Tokamak

A doughnut-shaped fusion device that confines plasma hotter than 100 million °C with strong, twisted magnetic fields.

• Fusing hydrogen isotopes needs about 150 million °C; the plasma, a gas of charged particles, is kept off the walls by magnetic fields.
• Coils around the ring plus a current flowing in the plasma itself twist the field into a helix, so particles circle without drifting out.
• It is the most studied fusion design; Europe's JET set a record of 69 megajoules of fusion energy in 2023, and ITER in France is the largest being built.
• Challenges: sudden 'disruptions' that dump energy into the walls, intense heat on the exhaust area, and running continuously rather than in pulses.