NUCLEAR FUSION

1. NUCLEAR FUSION

2. 3 simple questions: • why does the sun shine ? • why is gold more expensive than iron ? much rarer • what is radioactivity ?

3. 3 chapters • basic physics ofnuclear fusion • nuclear fusionin stars • nuclear fusionas energy supply on earth ?

4. basic physics of nuclear fusion

5. of elements

6. atomism explains chemistry: 92 elements (from H to U) increasing weight, regularities in properties (alkalines, halogenes, noble gases, ...) all of them found in nature ! WHY ? • job for the physicists heat up pure elements --> emission of light in characteristic colours „spectra“

7. all explained by quantum mechanics : atoms are not fundamental but composed of a small nucleus10-15 m and a cloud of electrons 10-10 m the electrons are negatively charged, the nucleipositively, cloud and nucleus are held together by the electromagnetic force analogy: electron cloud the size of a football stadium 100 m --> size of football (nucleus) at the centre point: 1 mm

8. nuclear physics • H (lightest nucleus) has 1 electron and • 1 positively charged particle as nucleus • = proton • 2 problems for heavier nuclei: • how can many protons stay together at such small distances ? • (repulsion of equal electric charges !) • nuclei with charge Z have a mass mZ ≥ 2Z mp • possible explanation: • need another neutral particle with mn ≈ mp • and a special force between n and p • neutron postulated 1920, experimentally proven 1932 (Chadwick) • new force: strong interaction • --> withp andnand strong int. nuclei can be built up

9. Epot n p „rules of the game“ for building up nuclei out of p and n : • quantum mechanics • special relativity • weak interactions quantum mechanics : p and nhave spin1/2 (fermions) no 2 fermions can be in the same state additional p and n are less tightly bound --> „shell structure“

10. special relativity : E = m.c2 combination of a and b to system S --> mS < ma + mb „mass defect“ ∆m = mS - (ma + mb) energy liberated = ∆ m.c2 = „binding energy“ weak interaction : p + e- <--> n + e n.b. also need to explain why some nuclei send out various types of radiation „radioactivity“ , , 

11. Epot n p „construction on paper“ of nuclei assume unlimited availability of p and n in either spin state (up or down ) notation:AZ , e.g. hydrogene =1H 2 nuclei: pp,nn,or pn S.I. is stronger for parallel spins not allowed for identical particles --> only pn bind ! = deuteron (p n+ p n) = 2H 3 nuclei: ppp andnnndon‘t bind, pnn OK = triton = 3H ppn OK =3He

12. Epot n p 4 nuclei: pppn and pnnndon‘t bind p p n njackpot ! = 4He all 4 nuclei in ground state total spin = 0 very high binding energy =  particle 5 nuclei: all unstable 6 nuclei: 3p3nOK =6Li (= ) 7 nuclei: 4p3nunstable 3p4nOK =7Li 8 nuclei: 4p4nunstable (=  

14. Epot Epot n p n p - radiation transition with radiation of high energetic photon + 

15. Epot Epot n p n p - radiation transition with transmutation of p into n + e++ e with emission of positron and neutrino pn + e++ e (also n p + e-+ e)

16. Epot Epot Epot n p n p n p - radiation nucleus splits + mostly in heavy nuclei

17. answer to „what is radioactivity“ ? • - radiation: split up ofnucleus resulting in a lighter nucleus + 4He •- radiation: transmutation of p (or n) into n (or p) and emission of e+ (or e-) and a (anti-)neutrino •- radiation: transition of a p or n from a higher to a lower energy level with emission of a  high energy photon (keV)

19. nuclear fusion in the universe

20. basic astronomical facts classification of stars: Hertzsprung-Russel diagram luminosity vs. colour index

21. spectra of starlight same spectra as for known elements on earth ! all the same stuff ..... expansion of the universe nebulae are other galaxies their light is the more shifted to longer wavelenghts the further they are away redshift soon interpreted as expansion of the universe (Einstein‘s GTR) sun BAS11 Z = 0.07

22. Big Bang nucleosynthesis big bangat t = 0 ??? expanding universeis cooling 1ms (300 MeV) free particles p, n, e-, e+, , s ( 1 MeV) e- p n  mn> mpneutrons decay d = (pn) forms, but destroyed by  s (100 keV) lifetime of d increases neutrons captured into 3T, 3He 4He 300 s ( 50 keV) nucleosynthesis finished ! 92%p, 7%4He, < 1% d, 7Li no free n left

23. m not much happens to universe of p, 4He, ande-between 5 min and 350000 y universe needs to cool below 3000° to form H and Heatoms nowGRAVITATION takes over enormous potential gravitational energy in H and Heatoms! expansion counteracted by the formation of gas clouds of various sizes contracting gas clouds rise of T at centre „proto-star“ if mass of cloud big enough ( > 0.1 ) after several million years (My) T will rise to reionisation and eventually to neutron production ! p + pp + n+ e++ free neutrons are back ! p + n d+ 

24. after 500 My: back to fusion of H to 4He the heavier the star the hotter in the centre the faster it will burn enormous stable energy release for billions of years we understand whythe sun shines

25. m m but we haven‘t found gold yet keep going ... stellar evolution study core: highest T production of 4He increases density and T fusion speeds up positive feed-back ! eventually all H in core burnt up fusion moves to shell, less dense „shell burning“ small star (m < 0.4 ) : H fusion will stop brown dwarf big star: m ≈ 20 H fusion will continue mass and density of 4He and T will further increase

26. helium burning „ 3  process“ 4He +4He 8Be OK, but 8Be unstable with very short lifetime need a third 4He to hit 8Be 4He +8Be 12C „helium flash“ enormous rise of E output and TH burning in outer shell also increases aside: nuclei with A = 4n (multiples of 4He) dominate fusion processes other nuclei produced as well but play little role nuclei with odd number of p or n much less stable only 5 stable odd - odd nuclei

27. helium burning will produce 12C (and 16O ) in core increase of T ignition of consecutive further burning stages helium burning 3 4He 12C ,16O carbon burning 2 12C 24Mg 20Ne ,16O neon burning 2 20Ne 24Mg oxygen burning 2 16O 32S 28Si ,24Mg silicium burning 2 32S 56Ni 56Co ,56Fe all processes at the same time in consecutive shells every new process shorter and with less E output

28. no more fusion processes beyond 56Fe / 56Ni ! beyond 56Fe binding energy of nucleons decreases formation of heavier nuclei requires energy instead of releasing it great job done ! all elements up to Fe , Ni produced but still nogold !!?

29. detail: once C, N and O are present a second Hfusion process contributes: „CNOcycle“ C, N and O just act as catalysts

30. m heavy star unstable: Fe accumulates in the core, no energy output whenmcore > 1.3 gravitational collaps e- p n  supernovaexplosion

31. supernova Fe core only n collaps to incompressible nucleus gigantic release of  nuclei falling in from outer shells stopped, partially disrupted made to bounce back as fragments release of p, n,  high energy collisions production of heavyn -rich nucleibeyond U

32. supernova collapsed core will form a neutron star or a black hole nuclei produced will settle down into full range of elements including gold !!! ejected matter: material for new star formation sun & solar system probably 3rd generation star heavy elements all from previous stars

33. chart of nuclei total nb. of known nuclei ≈ 3300 of which ≈ 250 (quasi-) stable

34. nAu ≈ 10-7 nFe relative abundance of elements in earth crust now we understand whygold is much rarer than iron

36. nuclear fusion as energy supply on earth ?

37. intriguing prospect: practically unlimited and cheap supply of fuel: 1H, 2D, 3T main problem: how to attain necessary high T to overcome electrostatic repulsion ? brute force solution: hydrogene bomb fusion triggered by fission bomb ! first „successful“ explosions by the US (1952) and the Soviet Union (1953)

38. controlled fusion 60 years of technical developments: fuel (almost) always 2D and 3T two techniques: - fusion of plasma enclosed in magnetic fields ( plasma = fully ionised atoms = bare nuclei ) - inertial fusion ignition of fuel by focussed lasers or particle beams 3 critical parameters to initiate fusion: - density - temperature of fuel (Lawson criterium ) - confinement time goal: keep fusion going for a time long enough so that energy output > energy input

39. plasma fusion enclose plasma in toroidal magnetic field „Tokamak“ „stellarator“ chamber circumference ≈ 20 m diameter ≈ 3 m magnetic coils: no problem, field ≈ 2-5 Tesla nuclei will spiral around magnetic field lines and around the torus plasma current will induce poloidal field --> instabilities injection of 2D and 3T and accelerationto fusion temperature fusion should keep up T n produced will escape and should heat up He gas in surrounding

40. large number of experimental sites all over the world most advanced: TFTR (US) first ignition 1986 JET (EU) first ignition 1991, energy O/I ≈ 0.65, 2 sec new international project(EU, US, Russia, Japan, China, South Korea, India) ITERInternational Thermonuclear Experimental Reactor in planning/construction in Cadarache (south of France) plans: test with hydrogene plasma in 2020 ignition with 2D and 3T in 2027 goals: energy output/input ≈ 10 burning time > 400 sec

41. inertial fusion main project: National Ignition Facility (Livermore, Cal., USA) driven by 192 high power lasers focussing on small target (mm) about to reach ignition (more difficult than expected ) has also military aims

42. advantages of fusion reactors w.r.t. fission reactors - fusion reactors cannot explode - unlimited supply of fuel - major radioactive material: only 3T with half-life 12.3 y disadvantages of fusion reactors - very expensive to construct and operate economically viable ?

44. summary we wouldn‘t exist • without the hydrogen and its gravitational potential produced during the cosmic evolution • without all the elements produced by nucleosynthesis in the stars • without the energy steadily supplied by the sunfor billions of years we would not exist without nuclear fusion