Patent Number: 043495060
Section: summary

A tokomak magnetic confinement fusion reactor confines a plasma within a toroidal plasma region by the use of a group of field coils that each encircle the plasma region. When the plasma, which may consist of deuterium and tritium, is "ignited," the plasma generates heat by nuclear fusion. Some of the heat escapes the plasma region and may be used to generate electricity. Successful operation of the reactor requires that sufficient heat escape to prevent such a high plasma temperature that the plasma becomes magnetohydrodynamically unstable and causes complete loss of plasma. At the same time, excessive heat should not escape from the plasma that could quench the fusion reaction. A mechanism is required to control heat loss from the plasma to maintain it at a desired operation point. A major source of heat loss from the plasma arises from ion heat conduction due to ripple in the toroidal magnetic field that confines the plasma. Toroidal field ripple is the amount of variation of the toroidal magnetic field as measured along a circular path extending along the toroidal plasma region. The principal contribution to field ripple is the geometrical arrangement of the field coils which encircle the toroidal plasma region, and arises because of the spacing between of the outer legs of the field coils from one another. It is generally desirable to enable operation of the reactor with minimal field ripple to minimize heat losses during starting up of the reactor. A mechanism which enables controllable variations of magnetic field ripple, and which does not require large amounts of additional space within the already-filled space of typical magnetic confinement reactors, would be of considerable value. OBJECTS AND SUMMARY OF THE INVENTION One object of the present invention is to provide an apparatus for controlling the energy state of the plasma in a magnetic confinement fusion reactor. Another object is to provide an apparatus for controlling magnetic field ripple in a tokamak type of fusion reactor. Another object is to provide an apparatus for controlling the heat flow out of magnetic confinement fusion reactors. Another object is to decrease the ripple in a magnetic containment fusion reactor during startup, and then to increase the ripple to enhance heat propagation from the plasma during normal operation. Another object is to provide a method for controlling the energy state of a magnetic confinement fusion reactor. In accordance with one embodiment of the present invention, an apparatus is provided for controlling the plasma energy state in a tokamak-type fusion reactor, which requires minimal additional space within the reactor. The apparatus includes a magnetic shield structure lying radially between the plasma region and the toroidal field coils of the reactor, with the shields constructed of a magnetic material which has a temperature-dependent saturation magnetization. The apparatus also includes a mechanism for controlling the temperature of the shields, to thereby vary their influence on the magnetic field ripple in the reactor and therefore the heat loss from the plasma. The shield can include primary shield sectors lying directly between the outer leg of each toroidal field coil and the plasma, so that the magnetic ripple produced by the primary sectors counters the ripple produced by the spaced toroidal field coils. The range of values of ripple achievable can be increased by means of additional secondary magnetic shield sectors placed in the spaces between the primary shield sectors. An entire shield structure which includes the primary and secondary shields, is a largely self supporting keystoned shell. In optimal designs, the attraction between adjacent magnetic shield sectors overcomes the outward force due to radially adjacent toroidal field coils, so that a relatively modest net inward force is created which keeps the sectors under compression. Temperature control of the shields can be produced by heat transfer fluids, such as cold water for rapidly cooling the shields and steam or pressurized hot water for heating them. The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description read in conjunction with the accompanying drawings.