Cyclotrons accelerate charged particles using oscillating electric fields generated between hollow metal chambers called dees. A positive ion is placed between the dees and accelerated toward the first dee when it is negatively charged. The magnetic field causes the ion to travel in a semicircular path, and it is accelerated again when the dee polarities reverse just as the ion reaches the gap. This process continues to accelerate the ion with each pass. Cyclotrons can be used to accelerate proton beams for nuclear physics experiments and cancer treatment through proton therapy.