# Stellarator Fusion Reactor How a stellarator uses twisted superconducting magnet coils to hold a 100\-million\-degree plasma, as seen in Germany's Wendelstein 7\-X reactor\. By Andi Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/LBRQoWDY6tvI) ## Transcript ### 1. A star in a magnetic bottle This is a stellarator, a machine built to do what the Sun does: join tiny atoms together and release energy\. The pink glow is a ring of gas hotter than the Sun\. The twisted grey hoops around it are magnets that hold it in place\. Drag to turn the model and look around\. The model is based on Wendelstein 7\-X in Germany, the largest stellarator in the world\. The real machine is about 16 metres across\. ### 2. Fusion needs fierce heat Fusion means squeezing the centres of light atoms, called nuclei, so close that they join and give off energy\. They push each other away, so they must move incredibly fast\. On Earth that takes gas at around 100 million degrees, far hotter than the centre of the Sun\. The Sun manages at about 15 million degrees because its enormous gravity squeezes its gas\. A machine on Earth has to make up for that with extra heat\. ### 3. A gas nothing can touch At that heat, atoms break apart into a glowing soup of charged particles called plasma\. No wall could hold it, so magnets do\. Charged particles are steered by a magnetic field: they spiral along its invisible lines like beads sliding on a wire\. The green line traces one magnetic field line\. The orange dots are hydrogen nuclei and the blue dots are electrons, drawn much larger than life\. ### 4. Why a plain ring leaks Bend the field into a simple ring and it is stronger on the inside than the outside\. That makes the particles slowly drift, nuclei one way and electrons the other, until they reach the wall\. Twist the field lines, and each particle takes turns drifting up and down, so the drifts cancel out\. Switch between the two and watch\. This twist is the key idea behind every machine of this kind\. ### 5. Coils that do the twisting In a stellarator, the twist comes only from the strange shapes of the magnet coils\. Wendelstein 7\-X has 50 wavy coils, in five identical sets of ten, plus 20 flat ones for fine tuning\. Each one had to be built to millimetre accuracy\. Its cousin, the tokamak, uses simpler coils and gets its twist by driving a huge electric current through the plasma itself\. ### 6. Ice\-cold magnets, red\-hot heart The coils are superconductors: cooled with liquid helium to minus 269 degrees, they carry electricity with no resistance at all\. A thick insulated shell called the cryostat keeps them cold, even though the hottest place in the solar system sits just inside them\. The cold part of the machine weighs about 425 tonnes, while the plasma inside it weighs less than a gram\. ### 7. Microwaves light the plasma The vessel is pumped almost empty and a tiny puff of hydrogen is let in\. Then powerful beams of microwaves heat its electrons, which heat everything else\. Switch on the heating and watch the plasma come to life\. Wendelstein 7\-X can heat its plasma with ten microwave beams of up to one megawatt each\. Frozen hydrogen pellets top up the fuel\. ### 8. Built to keep going Because its coils make the twist, a stellarator needs no current inside its plasma, so it could run steadily for a long time\. In 2025, Wendelstein 7\-X set a world record by holding its best plasma conditions for 43 seconds\. It makes no electricity yet; it is testing the magnetic cage for the power plants of the future\. In one run that year, the machine kept its plasma going for six minutes\.