Varenna Summer School Fusion energy

Transcription

1 1 Fusion energy F. Wagner Max-Planck-Institute for Plasma Physics, Greifswald Germany, EURATOM Association RLPAT St. Petersburg Polytechnic University Sun Fusion supplies the universe with energy In the core of the sun: 15 Mill C Matter: in the plasma state Energy-mass equivalence: E=mc 2 4 Mill tons of mass/sec to energy Fusion energy maintains fusion conditions: self-sustained burn for further 4.5 Mrd years

2 From the sun to the 1st fusion reactor Sun confinement by gravity p-p chain 4p He 4 + energy 2

3 From the sun to the 1st fusion reactor Sun confinement by gravity ITER tokamak magnetic confinement p-p chain 4p He 4 + energy D + T He n + energy 3

4 Fusion basics Technical fusion: hydrogen istopes deuterium (D) and tritium (T) D+T He 4 +n MeV (1 g fusion fuel = 10 tons coal) Process temperature: 150 Mill degrees Fusion: Binary collision process He (a-particle; E a = 3.5 MeV) provides internal heating self-sustained burn When cooled down: ash ash removal n carries its energy (E n = 14.1 MeV) to the outside. 4

5 The task of controlled nuclear fusion Confine a hot, high-pressure D-T plasma Heat it to high temperatures externally till inner a-particle heating takes over ( self-sustained burn) Provide plasma equilibrium, stability and good confinement Exhaust the He ash and maintain high plasma purity Provide D (from see-water) and T (via Lithium, from earth crust) Remove neutron-heat deposited into surrounding wall (blanket) Develop materials for high heat fluxes and high neutron damage Produce electricity by standard steam techniques Realisation: (1) Inertial confinement (use e.g. lasers to compress pellets) (2) Magnetic confinement (use magnetic fields) 5

6 Thermal isolation of high-temperature plasma - p gravity 6

7 7 sphere Magnetic confinement torus - p gravity

8 8 Magnetic confinement - p gravity Magnetic force - p

9 Tokamak: The most advanced system Tokamak (1951 Sacharov und Tamm) тороидальная камера в магнитных катушках toroidal chamber within magnetic coils Vertical field coil OH-system Iron yoke Plasma with current I p Toroidal field coils 9

10 Stellarator Toroidal field coils J h helical field coil Plasma: 3D Tokamak: internal confinement Stellarator: external confinement 10

11 JET: the largest tokamak 11 11

12 Devices of the Asian fusion programme KSTAR - Korea EAST- China SST-1 India JT-60 SA Japan 12 12

13 Wendelstein 7-X, Greifswald 13

14 Topics, discussed in more detail 1. What is a plasma? 2. Fusion in more detail 3. Physics of magnetic confinement 4. Why is the fusion reactor so large? 5. Status of fusion research 6. The ITER project 7. Fusion materials and technology 8. Safety of fusion reactors 9. Does fusion come too late? 14

15 1. What is a plasma? Answer: Partially or fully ionised gas; a mixture out of free electrons, ions and atoms: the 4 th state of matter plasma solid liquid gas ions electrons Plasmas develop at elevated temperatures: k B T e E ion T e (ev) > E k -E i : Plasmas are connected with strong light emission Ionisation: plasmas are electrically conductive currents F L = jxb Plasmas are open, non-equilibrium systems self-organisation Plasmas are quasi-neutral: n e = S Z i n i (not on small scale) Plasma particles interact via Coloumb collisions ~ 15

16 T-n space : natural plasmas Temperature (ev) Super novae Solar centre Pulsar magnetosphere Solar corona Ionosphere Aurora Density (m -3 ) Interstellar plasmas Flames white dwarfs 16

17 2. Fusion in more detail The governing role of the binding energy Energy gain from - fission of heavy nuclei or - fusion of light nuclei. Similiar to chemistry, also nuclear reactions set free the binding energy. For nuclei, unlike electronic transitions, the energy is MeV, not ev. 17

18 Nuclear reactions in more detail The nuclear force (strong interaction) is active only for distances in the order of the nucleus dimensions (fm). For larger distances, the repulsive Coulomb force dominates Potential wall: some 100 kev, large particle energy necessary to overcome 1928, Gamov : there is a finite probability for tunneling through the Coulomb wall: P tunnel exp{-2 Z 1 Z 2 e 2 /h v} E pot = e 2 /4 e 0 l db = ½ m v 2 E ~ 5 kev Highest reaction probability for light nuclei at high relative velocity! 18

19 s (cm 2 ) Cross-sections and reaction rates s Coulomb s fus E (kev) Coulomb-collisions more frequent than fusion collisions no beam-target fusion plasmas are thermal T thermo-nuclear fusion s - averaged over distribution - is of relevance reaction rate: <s fus v> (cm 3 /s) p-p cycle much too weak avoid weak interaction DT has the highest rate 19

20 Ignition conditions (Lawson Criterion) Power density balance of: D +T 4 He (E a = 3.5 MeV) + n (E n = 14.1 MeV); E fus = 17.6 MeV Steady-state: S sources = S sinks Sources: external heating p ext fusion power density p fus = n D n T <s fus v>e fus a-particle heating power density: p a = n D n T <s fus v>e a Sinks: radiation: p rad = c n e n Z Z 2 T e conductive and convective losses: p c = 3/2 (n e + n D + n T )T / t E t E = energy confinement time 20

21 Ignition conditions (Lawson Criterion) ignition Z eff = 1 ignition Z eff = 2 breakeven Curves of the Lawson Criterion BUT: profile effects Breakeven: Q = P fus /P ext = 1 Status of JET: Q = Fusion goal JET <T> (kev) Ignition: P a = P losses Plasma have to be clean He exhaust Fusion goal is clearly defined Triple-product: ntt E > m -3 kev s Qu: How to get there? 21

22 22 Fusion fuels Deuterium exists with a weight fraction of in water static range of billions of years. Tritium is a radioactive isotope and decays with a half life of years: T 3 He + e - + e no natural tritium available; in-situ production by fusion neutrons Breeding reaction: n + 6 Li 4 He + T MeV n + 7 Li 4 He + T + n` MeV self-sufficient tritium breeding. Lithium is very abundant and widespread (earth crust, water) sufficient for at least years.

23 Pro: D from sea water Li from earth crust inexhaustable energy source equal distribution of fuel on earth no CO 2 poduction no uncontrollable power excursions no critical afterheat no radio-active fusion products, no actinides Con: (1) tritium is radio-active pros and cons of fusion energy BUT: the production of tritium is in situ (2) neutrons activate structural materials BUT: radio-active by-products are well confined Their decay time is about 100 years 0.08 g D und 0.2 g Li put in a fusion reactor would supply a family of 4 with electricity for a year Source: FZJ 23

24 3. Physics of magnetic confinement Tasks and challenges: Confine a plasma magnetically with 1000 m 3 volume Develop heating techniques Maintain the plasma stable at 2-4 bar pressure and 150 Mill K in the core With 15 MA current running inside a fluid (tokamak) Find methods to maintain the plasma current steady-state (tokamak) Tame plasma turbulence to get the necessary confinement time Develop diagnostic techniques 24

25 3. Physics of magnetic confinement Charged particles in magnetic field: Lorentz force Confinement to B electron ion B Transport via Cb-collisions D ~ Dx 2 /Dt D ~ r L2 /t cb 25

26 3. Physics of magnetic confinement Charged particles in magnetic field: Lorentz force Confinement to B Confinement ll to B electron ion B Transport via Cb-collisions D ~ Dx 2 /Dt D ~ r L2 /t cb Geometry is a torus 26

27 Coordinates in toroidal system Energy 2050 Coordinate system Q R r, a R 0 F aspect ratio A = R 0 /a Geometry is a torus e = 1/A 27

28 The toroidal field is inhomogeneous B B B F (R,q) = m o S I sp / 2 R = R o B Fo / R = R o B Fo / (R o + r cos q) B Fo (1 e cos q) toroidal B field has a radial gradient: B t 1/R force F on magnetic moment of gyrating particle B and curvature drift separate electrons and ions 28

29 29 v m R B c 2 2 B v 2 2 1/ 2v q Rc B Drift in inhomogeneous field poloidal cut + + B B B charge separation results in electric field - B v E - E E B 2 B E x B loss of plasma by ExB drift no equilibrium

30 The remedy: Rotational transform Illustration: Honey spoon. B q B B F 30

31 Toroidal systems with rotational transform Tokamak Vertical field coils OH-system I p B F F q B q B Iron yoke Plasma current I p Toroidal field coils 31

32 Strongly shaped cross-section poloidal field coils allow flexible shaping Modern Tokamaks JET: R o = 3m, a = 1.2 m, b = 2 m Elongation k = b/a = 1.7 Triangularity d Size, a, R o : To provide t E fusion power Elongated shape k : To allow higher current more stability better confinement Plasma surface: separatrix with X-point and divertor impurity control particle and density control 32

33 33 Plasma heating Tokamaks: Ohmic heating (P W = R I p 2 = I p V loop ) of limited use: dr/dt < 0 Beam heating injection of energetic neutral beams E beam Reactor: 1MeV

34 Plasma heating Energy 2050 Tokamaks: Ohmic heating (P W = R I p 2 = I p V loop ) of limited use: dr/dt < 0 Beam heating Wave heating resonance injection of energetic neutral beams E beam injection of e.m. waves source Reactor: 1MeV Ion cyclotron resonance heating: w ci Electron cyclotron resonance heating: w ce 34

35 35 Global Confinement Times Plasma energy: W = 3/2 (n e T e + n i T i )d 3 r ~ 3V n T 0-d energy balance: W = P W/t E. Stationary: P = W/t E Power switched off: W = W/t E. t E and transport: Confinement and diffusivity:

36 The importance of t E Sensitivity study: t E Ht E standard H is the improvement factor Power amplification Triple product: Q = P fus /P ext ntt E H 2 ITER H V. Mukhovatov 36

37 Self-organisation of Plasmas: spatial structures in low temperature plasmas Simple arrangement for dielectric barrier discharge The planar gas-discharge systems self-organises in patterns; control parameter is the current. The non-linear element is the negative conductivity of the discharge Discharge The dielectric barrier prevents arc development The discharge current does not organise itself with homogeneous current density Research Group Purwins, UNI Münster,

38 The H-mode transition Historical diagramme from ASDEX, 1982, IPP Garching density L-mode H-mode b pol ~ <p>/b 2 energy heating The main features of the H-mode a spontaneous and distinct transition during the heating phase both energy- and particle confinement time increase the tracer for the transition is the Ha-radiation new instabilities appear in the H-phase: ELMs, edge-localised modes 38

39 Movie of edge turbulence S.J. Zweben et al., Phys. Plasmas 9 (2002)

40 Movie of edge turbulence S.J. Zweben et al., Phys. Plasmas 9 (2002)

41 41 Edge Transport Barrier in density and temperature Edge transport barrier

42 Understanding the H-mode transition The plasma self-organizes its turbulence level 1. Step: sheared flow de-correlates turbulence (Biglary, Diamond, Terry) q E B r v(q) marker Reduction of turbulence at L-H transition 42

43 Proxy, showing the spectral transport A. Shats, ANU, Canberra 43

44 Proxy, showing the spectral transport A. Shats, ANU, Canberra 44

45 Modelling of shear-flow decorrelation Gyrokinetic particle simulation of plasma microturbulence L H B Z. Lin at al., Science 45

46 Achievements in the H-mode 16.1 MW fusion power from JET H 200 Mill C in the core of JET 46

47 experimental t E in s Prediction from regression analysis ITER scaled t in s E 98(y,2) H-mode t E scaling 47

48 48 Modern stellarators Classical stellarator with helical windings Wendelstein 7-X with modular coils Optimised stellarator Modular coil

49 49 Modular stellarators Shown is the principle how modular coils can be constructed from helical ones. W7-X

50 Why tokamaks and stellarators? Tokamak Stellarator (classical) Conventional tokamak: pulsed operation current carrying plasma prone to instabilities As a consequence of internal confinement: Plasma is 2D good confinement for both free and trapped particles Steady-state operation with SC coils Plasma operation is quiescent As a consequence of external confinement: Plasma is 3D 3 rd class helically trapped particles lost The classical stellarator has no reactor relevance Loss orbit d/df = 0 50

51 4. Why is the fusion reactor so large? centre 51

52 Why is the fusion reactor so large? Critical level centre 52

53 Why is the fusion reactor so large? centre 53

54 Why is the fusion reactor so large? Sandpile experiment centre Osvanny Ramos Rosales Oslo University 54

55 Why is the fusion reactor so large? centre 55

56 Why is the fusion reactor so large? centre 56

57 5. Status of fusion research The geneology of tokamaks 57

58 Achievements with magnetic confinement Temperature T: 40 kev achieved (JT-60 U) Density n surpassed by factor 5 (C-mod, LHD) Confinement time t E : a factor of 4 is missing (JET) Fusion product ntt E : a factor of 6 is missing (JET) The first scientific goal is achieved: Q 1 (0.65) (JET) DT operation without problems (TFTR, JET) Maximal fusion power for short pulse: 16 MW (JET) Divertor developed (ASDEX) The design for the 1st experimental reactor complete: ITER The optimisation of stellarators (W7-AS, W7-X) 58

59 6. ITER (Latin: the path, the route) Central column Primary ohmic system Poloidal field coil Toroidal field coil Li test blanket Opening for Heating and diagnostics Divertor chamber cryostate Size scale 59

60 General requirements for ITER Goals: P fus ~ 500 MW; Q = P fus /P aux = 10 for 400 s; Q=5 for steady-state Ash removal in the core: Transport (D, v in ); t He */t E ~ 5 Ash removal from the system: divertor retention, recycling Low Z eff : fluxes (ELMs, fast particle losses) materials (C, Be, W); erosion mechanisms D I, v I.in, sawteeth Stable operation: limits which terminate operation (via disruptions) density limit (Greenwald): n GW ~ I p / a 2 (MA, m) ; n < 0.85 n GW beta-limit (Troyon): b = b N I p /ab current limit: q = 2.5 a 2 (B/RI p ) ((1+k 2 )/2) > 2 (q ITER ~ 3) vertical stability limit: k < 2 60

61 61 Design of ITER Fusion power 500 MW Power amplification Q=10 External heating 70 MW Pulse lenght > 8 Min. Plasma current 15 MA International partnership China, Europe, India, Japan, Korea, Russia, USA Plasma volume 840 m 3 Plasma energy 350 MJ Magnetic field 6 T (12 T) Energy of the field 10 GJ Size

62 7. Fusion technology and materials Tasks and challenges: Build a system with 200 Mill K in the plasma core and 4K about 2 m away Build magnetic system at 6 T (max. field 12 T) with 50 GJ energy Handle neutron fluxes of 2 MW/m 2 leading to 100 dpa Handle a-particle power of >10 MW/m 2 onto divertor targets Develop low activation material IFMIF Develop T breeding technology Provide high availability of a complex system 62

63 63 Materials for fusion Structural materials subjected to bombardment of 2 MW/m 2 from very energetic (14 MeV) neutrons Plasma facing materials subjected to an additional 500 kw/m 2 from hot particles and electromagnetic radiation (up to 20 MW/m 2 onto divertor ) Issues: Atoms knocked out of place several times a year ( >100 dpa over reactor life) dislocation loops, other damage hardening & embrittlement enhanced diffusion creep, rapid diffusion of impurities to grain boundaries etc. He (the fusion ash ) and H get embedded form nano-sized bubbles Some elements transmute by nuclear reactions Longer-term radioactive products (within 100 years limit)

64 8) Safety Energy pay-back time ~ 6 months, energy amplification factor > 40 Fuel costs can be ignored; COE ~ 6-10 Cent/kWh No chain reactions, fundamental physics laws prevent uncontrolled power excursions Radiation dosis during normal operation very small Low energy reservoir; no plant destruction in case cooling fails totally (LOCA) In case of major accident no evacuation necessary, no exchange of soil No release of environmentally relevant gases, no CO 2, low external costs Volume of radioactive waste is comparable to that of fission (same power, same lifetime) About 99% of the activated material can be recycled About 1% of the activated material contains long-living waste (decay-time ~ 100 years) Waste does not have to be actively cooled Fusion has low remaining radiotoxicity EU safety study SEAFP 64

65 Temperature rise after LOCA fusion process stops immediately additional heat only from reactive decay processes no melting of a power plant long involved time scales enough time to react. SEAFP, Model 2, available materials 65

66 Relative Units (arb. units) Activated waste from a fusion power plant Total radioactive inventory of different nuclear reactors 1.E+00 radio-toxicity Pressurized water reactor Decay by factor 10 in 500 years Fusion power plant: 4 orders in magnitude in 50 years. 1.E-01 1.E-02 1.E-03 Pressurized water R. Fusion: Vanadium Fusion: Steel, He, Be after 400 years: remaining radio-toxicity comparable with ash from coal power Station recycling of most of the material possible 1.E-04 1.E-05 1.E-06 coal possibly no long-term storage 1.E Time after termination (years) 66

67 Technology Installatoins Plasma physics 9. Does fusion come too late? road map to a fusion power station Tokamak physics programme Electricity production Commercial availability Stellarator development Decision point JET ITER DEMO IFMIF, 14 MeV neutron source ITER- Technology DEMO Technology

68 Does fusion come too late? Change in technology from red to blue Increase in world population Increase in energy consumption Blue: RE, fission with breeder fusion The 1st half of this century will experience dynamic developments There are many uncertainties Increase of CO 2 Oil peak keep all options open now

69 69 Final comment Research into high temperature plasmas is an intellectually rewarding field Fusion has a tremendous potential facing the future uncertainties - the risks of fission, storage of RE the fusion development has to be accelerated There is a clear road-map to commertialize fusion (of course, there is still no guarantee of final success) ITER will answer open physics questions related to burning plasmas W7-X will demonstrate the quality expected from stellarator optimisation ITER, IFMIF, DEMO: The programme will move away from plasma science more toward technology orientation After the ITER physics and technology programme - if successful fusion can be placed into national energy supply strategies With fusion, we hand over to future generations a clean, safe and - in our expectations - economic power source

70 Inertial confinement fusion (ICF) Topics and issues: To compress and heat a small (R o ~2mm) solid D Tpellet by driver = lasers, beams, or X-rays from a Z-pinch such that they ignite and deliver fusion energy. The physics of the conventional approach: Ignition via central hot-spot formation Indirect drive Hot-spot development pressure: 250 Gbar p therm /p degen ~ 5 Core heating by confined a-particles Burn wave propagates outward into shell of high n target 70

71 NIF and LMJ facilities NIF Target chamber LMJ: 240 lasers 3 rd harmonic of Nd-Yag 1.8 MJ -> 20 MJ thermonuclear yield 550 TW 71

72 High-performance discharges: Tokamak Ip (x10) MA P NBI MW DIII-D Magn. perturbation n=3, n=2, n=1 D a, upper divertor b N 4l i q min q(0) n e Z eff n e = m -3 P NBIabs = 4.8 MW. b ~ 3% b N = 2.7 H 89 = 2.6 n e /n egw = 0.4 Mapped to ITER Q=10 Steady-state Dt ~ 36 t E 72

73 Long-pulse HDH discharge of W7-AS HDH regime B = 0.9 T n e = m -3 P NBIabs = 2.5 MW b = 3.4% b N ~ 9.3 H ISS95 = 1.4 n e /n egw = 2.5 t I /t E ~ 2 Dt ~ 36 t E 73

74 The role of symmetry Tokamak Classical Stellarator I hel.coil I p B q ds Ampere s law Tokamak: curl B = m 0 j Stellarator: curl B = 0 3D: B = B(y,f,q) 2D: B = B(y,q); d/df=0 sufficient confinement no continuous symmetry no associated constant of motion (canonical momentum p f ) insufficient confinement 74

75 75 Stellarator optimisation: W7-X The multiple helicity stellarator W7-X Variation of plasma cross-section in one semi-module

76 Space and time scales which govern plasma physics Space scales: Plasmasize a, R, L n n/ n ~ m Volume, gradient length Debye length l D ~ T/n ~ m Plasma definition: size >> l D Mean-free-path l ~ 10 3 m transport II B l >> 2 R reactor conditions: T=15 kev, n=10 20 m -3 : l ~ 40 km ~ 1000 passes around the torus: criterion for confinement quality Larmor radius r L ~ T/B ~ m transport B r L << L n Time scales: Collision frequency ~ n T 1/2 ~ 10 4 s -1 transport, dissipation Plasma frequency w pe ~ n ~ s -1 e.m. wave propagation Cyclotron frequency w c ~ B ~ s -1 propagation, resonance, heating Confinement time t E ~ 1 s thermal insulation: t E ~ a 2 /D 76

77 The toroidal field is inhomogeneous B B Magnetic mirror: W tot = const m ~ W /B = const (1) free particles (2) trapped particles D ~ D banana /t coll * B F (R,q) = m o S I sp / 2 R = R o B Fo / R = R o B Fo / (R o + r cos q) B Fo (1 e cos q) toroidal B field has a radial gradient: B t 1/R force F on magnetic moment of gyrating particle B and curvature drift separate electrons and ions 77

78 78 Plasma particles drift in the inhomogeneous field Bounce movement between two mirror points trapped particles gyration in field

79 79 Plasma particles drift in the inhomogeneous field Bounce movement between two mirror points trapped particles Drift off the field line gyration in field

80 Strongly shaped cross-sections poloidal field coils allow flexible shaping Modern Tokamaks JET: R o = 3m, a = 1.2 m, b = 2 m Elongation k = b/a = 1.7 Triangularity d Size, a, R o : To provide t E fusion power Elongated shape k : To allow higher current more stability better confinement Plasma surface: separatrix with X-point and divertor impurity control particle and density control 80

81 Strongly shaped cross-sections poloidal field coils allow flexible shaping Modern Tokamaks JET: R o = 3m, a = 1.2 m, b = 2 m Elongation k = b/a = 1.7 Triangularity d Size, a, R o : To provide t E fusion power Elongated shape k : To allow higher current more stability better confinement Plasma surface: separatrix with X-point and divertor impurity control particle and density control 81

82 Important steel ingredients vital for steels useful for steels S. Roberts 82

83 Materials for fusion SiC composits S. Roberts Only the elements in green can be used: anything else transmuted by highenergy neutrons to something VERY radioactive. Problems: Mn, Mo, Ni, N... Divertor material New standard: Eurofer: Fe - 8.9%Cr 1%W - 0.2%V %Ta %C 83

84 84 Tritium Inside plasma: a few grams Inventory in walls is the problem. T 3 He + e - + e E max = 18 kev, <E> = 5.8 kev, T 1/2 = 12.3 Jahre 1 g Bq Incorporated HTO Fast component t 1/2 10 Tage, Watercycle of the human body Slow component: 30 and 300 Tage Organically bound Tritium, responsible for about 10 % of dosis Penetration depth in organic material 60mm. Molecular form in ambient nature: HT oder HTO: Incorporated HTO : t 1/2 = 10 Tage 1 Bq Sv, HT is about x less poissonous than HTO

85 The benefit in low radio-toxicity Dose rate of different materials after irradiation to 10 MWy/m² 85