Tokamaks: Holding Star-Hot Fuel in a Magnetic Doughnut
1/ How do you hold fuel at 100 million degrees without wrecking its container? You keep the hottest part away from the walls. A machine called a tokamak uses twisted magnetic fields to do it. Its goal: energy from joining atoms’ tiny centers. ☀️
2/ Meet two possible fuels: deuterium and tritium, heavier forms of hydrogen. Each has one proton at its center. Deuterium adds one neutron; tritium adds two. When their centers join, they release energy, a helium center and a neutron. That’s fusion.
3/ Heat the fuel enough and electrons break free from atoms. You get plasma: a gas of electrically charged particles. Magnetic fields bend their paths into spirals around invisible field lines. Stronger fields make those spirals tighter.
4/ Why a doughnut? Imagine joining the ends of a racetrack so runners never reach a dead end. A tokamak’s doughnut shape lets magnetic paths loop around. Large coils make this looping field—but by itself, it cannot hold the plasma well.
5/ The missing ingredient is a twist. The doughnut’s uneven, curved field makes particles drift. An electric current through the plasma adds a second field, twisting the paths around the doughnut. This helps drift effects partly cancel along the trip.
6/ Picture invisible doughnut-shaped layers nested inside one another. The twisted magnetic paths lie on these layers. Particles move easily along them but much less easily across them. Keeping particles from crossing layers is the heart of confinement.
7/ Keeping plasma steady is a balancing act. Hot plasma pushes outward; magnetic forces balance that push. Extra coils shape and position it, often into a D-shaped cross-section. Sensors and fast controls adjust the fields when the plasma starts moving.
8/ Your magnetic cage still leaks. Collisions and tiny swirls can carry heat and particles across its layers. Bigger wobbles can cause a sudden loss of confinement, called a disruption. Scientists must control both the little swirls and the big shakes.
9/ Even a good cage needs an exhaust. A device called a divertor guides escaping particles along edge magnetic paths to tough plates. It handles escaping heat and helps remove helium made by fusion and unwanted material that could cool the fuel.
10/ Magnets hold the fuel; heating systems make it hot. Electric current, beams of fast atoms and powerful radio waves add energy. Once enough fusion happens, the charged helium centers it creates can help heat the plasma from inside.
11/ Remember the neutron fusion releases? It has no electric charge, so magnets cannot trap it. In a future power plant, it would heat a surrounding “blanket.” Lithium there could also make fresh tritium, which is rare and radioactive.
12/ Would that heat go straight into water? Usually, a coolant would carry it from the blanket to a system that makes electricity—perhaps using steam and a turbine. Today’s tokamaks are research machines. The goal: turn a magnetic doughnut into a power source.