Chapter 12. Magnetic Fusion Toroidal Machines: Principles, results, perspective

Transcription

1 Chapter 12 Magnetic Fusion Toroidal Machines: Principles, results, perspective S. Atzeni May 10, 2010; rev.: May 16, 2012 English version: May 17,

2 Magnetic confinement fusion plasmas low density ( m -3 ) plasma, In nearly steady-state conditions, confined by appropriately shaped magnetic fields 2

3 Open and closed magnetic configurations Open configurations: magnetic mirrors (see lectures on charged particle motion in external fields, Ch. 2 of this course) Closed configurations (in practice toroidal configurations): - Tokamak - Stellarator - Reversed Field Pinch These notes mainly concern the tokamak device, because so far this is the most studied and best performing device. We shall say a few words about stellarators, too. 3

4 Toroidal device Magnetic field Toroidal + Poloidal + Vertical Main concepts: - Tokamak - Stellarator source: Chen, Introduction to Plasma Physics,

5 Rotational transform (i.e. poloidal field + toroidal field) required for stability 5

6 Tokamak configuration The tokamak is a toroidally stabilized (see Lectures on equilibrium ), in which toroidal field is generated by external magnets poloidal field is generated by the electrical current flowing in the plasma (plasma = secondary loop of a transformer) => the (conventional) tokamaks is a pulsed machine A vertical field B v is also required to generate a force (j x B v ) balancing the radial expansion force due to the gradient of magnetic pressure (since toroidal field decreases with distance from major axis as1/r) 6

7 Tokamak source: Pease, in Dendy

8 Tokamak 8

9 Textor Tokamak (Julich, Germany) courtesy of M. Mangels, Forschungszentrum Jülich GmbH

10 dal sito ENEA FTU, Frascati Torus Upgrade (ENEA, Frascati0

11 Stellarator both toroidal and poloidal fields created by external magnets (with elicoidal coils in the simplest case) source: Pease, in Dendy

12 Tokamak vs stellarator Tokamak: - simpler (axisymmetric) - better confinement (at least so far) - instrinsically pulsed (induced current) but a non-inductive current can be generated, by auxiliary power sources (micro-waves, fast particle beams) - steady-state Stellarator: but a modular structure can be built (see next) 12

13 Modular stellarator Conceptual scheme of Weldenstein 7 AX, operating at Greinswald (Germany) since 2016 (see 13

14 Modular Stellarator Weldenstein 7 X [Greinswald, Germania]

15 Diagnostics (I) plasma conditions: n m -3 T = kev T = 5-15 T L = m required resolution Δx = cm Δt = µs - ms detailed knowledge of plasma composition essential electrical & magnetic measurements, measure of electron and ion densities, electron and ion temperatures, radiation emission, fluctuations,... 15

16 Diagnostics (II) probes for electrical and magnetic measurements interferometry, polarimetry, reflectometry to measure electron density (multi-channel) Thomson scattering (multicanale), electron cyclotron radiation spectra to infer electron temperature and features of the electron distribution function neutral ion emission and neutron spectra to measure ion temperature bolometry, X-ray spectroscopy e..m. probes, refelctometry, heavy ion scattering to diagnose fluctuations 16

17 Physics: issues and achievements (I) equilibrium: well understood (MHD + control) macroscopic stability: OK (MHD + control + enforcement of opeational limits ) microscopic stability: a number of different processes; some of them not yet fully understood, not fully predictable and controllable; kinetic theories with detailed account of magnetic topology required Confinament: not fully understood (turbulence, field errors, microinstabilities,...) ==> experimental scaling laws, e.g. t E I 2 nt f geom = (3 f geom )1/ 2 I (P input ) 1/ 2 t E ==> enlarge the machine to improve confinement I 0.85 R 1.2 (P input ) 1/ 2 (a0.3 B 0.2 R 0.2 n 0.1 ) 17

18 Source: European Fusion Development Agency (EFDA) 18

19 note: Major simulation effort The problem is a real Grand challenge: (3+3)-dimensional kinetic problem, with complex field topology, multiple space- and-time scales So far, a lot of physics insight, not yet fully predictive transport simulations Promising results for fast fusion particle plasma interactions 19

20 Confinement time scaling and extrapolation to ITER Reactor 20 courtesy of G. Mazzitelli, ENEA

21 Physics: issues and achievements (II) Ohmic heating (ηj 2 ): insufficient beacuse η T -3/2 ==> auxiliary heating: fast neutral particle ( kev) injection microwaves (100 MHz GHz), at ion cyclotron frequency, electron cyclotron frequency, lower-hybrid frequency, etc Fusion alpha-particles: Coulomb slowing down particle-wave interactions? (v α v alfven ) [major physics unknown for thermonuclear high-q device ==> simulation; scaled down experiments? plasma-wall interactions (sputtering, heating, etc.) impurity poisoning (P rad n e <Z 2 >) 21

22 Fisica: problemi e risultati (III) record performance (JET device): n τ T = m -3 s kev Q = 0.3 (per 1 s) P fus = 22 MW [Phys Rev Lett 80, 5548 (1998)] other devices, with superconducting coils, operate with pulses of several minutes tokamak physics is studied on a number of devices; such devices have contributed to sstudies on confinement, plasma-wall interaction, magnetic configuration optimization, stability control, auxiliary heating, divertor physics,... 22

23 JET (Joint European Torus; see ) Major radius = 3 minor radius = 1 m; B = 3.8 T, I = 7 MA, auxiliary heating (rario-frequency + neutral injection) up to 50 MW 23 (courtesy of EFTA-JET)

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25 Next step: ITER (see Goal: Fusion power: 500 MW Q 5 Pulse duration 400 s Main parameters dimensions: JET x 2 major radius: 6.2 m minor radius : 2 m elongation: 1.8 plasma current: 15 MA magnetic field: superconducting magnets auxiliary heating: MW plasma volume = 800 m T on axis 12 T maximum, on conductors 25

26 international collaboration (EU, Russia, Japan, USA, India, Cina, S. Corea) cost: 30 G (?) under construction first plasma in 2024 integrated expts in 2028 Deuterium-tritium expts in 2036 (source D.J. Campbell, Iter organization at the American Physical Society Plasma Physics Division Conf., Nov. 2016)

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29 Iter experiments / The unknowns confinement α-particle plasma interactions macroscopic disruptions divertor localized heat loads on the first wall 29

30 Reactor dimensioning Temperature is more or less fixed (20 kev, see Lawson criterion) 3.5 MeV alpha-particle containement => current stability: once current and aspect-ratio fixed => toroidal field toroidal field (& cross section shape) ==> pressure ==> plasma density density and temperature ==> power density heating and confinement => dimensions, auxiliary heating power Additional constraints to dimensions set both by allowable thermal and neutron wall loads, and by confinement 30

31 From Iter to the reactor Fusion power x 6 pulse duration: from tens of minutes to hours (or even steady-state) tritium breeding (ITER does not include a full blanket) costs must be reduced to be proved: reliability, duration, mantainance (e.g. wall replacement via remote handling, access to magnets,...) 31

32 DTT: una proposta italiana tokamak di supporto ad ITER per lo studio di sistemi di rimozione del calore e di riduzione dei carichi termici sulla prima parete A. Pizzuto (a cura di), ENEA (2015), ISBN

33 A Fusion Power Plant A Lithium Blanket produces Tritium D + T 4 He + n + Energy n + 6 Li 4 He + T A heat exchanger in this blanket produces steam that drives turbines! Electricity 33

34 Any alternative to ITER? In principle, high-q operation could be demontrated with devices employing higher magnetic field, and with substantially smaller size. Such devices could allow studying the peculiar physics of thermonuclear plasmas (low collisionality plasmas, with fusion reactions, α-particle plasma interactions, etc.) at lower cost and on a shorter time scale. A long-standing proposal: Ignitor (B. Coppi) However, it seems difficult to conceive a high-magnetic field power-producing reactor. 34

35 dimensions: major radius: 1.32 m minor radius : 0.47 m plasma current: 11 MA on-axis magnetic field: 13 T Copper magnets auxiliary heating power: 15 MW Fusion power: 100 MW 35 source: Detragiache, ENEA

36 Bibliography Elementary discussion of tokamak configuration: G.Pucella e S.E. Segre, Fisica dei Plasmi, Zanichelli (2010), par. 2.4 (in Italian). Elementary presentation of tokamak configuration and tokamak reactor: J. Wesson, Tokamaks, 3rd Ed., Oxford University Press (2004), Sec. 1.6 e Secr A treatise on tokamaks: J. Wesson, op. cit. (749 pp.). Intermediate level lectures: Chapters 8, 17 and 18 of R. Dendy (Ed.): Plasma Physics, Cambridge University Press (1993). 36

37 Bibliography status of research [Nature Physics 12 (May 2016)]