9. Nuclear Fusion 9.1 Nuclear Fusion Principles 9.2 Approaching the Fusion Conditions 9.3 Controlled Fusion Experiments
9.2 Approaching the Fusion Conditions 9.2.1 Rationale for Fusion Energy 9.2.2 Uncontrolled Nuclear Fusion: The H-bomb 9.2.3 Approaches to Nuclear Fusion 9.2.4 Controlled Fusion: Gravitational Confinement 9.2.6 Controlled Fusion History 9.2.7 The Lawson Criterion 9.2.8 Breakeven and Ignition
9.2.1 Rationale for Fusion Energy Searching for clean energy sources….
Rationale for Fusion Energy (II) The Fusion Energy Hope
Rationale for Fusion Energy (III) • Fusion Energy vs. other sources
Rationale for Fusion Energy (IV) • Current Alternatives to Fossil Fuels
Rationale for Fusion Energy (V) • Environmental impact of energy sources
Rationale for Fusion Energy (VI) Fuel requirements
9.2.2 Uncontrolled Fusion Thermonuclear bomb (“Ivy Mike”, 1952)
9.2.4 Controlled Fusion: Gravitational Confinement The Sun: A Very Old Fusion Reactor
9.2.5 Controlled Fusion History • Serious research towards obtaining useful power by "controlled thermonuclear reactions" (nuclear fusion), began about fifty years ago at the end of the second world war • Deuterium was discovered in 1932 (Urey et al.) • Thermonuclear reactions had been looked for in gas discharges and exploding wires loaded with deuterium before the WWII, but none had been found.
Controlled Fusion History (II) • Nuclear reactions from colliding Deuterons (Rutherford, 1934) • Pinch effect (Bennett, 1934) • Star fusion cycle theory (Bethe, 1939) • Burning Deuterium suggested as source of energy (Fermi, 1942) • By the end of the II WW fusion power started to be considered as a serious possibility • Preliminary studies at Los Alamos in connection with the possibility of a “Super” fusion bomb
Controlled Fusion History (III) • Fusion research programs started in England investigating toroidal systems, with the aim of confinement by the pinch effect (G.P. Thomson, P. Thonemann, 1946) • The fusion related work became classifiedbecause of its potential as a powerful neutron source for making plutonium for weapons • In 1949 Reynolds and Craggs searched for neutrons in sparks in deuterium at atmospheric pressure, but none was found.
Controlled Fusion History (IV) • Russian fusion program: Sakharov and Tamm suggested "magnetic insulation" (1950) • Development of H-bomb (Teller et al., 1951) • Figure-of-eight "Stellarator” built in Princeton (Spitzer, 1951) • First tokamak-like experiment (Artsimovich, 1955) • Russian work on D-T reported to the UK Atomic Energy Research Lab. (Kurciatov, 1956)
Controlled Fusion History (V) • By 1958 the fusion had gathered enormous momentum and it was no longer classified • Fusion made a major impact at the 1958 "Atoms for Peace" conference in Geneva, where an impressive exhibition, including working models, was assembled • Initial enthusiasms were damped in the 60’s from the results of more challenging experiments
Controlled Fusion History (VI) • During the 1973 "Energy Crisis" large scale tokamak and mirror machines were planned and built in the years that followed. • The "Inertial confinement" concept emerged from the American weapons program: a succession of miniature hydrogen bombs, millimeters in diameter ignited by intense lasers or heavy ion beams • JET, the Joint European Torus first came into operation in 1983, still the largest fusion device in the world
9.2.6 The Lawson Criterion • A fusion plasma in equilibrium must have its energy losses made up by continuing fusion in its core. • The loss rate is estimated by the total thermal content in the electrons as well as the ions, divided by the energy confinement time. • The heating rate due to fusion reactions is a volume integral of the fuel ion densities, times the cross section, times the energy liberated. • If the energy of the 14 MeV neutrons is captured for further use, it can be counted in the energy gain.
The Lawson Criterion (II) • Once a critical ignition temperature for nuclear fusion has been achieved, it must be maintained at that temperature for a long enough confinement time at a high enough ion density to obtain a net yield of energy. • In 1957, J. D. Lawson showed that the product of ion density and confinement time determined the minimum conditions for productive fusion, and that product is commonly called Lawson's criterion.
The Lawson Criterion (III) • The Lawson criterion for fusion energy breakeven is found by evaluating the cross section at the “optimum temperature” for fusion energy yield • This temperature is not necessarily equal to the temperature at peak cross section. • The Lawson criterion states that n t> 1020 s/m-3
9.2.7 Breakeven and Ignition • On the path to show the feasibility of a commercial reactor, there are two typical steps: • breakeven, where the energy obtained in the core of the plasma by fusion reactions is equivalent to the input energy in the plasma • ignition, where the helium particles produced in the reactions are supporting energetically further reactions of fusion of fresh D-T fuel injected in the plasma
Breakeven and Ignition (II) • Temperature, density and confinement time are the fundamental parameter to characterize a fusion reactor • Good thermal insulation of the plasma as well as physical containment of the plasma in the reactor must be assured • Breakeven condition is of less stringent then breakeven and has, in magnetically confined plasma, an T.n.tE, 6 times smaller than the ignition criteria • "Ignition Condition ":n.T.tE > 5.1028 K . m-3s
Breakeven and Ignition (III) • For a viable magnetically confined D-T reactor the typical parameters for the core of the plasma are: • temperature of the plasma has to reach over 100 million degrees • plasma density, n, in the order of 1020 particles per cubic meter • confinement time, tE, which measures the rate of energy loss of the plasma, has to be over one second; typically 1-2s
Breakeven and Ignition (IV) • The total fusion power produced in the plasma can be computed from the energy produced by each fusion reaction and from the fusion cross section • The number of fusion reactions per second per each particle of the species A are given by (analogous to the collision frequency) where s is the fusion cross section and u is the relative collision velocity • Typically the product su is averaged over a Maxwellian distribution
Breakeven and Ignition (V) • Since fusion occurs by collisions with particles of species B the number of fusion reactions per second per volume unit is given by • If Efusis the energy released by each fusion reaction the fusion power per volume unit (fusion power density) produced in the plasma will be
9.3 Controlled Fusion Experiments 9.3.1 Inertial Fusion Energy (IFE) 9.3.2 Magnetic Fusion Energy (MFE) Research 9.3.3 The TOKAMAK 9.3.4 The ITER Fusion Reactor 9.3.5 National Spherical Torus Experiment 9.3.6 Field Reversed Configuration 9.3.7 Progress of Fusion Research
9.3.1 Inertial Fusion Energy (IFE) • An IFE power plant must compress a few milligram of D-T fuel to a density approximately thirty times the density of lead • A smaller fraction of the fuel must be heated to a temperature over 100-million degrees Celsius to ignite a propagating fusion burn • Each milligram that burns releases 340 MJ, the energy equivalent of burning over 10 kilograms of coal.
Inertial Fusion Energy (II) • Hollow spherical capsule filled with D-T gas is chilled using cryogenic liquid helium • Thin layer of D-T ice formed inside the shell. • Tritium radioactive decay vaporizes ice producing a layer of highly uniform thickness
Inertial Fusion Energy (III) • Schematic illustration of a heavy-ion driven IFE target showing the a hollow plastic capsule containing a layer of D-T ice, suspended inside a cylindrical metal-lined (“hohlraum”)
Inertial Fusion Energy (IV) • High-velocity heavy ions penetrate into absorber heating it to a high temperature • Most heating occurs deep inside the absorber
Inertial Fusion Energy (V) • Hot absorber heats the inside surfaces of the hohlraum by radiation, with the hohlraum surfaces reaching temperatures three-million degrees • Outside surface of the capsule vaporizes into a high-pressure plasma
Inertial Fusion Energy (VI) • Expanding material from the capsule surface accelerates the inside layer of D-T radially inward
Inertial Fusion Energy (VII) • The compressed D-T fuel reaches maximum density allowing ignition and a propagating burn out into the denser compressed D-T fuel
Inertial Fusion Energy (VIII) • The outside of a NOVA (LLNL) hohlraum glows as 10 laser beams heats the inside surface
Inertial Fusion Energy (IX) • The targer chamber or the 50 kJ NOVA laser (LLNL)
Inertial Fusion Energy (X) • The 192 laser beams in the National Ignition Facility (LLNL) will heat the inside surface of a hohlraum with high uniformity
Magnetic Fusion Energy Research (II) • The magnetic fusion energy confinement is to date the most developed approach for fusion research • Experiments are presently considering mostly toroidal geometries • Older (linear) mirror concepts are not part of the mainstream fusion research anymore • The research emphasis is now slowly shifting from very large devices (extremely expensive) to medium/small-sized concepts with more attractive potential for the economics of future power plants
9.3.3 The TOKAMAK • TOKAMAK is an acronym from the Russian words “toroidal chamber magnet coil” • The TOKAMAK is the most developed (and funded) experimental approach to fusion • First experiments were toroidal devices of circular cross sections (up to the TFTR in Princeton, now decommissioned • More modern concepts feature a “D” shaped cross section for increased stability: JET (EU), DIII-D (USA), JT60 (Japan)
The TOKAMAK (III) • The tokamak-principle is characterized by a torus-shaped plasma chamber which confines the plasma by three overlapping magnetic fields: • The toroidal magnetic field generated by the so-called toroidal field coils • The poloidal magnetic field generated by the poloidal field coils (therefore a tokamak cannot be operated stationary but only in a pulsed mode), and • The vertical magnetic field generated by the vertical field coils (non shown in the picture)
The TOKAMAK (IV) • The combined toroidal and poloidal magnetic fields produce the desired twist of the magnetic field lines to compensate for the particle drifts • The poloidal field is produced by the toroidal current induced in the plasma (like a transformer) by the primary poloidal field coils • The vertical magnetic field is added to compensate for the natural tendency of the plasma current ring to expand (like any coil that carries current)
The TOKAMAK (V) Joint European Torus (JET), Culham, UK
The TOKAMAK (VI) The DIII-D Tokamak (San Diego, CA, USA)
ITER (II) • ITER design parameters
ITER (III) • International collaboration project: Canada, Europe, Japan, and Russia (formerly also USA) • ITER: International Thermonuclear Experimental Reactor (“iter” means also “the way” in Latin) • Largest fusion experiment (TOKAMAK type) designed in detail so far • First machine designed to actually produce positive net fusion power • Superconducting magnet design • Construction and future direction are still being evaluated
9.3.5 NSTX • National Spherical Torus Experiment (NSTX) constructed by the Princeton Plasma Physics Lab. (PPPL) • Based on a TOKAMAK-like principle • NSTX will be used to study the physics principles of spherically shaped plasmas • NSTX is a "proof of principle" experiment and therefore will employ deuterium plasmas only • NSTX will produce a plasma that is shaped like a sphere with a hole through its center, different from the "doughnut" shaped plasmas of conventional tokamaks
NSTX (II) • The NSTX design