When hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). If you've ever tried to place two magnets together and felt them push apart from each other, you've experienced this principle first-hand.
To achieve fusion, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:
High temperature - The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons.
- Fusion requires temperatures about 100 million Kelvin (approximately six times hotter than the sun's core).
- At these temperatures, hydrogen is a plasma, not a gas. Plasma is a high-energy state of matter in which all the electrons are stripped from atoms and move freely about.
- The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core. We must use energy from microwaves, lasers and ion particles to achieve these temperatures.
High pressure - Pressure squeezes the hydrogen atoms together. They must be within 1x10-15 meters of each other to fuse.
- The sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core.
- We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams.
With current technology, we can only achieve the temperatures and pressures necessary to make deuterium-tritium fusion possible. Deuterium-deuterium fusion requires higher temperatures that may be possible in the future. Ultimately, deuterium-deuterium fusion will be better because it is easier to extract deuterium from seawater than to make tritium from lithium. Also, deuterium is not radioactive, and deuterium-deuterium reactions will yield more energy.