tesla is the SI unit of magnetic flux density — in everyday usage, magnetic field strength — equal to one weber of magnetic flux per square meter; fusion-grade high-field magnets reach roughly 20 tesla.
The unit is named for the electrical engineer Nikola Tesla and was adopted into the International System of Units in 1960. Its older near-neighbor is the gauss, still common in astronomy and materials work: one tesla equals 10,000 gauss. The scale runs steep — Earth's magnetic field measures about 50 microtesla, a hospital MRI magnet operates at 1.5 to 3 tesla, and the strongest steady laboratory fields sit in the mid-forties.
In magnetic confinement 📝fusion the tesla is the number the whole enterprise turns on, because 📝fusion power density in a 📝Tokamak scales roughly with the fourth power of field strength: doubling the field buys about sixteen times the power from the same 📝plasma volume. The 📝low-temperature superconductor magnets of earlier machines plateau around 12 to 13 tesla, while 📝High-Temperature Superconducting (HTS) Magnets wound from 📝REBCO reach roughly 20 — the step that shrinks a net-energy machine to the size of 📝SPARC.