MadSci Network: Physics

Re: How are very high temperatures generated.

Date: Tue Dec 21 17:09:38 2004
Posted By: David Lien, Staff, Planetary Science, Planetary Science Institute
Area of science: Physics
ID: 1103295005.Ph


I hope the following helps you better understand how the high temperatures needed for fusion 
are generated. It is a bit lengthy, but your question does not have a simple answer. Since you 
also mentioned that what you have found about Fusion has so far been a bit confusing, I wanted 
to (hopefully) give you some background about Nuclear Fusion so that my discussion of high 
temperatures would make a bit more sense.  

First, however, I want to clear up what may be a misunderstanding based on your question.

Hydrogen is used in two very different ways to generate energy.  The first method is by using a 
device called a Fuel Cell (or sometimes a Hydrogen Fuel Cell), where Hydrogen atoms breaks 
away from nearby atoms (but leave behind their electrons), passes through a specially made wall 
(called a semi-permeable membrane), and chemically reacts with Oxygen to form water. The 
electrons cannot pass through this semi-permeable membrane, so the result is a container with 
positive charges on one side of the semi-permeable membrane, and negative charges on the 
other. The Fuel Cell is now just like any other battery, and connecting a wire between two halves 
of the cell will result in the flow of electric charge, which can run a motor, light up a room, or do 
any other kind of job that a battery can do.  

The second method of energy generation which uses Hydrogen is known as Fusion (it can also be 
called Hydrogen Fusion or Nuclear Fusion). Unlike the  Fuel Cell, which uses chemical reactions to 
generate electricity, Hydrogen Fusion relies on nuclear reactions to generate energy: the nuclei of 
two atoms actually stick together (in a chemical reaction, the nuclei never get anywhere near 
each other). After a Hydrogen Fusion reaction, there is no Hydrogen left! It's actually turned into 
another element: Helium.

To be clear, then, neither of the two methods of energy generation actually produce Hydrogen.   
We have to use the Hydrogen that is already available to us. In fact, most Fuel Cells actually run 
on fossil fuels like natural gas or even the gasoline that cars use. Some  Fuel Cells can run on 
pure Hydrogen, which can be extracted from water (H2O). Unfortunately, it takes energy to break 
apart water, and the Laws of Thermodynamics say that we will always lose some of the energy 
when we break apart water, and then reform it while trying to do something useful with the 
energy we've liberated. The main interest in Hydrogen Fuel Cells is that they do not pollute at all 
since only water is produce in the reaction. 

Now, to return to your original question - how are the high temperatures needed to create 
Hydrogen Fusion generated?

The nucleus of most Hydrogen atoms consist of just a single proton. One out of every 100,000 
atoms of Hydrogen has a neutron attached to the proton. Scientists call nuclei with the same 
number of protons but different numbers of neutrons "isotopes," and Hydrogen has three of 
them. Normal Hydrogen has only the single proton. Deuterium is the name we give to Hydrogen 
when it has one neutron stuck onto the proton, and Tritium is Hydrogen with two neutrons stuck 
onto the single proton  (Tritium is also radioactive,and turns into Deuterium when it loses one of 
its neutrons ). 

Nuclear Fusion occurs when two nuclei come close enough for a nuclear reaction to occur. The 
result of a Fusion nuclear reaction is usually one or two new nuclei and energy. Most 
experimental Hydrogen Fusion reactions combine Deuterium and Tritium together. The result of 
this nuclear reaction is the formation of a Helium nucleus (2 protons and 2 neutrons) and one 
extra stray neutron.  If you were to weigh the Deuterium and Tritium nuclei before they react, 
then weight the Helium nucleus and the neutron after the reaction, you would find that some 
mass is "missing" (less than 1% of the total mass of the original two nuclei is "missing").  It is this 
"missing" mass which is turned into the energy we are interested in, and the amount of energy 
can be calculated from Einstein's famous formula, E=mc^2 (where m is the "missing" mass). Note 
that although only a very small amount of matter is converted into energy,  the amount of energy 
we get is very large since we are multiplying a small number by a much larger number (the speed 
of light squared).

The biggest problem with Hydrogen Fusion is that the two nuclei have to get very close to each 
other in order to for a nuclear reaction to occur. However,  both nuclei are positively charged, 
and we know that like charges repel (and the closer the two nuclei get, the more they repel each 
other). There are two ways to get the two nuclei close to each other - squeeze them really hard 
or throw them at each other very fast (or, of course, a combination of the two).  This is where the 
temperature becomes important, since the temperature of an atom is related to how fast the 
atom is moving (there are a number of different ways to define temperature, this one is known as 
the kinetic temperature and is the most important one for Hydrogen Fusion).

Unlike Hydrogen Fuel Cells, which are now used in a variety of applications (NASA's Space Shuttle 
uses them), Hydrogen Fusion reactors are still in the experimental stage, and there are two 
competing methods now being investigated.  The first is called Plasma Fusion and the second is 
called Inertial Confinement Fusion (since these methods use Hydrogen Nuclear Fusion, the words 
Hydrogen and Nuclear are usually left off so that the names don't get too cumbersome).

A Plasma is a gas made up of atoms which have had one or more of their electrons removed (this 
is called an ion).  Since every isotope of Hydrogen  has only a single electron, a Hydrogen Plasma 
is a gas made up of free electrons and the positively charged nucleus (proton, proton+neutron, 
or proton+2 neutrons depending on whether the isotope is Hydrogen, Deuterium, or Tritium).

Some sort of container is needed to keep the Plasma from escaping, but the cool container walls 
make it very difficult to keep a Plasma hot (place your hand near a window on a cold morning, 
and you'll notice the air temperature is much cooler near the window than in the center of the 
room).  Scientists use a neat trick to keep the Plasma away from the cold metal walls: magnets!  
Charged particles do not travel in straight lines when they encounter a magnetic field. Instead, 
they spiral around the magnetic field lines. One way of picturing this motion is to partially fill a 
bathroom sink with water and float some tiny scraps of paper on the surface.  Then unplug the 
sink and hold a pencil right above the drain so it points straight out.  This pencil represents a 
line of magnetic force and the paper represents the Plasma. As the sink drains, a funnel forms, 
and as the paper gets closer to the funnel it moves faster and faster around the pencil while at 
the same time getting closer to the bottom of the sink.

In a Plasma, the ions and electrons are trying to move "down" the drain, but travel around in 
circles at the same time.  And as the field gets stronger (closer to the drain), the Plasma moves 
faster.  However, unlike the water and paper, most of the Plasma actually gets reflected back 
(think of the bubbles created at the throat of the drain which float back out of the drain into the 
sink).  If we are clever with our magnets, we can create a "magnetic bottle" out of which the 
Plasma cannot escape. But unlike water in a  water bottle, the Plasma in a magnetic bottle is 
constantly bouncing back and forth inside the bottle. Occasionally the ions hit each other, and if 
they hit hard enough, the two colliding nuclei will get close enough for a nuclear reaction to 

How do we get the ions to move fast enough?  This is your original question about how we 
generate high temperatures, but re-stated in terms of high speed instead of high temperature.  
For Plasma Fusion, we can shoot very fast electrons into the magnetic bottle (your television tube 
is shooting somewhat fast electrons onto a chemical painted onto the glass which emits light 
when hit by an electron, so it's pretty easy to create a device to create fast electrons).  These fast 
electrons hit the Deuterium and Tritium ions, causing them to speed up.  Radio waves are a form 
of electromagnetic radiation (a blend of electric and magnetic fields which oscillate over time). 
Since the ions are charged particles, they can interact with the oscillating electric waves from a 
strong radio wave transmitter.  The ions pick up speed by "surfing" the electric field waves. In 
either case, the speed of the ions is increased enough so that a nuclear reaction occurs when a 
Deuterium ion hits a Tritium ion (D-D and T-T collisions need much higher temperatures (or 
speeds) before nuclear reactions occur).  This is a very simplified description of Plasma Fusion, 
and the details I've left out are the ones which  make bottling and heating the Plasma very 
difficult in practice.  Nature likes to tempt us with possibilities, then tease us by hiding the 
important details.  Scientists always love a challenge, and one of our greatest joys is when we 
uncover one of Nature's hidden secrets.

The second method of Hydrogen Fusion, Inertial Confinement Fusion, uses a very different 
method to get the Deuterium and Tritium nuclei close enough to fuse. It starts not with a hot 
ionized Plasma, but with a small sphere of solid Deuterium and Tritium encased in a thin metal 
shell. This small fuel pellet has a diameter of about 5 mm and has a starting temperature of only 
a few degrees above absolute zero! It seems odd, but in order to create the very high 
temperatures needed for Hydrogen Fusion we must first start off the the coldest Hydrogen we 
can create.  Is it necessary for the Hydrogen to be cold? Not really. However, it is necessary for 
the Hydrogen to be solid,  and on the surface of the Earth, solid Hydrogen can exist only at 
temperatures slightly above absolute zero.

So how does this work?  We take this very cold fuel pellet and throw it into the center of a  
vacuum chamber.  This central point is also the focal point of many high energy lasers. As soon 
as the pellet reaches the center of the vacuum chamber the lasers turn on, and the outer shell of 
metal on the pellet absorbs the energy almost instantaneously.  This causes the metal surface to 
literally explode. The shock wave from the explosion travels both outwards and, more 
importantly, inwards.  This inward travelling shock wave compresses and heats the solid mixture 
of Deuterium and Tritium, which causes the nuclei to get close enough to each other for  nuclear 
reactions to occur (as I mentioned before, a nuclear reaction can occur even at low temperatures 
if we squeeze the two nuclei together hard enough). The trick here is that the shock wave 
compresses and heats the atoms before they have a chance to move away from each other.  
Inertia is the property of matter which resists changes in motion (Newton's First Law: objects 
resist acceleration).  Since the lasers are hitting the pellet from all angles, the pellet is uniformly 
compressed before the atoms have a chance to react to their newly acquired temperature - the 
atoms are confined by their inertia, hence the name "Inertial Confinement" Fusion.    

I hope that my discussion has helped you better understand Hydrogen Fusion. It's a fascinating 
subject which has the potential of solving most of the world's energy supply problems, although 
it will probably be at least twenty years before the first commercial Fusion reactors are built. 
Scientists have made great strides in developing the science and technology behind both Plasma 
Fusion and Inertia Confinement Fusion, but much more experimental and theoretical work is 
needed. For example, although Hydrogen Fusion releases a tremendous amount of energy, the 
current methods for capturing this energy and converting it into a more useful form are very 
inefficient (for example, using the energy to turn water into steam, which then can be used to 
power an electric generator). It will likely be the next generation of scientists (that's you, by the 
way) who will ultimately solve these problems.  

Links:  Plasma fusion basics,  Princeton 
University    Interactive  Plasma Physics Education Experience 
(plasma fusion info)   General Atomics    National Ignition Facility, University of 
Californa, Berkeley  Links to different ways to heat 
ICF pellets

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