STEREO  Images  of  Extreme  UV  Radia6on  at  1  Million  C
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Fission  is
•   the  breaking  apart  of  a  nucleus
•   what  occurs  during  radioac6ve  decay
•   naturally  occurring  and  happens  in  power  plants
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Fusion  is
•   the  combining  of  two  nuclei  to  form  a  heavier  nucleus
•   what  occurs  inside  the  core  of  the  Sun
•   the  magic  bullet  for  solving  human  energy  problems…   or  maybe  not
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(1920s) Sir Arthur Eddington: Puts it all together….
1) Gravitational forces pressurize the center of the Sun.
2) Compression heats the interior (he thought to 40 million K).
3) Thermal collisions strip electrons from their nucleus, creating a plasma .
4) Free protons collide and stick to form helium nuclei.
5) The mass difference between H and He is converted to energy and
released as light!
There are problems with this idea: Protons are VERY hard to make fuse! Where does all the extra positive charge go????
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There was still a problem though.
The beginning of the process requires: p + p è D + e +
The amount of thermal energy, even in the core of the Sun, is NOT enough to overcome repulsion between the two protons.
How does it happen then?
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Classical  Mechanics   vs.  Quantum  Mechanics
Classical  Picture:  Think  of  the  electron  as  a  point  par6cle.
Electron  moves  towards  the  barrier.
Electron  bounces  off  of  the  barrier.
This  is  NOT  what  happens  quantum  mechanically!!!
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Classical  Mechanics   vs.  Quantum  Mechanics
Quantum  Picture:  Think  of  the  electron  as  a  wave,  which  describe   the  probability  an  electron  will  be  found  at  a  given  loca6on.   probability  wave
Electron  wave  moves  towards  the   barrier.
Electron  has  large  probability  of  not   passing  through  the  barrier.
But  it  has  a  small  probability  of   passing  through  the  barrier
Most  of  the  6me,  the  electron  does   not  pass  through  the  barrier.
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Classical  Mechanics   vs.  Quantum  Mechanics
Quantum  Picture:  Think  of  the  electron  as  a  wave,  which  describe   the  probability  an  electron  will  be  found  at  a  given  loca6on.   probability  wave
Electron  wave  moves  towards  the   barrier.
Electron  has  large  probability  of  not   passing  through  the  barrier.
But  it  has  a  small  probability  of   passing  through  the  barrier
Quantum  Tunneling :
Occasionally  the  electron  passes   through  the  barrier.
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How likely is it for two protons to combine in the core of the Sun?
P + P è D + e + has ~1 chance in 4 x 10 16 , each second!
Your chances of winning the lottery are 40 BILLION times greater .
So how does this happen?
•   Density in the solar core = 150 g/cm 3 .
•   Mass of a proton = 1.7 x 10 -30 g.
•   Proton density in core = 8 x 10 31 p/cm 3 .
•   Thus there are ~ 10 15 reactions/s-cm 3 .
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•   Hans Bethe (~1940): Developed a theory for the ways in which fusion (4H è He) could work in stars.
•   What kind of fusion happened depended on how hot and dense the core is and how massive the star is.
•   Very massive stars have one kind of fusion, called the CNO cycle (Weizsäcke r ), that involved `rare` elements (carbon, nitrogen, and oxygen).
•   Less massive stars (like the Sun) are neither dense or hot enough for CNO fusion. Instead they fused He directly from H.
He called this the Proton-Proton
(P-P) chain.
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(p+n)
Step 1: p + p ⇒ D + e + + ν e e + + e -
⇒ 2 γ
(neutrino)
Reaction Rate = 10 15 reactions/s-cm 3 ;  once  in  every  1.4  x  10 10  yr  per  pair  of  Ps
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Step 2: D + p ⇒
3 He + γ
(p+n) (2p+n)
Reaction Rate = occurs once every 0.6 s per D and P pair
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Step 3: 3 He + 3 He ⇒
4 He + 2P
(2P+2n=alpha particle)
Reaction Rate = once every
10 6 yr per 3 He pairs
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Step 1: p + p
⇒
D + e + +
ν e
(0.42 MeV) e + + e -
⇒ 2 γ (1.02 MeV)
Step 2: D + p
⇒
3 He +
γ (5.49
MeV)
Step 3: 3 He + 3 He ⇒
4 He + 2p (12.86 MeV)
Need  two  of  Step  1  &  2  to  have  one  of  Step  3
Net: 4p + 2e -
⇒
4 He + 6
γ
+ 2
ν e
(~ 26 MeV)
(Where 1 MeV = 10 6 eV = 1.6 x 10 -13 J)
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The P-P chain is very hard to do on the Earth.
Instead we can do:
1) D + D è T (1 MeV) + p (3 MeV)
è He 3 (0.8 MeV) + n (2.5 MeV) split
50% - 50%
2) D + T è He 4 (3.5 MeV) + n (14 MeV)
3) D + He 3 è He 4 (3.7 MeV) + p (14.7 MeV)
These three paths are not equal in usefulness
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•   Paths 1) and 2) produce fast neutrons that irradiate the containment vessel and reduce efficiency.
•   Path 3) produces the most energy, is the most efficient, and is cleanest.
There’s a problem However:
3 He is not found on Earth!
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•   The Apollo Missions found 3 He in the lunar soil. Estimates range up to 10 6 Tons of it.
•   The energy content of 3 He is:
Energy ( 3 He)= 2x10 8 kWh/kg
•   The average person uses 250 kWh each month:
1 kg powers 10 5 people for a YEAR!!
•   1 kWh of Energy “Costs” about $0.10
3 He is worth $600,000 an ounce!
10 6 Tons = 300,000 years of power!!!
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•
•
Lawson  Criterion
Plasma  Density  (n i
)
Plasma  Temperature  (T i
)
•   Energy  Confinement  Time  (T
E
)
=  Fusion  Triple  Product
ITER:  D  +  T     hap://www.6nt.or.th/adv/fusion/edu/tripleprod.html
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•   Magne6c  field  cause  moving   charges  to  perpendicular  to   the  field  lines
•   Charges  are  bound  to   magne6c  fields
•   Charges  can  move  along   field  lines
Electron
=  Magne6c  field  into  the  page
Plasma  is  bound  to  the  magne6c  field.
AND
Magne6c  fields  are  bound  to  the  plasma.
B
Posi6ve  ion
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B

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•   Uses  magne6c  fields  to   confine  hot  plasmas
•   Types  of  expt
–   Tokamaks  (ITER,  NSTX,
Alcator…)
–   Stellerators
–   Compact  Torus  (HIT-­‐SI…)
–   Innova6ve  Confinement
Concepts
•   A  lot  of  work  s6ll  needs  to   be  done
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•   Uses  lasers  to  crush  a   full  target  very  rapidly
•   Types  of  facili6es:
–   Solid  State  Lasers(NIF,
NRL…)
–   Gas  Laser  (NRL,  PALS…)
•   Lots  of  work  here  too
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