Nuclear Fusion - Bringing the Sun to Earth

Essay, 2011

10 Pages



1. Introduction

2. Nuclear fusion in sun

3. Nuclear fusion on earth
3.1 Fusion reactions
3.2 The conditions of ignition

4. The magnetic confinement
4.1 The Tokamak
4.2 The Stellarator

5. Nuclear fusion as energy source
5.1 The fusion power plant
5.2 Safety and environmental issues

6. The progress of fusion research

1. Introduction

The dream that a bathtub of water and 100 g lithium could supply a family for 50 years with electricity1 stimulated scientists since the 1940s all over the world to make every effort to construct a working fusion reactor that uses the most fundamental of all energy sources: the nuclear fusion that fuels sun.

However, the history of fusion research started a century earlier when scientists were trying to understand the process that fuels the sun.

2. Nuclear fusion in sun

A long time, it was an unsolved riddle to scientists, how the sun produces its energy. The sun can’t simply be made of stone coal and oxygen, otherwise it have existed for about 5000 years.2 Hermann von Helmholtz (1821-1894) proposed that it could heat up by consistent shrinking because gravitational energy can be converted into heat by contraction. If however the sun would have emerged from an interstellar nebula and would have contracted to its contemporary size, it would be roughly 22 million years old and geologists have proved that the earth is more than four billion years old and the sun can’t be younger than its planet.3 In 1920, Sir Arthur Stanley Eddington wondered wether the energy of stars was produced by fusion of hydrogen atoms to heavier atoms. And this principle of nuclear fusion is what we nowadays assume to be the motor of the stars including our sun.

In the core of the sun, the temperature is about 15 million °C and there is a pressure of 134 g cm-3. The temperature quickly decreases to the outside: the radiative zone has a temperature of 3 million °C, the photosphere of 5500 °C. The average density of the sun is 1.4 g cm-3, so most of the mass is concentrated in the core. Only in the core the conditions are such that nuclear fusion can take place as very high temperature and pressure is needed to let particles collide very often which allows their fusion.

illustration not visible in this excerpt

Fig. 1: The proton-proton cycle.

Per cycle, 26.7 MeV are released. The conversion of a single gram hydrogen to helium gives an energy of 280,000 kWh. To maintain its luminosity, four million tons of matter are used for nuclear fusion each second. One more million ton is lost by the solar wind, a flow of charged particles such as protons, α-particles (a4 He nucleus) and electrons that emanates from the sun. Despite these large quantities, the sun has not even lost 1% of its mass since its origin five billion years ago.

The release of energy by nuclear fusion can be explain using the relativity equation E=Δmc2, where Δm is the mass loss and c the speed of light. A proton has a mass of 1.007825 u, a4 He nucleus has a mass of 4.002603 u and the mass difference between four hydrogen nuclei and an α-particle is 0.028697 u. Using E=Δmc2, the energy released is 26.57 MeV. The energy released is given in MeV, mega electron volts. 1 eV is equal to the amount of kinetic energy given to a single unbound electron when it accelerates through 1 volt of electric potential difference.4 The e is just the elementary charge, therefore 1 eV = 1.6×10-19 J and the units cancel as follows:[Abbildung in dieser Leseprobe nicht enthalten]

3. Nuclear fusion on earth

In the late 1940’s scientists began to investigate if it was possible to use the nuclear fusion, that had been discovered to be the sun’s fuel, as an energy source on earth.

3.1 Fusion reactions

The source of fusion energy is the binding energy of the atoms. Splitting the nucleus of an atom into smaller parts requires energy; if two or more nuclei join together, energy is released. The most stable nuclei are those of iron, cobalt, nickel and copper which have an atomic mass of around 60. Nuclear fission of very heavy nuclei such as uranium, atomic mass 235, and fusion of light nuclei such as hydrogen and its isotopes2 H (deuterium) and3 H (tritium) can be used as energy sources. Nuclear fission is used since the early1950s.

illustration not visible in this excerpt

D stands for deuterium, T for tritium, n for a neutron and p for a proton. The fusion of deuterium and tritium is the one with the highest reaction rate at the lowest temperature and has therefore the largest energy spoil.


1 Die Fusion dringt zum Kern vor, MaxPlanckForschung, No. SP, 2010, pp. 6-11, German

2 Keller, Hans-Ulrich: Das hei ß e Herz der Sonne, in: Kosmos Himmels Jahr 2010, pp. 153-161

3 John Wesson: The science of JET, 2000

4 Dictionary of Physics, Oxford University Press, 2005

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Nuclear Fusion - Bringing the Sun to Earth
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David Brückner (Author), 2011, Nuclear Fusion - Bringing the Sun to Earth, Munich, GRIN Verlag,


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