What must be present in order for a fusion to occur?

Answer 1

In fusion, there needs to enough energy to overcome the Coulomb forces of the particles you are trying to fuse, and get the particles close enough that nuclear attraction takes over. This is the short answer.

Answer 2

Fusion conditions

How to light a fire:

The most efficient fusion fuel is a mixture of deuterium and tritium

• Temperature: between 100 and 200 million degrees Celsius
• Density: approximately 1 milligram per m3(about one millionth of the density of air)
• Time: maintain these conditions for as long as possible! Breakeven energy occurs If particles are confined in the plasma for longer than a couple of seconds.

Fusion releases an enormous amount of energy throughout the universe, but the challenge is to create conditions similar to those in a star here on an earthly scale.

Fuel: A fusion power plant will choose only one of the many processes going on in the sun; it turns out that the most suitable reaction for an earth-bound fusion is between two isotopes of hydrogen, deuterium and tritium. Deuterium is normal hydrogen (a proton) with an added neutron, and tritium is hydrogen with two added neutrons. When these two nuclei combine, a neutron gets shot out at high speed. This hot neutron can then be captured, and its energy used just as in a conventional powerplant, e.g. to heat water to turn a turbine. The other product of the reaction is a helium nucleus (two protons and two neutrons), which ends up with about one quarter of the energy that the neutron carries. Because the helium nuclei are charged they cannot escape the magnetic field that confines the plasma, unlike the neutrons.

Temperature: Deuterium-tritium fusion might be the most efficient reaction, but it still requires temperatures in excess of 100 million degrees. To achieve these remarkable temperatures, four separate heating systems are in use on JET, each capable of delivering well over a million watts of power to the fuel. Together they generate and sustain plasma that is easily hot enough for the high energy collisions required for fusion to occur.

Density: The constituents of a 100 million-degree plasma are moving about really fast, and, if left alone would soon be so far apart as to render collisions extremely unlikely. To keep the density of the plasma high enough to ensure collisions do actually occur, the plasma vessel is surrounded by huge electromagnets. These create magnetic fields 10,000 times stronger than the Earth's magnetic field and confine the plasma to perpetually circulating within the ring-shaped vessel. However if the plasma gets too dense then collisions of a different kind - between nuclei and electrons - begin to create large amounts of radiation. This radiation, called bremsstrahlung, saps energy from the plasma and prevents fusion from occurring - the optimum density value is around one millionth of the atmosphere.

Time: Eighty percent of the fusion energy is carried away by the neutrons, but the 20% carried by the helium nuclei remains in the plasma. The newly formed helium ricochets around the vessel colliding with unburnt fuel nuclei, heating them up, thereby reducing the need for the external heating systems. However it takes time for this to happen - depending on the density and temperature of the plasma.

Plasma physicists combine these three parameters - temperature, density and time - by multiplying them together to form what is known as the fusion product (or triple product).

At a certain value of the fusion product, called ignition, the reaction becomes self-sustaining: the heat generated by the reaction is enough to keep the plasma hot and so the external heating systems can be turned off.

In practice, however, reactors would not be run this hot. Even below the ignition conditions the energy carried by the neutrons provides output well above break-even, but the added control resulting from having a plasma that can be easily turned off is very advantageous.

Answer 3

Extremely high temperatures and extremely high pressures. And by extremely high I mean that we have not been able to sustain such an environment on Earth and probably will not be able to sustain such an environment for at least another 50 or 100 years.

We "can" do it. We just can't sustain it. The tokamak (ITER) is not scheduled for first plasma until 2019, and that will probably not be for more than a few seconds.

Answer 4

nuclear fusion begins when the temperature and pressure rise high enough to force two hydrogen nuclei together to form a single helium nuclei, this process releases massive mounts of energy, an order ( several orders?) of magnitude larger then the energy input to begin the process.

Answer 5

• high temperature
• high pressure
• a fuel that can undergo fusion