Patent Number: 042630967
Section: summary

This invention relates generally to devices which utilize a toroidal magnetic field and more particularly to toroidal coil systems with demountable toroidal coil elements disposed about a toroidal containment vessel. One of the more challenging problems in the design of toroidal plasma devices is the development of suitable assembly/disassembly and maintenance methods. The fact that the toroidal field coils of such a device link a toroidal plasma containment vessel makes access to the interior of the device very difficult. The present invention finds particular utility in experimental devices and the use thereof in experimentation and investigation in respect to toroidal plasma devices like those of the tokamak type. In some devices, poloidal coils are located inside the toroidal coils. In such cases, assembly has been performed by winding the poloidal coils in place or by employing demountable toroidal coils. This presents some difficult manufacturing and maintenance problems. In addition, for devices employing superconducting toroidal coils, demountable joints would be very difficult if not impossible. In any event, a high incentive exists to develop toroidal coil concepts which allow easy access to the envelope formed by such coils. There are at present two basic approaches for providing access through the toroidal coils. One is to remove an entire sector of the device, including a portion of the toroidal coil system. This involves severing the toroidal plasma containment vessel, moving massive sections of the assembled magnets, and breaking hundreds of connections. The other approach is to make the magnet very large in order to provide access between coils. Large coils are required when their number is reduced in order to maintain an acceptable field ripple at the plasma surface. Unfortunately, this implies poor utilization of the available magnetic volume. In accordance with the present invention, a toroidal coil system is provided which utilizes two different types of coils; one in which field generating coils link the toroidal containment vessel and one in which field correcting coils do not link any component of the device. The unlinked coils can be easily pulled back from the device, providing much improved access. Consider first the field generating coils which link the plasma containment vessel and other components of the device. They can be of arbitrary shape, such as circular or D-shaped. The total ampere-turns (NI).sub.L of these coils is given by ##EQU1## where R.sub.o is the major radius of the torus, B.sub.o is the magnetic field at R.sub.o, and .mu..sub.o is the permeability of free space. One configuration for maximizing access is to employ two large fixed return legs each carrying 1/2 (NI).sub.L ampere-turns. This structure is relatively fixed about the vessel. This arrangement provides convenient access to the toroidal containment vessel, but unfortunately a large field ripple will result. To reduce the large field ripple, current in the opposite direction is provided to cancel partially the current around the outside of the toroidal containment vessel in the large return legs of the linking coils. This is accomplished by the field correcting unlinked coils. The unlinked coils have two C-shaped sections or legs joined at their open ends. One leg overlaps the outer sections of the linking coils and carries current in the direction opposite to that in the outer sections of the linking coils. The other section or leg is placed at a different azimuthal position. The azimuthal angle between the bights of the two C-shaped sections will be multiples of 360.degree./N where N is the total number of coils in a conventional magnet design which are required to obtain a given field ripple. These C-shaped coils can be relatively easily moved into place around the torus, and removed for maintenance, for they do not link the toroidal vessel. When clustered in position around the torus, the configuration will resemble the usual magnet configuration. The exact location of the return legs will be chosen to obtain the desired cancellation of the effect of the field ripple in the plasma region. Therefore, it is a primary object of the invention to provide a demountable coil system for a toroidal device for ready access and maintenance. A further object is to provide a toroidal coil system for such devices wherein field generating coils at a few locations link a toroidal containment vessel, with smoothing of the toroidal field provided by removably mounted coils not linking the vessel.