Tokamak physics and thermonuclear perspectives Alexei Dnestrovskij

1. Tokamak physics and thermonuclear perspectives Alexei Dnestrovskij Kurchatov Institute, Moscow Russia This lecture was prepared during the visit to the Culham Science Centre, UK

2. Outline • Requirements for fusion energy release, Lawson criterion • Tokamak device • Tokamak basic physics • Examples of Tokamak experiment • The next step – ITER

3. D+T=He + n 3.5MeV 14.1MeV What is FUSION? It occurs when two light nuclei are forced together, producing a larger nucleus The combined mass of the two small nuclei is greater than the mass of the nucleus they produce The extra mass is changed into energy We can calculate the energy released using Einstein’s famous equation: E = mc2

4. Great Balls of Fire! Fusion in the 21st Century Fusion is the Process Powering the Sun To overcome strong repulsive forces, fusion nuclei require very high energies - matter becomes a … PLASMA

5. Cross section: D-T <v> D-D D-He3 Ion Temperature /keV Fusion factor: Q= Fusion power External power Fusion requires high plasma pressureand confinement (p=n*T and E) 10 1 100 1 10 100 High temperatures(Te and Ti)are required for fusion events< v > High density(n ) is requiredfor reaction rate Fusion power  n2<v> Echaracterises energy loss time = Requirement forignition (Lawson criterion): nE > 1.5x1020m-3s At T~30keV ~300MoC stored energy loss rate

6. … on some history • Anomalous Bohm confinement During 50s- late 60s it correlated with wide variety of data on radial diffusion in different devices. • Tokamak experiments in 1965 Artsimovich and colleagues reported an excess of Bohm confinement by a factor approximately 3 • So it was the beginning of TOKAMAK ERA in fusion

7. What is the TOKAMAK ? meaning “toroidal chamber” and “magnetic coil” • Tokamak, from the Russian words: toroidalnaya kamera and magnitnaya katushka A tokamak is a toroidal plasma confinement device with: to provide atoroidal magnetic field • Toroidal Field coils • Transformer with a primarywindingto produce a toroidal currentin the plasma • The current generates a poloidal magnetic field and therefore twisted field lines which creates a perfect “trap” • Other coils shape the plasma

8. Fusion factor: Q= Fusion power External power Major Progress Towards Fusion Power Q=0.65 achieved in JET Q~1 in Neutron Source Q~10 in ITER Q>50in Power Plant

9. Magnetic fusion activities Future steps • ITER (International Thermonuclear Experimental Reactor), project is ready • 3 medium size tokamaks under construction: KSTAR (S.Korea), HT-7U (China), SST-1 (India). Main aims: steady-state long pulse operations • CTF (Component Test Facility), preliminary studies In operation • 3 large fusion devices operational (JET, JT-60U, LHD), a big stellarator under construction (W7-X) • 11 medium size tokamaks are operational • … plus ~ 50 small size devices

10. Progress on JET (Joint European Torus) JET parameters: Major radius 3m Minor radius 1m Plasma height 3.5m Plasma current 3MA Toroidal B = 2.7T

11. electrons B B B B • Outwards ExB drift - - - - - - - - - - - - - - B ExB E E B + + + + + + + + + + Tokamak Basics Drifts in nonuniform plasma Example: B drift • Magnitude of B varies with position • Larmor radius varies as 1/B ions • What happens in tokamak plasma due to the drift? • Charge separation • Vertical electric field • ExB plasma losses

12. Important parameter for stability analysis safety factor q toroidal turnspoloidal turns q= Edge q >2 for stability Tokamak Basics Toroidal current in tokamak produces poloidal magnetic field Plasma current Poloidal magnetic field Toroidal magnetic field • Field lines become helical • Particles can move to short out any E field produced by gradient/curvature drifts

13. Tokamak Basics ASDEX-Upgrade in Germany Plasma forms magnetic surfaces Quasineutrality of plasma Σnjej=0 provides to use fluid MHD equations Strong parallel transport : V||/V┴ ≈ 106 gives the formation of magnetic surfaces MAST in UK • Fast processes (10-8 – 10-7 s) • Plasma pressure becomes a function of magnetic surfaces • p=p(ψ) • where ψ – poloidal magnetic flux • Equilibrium installed • p = jxB

14. Magnetic confinement problems Magnetically confined Fusion plasmas suffer from: 1) Instability - difficult to confine a high density and temperature plasma with low magnetic fields. 2) Turbulence - limits the confinement time for a given sized machine. 3) Power loading - high volume to surface area ratio means power loading on surfaces is high. 4) Neutron activation - materials must withstand high neutron fluxes

15. TS Examples of plasma behaviour: sawteeth • Oscillations of plasma parameters • Name from SXR trace • Shown by most tokamaks • appear when q<1 • 2 very different time scales (crash ~ µs, ramp ~ 10’s ms) What is the sawtooth? Kadomtsev model

16. Examples of plasma behaviour: Plasma heat and particle losses • Collisional transport • Fluctuations driven transport • δn/n ~ δT/T ~ eδ/T < 50% • δBr/B ~ 10-4 • It is commonly accepted: the enhanced transport is the result of fluctuations • Flux due to the electrostatic fluctuations : • Γ=< δn δv>~< n c δE/B> ~< n c δ/L /B> ~< n/L cT/(eB)> • Flux due to the electromagnetic fluctuations: • Γ=n/B < δv||δBr> • Can the fluctuations be suppressed ? Bohm’s like scaling law.

17. Examples of plasma behaviour: H-mode • Upon exceeding a critical heating power (PL-H) transition from L-mode to H-mode occurs • Transition occurs with spontaneous formation of an Edge Transport Barrier (ETB) • thin, situated at edge of plasma, just inside of the scrape off layer (region II on picture)

18. Examples of plasma behaviour: H-mode • Drop in D radiation indicating decrease in particle flux with formation of transport barrier • Many evidences of fluctuation suppressing overall the plasma volume • No commonly accepted complete physical model Dα line intensity

19. plasma current H-mode 50 ms Dα - signal H-mode and ELMs:movie from MAST shot

20. Examples of plasma behaviour: Modes with Internal Transport Barrier (ITB) JT-60 1994 The role of q-profile Ne • An ITB is in essence similar to the H-mode ETB, however it is not restricted to the edge • ITBs can be formed at almost any point in the plasma • ITBs can dramatically increase plasma performance • ITBs are obtained by manipulating plasma’s q-profile • produce regions of weak or reversed magnetic shear q’=rdq/dr • ITBs form at min q or on a rational q close to the minimum Ti Te q

21. The Future for Fusion Power Fusion Power Pulse duration Q When? JET 1997 16MW ~1 second <1 2015-2020 500-700MW <30 minutes >10 ~2050 ~3000- ~1 day ~50 4000MW ITER Power Plant

22. The next step - ITER • To demonstrate integrated physics and engineering at ~GW level at min. cost • Superconducting coils, power-plant-level heat fluxes, “nuclear” safety • Its design is realistic, detailed and reviewed like no other fusion device • Negotiations in final phase for ITER • Unofficially Europe now made a decision to built ITER in South France

23. $$$$$$ Fusion economics ITER cost 6 billion $(1989) per 500-700MW thermal energy (over $1billion was spent for project design) Usual fission $0.7billion per thermal gigawatt power plant Oil fuel $100 billion in last year spent exploration and development by 30 top oil firms (not for production)

24. To summarize the reviewed problems … The tokamak device picks up many physical problems together: • Plasma confinement: • Different kinds of instabilities • Plasma transport across the magnetic surfaces • Disruptions • Diagnostics • External Heating • Tokamak-reactor problems (challenge for ITER): • Power exhaust • Neutrons