Thermonuclear Fusion

Wahyu Yon
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Wahyu Yon
► PHYSICS
thermonuclear fusion

Nuclear fusion is a reaction in which two or more atomic nuclei
are combined to form one or more different atomic nuclei
and subatomic particles (neutrons or protons).

The difference in mass between the reactants and products is manifested
as either the release or absorption of energy.

This difference in mass arises due to the difference in nuclear binding energy between the atomic nuclei before and after the reaction.

Nuclear fusion is the process that powers active or main sequence stars
and other high-magnitude stars, where large amounts of energy are released.

A nuclear fusion process that produces atomic nuclei lighter than iron-56
or nickel-62 will generally release energy.

These elements have a relatively small mass and a relatively large binding energy
per nucleon.

Fusion of nuclei lighter than these releases energy (an exothermic process),
while the fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is endothermic.

The opposite is true for the reverse process, called nuclear fission.

Nuclear fusion uses lighter elements, such as hydrogen and helium,
which are in general more fusible; while the heavier elements, such as uranium, thorium and plutonium, are more fissionable.

The extreme astrophysical event of a supernova can produce enough energy
to fuse nuclei into elements heavier than iron.

In 1920, Arthur Eddington suggested hydrogen-helium fusion
could be the primary source of stellar energy.

Quantum tunneling was discovered by Friedrich Hund in 1929,
and shortly afterwards Robert Atkinson and Fritz Houtermans
used the measured masses of light elements to show that large amounts
of energy could be released by fusing small nuclei.

Building on the early experiments in artificial nuclear transmutation
by Patrick Blackett, laboratory fusion of hydrogen isotopes
was accomplished by Mark Oliphant in 1932.

In the remainder of that decade, the theory of the main cycle of nuclear fusion
in stars was worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project.

Self-sustaining nuclear fusion was first carried out on 1 November 1952,
in the Ivy Mike hydrogen (thermonuclear) bomb test.

Research into developing controlled fusion inside fusion reactors
has been ongoing since the 1940s, but the technology is still
in its development phase.

Nuclear fusion in stars

An important fusion process is the stellar nucleosynthesis that powers stars, including the Sun. In the 20th century, it was recognized that the energy
released from nuclear fusion reactions accounts for the longevity of stellar heat
and light.

The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei.

Different reaction chains are involved, depending on the mass of the star
(and therefore the pressure and temperature in its core).

Around 1920, Arthur Eddington anticipated the discovery and mechanism
of nuclear fusion processes in stars, in his paper

The Internal Constitution of the Stars.[9][10]

At that time, the source of stellar energy was a complete mystery;
Eddington correctly speculated that the source was fusion of hydrogen
into helium, liberating enormous energy according to Einstein's equation E = mc2.

This was a particularly remarkable development since at that time fusion
and thermonuclear energy had not yet been discovered, nor even that stars
are largely composed of hydrogen (see metallicity).

Eddington's paper reasoned that:

1_The leading theory of stellar energy, the contraction hypothesis,
should cause stars' rotation to visibly speed up due to conservation
of angular momentum.

But observations of Cepheid variable stars showed this was not happening.

2_The only other known plausible source of energy was conversion
of matter to energy;

Einstein had shown some years earlier that a small amount of matter
was equivalent to a large amount of energy.

3_Francis Aston had also recently shown that the mass of a helium atom
was about 0.8% less than the mass of the four hydrogen atoms
which would, combined, form a helium atom
(according to the then-prevailing theory of atomic structure which held atomic weight to be the distinguishing property between elements;
work by Henry Moseley and Antonius van den Broek would later show
that nucleic charge was the distinguishing property and that a helium nucleus, therefore, consisted of two hydrogen nuclei plus additional mass).

This suggested that if such a combination could happen,
it would release considerable energy as a byproduct.

4_If a star contained just 5% of fusible hydrogen,
it would suffice to explain how stars got their energy.
(We now know that most 'ordinary' stars
contain far more than 5% hydrogen.)

5_Further elements might also be fused, and other scientists
had speculated that stars were the "crucible" in which light elements
combined to create heavy elements, but without more accurate measurements
of their atomic masses nothing more could be said at the time.

All of these speculations were proven correct in the following decades.

The primary source of solar energy, and that of similar size stars,
is the fusion of hydrogen to form helium
(the proton–proton chain reaction),
which occurs at a solar-core temperature of 14 million kelvin.


The net result is the fusion of four protons into one alpha particle,
with the release of two positrons and two neutrinos
(which changes two of the protons into neutrons), and energy.

In heavier stars, the CNO cycle and other processes are more important.

As a star uses up a substantial fraction of its hydrogen,
it begins to synthesize heavier elements.

The heaviest elements are synthesized by fusion that occurs
when a more massive star undergoes a violent supernova
at the end of its life, a process known as supernova nucleosynthesis.