A 'tokamak' is a device associated with:

A tokamak is a device designed to confine extremely hot plasma using powerful magnetic fields, in order to achieve controlled nuclear fusion. The word "tokamak" comes from a Russian acronym meaning "toroidal chamber with magnetic coils". It was invented by the Soviet physicists Igor Tamm and Andrei Sakharov in the 1950s, and the first tokamak was built in the Soviet Union in 1958.

For fusion to occur, the fuel, usually deuterium and tritium (isotopes of hydrogen), must be heated to temperatures of over 100 million °C, about ten times hotter than the core of the Sun. At such temperatures, the fuel becomes plasma, a state of matter made of charged particles. No material container can withstand such heat. In a tokamak, the plasma is held in a doughnut-shaped (toroidal) chamber by strong magnetic fields, which keep it away from the walls.

Important fusion projects:

ITER (International Thermonuclear Experimental Reactor), being built at Cadarache in southern France, is the world's largest tokamak. Its members are the European Union, India, China, Japan, Russia, South Korea and the USA. India joined in 2005 and contributes about 9% of the cost, supplying parts such as the cryostat, the world's largest stainless-steel vacuum chamber, made by Larsen and Toubro.

India's own tokamaks, ADITYA (1989) and SST-1 (Steady State Superconducting Tokamak, 2013), are at the Institute for Plasma Research (IPR) in Gandhinagar, Gujarat.

China's EAST tokamak and South Korea's KSTAR have set records for holding hot plasma for long periods.

Fusion is considered a possible future source of almost limitless clean energy, because its fuel is abundant (deuterium can be extracted from seawater) and it produces no greenhouse gases and little long-lived radioactive waste.

Why not the other options?

• ✗Nuclear fission reactors use control rods and moderators, not tokamaks.
• ✗Particle detection uses devices such as Geiger counters and cloud chambers.
• ✗Solar power generation uses photovoltaic cells.