Fusion: Energy of the Future

1. Fusion: Energy of the Future Part 2 By Joel Barfield and Marcus Krefting

2. Questions • Why is ohmic heating not effective at high plasma temperatures? • What is ITER’s mission?

3. Topics covered • Ohmic Heating • Plasma resistivtity • Electron Cyclotron Resonance Heating • ITER • Superconducting Coils • Vacuum Vessel

4. Heating of plasma • Ohmic Heating • Electron Cyclotron Resonance Heating

5. Ohmic Heating • What is the resistance of a plasma? • Up to now we have talked about current flowing though a plasma as though it was a perfect conductor. • The truth is that plasma does have a resistance to current flow.

6. Resistance of a Plasma • What causes the Plasma to have a resistance? • Collisions between Electrons and Ions • Since Fusion takes place in a fully ionized plasma, we will restrict our discussions to that case. • In a fully ionized plasma, there are only charged particles, so we will only need to look at the collisions of charged particles Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

7. Plasma Resistivity • The fluid equations of motions are Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

8. Momentum • As the terms which represent the the momentum gain of the electron fluid caused by the collisions with ions and vice versa. • Because momentum must be conserved Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

9. Momentum • Because Pei must be proportional to the Coulomb force, density of electrons and ions, and the relative velocity of the two fluids. It can be written like this • is the specific resistivity of the plasma. • And so the collision frequency is Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

10. Collision of an electron and ion

11. Collision of an electron and ion • The Coulomb Force • The change in momentum is approximately Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

12. Collision • After some simple algebra it can be shown that the collision frequency is • After some simple substitutions, we see that is equal to Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

13. Ohm’s law for Plasma • Since , it can be shown that . • This is simply Ohm’s Law • The problem is with ohmic heating is that because the resistance is proportional to ,heating at temperatures near the ones required for fusion is a very slow process Chen, Francis; "Introduction to Plasma Physics and Controlled Fusion Volume 1" pg178-185

14. Electron Cyclotron Resonance Heating • Electron Cyclotron Resonance Heating, or ECRH, employs the resonance of electron gyromotion with high power, high frequency electromagnetic waves to deposit additional power into magnetized plasma. http://www.rijnh.nl/n3/n3/f4.htm

15. Electron Cyclotron Resonance Heating • The principle of electron cyclotron resonance. In red is shown the gyromotion of an electron in the magnetic field, B. The blue arrows indicate the electric field of a resonant, right handed polarized electromagnetic wave at four different phases of the particles motion. http://www.rijnh.nl/n3/n3/f4.htm

16. Electron Cyclotron Resonance Heating • Because the wave satisfies the electron cyclotron resonance condition, w - k|| v|| = wce, the electron experiences a continuous acceleration of the velocity perpendicular to the magnetic field. Here w and wce are the wave frequency and frequency of the electron gyromotion ("electron cyclotron frequency"), respectively, while k|| is the wave vector parallel to B and v|| the parallel velocity of the electron http://www.rijnh.nl/n3/n3/f4.htm

17. Electron Cyclotron Resonance Heating • The combination of high frequency and, consequently, short wavelength (typically a few mm), high power waves that can be focused in a beam with a cross section of only a few centimeters, and the resonant plasma-wave interaction provides a way to deposit several hundreds of kWs into a plasma volume as small as 10 cubic centimeter. http://www.rijnh.nl/n3/n3/f4.htm

18. ITER • What is ITER • International Thermonuclear Experimental Reactor • ITER's mission is to demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes. To do this, ITER will demonstrate moderate power multiplication, demonstrate essential fusion energy technologies in a system integrating the appropriate physics and technology, and test key elements required to use fusion as a practical energy source. • ITER will be the first fusion device to produce thermal energy at the level of an electricity-producing power station. It will provide the next major step for the advancement of fusion science and technology, and is the key element in the strategy to reach the demonstration of an electricity-generating power plant (DEMO) in a single experimental step. www.iter.org

19. ITER • ITER is an experimental fusion reactor based on the "tokamak" concept - a toroidal (doughnut-shaped) magnetic configuration in which to create and maintain the conditions for controlled fusion reactions.The overall ITER plant comprises the tokamak, its auxiliaries, and supporting plant facilities. • In ITER, superconducting magnet coils around a toroidal vessel confine and control a mix of charged particles - the "plasma" - and induce an electrical current through it. www.iter.org

20. ITER www.iter.org

21. Coils • Superconducting toroidal and poloidal magnetic field coils confine, shape and control the ITER plasma. The ITER superconducting magnet systems consist of 18 Toroidal Field (TF) coils, 6 Poloidal Field (PF) coils, a Central Solenoid (CS) coil, Correction Coils, and related structures. They are combined in an integrated overall assembly which simplifies the equilibration of electromagnetic loads. All coils are cooled by a supercritical helium flow maintained by cryogenic circulation pumps. • The CS coil weighs about 840 t, and is about 12 m high and 4 m in diameter. It consists of a stack of 6 electrically-independent modules to allow good control of the inboard plasma shape. The stack is compressed to maintain its integrity under all operating conditions. The coil is wound in hexa- or double-pancakes, with all the joints in low field regions. • Each TF coil weighs about 290 t, and is about 14 m high by 9 m wide. Its manufacturing process is being developed and tested in the TF Model Coil Project (L-2). • Both CS and TF coils use a similar superconductor configuration. The superconductor is a Nb3Sn cable-in-conduit type. For the TF coils, some 1100 wires, about 0.7 mm in diameter, are twisted together inside a metal tube about 4 cm in diameter to form the conductor in lengths of 820 m. In use, supercritical helium flows inside the tube around the wires and down a central gap to cool them. www.iter.org

22. Coils • The superconducting Nb3Sn compound is brittle, and initially the wires contain separated Nb and Sn (as well as a copper matrix) which react together after a 200 hour heat treatment at 650°C. This can only be performed after all cabling and conductor bending operations are complete, but before any temperature-sensitive coil components are added (such as the coil electrical insulation). • After the conductor has been wound into the shape required for the coil and heat-treated, it is electrically insulated with a wrap of glass fibre and kapton. Extra structural material is added outside the insulation and the glass-kapton is filled with liquid epoxy resin which is then cured. Conductor, insulation and structural reinforcement are bonded together to form pancakes and then a rigid winding pack. This winding pack is then further reinforced by putting it inside a steel housing. • Because of the brittleness of Nb3Sn, its manufacture is a relatively expensive process. However, the PF coils occupy a field region where NbTi strand can be used, helping to keep their costs down. The PF coils are, however, geometrically linked with many systems, making their replacement difficult, so each coil is arranged with redundant turns so that "incipient" short circuits can be detected before they cause damage, and faulty double pancakes isolated. All coils can operate at their nominal current in this "backup" mode, although a lower cooling inlet temperature may be needed. As a further precaution, the coils can be rewound in situ or removed/replaced, although at a price in machine down-time. www.iter.org

23. Vacuum Vessel • The main vessel and its internal components have a number of functions: • to attenuate and absorb neutron energy and flux from the plasma to a level tolerable for the magnets and surrounding equipment, as well as to allow personnel access near the machine a short time after plasma shutdown; • to provide an ultra-high-quality vacuum to allow the plasma to operate; • to divert helium from the fusion reactions plus impurities coming from the surroundings to locations where the power and particles can be unloaded and pumped away; • to allow access for plasma heating, fuelling, and diagnosis, and to permit testing of DEMO-relevant blankets and materials in special test modules, as well as remote maintenance of all in-vessel components; • to act as the first line of defence to confine any coolant leaks so that radioactive materials are not spread outside the plant. • The blanket absorbs heat radiating from the plasma as well as providing neutron shielding. The divertor exhausts the flow of energy from charged particles produced in the fusion reactions and removes helium and other impurities. Port plugs access the plasma for heating, diagnostics, reactor blanket testing, and remote maintenance. All these are attached in various ways to the vacuum vessel which can remove decay heat from itself and all in-vessel components to the environment by the passive process of natural water convection even when all other cooling systems are not working. www.iter.org

24. Vacuum Vessel www.iter.org

25. Answers to the Questions • Why is ohmic heating not effective at high plasma temperatures? • because the resistance is proportional to • What is ITER’s mission? • ITER's mission is to demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes.

26. Any Questions?