You can see the alternating magnets in a wiggler insertion device here. See below for an explanation.
I went to an open day at the Diamond Light Source at Harwell in Oxfordshire. I'd been before but this was an even better tour because we got to go into the storage ring. A synchrotron light source is a ring particle accelerator that accelerates the electrons up to speed and then makes them change direction using magnetic fields to put forces on the electrons. Remember that current in a wire is a flow of electrons and creates a moving magnetic field. There is also an electric field around an electron. So by making the electrons change direction, you can make the moving electric and magnetic fields of an electromagnetic wave. Web sources say that what happens with the electrons in the synchrotron is pretty much the same principle as using moving electrons to make radio waves in a radio antenna. The difference is that the speed of the electrons is so big here that relativity theory is needed for the calculations. The synchrotron uses sets of strong magnets about as big as the keys on the piano. They are alternately placed to make the beam bend backwards and forwards. If they are in the horizontal plane they are called wigglers. In the vertical plane they are called undulators. The X-ray light will therefore be polarised either horizontally or vertically.
