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”
  – Wikipedia: “Nuclear fusion”
  
• An ideal and clean source of electricity
• The process used by the Sun
• It’s the “opposite” of Nuclear fission (which releases power by splitting big atoms)
• Mass difference \(\Rightarrow\) \(E=mc^2\) \(\Rightarrow\) voila

Fusion

• Specifically, are there 2 small things which we can get to join together, Step ??? = Profit
• Nuclei are + charged, repel each other
  • Must hit each other with high energy to convince them otherwise
    
• “DT” wins: Deuterium (\(^2\)H) = heavy hydrogen; Tritium (\(^3\)H) = extra-heavy hydrogen
  • Gives us our target fusion reaction: \(^2\)D + \(^3\)T \(\Rightarrow\) \(^4\)He + \(^1\)n + 17.6 MeV
  • Peak at temperatures > 100 M °C

N.B. Different from the fusion process in the Sun

Fusion

• In summary:
  • Get some Deuterium and Tritium
  • Heat them to >100 M °C
  • Make sure they stay put
  • Fusion happens
  • Profit

Fusion & Plasma

• Hydrogen at 100 M °C
  • c.f. solar core (15 M °C); solar surface (5000 °C)
  • Fusion labs are hottest places in the solar system
• Plasma – 4th state of matter (solid, liquid, gas, plasma):
  • Gases break apart at very high temperatures / low densities
  • Electrons and ions separate \(\Rightarrow\) all particles in a plasma are charged!
    • Charged particles interact with electromagnetic fields
  • Most of “stuff” in space, plus the outer planets, is a plasma
• A lot of fusion physics is plasmas physics

Idea: use magnetic fields to “hold” our plasma
\(\to\) Magnetic Confinement Fusion


“We say that we will put the sun into a box. The idea is pretty. The problem is, we don’t know how to make the box.”
– Pierre-Gilles de Gennes

Magnetic confinement

Magnetic confinement