly that mode rings now; the dashed line is the beam, which rings the mode it keeps step with. The bars at the right are the higher-order passbands, TM011 to TM120, against the accelerating mode.

Each passband is a cluster of nine lines. The higher-order ones, damped by their couplers, die first and leave the accelerating mode; a held key's bunch train pulls every mode onto the train's harmonics.