Electrons in beam are repelled by negative charges and attracted by positive charges on walls.

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The passage of the electrons is perfectly synchronized with the oscillation of the standing waves.

When the electron bunch moves into the next cavity section, the charges on the walls have switched position, so that the electron bunch is always pushed forward.

Each electron in the bunch gains an energy equal to twice the rest mass energy of the electron (mc²=0.5 MeV) as it passes through each cavity ellipsoid.