How does an actual quadrupole work?
The image above shows a quadrupole that is ready to be installed in the SPS machine. You can see the shape of the poles, and the
coil where the current flows, creating the magnetic field between the poles.
We said earlier that the nature of the magnetic field implied that if a quadrupole was horizontally focusing it would be
vertically defocusing. Indeed, here is a simulation of a quadrupole which will vertically focus the positive particles entering the plane of the drawing.
Move the mouse pointer around between the poles and see how the direction and intensity of the magnetic field (green line) and
of the magnetic force (yellow line) vary, depending on the position in the quadrupole.
You will see that
- a horizontal offset will result in a force that will drag the particle even further away from the center
- a vertical offset results in a vertically restoring force
- a particle which arrives at the center of the quadrupole experiences no field, hence no force
Question: How would you make a quadrupole to horizontally focus the positive particles?
Answer: at the bottom of the page
If you rotate the quadrupole by ninety degrees along its axis, an N pole will replace an S pole and vice-versa. A horizontal offset will result in a
restoring horizontal force. You can experiment by putting your computer monitor on its side :)
last update on Friday, December 19, 1997 - 2:25:58 PM