cyclotron
/ SY-kluh-tron /
Suppose you want to give a particle many pushes but you do not have room for a kilometre-long corridor. The clever trick the cyclotron uses is to make the particle go in circles, so it passes the same accelerating gap over and over. Invented in 1930 by Ernest Lawrence, the cyclotron was the first machine to coil a particle's path into a compact spiral, and its descendants still run in hospitals and labs today.
Here is how it works. A magnet fills the machine with a steady vertical field, which bends the path of any charged particle into a circle. In the middle there is one accelerating gap fed by an oscillating electric field. Each time the particle crosses the gap it gets a kick and speeds up; going faster, it swings out into a slightly bigger circle, so its path traces an ever-widening spiral from the centre outward. The beautiful coincidence that makes the simplest cyclotron work is that, for a given magnet, the time to go once around stays the same no matter how big the circle gets — so a fixed-rhythm electric field keeps perfect time with the particle automatically.
That elegant coincidence is also the cyclotron's ceiling. Once a particle approaches the speed of light, relativity makes it effectively heavier, the timing slips, and the simple cyclotron falls out of step. Fixes exist (varying the field or the rhythm), but for truly high energies physicists moved to the synchrotron. Still, cyclotrons remain workhorses for moderate energies: they are the standard machines that make the short-lived radioactive isotopes used in PET medical scans and in many cancer treatments.
A hospital cyclotron the size of a small room spirals protons outward and then slams them into a target to manufacture fluorine-18, the short-lived isotope that lights up tumours in a PET scan.
One gap used many times: the particle spirals outward as it speeds up.
The simple cyclotron stops working at high energy because relativity makes a near-light-speed particle behave as if heavier, throwing off the timing; this very limitation is what motivated the synchrotron, which adjusts as the beam speeds up.