Method of sustaining a radial electric field and poloidal plasma rotation over most of the cross-section of a tokamak

#The present invention relates generally to a method for increasing energy confinement and controlling transport in the plasma of a tokamak. More particularly, the present invention relates to a method of creating and sustaining a radial electric field throughout a substantial portion of the cross-section of the plasma. The configuration and magnitude of the radial electric field that may be achieved is determined by the apparatus used to create the radial electric field and plasma and by the relationship of apparatus parameters to the resulting radial electric field.

#The apparatus for toroidal magnetic confinement that is most popular in controlled fusion research today is the tokamak device. To date, the experiments that have been performed with tokamaks to create high temperature plasmas have been of short duration. The duration and energy confinement of the tokamak plasma must be increased to produce useful amounts of energy with this device. A radial electric field within the plasma of a tokamak has been experimentally shown to increase energy confinement. Theoretical work has been the basis of the proposition that radial electric fields can reduce particle and energy loss from tokamak plasmas. Experimental results reported by Oren et al, J. of Nuclear Materials 111 & 112, 34, (1982) demonstrated that overall confinement time would increase by a factor of 10 when a radial electric field was created in the plasma of a tokamak by a cold cathode or a tungsten filament. Particle and energy confinement are known to be closely related. The radial electric field that was created in the plasma by this method was of significant magnitude only at the extreme edges of the plasma. A problem which arises from longer duration plasmas is the accumulation of impurities and if fusion is occurring, fusion waste products such as helium ash. Impurity accumulation within the plasma was demonstrated by Oren et al. when a negative potential was induced within the plasma. The accumulation approached a constant value as the radial electric field decreased.

#Taylor et al. Phys. Rev. Letters. 42. 446 (1979) reported that electron injection from a tungsten filament would suppress impurity influx. The injection of cold electrons was reported to reduce the sheath potential and decelerate ions before they hit the wall reducing impurity production at the wall. Control of the potential at the edge of the plasma was recognized as a mechanism for controlling production of impurities.

#The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an embodiment of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:

#FIG. 1 illustrates the upward path of the unidirectional electron beam.

#FIG. 2 illustrates a poloidal cross section of a tokamak during operation of the present invention.

#To further understand the present invention, it should be realized that the potential gradient that is created within the plasma is the result of a unidirectional electron beam which circulates around the main plasma along magnetic field lines. The preferred embodiment of the invention utilizes the injected electron beam to create and sustain the tokamak plasma configuration as well as to create the radial electric field within the plasma.

#FIG. 1 illustrates the path of the unidirectional electron beam injected from a cathode, 1, in a toroidal direction. The beam path, 3, follows an upward helical path of radius approximately that of the major radius of the tokamak. The path terminates at an anode, 5. Center line, 7, is the center of the major radius of the tokamak. A toroidal magnetic field within the tokamak creates circular lines of magnetic force. A circular path would allow a beam to traverse one circuit and then encounter the cathode. In order to avoid the cathode, a vertical magnetic field was imposed of strength 7-10 G near the cathode. This strength was shown to be sufficient for an electron beam injected at energy between 250 ev and 500 ev into a tokamak of major radius 59 cm and minor radius of 10 cm with a toroidal magnetic field between 3.43 KG and 4.48 KG.

#FIG. 2 illustrates a poloidal cross-section of a tokamak during operation with an electron beam injection. FIG. 2 illustrates the positions within the vacuum vessel of the cathode, 1, top limiter, 19, side limiter, 21, diverter coils, 17, and external vertical field coils, 25. The cathode, 1, is positioned at one location in the vacuum vessel. Several top and side limiters, 19 and 21, are positioned around the major circumference of the vacuum vessel. Diverter coils, 17, and external vertical field coils, 25, are continuous around the vacuum vessel 23 at the position illustrated by FIG. 2. The center line of the major radius of the tokamak is located at the distance equal to the major radius of the tokamak from the center of vacuum vessel, 23, in the direction away from side limiter 21. The cathode, 1, emits electron beam 15. The cathode that is used for the preferred embodiment of the invention comprises close fitting cylindrical shells of lanthanum hexaboride on a carbon rod. The carbon rod is heated by a large ac current and the entire assembly is biased between -250 volts and -600 volts with respect to the anodes. The magnetic field that directs the beam vertically is produced by diverter coils 17, and vertical field coils external to the vacuum chamber, 25. The main plasma 11, carries plasma current of 330 amps to 1000 amps. Top limiter, 19, and side limiter, 21, are anodes, and vacuum vessel, 23, is connected to the