In order to study ions, we need to hold them in one spot for extended periods of time.
For this we use an ion trap. Here you can see one such ion floating in an ion trap in our lab.
To truly trap an object, we need to control it so that if it rolls in any direction,
it will move back toward the center.
While we are dealing with charged particles and electric potentials [in the lab], we can imagine this problem as a ball rolling on a hill -- a gravitational potential.
A good trap would be a bowl. No matter where the ball is, it is pushed back toward the center.
That's fine for a ball on a hill, but
electromagnetism prevents us from making such a bowl for charged particles.
The best we can do is have a bowl in one direction and an "anti-bowl" -- or a hill -- in the other.
While the bowl clearly traps the particle, the hill just pushes the particle off to one side. It's an unstable equilibrium.
This shape is described as a saddle, shown here.
We 3d printed the saddle and are using a ping-pong ball as our ion.
When placed on the saddle the ball just rolls down the downhill portion.
If we then spin the saddle, however,
as the ball begins to roll down the downhill portion the uphill swings into place to catch it.
The ball is now trapped. The saddle has to be spun fast enough to catch the ball,
but not so fast that it kicks the ball out of the trap.
Using this method, albeit with electric potentials instead of a surface, we're able to trap and study ions for long periods of time.
