Nuclear(Fusion( - University of Washington...fusion (4H ! He) could work in stars. • What kind of fusion happened depended on how hot and dense the core is and how massive the star

Nuclear  Fusion  

STEREO  Images  of  Extreme  UV  Radia6on  at  1  Million  C  

1  

2  

Fusion  vs.  Fission  Fission  is    •  the  breaking  apart  of  a  nucleus    •  what  occurs  during  radioac6ve  decay  •  naturally  occurring  and  happens  in  power  plants    

3  

Fusion  vs.  Fission  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  

Eddington’s Sun:

(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???? 4  

Overcoming the Impossible:

p + p è D + e+

There was still a problem though.

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?

5  

The beginning of the process requires:

Classical  Mechanics  vs.  Quantum  Mechanics  

Electron  moves  towards  the  barrier.  

Electron  bounces  off  of  the  barrier.  

Classical  Picture:  Think  of  the  electron  as  a  point  par6cle.  

This  is  NOT  what  happens  quantum  mechanically!!!  

6  

Classical  Mechanics  vs.  Quantum  Mechanics  

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  Picture:  Think  of  the  electron  as  a  wave,  which  describe  the  probability  an  electron  will  be  found  at  a  given  loca6on.  

probability  wave  

Most  of  the  6me,  the  electron  does  not  pass  through  the  barrier.  

7  

Classical  Mechanics  vs.  Quantum  Mechanics  

Electron  wave  moves  towards  the  barrier.  

Quantum  Picture:  Think  of  the  electron  as  a  wave,  which  describe  the  probability  an  electron  will  be  found  at  a  given  loca6on.  

probability  wave  

Quantum  Tunneling:  Occasionally  the  electron  passes  through  the  barrier.  

Electron  has  large  probability  of  not  passing  through  the  barrier.    But  it  has  a  small  probability  of  passing  through  the  barrier  

8  

A Numbers Game:

How likely is it for two protons to combine in the core of the Sun?

Your chances of winning the lottery are 40 BILLION times greater.

P + P è D + e+ has ~1 chance in 4 x 1016, each second!

So how does this happen?

•  Density in the solar core = 150 g/cm3.

•  Mass of a proton = 1.7 x 10-30 g.

•  Proton density in core = 8 x 1031 p/cm3.

•  Thus there are ~ 1015 reactions/s-cm3.

9  

Hans Bethe:

•  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 (Weizsäcker), 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. 10  

Solar Fusion: The P-P Chain

e+ + e- ⇒ 2γ

Step 1: p + p ⇒ D + e+ + νe

(neutrino)

Reaction Rate = 1015 reactions/s-cm3;  once  in  every  1.4  x  1010  yr  per  pair  of  Ps

(p+n)  

11  

Solar Fusion: The P-P Chain

Step 2: D + p ⇒ 3He + γ (p+n) (2p+n)

Reaction Rate = occurs once every 0.6 s per D and P pair

12  

Solar Fusion: The P-P Chain

Step 3: 3He + 3He ⇒ 4He + 2P (2P+2n=alpha particle)

Reaction Rate = once every 106 yr per 3He pairs

13  

The  P-­‐P  Chain  in  the  SUN  

Step 1: p + p ⇒ D + e+ + ν e (0.42 MeV)

Step 2: D + p ⇒ 3He + γ (5.49 MeV)

Step 3: 3He + 3He ⇒ 4He + 2p (12.86 MeV)

Net: 4p + 2e- ⇒ 4He + 6γ + 2ν e (~ 26 MeV) (Where 1 MeV = 106 eV = 1.6 x 10-13 J)

e+ + e- ⇒ 2γ (1.02 MeV)

14  

Need  two  of  Step  1  &  2  to  have  one  of  Step  3  

(2*0.42MeV)  +  (2*1.02MeV)  +  (2*5.49MeV)  +  (1*12.86MeV)  =  26.72  MeV  

The  P-­‐P  Chain  in  the  SUN  

15  

Fusion Energy on Earth

2) D + T è He4 (3.5 MeV) + n (14 MeV)

3) D + He3 è He4 (3.7 MeV) + p (14.7 MeV)

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 He3 (0.8 MeV) + n (2.5 MeV)

split 50% - 50%

These three paths are not equal in usefulness 16  

There’s a problem However:

•  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.

Fusion Energy on Earth

3He is not found on Earth! 17  

Mining the Moon?

•  The Apollo Missions found 3He in the lunar soil. Estimates range up to 106 Tons of it.

•  The energy content of 3He is:

Energy (3He)= 2x108 kWh/kg

•  The average person uses 250 kWh each month:

1 kg powers 105 people for a YEAR!!

•  1 kWh of Energy “Costs” about $0.10

3He is worth $600,000 an ounce!

106 Tons = 300,000 years of power!!! 18  

Fusion Energy on Earth Lawson  Criterion  •  Plasma  Density  (ni)  •  Plasma  Temperature  (Ti)  •  Energy  Confinement  Time  (TE)  

19  hap://www.6nt.or.th/adv/fusion/edu/tripleprod.html  

=  Fusion  Triple  Product  

ITER:  D  +  T    

Plasma  and  Field  Interac6ons  

•  Magne6c  field  cause  moving  charges  to  perpendicular  to  the  field  lines  

•  Charges  are  bound  to  magne6c  fields  

•  Charges  can  move  along  field  lines  

B  

Posi6ve  ion  

Electron  

B  

Plasma  is  bound  to  the  magne6c  field.  AND  

Magne6c  fields  are  bound  to  the  plasma.  

=  Magne6c  field  into  the  page  

20  

Tokamaks  

21  

Fusion  Experiments:  Magne6c  Confinement  

•  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  

22  

Fusion  Experiments:  Iner6al  Confinement  

•  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  

23