SA, 3/2017 Chapter 5 Notes on fusion reactions and power balance of a thermonuclear plasma! Stefano Atzeni See S. Atzeni and J. Meyer-ter-Vehn, The Physics of Inertial Fusion, Oxford University Press (2004, 2009), Chapter 1 and Chapter 2.
Exoenergetic reactions: fusion and fission reactions Exoenergetic reactions = energy releasing reactions 1 + 2 = > 3 + 4 +... + Q, with energy Q > 0 For Einstein mass-energy relation, Q > 0 if the total mass of the reaction products is smaller than the mass of the reactants Equivalently, if we consider the nuclear binding energies B: Exoenergetic reactions < = => average binding energy per nucleon increases 2
a few fusion reactions the easiest deuterium cycle D + 3 He --> α (3.67 MeV) + p (14.7 MeV) a dream in the Sun (first of a chain) p + p --> D + e + + ν
5
Fusion reaction cross sections pp cross-section extremely small: 3 x 10-26 barn at energy of 10 kev 6
For energy production: thermonuclear fusion beam-target reactions (between an accelerated nucleus and target at rest) or! beam-beam reactions cannot be used for net energy production, because scattering and slowing down dominate over fusion (accelerated nuclei are slowed down before they fuse; the energy they lose cannot be recovered) energy production requires a hot plasma => thermonuclear fusion! In a hot plasma fast nuclei (ions) collide with other nuclei and electrons; the energy lost by one particle is gained by another particle within the same thermal distribution
Thermonuclear reaction rate Volumetric reaction rate for the reaction between nuclei of species 1 and 2 = number of reactions per unit volume and unit time where n 1 = number density of nuclei 1 n 2 = number density of nuclei 2 δ 12 = Kronecker δ <σv> = Mawellian averaged reactivity Maxwellian averaged reactivity: product of the cross section σ(v) times the relative velocity of the reacting nuclei, averaged over the Maxwellian velotityy distribution <σv> depends on temperature only (for a given reaction) 8
Maxwellian averaged reactivities D + T => α + n + 17.6 MeV has by far the largest reactivity DT reactivity is maximum at T = 64 kev At T < 60 kev is at least 10 times larger than the reactivity of any other reaction 9
10
Even for DT, temperature > 5 kev required for thermonuclear plasma self-sustainement A (simplified) necessary condition for DT plasma self-sustainement: at 4.2 kev fusion alpha particle power exceeds bremsstrahlung power D + T => α + n + 17.6 MeV 14.1 MeV neutrons: leave the plasma; 3.5 MeV α-particles: can be slowed in the plasma. Plasma emits bremsstrahlung Plasma temperature self-sustainement: fusion α-particle power Bremsstrahlung n D n T < sv > Q DT 5 > n e n i C b T < sv > Q DT 20 > C b T (assuming equimolar pure DT plasma) ideal ignition temperature 11
Steady-state plasma power balance power balance (per unit volume): losses: bremsstrahlung n 2 T 1/2, where n is the electron density other, characterized by an energy confinement time τ: 3 n T/τ (assuming equal densities and temperatures for electron and ions) input: fusion-α power = (1/5) fusion power n 2 <σv> auxiliary heating: (fusion power)/q Q: energy multiplication [1/Q: fraction of re-cycled fusion energy] Q = : ignition (self-sustainment of plasma temperature) 12
13
Lawson criterion T = 10-20 kev nτ 2 x 10 20 m -3 s
Triple product n τ T 15
Magnetic and inertial plasma confinement magnetic confinement fusion (MCF): quasi-steady state fusion power release from low density plasma (10 20-10 21 m -3 ), in quasi steady-state, contained by appropriately shaped magnetic fields inertial confinement fusion (ICF): explosive fusion energy release from a strongly compressed plasma; theplasma pressure cannot be sustained by any means. Therefore the compressed plasma is confined by its inertia only, and remains compressed for a time τ R/c s, where R is a characteristic dimension (e.g. radius of a plasma sphere, and c s is the sound speed). 16
Magnetically confined thermonuclear fusion plasma T > 10 kev plasma pressure (p = 2nk B T) contained by magnetic field pressure B 2 /2µ 0 ; in practice p = 2 n k B T = β B 2 /2µ 0 with β 0.1 for T = 5 tesla, n few units x 10 20 m -3 energy confinement times τ Ε 1-5 s T = 10-20 kev n 10 14 cm -3 τ 1-5 s
18
19