Fusion

Discussion

introduction

The sun has been around for some five billion years and is expected to shine for another five billion years to come.

Size is related to energy. Nuclear energy is to chemical energy as atomic dimensions (10−10 m) are to nuclear dimensions (10−15 m). Nuclear reactions have energies on the order of 100,000 times the energy of chemical reactions.

Paraphrase needed …
F. W. Aston discovered in 1920 the key experimental element in the puzzle. He made precise measurements of the masses of many different atoms, among them hydrogen and helium. Aston found that four hydrogen nuclei were heavier than a helium nucleus. This was not the principal goal of the experiments he performed, which were motivated in large part by looking for isotopes of neon. The importance of Aston's measurements was immediately recognized by Sir Arthur Eddington, the brilliant English astrophysicist. Eddington argued in his 1920 presidential address to the British Association for the Advancement of Science that Aston's measurement of the mass difference between hydrogen and helium meant that the sun could shine by converting hydrogen atoms to helium. This burning of hydrogen into helium would (according to Einstein's relation between mass and energy) release about 0.7% of the mass equivalent of the energy. In principle, this could allow the sun to shine for about a 100 billion years. In a frighteningly prescient insight, Eddington went on to remark about the connection between stellar energy generation and the future of humanity:

If, indeed, the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfillment our dream of controlling this latent power for the well-being of the human race — or for its suicide.

Bethe described the results of his calculations in a paper entitled "Energy Production in Stars".


[slideanimate]

Light nuclei join to form a heavier nucleus. Energy is released in the process. Fusion powers the stars and high yield thermonuclear weapons.

solar fusion

Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. The sun, 107 K core, hydrogen fuses to form helium through a process known as the proton-proton chain (often shortened to the p-p chain).

stages
2( 11H  +  11H →  21H  +    0+1e  +  00ν)   0.4 MeV + 1.0 MeV
2( 11H  +  21H →  32He  +    00γ ) 5.5 MeV
  32He  +  32He →  42He  +  2 11H   12.9 MeV
overall
    4 11H →  42He + 2(0+1e + 00γ + 00ν) 26.7 MeV


[slide]

More on stellar fusion in another section of this book.

thermonuclear weapons

The first fusion bomb used liquefied deuterium (heavy hydrogen). Current "h-bombs" are dry thermonuclear weapons. The fuel of choice is lithium deuteride (lithium-6 deuteride to be more precise).

lithium 6   63Li  +  10n  →  31H  +  42He

 ⇒  63Li + 21H → 2(42He)
deuteride   21H  +  31H  →  42He  +  10n


[slide]

More on fusion bombs in another section of this book.

fusion reactors

magnetic confinement
tokamak — toroidal chamber and magnetic coil

inertial confinement?
laser systems

method density (kg/m3) temperature (K) confinement time
magnetic confinement 0.000001 100 million several seconds
inertial confinement 1,000,000 100 million 10−11 s
solar core 100,000 16 million as old as the sun
hydrogen bomb ? ? ?
Approaches to nuclear fusion Source: LLNL
Z element A mass (u) abundance   Z element A mass (u) abundance
–1 [electron] 0 0.000549     5 boron 8 8.024605  
      9 9.013328  
0 [neutron] 1 1.008665         10 10.012937 19.9
      11 11.009305 80.1
+1 [proton] 1 1.007276         12 12.014352  
      13 13.01778  
1 hydrogen 1 1.007825 99.985   6 carbon 10 10.01686  
  [deuterium] 2 2.0140 0.015       11 11.01143  
  [tritium] 3 3.01605         12 12 98.9
      13 13.003355 1.1
2 helium 3 3.01603       14 14.003241  
    4 4.00260 100       15 15.010599  
    5 5.01222     7 nitrogen 12 12.018613  
      13 13.005738  
3 lithium 5 5.01254         14 14.003074 99.63
    6 6.015121 7.5       15 15.000108 0.37
    7 7.016003 92.5       16 16.006099  
    8 8.022485         17 17.008450  
    9 9.026789     8 oxygen 14 14.008595  
      15 15.003065  
4 beryllium 7 7.016928         16 15.994915 99.76
    8 8.005305         17 16.999131 0.04
    9 9.012182 100       18 17.999160 0.20
    10 10.013534         19 19.003577  
    11 11.021658         20 20.004075  
Selected isotopes of the light elements