Re: What 's the principle of fusion reactor and how many type of them?

Fusion reactors will use energy released in nuclear fusion reactions, 
where light element nuclei are fused to form a heavier element nucleus. 
In that proccess, some amount of original nuclei mass is transformed 
into energy (remember E=mc2 ?). You can find a more detailed 
explanation of nuclear reactions on MadSciArchives (answers to 
questions 970785342.Ph, 853464363.Ph  or 899578182.Ph.).

About Fusion Reactors, it is important to state that there is no one 
working by now, after more than forty years and billions dollars spent. 
Nonetheless, there are thousands of scientists all over the World 
designing future reactors and running dozens of experimental machines 
to learn about nuclear fusion and reactors design (although none of 
them can actually produce energy). 

An hypothetical Fusion Reactor would work as actual Fission Reactors 
do. During a nuclear reaction some neutrons fly away from original 
nuclei, taking with them a portion of the energy released in that 
reaction. Those neutrons are slowed down, passing their kinetic energy 
to some coolant which results heated. Heat absorbed by the coolant is 
used to produce steam from water, which will later be used to move a 
turbine linked to an electric generator, producing electric energy in 
that way.

That's the principle, and it looks simple. One of the main problems 
with fusion reactors design is that in order to obtain nuclear fusion 
the fuel to be used (hydrogen or its isotopes) must be at a temperature 
as high as 100 million Centigrade Degrees. As you can imagine any known 
material would result volatilized at a temperature one thousand times 
lower than this, so a reactor must be designed as to keep thermonuclear 
fuel away from its walls.

There are two approaches to this idea. 

One of them is what is called ''Magnetic Confinement''. At such a high 
temperature as is needed to obtain fusion reactions, matter appears 
completely ionized. Charged particles (electrons and nuclei) can be 
trapped in a magnetic field, because a moving charged particle follows 
magnetic field lines. If you design a device where magnetic field lines 
don't cross the walls, charged particles will follow them and will 
never touch the walls, avoiding the risk of damaging them. Such a 
device has a vacuum hollow doughnut shaped chamber, and magnetic field 
lines go around it (and particles following them). Then fuel can be 
heated by any means (microwaves, neutral beams, ohmic heating ...) to 
reach nuclear fusion temperatures, and the reactor will then be 
working. 

There are two types of these devices: Tokamaks and Stellarators.
Both are almost the same, and the main difference is that in Tokamaks 
nuclear fuel is heated and confined by driving an electric current 
through it (remember it is ionized, so it is an electric conductor) 
while in Stellarators no electric current is driven through the plasma 
(the name that is given to an ionized gas).

The other approach to nuclear fusion reactors is ''Inertial 
Confinement''. An inertial confinement device consists in a vacuum 
chamber (usually spherical in shape) where small fuel pellets are 
injected, as if shooted by a gun. When those pellets reach the 
chamber's centre, extremely powerful laser or particle beams are 
shooted to it from all directions. Those beams make the pellet to be 
compressed and heated till a point where fusion reactions begins, with 
subsequent emission of neutrons. After the first pellet has been burned 
another one is shooted into the chamber, and the proccess continues.

This is a rough view of fusion reactor designs. You can find much more 
information visiting the websites listed below.

-> Magnetic Confinement:
   * Tokamaks: 
      http://www.iter.org
      (International Thermonuclear Experimental Reactor - ITER.
       This is a project involving countries and scientists all
       around the World, to design and build the first reactor
       where ignition is expected to be reached.)
 
      http://www.jet.efda.org
      (Joint European Torus - JET.
       One of the largest tokamaks in the World. It has been
       running and producing many valuable scientific results for
       twenty years.)

   * Stellarators:
      http://www.ipp.mpg.de/ipp/ipp.eng.html
      (Max-Planck Institute for Plasma Physics. Germany.
       They have one of the two largest stellarators ever built:
       W7AX. Now they operate a smaller one: W7AS.)

      http://www.nifs.ac.jp/~LHDhp/homepage.html
      (Large Helical Device - LHD. Japan.
       The other largest stellarator ever built: LHD.)


-> Inertial confinement:
   * Lasers beams:
     http://www.llnl.gov
     (Lawrence Livermore National Laboratory - USA.
      They have the World's largest and most powerful laser: NOVA)

   * Ion Beams:
     http://www.sandia.gov
     (Sandia National Laboratories - USA)
         
 

You have some information about Nuclear Fusion at MadSci Archives. 
Search for ''Nuclear Fusion'' or ''Nuclear reactors'' using MadSci 
Search Engine.

You can also surf through the following websites, and find all the 
information you want about nuclear fusion and plasma physics.

* http://fusion.gat.com/PlasmaOutreach/  
(Various USA Universities and Laboratories have developed a program 
called 'Plasma Sciences and Technologies Education Outreach' with the 
aim of getting ready a place to learn about Plasma Physics and Nuclear 
Fusion).  

* http://FusEdWeb.pppl.gov/ 
(Princeton University Plasma Physics Laboratory)

* http://www.lbl.gov/abc/
(Lawrence-Berkeley National Laboratory.
Here you have a site with introductory information about nuclear and 
particle Physics.)

* http://www.ornl.gov/
(Oak Ridge National Laboratory, Tennessee. They work in nuclear fusion, 
fision and many more things ...)


Enjoy, ... and keep on thinking.