Fusion energy and plasma physics

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1 Fusion energy and plasma physics Fusion is the way the Sun and stars produce energy It is one of the few new ways of producing energy on Earth The energy reservoir is practically infinite The process is almost clean There are still big technical and scientific issues to solve

2 Nuclear binding energy: total energy required to break up a nucleus into its constituent protons and neutrons binding energy per nucleon: divide the binding energy of a nucleus by the number of protons and neutrons (i.e. number of nucleons)

3 Fusion: two light nuclei forced together Fission: breaking apart a heavy nucleus

4 When two light nuclei are forced together they will fuse energy because the mass of the combination will be less than the mass of the individual nuclei. nuclear binding energy = mc 2 Compare: Binding energy of the electron in a hydrogen atom: 13.6 ev Binding energy of the alpha particle: 28, ev

5 Energy production in the Sun Proton-proton cycle fuels the Sun and other stars that have temperature less than ~15 million Kelvins (for massive starts Helium fusion, carbon cycle) ppi : (~85%) (a) p p d e (b) d p 3 He e (c) 3 He 3 He 2p ppii (alternative to ppic) (a) (b) (c) He Be e - Be Li Li p 7 7 e ppiii (alternative to ppiib) (a) (b) (c) Be p B Be * 8 Be * 8 B e e

6 Proton-cycle

7 In massive stars: CNO cycle Hans Albrecht Bethe: Nobel prize 1967: For his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars"

8 How to realize the fusion on Earth? To reach fusion the nuclei must be fast enough to overcome the Coulomb repulsion (high enough T) and there must be a sufficient amount of nuclei (large enough n). The solar fusion is possible due to the extreme density resulting from gravitational collapse, which cannot be reached here. What are the alternatives? Deuterium: heavy hydrogen, one proton, one neutron Tritium: hydrogen-3 one proton, two neutrons

9 Reactivity fuel selection 150 MK Reactivity = cross-section x velocity To have an efficient fusion reactor A temperature of about kev ( 150 MK) is required the fuel should evidently be D+T Huge sources: Deuterium from D 2 0 heavy water, lots in oceans Tritium from Lithium (half-life of Tritium ~10y no sizable natural source) n + Li 6 (Li 7 ) reaction at the plant Li very common in earth n is the main product of the DT-fusion! The fuel is practically fully ionized plasma.

10 ( D+T 4 He (3.5 MeV) + n (14.1 MeV) 6 4 n+ Li He (2.1 MeV) + T (2.7 MeV)

11 Sun: Only 15 MK, but very high pressure (due to high density) Proton-proton fusion The preferred solution for fusion reactors: Reachable plasma density but higher temperature > 100 MK Energy output in form of energetic neutrons. (Note that the neutrons make the vessel weakly radioactive.) Deuterium tritium fusion

12 Conditions for fusion? When the critical temperature (T, kev) to ignite fusion has been achieved, it must be maintained at that temperature for a sufficiently long confinement time ( E, s) at a high enough ion density (n e, m 3 ) in order to be able to produce net energy. E the time the plasma is maintained at a temperature above the critical ignition temperature to yield more energy from the fusion than has been invested to heat the plasma E W P loss The Lawson criterion: requirement that the fusion heating exceed the losses Product of n e, T and E determine minimum condition for fusion e.g for D-T fusion n e T E > m 3 s kev (not yet achieved by any reactor!) E.g., for plasma density of m 3 and temperature 10 kev the required confinement time is 5 s.

13 How to confine such hot plasma? Inertial confinement seeks to fuse nuclei so fast that they don t have time to move apart. Fusion Initiated by heating and compressing a fuel target (typically D-T). Energy is delivered to outer layers of the target using e.g. laser light Short confinement time high density and very efficient beam required. Chain reaction seeked

14 Magnetic confinement Twist for toroidal magnetic field - Axial current through plasma (tokamak) - external helical current-carrying winding (stellarator)

15 Tokamak Tamm and Sakharov in the 1950s Today the leading technology, but not necessarily the final solution! poloidal field is created by a current in the plasma itself!

16 JET, Joint European Torus Culham, UK 32 toroidal coils Observations began 1983

17 Stellarators No plasma current, no charge separation more stable operations than in tokamaks complex coils, hard to manufacture Invented by Lyman Pitzer Large Helical Device, Japan

18 How plasma is heated? Ohmic heating - passing current through plasma - becomes less effective as temperature increases Neutral beam injection - high energy neutral atoms to ohmically heated plasma. - atoms immediately ionized and trapped by the magnetic field and transfer part of their energy to the plasma particles Radio wave heating - high frequency waves generated outside the torus - energy can be transferred to charged particles (electron, ion cyclotron resonance)

19 Progress in fusion self-sustained energy production n e T E > m 3 s kev JT = Japan Torus TFTR = Tokamak Fusion Test Reactor, Princeton, New Jersey there has been enormous progress in fusion performance during the last 30 years

20 ITER Tokamak research project to "demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes first proposed 1985 by Soviet Union global scale project: European Union, India, Japan, China, Russia, South- Korea, USA cost 9.3 billion dollars goals: MW of fusion power sustained for seconds (compare with JET peak: 16 MW for less than a second) times more thermal energy from fusion than consumed by heating Deuterium-Tritium cycle

21 ITER will be located at Cadarache close to Marseilles To be ready

22 The task of ITER is to demonstrate the net energy production (ignition). Only after ITER operational fusion energy will become possible (2050?) DEMO

23 Parameters of ITER Major radius of plasma Minor radius of plasma Plasma current Magnetic field Fusion power Heating power (RF and neutral beams) Plasma density Plasma temperature Neutron load on the inner wall 6,2 m 2,0 m 15 MA 5,3 T MW max 75 MW m -3 1, K 0,57 MW/m 2 Price: > 5 x 10 9 EU 40%, France, USA, Russia, China, Japan, South Korea, each 10%