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Nuclear fusion is a process where two atomic nuclei combined to form heavier nuclei, releasing a large amount of energy in the process. This process occurs at extremely high temperatures because the positively charged atomic nuclei (or ions) need to overcome their electrostatic repulsion. Electrostatic repulsion is the force that prevents two objects with the same charge from coming together. At high temperatures, the ions collide with more energy and are more likely to overcome their electrostatic repulsion, thus getting close enough for the strong nuclear force to bind them together.

Nuclear fusion, as it occurs in the core of the sun, requires temperatures of about 15 million degrees Celsius. At these temperatures, the gas becomes a plasma consisting of free electrons and positively charged ions. When the plasma reaches sufficient temperature, density, and confinement time, a self-sustaining fusion reaction occurs where the energy produced generates enough heat to keep the reaction going. In order to achieve controlled nuclear fusion on Earth, even higher temperatures (100 million degrees Celsius or more) are required due to the less favorable conditions compared to the sun's core.

A tokamak reactor is a device designed to confine and control the hot plasma necessary for nuclear fusion. The plasma is held by magnetic fields created by external coils of wire and the current flowing within the plasma. The magnetic fields prevent the plasma from making direct contact with the reactor walls, thus maintaining the high temperatures required for fusion. In a tokamak reactor, the plasma is shaped like a torus (a donut shape) to allow a continuous flow of particles, maintaining stability, and avoiding any disruption of the fusion process.

Heat is contained within the tokamak reactor by using several techniques: 1. Magnetic confinement: The magnetic field confines the plasma, preventing it from making contact with the reactor walls. This confinement minimizes heat loss and ensures that the temperature remains high. 2. Heating systems: Additional heating methods, such as neutral beam injection, radiofrequency heating, or microwave heating, can provide external energy to the plasma, ensuring it maintains the necessary high temperature for fusion reactions to occur. 3. Continuous fueling: The constant addition of fusion fuel, such as deuterium and tritium, sustains the reaction and helps maintain the required high temperature. By combining these techniques, tokamak reactors can maintain the necessary high temperatures for nuclear fusion to occur, paving the way for controlled fusion energy production on Earth.