Wednesday, August 5, 2015

Fusion

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Fusion
Multiple Responses:
1.
WHAT IS FUSION?
Fusion is the process that powers the sun and the stars. It is the reaction in which two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process some of the mass of the hydrogen is converted into energy. The easiest fusion reaction to make happen is combining deuterium (or “heavy hydrogen) with tritium (or “heavy-heavy hydrogen”) to make helium and a neutron. Deuterium is plentifully available in ordinary water. Tritium can be produced by combining the fusion neutron with the abundant light metal lithium. Thus fusion has the potential to be an inexhaustible source of energy.

To make fusion happen, the atoms of hydrogen must be heated to very high temperatures (100 million degrees) so they are ionized (forming a plasma) and have sufficient energy to fuse, and then be held together i.e. confined, long enough for fusion to occur. The sun and stars do this by gravity. More practical approaches on earth are magnetic confinement, where a strong magnetic field holds the ionized atoms together while they are heated by microwaves or other energy sources, and inertial confinement, where a tiny pellet of frozen hydrogen is compressed and heated by an intense energy beam, such as a laser, so quickly that fusion occurs before the atoms can fly apart.

Who cares? Scientists have sought to make fusion work on earth for over 40 years. If we are successful, we will have an energy source that is inexhaustible. One out of every 6,500 atoms of hydrogen in ordinary water is deuterium, giving a gallon of water the energy content of 300 gallons of gasoline. In addition, fusion would be environmentally friendly, producing no combustion products or greenhouse gases. While fusion is a nuclear process, the products of the fusion reaction (helium and a neutron) are not radioactive, and with proper design a fusion power plant would be passively safe, and would produce no long-lived radioactive waste. Design studies show that electricity from fusion should be about the same cost as present day sources.

We’re getting close! While fusion sounds simple, the details are difficult and exacting. Heating, compressing and confining hydrogen plasmas at 100 million degrees is a significant challenge. It has taken a lot of science and engineering research to get fusion developments to where they are today. Both magnetic and inertial fusion programs are conducting experiments to develop a commercial application. If all goes well, commercial application should be possible by about 2020, providing humankind a safe, clean, inexhaustible energy source for the future.

2.
What is Fusion?
Fusion is the energy source of the Universe, occuring in the core of the Sun and stars. (Click to view larger version...)
Fusion is the energy source of the Universe, occuring in the core of the Sun and stars.

Fusion is the process at the core of our Sun. What we see as light and feel as warmth is the result of a fusion reaction: hydrogen nuclei collide, fuse into heavier helium atoms and release tremendous amounts of energy in the process.

The gravitational forces at play in the Universe have created the perfect conditions for fusion. Over billions of years, these forces caused the hydrogen clouds of the early Universe to gather into massive stellar bodies. In the extreme density and temperature of their cores, fusion occurs.

How does fusion produce energy?
 (Click to view larger version...)
Atoms never rest: the hotter they are, the faster they move. In the core of our Sun, temperatures reach 15,000,000° Celsius. Hydrogen atoms are in a constant state of agitation, colliding at very great speeds. The natural electrostatic repulsion that exists between the positive charges of their nuclei is overcome, and the atoms fuse.  The fusion of light hydrogen atoms (H-H) produces a heavier element, helium.

The mass of the resulting helium atom is not the exact sum of the two initial atoms, however—some mass has been lost and great amounts of energy have been gained. This is what Einstein's formula E=mc² describes: the tiny bit of lost mass (m), multiplied by the square of the speed of light (c²), results in a very large figure (E), which is the amount of energy created by a fusion reaction.

Every second, our Sun turns 600 million tons of hydrogen into helium, releasing an enormous amount of energy. But without the benefit of gravitational forces at work in our Universe, achieving fusion on Earth has required a different approach.

Fusion on Earth
Three, two, one ... We have plasma! Inside the European JET Tokamak, both before and during operation. Photo: EFDA, JET. (Click to view larger version...)
Three, two, one ... We have plasma! Inside the European JET Tokamak, both before and during operation. Photo: EFDA, JET.

Twentieth-century fusion science has identified the most efficient fusion reaction to reproduce in the laboratory setting: the reaction between two hydrogen (H) isotopes deuterium (D) and tritium (T).  The D-T fusion reaction produces the highest energy gain at the 'lowest' temperatures. It requires nonetheless temperatures of 150,000,000° Celsius to take place—ten times higher than the H-H reaction occurring at the Sun's core.

At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma—a hot, electrically charged gas. In a star as in a fusion device, plasmas provide the environment in which light elements can fuse and yield energy.

 (Click to view larger version...)
In ITER, the fusion reaction will be achieved in a tokamak device that uses magnetic fields to contain and control the hot plasma. The fusion between deuterium and tritium (D-T) will produce one helium nucleus, one neutron, and energy.

The helium nucleus carries an electric charge which will respond to the magnetic fields of the tokamak and remain confined within the plasma. However, some 80 percent of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, transferring their energy to the walls as heat.

In ITER, this heat will be dispersed through cooling towers. In the subsequent fusion plant prototype DEMO and in future industrial fusion installations, the heat will be used to produce steam and—by way of turbines and alternators—electricity.

3.
What is Fusion?
Fusion is the reaction where two lighter atoms join to form a heavier one. When under the right conditions two nuclei fuse together, a remarkable thing can happen. The total mass of the resulting fused nucleus is just slightly less than the separate masses of its components, and the difference is released as energy. This makes fusion very powerful. It is the reaction that powers the sun and that occurs naturally in all stars.

The simplest fusion reaction starts with a Hydrogen nucleus, a single proton. Protons have positive electric charge and repel each other. But if you squeeze them together hard enough, the 'strong' binding force overcomes the 'weak' electrical repulsion, and the two protons fuse to form a hydrogen isotope deuterium.
A slightly more complicated fusion reaction can take place between deuterium and tritium - rare isotope of hydrogen. The fusion of deuterium with tritium is particularly efficient in the conditions we can achieve in fusion reactors. As can be seen in the illustration on the right, fusion of deuterium and tritium creates helium-4, thereby freeing a neutron, and releasing 17.59 MeV of energy.
The power that is released when fusing two nuclei has led to many experiments in the laboratory. Scientists from all over the world develop fusion power on earth, as it offers the prospect of a long-term, safe, environmentally benign energy option to meet the energy needs of a growing world population. We have now succeeded in achieving fusion reactions, but it has proven difficult to control the reaction enough to make it a practical source of electrical power.
So what is so difficult about harvesting energy from a fusion reaction? As we mentioned before, to make fusion happen, two nuclei must come very close together. That only happens when they collide with a very high speed, which means that the temperature of the gas must be very high - so high that is becomes a Plasma.
The hot plasma needs to be confined, but the temperature of the ions is so high that there is no possibility to contain it in any normal vessel, as it would collide with the wall and cool down. To overcome this problem, people started to experiment with intricate methods of confining plasma.

4.
With its high energy yields, low nuclear waste production, and lack of air pollution, fusion, the same source that powers stars, could provide an alternative to conventional energy sources. But what drives this process?

What is fusion?
Fusion occurs when two light atoms bond together, or fuse, to make a heavier one. The total mass of the new atom is less than that of the two that formed it; the "missing" mass is given off as energy, as described by Albert Einstein's famous "E=mc2" equation.

In order for the nuclei of two atoms to overcome the aversion to one another caused their having the same charge, high temperatures and pressures are required. Temperatures must reach approximately six times those found in the core of the sun. At this heat, the hydrogen is no longer a gas but a plasma, an extremely high-energy state of matter where electrons are stripped from their atoms.

Fusion is the dominant source of energy for stars in the universe. It is also a potential energy source on Earth. When set off in an intentionally uncontrolled chain reaction, it drives the hydrogen bomb. Fusion is also being considered as a possibility to power crafts through space.

Fusion differs from fission, which splits atoms and results in substantial radioactive waste, which is hazardous.

Cooking up energy
There are several "recipes" for cooking up fusion, which rely on different atomic combinations.

Deuterium-Tritium fusion: The most promising combination for power on Earth today is the fusion of a deuterium atom with a tritium one. The process, which requires temperatures of approximately 72 million degrees F (39 million degrees Celsius), produces 17.6 million electron volts of energy.

Deuterium is a promising ingredient because it is an isotope of hydrogen, containing a single proton and neutron but no electron. In turn, hydrogen is a key part of water, which covers the Earth. A gallon of seawater (3.8 liters) could produce as much energy as 300 gallons (1,136 liters) of gasoline. Another hydrogen isotope, tritium contains one proton and two neutrons. It is more challenging to locate in large quantities, due to its 10-year half-life (half of the quantity decays every decade). Rather than attempting to find it naturally, the most reliable method is to bombard lithium, an element found in Earth's crust, with neutrons to create the element.

Deuterium-deuterium fusion: Theoretically more promising than deuterium-tritium because of the ease of obtaining the two deuterium atoms, this method is also more challenging because it requires temperatures too high to be feasible at present. However, the process yields more energy than deuterium-tritium fusion.

With their high heat and masses, stars utilize different combinations to power them.  [VIDEO: Sun to Sun – The Need for Fusion Energy]

Proton-proton fusion: The dominant driver for stars like the sun with core temperatures under 27 million degrees F (15 million degrees C), proton-proton fusion begins with two protons and ultimately yields high energy particles such as positrons, neutrinos, and gamma rays.

Carbon cycle: Stars with higher temperatures merge carbon rather than hydrogen atoms.

Triple alpha process: Stars such as red giants at the end of their phase, with temperatures exceeding 180 million degrees F (100 million degrees C) fuse helium atoms together rather than hydrogen and carbon.

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