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4. Magnetic Confinement Fusion - MAGNETIC CONFINEMENT...

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MAGNETIC CONFINEMENT FUSION © M. Ragheb 11/22/2008 INTRODUCTION The goal of the fusion research effort is to derive energy from the fusion of light atomic nuclei. Nuclear fusion is an important natural process. Many chemical elements originate from hydrogen through fusion in the process of nucleo-synthesis. Fusion is the energy source of the sun and stars. Under terrestrial conditions the two hydrogen isotopes, deuterium D and tritium T, fuse most readily. In the process, a helium nucleus is produced. It is accompanied by the release of a neutron and energy. Just 1 single gram of fusion fuel could generate 90,000 kW.hrs of energy in a power plant equivalent to the combustion heat of 11 metric tonnes or 11,000,000 grams of coal. The magnetic confinement approach exploits the interaction of charged particles with magnetic fields. Charged particles are deflected by a magnetic field and, if the field is strong enough, particles will orbit around the field lines, gradually progressing along it if they have some longitudinal velocity component. This feature forms the basis of magnetic confinement fusion, which has been under investigation since the 1950s. PLASMA AMPLIFICATION FACTOR Q An important design parameter for a magnetic fusion reactor is its plasma amplification factor: Q, defined as the ratio of the thermonuclear power produced to the power input to generate and heat the plasma to thermonuclear fusion temperatures: thermonuclear input P Q = P (1) In a closed system, such as a toroidal device, large values of Q are possible. Once the energy in the ions released from the fusion reactions in the plasma exceeds the radiation and other losses, plasma ignition should occur. The use of neutral ion beams injection or radiofrequency heating can thus be stopped, and the plasma burn becomes self-sustained, as long as the fusion fuel continues being supplied, and the plasma remains confined. In an open system such as a mirror fusion reactor, because of the inevitable end losses, it is unlikely that ignition can be achieved. Such devices would have to be used as a driven power amplifier. With a low value of Q, energy would have to be supplied continuously and the fusion reaction would amplify the energy input to the plasma by the factor Q. In a single cell plasma, Q may not exceed about 1.2. The magnetic fusion approach attempts at developing devices with a high value of Q. The enhancement of Q involves the reduction of end losses in open systems, particularly of high energy ions. IGNITION CONDITION: LAWSON BREAKEVEN CRITERION
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