Patent Number: 043549985
Section: description

Generally, the present invention is directed to a method and apparatus for removing ions trapped in a thermal barrier formed between a mirror coil and an end plug in a fusion reactor apparatus, such that these trapped ions are caused to drift into a divertor positioned in the path of said ion drift. The ions are caused to diffuse across the magnetic field and drift across said thermal barrier region due to a bend formed along the normal path of the reactor plasma at a point in the area between the end plug and an adjacent mirror corresponding to the thermal barrier region. The bending of the plasma along a curvature of radius R is generated by one or more magnetic turning coils, such that ions trapped in said regions are caused to drift perpendicular to the centrifugal lines of force generated thereby and the direction of the incident magnetic field. Thus, the trapped ions are caused to drift perpendicular to the plane of the bending. The trapped ions continue to be forced away from the plane of plasma bending along a perpendicular path until they come in contact with a divertor, which thereby acts to remove such ions from the plasma chamber. Although removal of all trapped ions constitutes a continuing power loss to the plasma, it is deemed to be, at worst, equivalent to the energy required to operate the magnetic pump required by the prior art to eliminate such trapped ions. For particles in the plasma which are not trapped in the thermal barrier, but continue to be reflected by magnetic mirrors back and forth in the central cell of the fusion reactor apparatus, the plasma column in the thermal barrier regions on each end of the central cell must be bent in opposite directions. This is so that the displacements of orbits due to the curvature drift imparted to ions in each barrier region are cancelled out for such untrapped, i.e., passing, ions. The particular invention may be more clearly understood with reference to the figures, in which FIG. 1 is a top plan view of a fusion reactor apparatus 10 according to the present invention. As seen in FIG. 1, the fusion reactor apparatus 10 includes a central cell or chamber 12 for plasma generation and confinement. A plasma formed in the central cell 12 is maintained therein by means of a plurality of solenoid coils 14 positioned along the length of the central cell 12. As can be seen, the central cell 12 is an elongated chamber preferably in a cylindrical shape. Coils 14 are positioned with respect to chamber 12 by means of supports 15. To impede particles in the plasma from escaping out the open ends 16 and 18 of the central cell 12, mirror means are provided to cause the plasma to be reflected back into the central cell 12. The mirror means at each end includes a respective mirror coil 20, 22 and an adjacent baseball minimum-B magnet 24, 26. The baseball magnets 24, 26 are so called due to their shape, which approximates the shape of the seam on a conventional baseball. Each mirror coil 20, 22 acts to throttle down the flow of plasma from the central cell 12 towards its adjacent baseball magnet. The baseball magnets 24, 26 act as the end plugs for the plasma. In the absence of particle collisions, the plasma density drops as it expands in cross section as it emerges from the high magnetic field at the throat of the mirror coil. The density drop creates a depression .phi..sub.b in the positive potential. As previously described, this creates a potential barrier to the negatively charged electrons, and therefore serves as the electron thermal barrier between each mirror 20, 22 and its respective end plug 24, 26. The area within each reactor chamber 12 in which such thermal barrier regions are created is marked of at 28 and 30. FIG. 2 illustrates axial profiles electrostatic potential and particle density in the thermal barrier and end plug regions at one end of the cell 12. As seen in the electrostatic potential curve, .phi..sub.e is the base potential of the central cell 12 and .phi..sub.c is the increased potential enabled in the end plug due to the existence of the thermal barrier potential, identified as .phi..sub.b. As can be seen, electrons (e.sup.-) tend to fall upward in this curve into the end plug due to their negative charge, while ions tend to fall down into the potential well .phi..sub.b in the thermal barrier due to their positive charge. An exemplary plasma density curve is also shown in FIG. 2, wherein n.sub.c is the central cell plasma density, n.sub.p is the end plug plasma density and n.sub.b is the plasma density in the thermal barrier. Since energetic electrons are trapped in the end plug, n.sub.p .ltorsim.n.sub.c rather than n.sub.p &gt;&gt;n.sub.c. The present invention is directed at keeping n.sub.b at a minimum, to prevent it from equalling or exceeding the density of passing ions. Referring again to FIG. 1, to deposit heating power in the plasma, conventional high energy beams of neutral hydrogen atoms are coupled thereto at points 32 and 34 on respective end plugs 24, 26. These neutral atoms freely enter the plasma magnetic envelope and are then stripped of their electrons by collisions and retained in the plasma as energetic ions. The retained ions heat the plasma as they gradually slow down, transferring energy to the plasma particles. Also included at the end plugs are ducts 36 and 38 for vacuum pumping and microwave heating of the end plug electrons. As previously described, a difficulty with the thermal barrier concept is that collisions of particles passing across the thermal barrier cause some of these particles to lose energy and be trapped in the thermal barrier region. In time, the trapped particle density would grow until the total pressure would equal or exceed the pressure in the central cell 12. The present invention provides a bend in the plasma column at the thermal barrier region to prevent such an increase in trapped particle density. The bend in the plasma is accomplished by means of turning coils, coils 40 and 42 for bending of the plasma in the thermal barrier region 28, and coils 44 and 46 for bending of the plasma in the thermal barrier region 30. This bend in the thermal barrier, having a radius R as seen in FIG. 1, creates a centrifugal force F on the particles, with F being proportional to the inverse of radius R of bending. The effect of this force, in conjunction with the incident magnetic field, is to create a drift velocity in the plasma ions which is perpendicular to the plane of the bending. That is, the direction of the drift velocity V.sub.d is given by the cross product of the centrifugal force F and the magnetic field B, or: EQU V.sub.d .parallel.F.times.B (1) FIG. 3 is a cross-sectional view of the thermal barrier region 30 of FIG. 1. A plasma zone comprising a predominant amount of plasma is also shown in cross section as plasma region 50. The flux lines of the incident magnetic field flow in a direction into the plane of the cross section, as indicated at B. The centrifugal force exerted in plasma region 50 due to the bending of the plasma in this region is shown diagrammatically at F. The direction of ion orbital drift is upward out of the plasma region 50, as shown at a in FIG. 3. FIGS. 1 through 3 also illustrate the position of divertor means, comprising a divertor 52 positioned with respect to thermal barrier region 28 and a divertor 54 positioned with respect to thermal barrier region 30. As seen more clearly in FIG. 3, the divertor 54 is positioned at the top of the thermal barrier region 30 such that it is in the path of ions as they are caused to drift upwards as a result of the bend in the plasma region 50. The divertor 54, as seen in FIG. 3, strips off impurities from the fusion reactor, and is of a conventional design. The divertor includes particle collection vanes 56 and cryopanels 58. The collection vanes 56 are maintained at approximately room temperature, and the cryopanels 58 are kept at a substantially reduced temperature to trap the gas given off by the collection vanes. The reason for this structure is that the particles which hit the divertor 54 are very energetic. Thus, surface heat loading on said vanes 56 is very large. The vanes are kept at room temperature to help prevent overheating of the cryopanels and not desorb trapped gas on cold vanes such that when particles hit vanes, gas desorbs. Therefore, in operation the divertor 54 pumps ions out of the thermal barrier region 30 by neutralizing the ions that hit the divertor. The ions recombine with electrons in the divertor collection vanes 56 forming a neutral gas. To prevent the gas from drifting back into the plasma region 50, it is trapped in the cryopanels 58 by freezing the gas out. Such cryopanels are used since they are the only structure that has the speed needed to effectively take the gas out of the system. The cryopanels are periodically warmed to enable the gas trapped thereon to be pumped off using conventional pump means. An alternative to the divertor means 54 is the use of the gettering material, a metal that has a chemical affinity for the gases that are coming off. Certain metals, e.g., tantalum or titanium, can absorb enormous quantities of foreign gas. The problem with such materials is that they must constantly be replaced or removed and cleaned and then reinserted. Additionally, such materials also tend to inject other impurities into the plasma. FIG. 4 illustrates a perspective view of the thermal barrier region 30, respective turning coils 44 and 46, and the divertor 54 positioned with respect thereto. The fact that ions trapped in a thermal barrier region are eliminated therefrom, i.e., lost to the divertor, in a drift time much less than the filling time of the thermal barrier, is seen from the following equations. Drift velocity V.sub.d is given by: EQU V.sub.d =T.sub.i /ZeBR (2) where Z is the charge number of the trapped ion, R is the radius of curvature of the bend in the plasma in the thermal barrier region, B is the magnetic field strength, and T.sub.i is the ion temperature. Ions are lost from the barrier region in the period .tau..sub.d, the "drift time", given by: EQU .tau..sub.d =ZeBR.alpha./T.sub.i (3) where ".alpha." is the radius of the thermal barrier region, i.e., the distance an ion must travel from the plasma to the divertor. FIG. 3 illustrates this dimension ".alpha." in the exemplary plasma region 50. The filling time of ions into the thermal barrier region, .tau..sub.g, is given roughly by: ##EQU1## where A is the mass number of the ion and .nu..sub.ii is the proton -proton collision frequency. Under typical operating conditions as set forth by Baldwin et al., and at reasonable radii R, these equations show that .tau..sub.d &lt;&lt;.tau..sub.g, so that the thermal barrier regions 28, 30 do not fill up with trapped ions. Further, since .tau..sub.d &lt;&lt;.tau..sub.g, the loss time is determined by .tau..sub.g. Thus, for impurity ions at a high temperature, equation (4) shows that ##EQU2## e.g., for oxygen with Z=8 and A=16, Z.sup.2 /.sqroot.A=16, whereas for hydrogen Z=1 and A=1, so that Z.sup.2 /.sqroot.A=1. As seen from these examples, impurity ions are removed from the thermal barrier region at a much faster rate than hydrogen ions, thereby enhancing the operation of the associated divertor in controlling the level of impurities in the plasma. Divertor functioning is further enhanced for impurity ions, due to the effects of the electrostatic potential on such ions in the thermal barrier region. This electrostatic potential produces an electric field mainly parallel to the magnetic field in the plasma region 50. Since the potential well for ions is proportional to the atomic weight Z of the ion, it is much deeper for the higher atomic weight impurity ions than for hydrogen ions. As a result, the effective mirror ratio for the impurity ions, a measure of the ease with which particles are reflected from a given region, is correspondingly larger. The trapping and subsequent ejection of impurity ions from the plasma are thus further improved. An exemplary trapped ion drift path in the thermal barrier region 30 is illustrated diagrammatically in FIG. 5. The particle path is shown starting at point 1 in the midst of the plasma region 50, and ending at point 11 against a particle collection vane 56 in the divertor 54. Keep in mind that as the trapped ion is reflected back and forth between the mirror 22 and the adjacent end plug 56 while being caused to drift progressively further away from the plasma region 50, it remains stuck in the potential well of the thermal barrier region 30. The orbit of the ion in the well is merely shifted until the particle hits divertor 54, which comprises an obstruction in the well at this outer ion orbit radius. Lastly, as seen in FIG. 1, for the passing plasma ions not trapped in either thermal barrier region 28 or 30, means must be provided so that the drift velocity imparted to the untrapped ions at each end of the plasma cell 12 is prevented from being additive. This means comprises bending of the two ends of cell 12 in opposite directions with respect to one another. Consequently, the displacement of the ion orbits due to the drift velocity imparted to such ions in each thermal barrier region are opposite in direction, thus cancelling each other out. It is to be understood that the foregoing description merely illustrates a preferred embodiment of the present invention, and that various modifications, alternatives and equivalents thereof will become apparent to those skilled in the art. Accordingly, the scope of the present invention should be defined by the appended claims and equivalents thereof .