Patent Number: 053533143
Section: description

DETAILED DESCRIPTION OF THE INVENTION The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. Referring first to FIG. 1, there is shown a diagrammatic view of the main elements of a tokamak 20, with a portion thereof cutaway. The design and operation of such a tokamak is well described in the art, see, e.g., Artsimovich and/or Furth, supra. Only a very cursory overview of the tokamak's construction and operation is thus presented herein. Basically, the tokamak 20 includes a toroidal vacuum vessel 22 that is centered about a major axis 24. A minor axis 25, centrally located within the toroidal vessel 22, encircles the major axis 24. The relationship of the major and minor axes 24 and 25 is best seen in FIG. 1A. The vessel 22 is made from a conductive material, such as non-magnetic stainless steel or inconel, and is constructed with sufficiently thick walls to withstand the vacuum pressures that are developed therein. A large number of toroidal field magnetic coils 26 are equally spaced around the vessel 22, each encircling the minor axis 25 and a respective segment of the vessel 22. Eighteen such coils 26 are illustrated in FIG. 1, but this number is only exemplary. When energized with an electrical current, the toroidal coils 26 combine to produce a toroidal magnetic field B.sub.T, represented by the arrow 28, that encircles the major axis 24 within the vacuum vessel 22. A plurality of poloidal field magnetic coils 30 are positioned inside of the toroidal field coils 26, yet still outside of the vacuum vessel 22, so as to encircle the major axis 24. As depicted in FIG. 1, the windings of the poloidal field coils 30 are substantially perpendicular to the windings of the toroidal field coils 26. When energized with an appropriate electrical current, the poloidal field magnetic coils 30 combine to produce a poloidal magnetic field B.sub.P, represented by the arrow 32, that encircles the minor axis 25 of the vacuum vessel 22. Ohmic heating primary windings 34 are positioned inside of the toroidal field coils 26, in close contact with the vacuum vessel 22, so as to encircle the primary axis 24, much like the poloidal field coils 30. When energized with an electrical current, the field coils 30 (acting as a transformer primary winding) induce an electrical current, I.sub.p, 36 in the plasma (acting as a transformer secondary winding) to heat the plasma. Not shown in FIG. 1, but understood to be part of any tokamak or similar plasma-confining structure are conventional means for establishing a desired vacuum pressure within the vessel 22, and means for injecting the appropriate gases into the vessel from which plasma may be formed. Because plasma is an ionized gas, it is also an electrical conductor, with the movement of electrons (negatively charged particles) in one direction and the movement of positively charged ions in the other direction representing the flow of electrical current. An important part of the operation of a tokamak is the creation of axial current flow through the plasma contained within the vessel 22. Such current flow follows the minor axis 25 and is depicted in FIG. 1 by the arrow 36. In operation, appropriate gases are introduced into the vacuum vessel at the appropriate pressure. These gases, e.g., .sup.2 H and .sup.3 H, are heated to extremely high temperatures in order to form a hot plasma. The toroidal magnetic field B.sub.T confines the plasma to a toroidal volume inside of the vessel 22 that does not touch the walls of the vessel. This occurs because the toroidal magnetic field B.sub.T has lines of magnetic force coincident or parallel with the minor axis 25, and plasma, as a whole, is substantially confined to and follows magnetic lines of force, forming as it were a plasma ring. The poloidal magnetic field B.sub.P is needed to complete the plasma confinement against drifts caused by gradients in B.sub.T. The combined fields form, as it were, a plasma and magnetic vortex. The externally applied component of B.sub.P is also used to shape the cross sectional area of the plasma ring within the toroidal plasma volume to a desired shape. For example, at some points within the vessel, or at some times when the plasma is within the vessel, the cross sectional area of the plasma cloud may be "squeezed" thereby compressing the plasma into a smaller volume, and further increasing its temperature. At other points within the vessel, or at other times, the cross sectional shape of the plasma cloud may be expanded, with some of the plasma particles being diverted away from the main plasma body. Such control of the cross-sectional shape of the plasma cloud is, as indicated, controlled by the poloidal field coils 30. For this reason, such coils are sometimes referred to as the "shaping field coils" or "shaping field windings", Of particular relevance to the present invention, the poloidal magnetic field B.sub.P is also used as a secondary magnetic field to divert some of the plasma out of the main plasma cloud or body to a suitable target, where the plasma can be neutralized and removed from the vessel 22. FIG. 2 diagrammatically illustrates a cross section of the vacuum vessel 22 of the tokamak of FIG. 1, or similar tokamak, and shows representative magnetic poloidal field lines 38 and 39 that confine the plasma to the inner portions of the vacuum vessel. Most of the plasma is thus confined to an area shaped by the closed magnetic field lines 38 or 39. This portion of the cross section is thus often referred to as the plasma core. Because the plasma torus is symmetric about the major axis 24, the poloidal magnetic field lines also equivalently define toroidal surfaces, called magnetic surfaces. The circle-dot " " 28 shown in FIG. 2 is conventional vector notation to show the direction of the toroidal magnetic field. In FIG. 2, the shows the standard B.sub.T direction in the DIII-D tokamak, which is pointing out of the plane of the paper toward the viewer. The magnetic field line 40, which represents the transition between magnetic field lines that are closed and magnetic field lines that are open, is known as the "separatrix". The separatrix 40 thus defines the boundary or magnetic surface separating the plasma that is confined within the core of the tokamak 20 (or other plasma-confining structure) and the plasma that is not confined within the tokamak. As seen in FIG. 2, the separatrix 40 crosses near the bottom of the vessel 22 at a cross point 42. The cross point 42 is also referred to as the separatrix "X-point". Any plasma on the outside of the separatrix 40 is thus not confined, and will eventually be diverted, following other magnetic force lines, such as the open magnetic force lines 43 and 44, or equivalently, open magnetic surfaces, away from the main body of plasma to the inside edges of the vessel 22. In practice, the plasma that escapes across the separatrix flows along the magnetic field lines in a thin layer 46 just outside of the separatrix 40, known as the "scrape-off layer" (SOL), until it reaches the divertor targets 48 and/or 50. The divertor targets 48 or 50 are made from any suitable material, such as graphite, adapted to absorbed the heat of the diverted plasma and neutralize the plasma to form a gas. The gas is then collected in a plenum 52. Referring next to FIG. 3, a diagrammatic representation of a divertor electrode and pumping plenum made in accordance with the present invention is illustrated. The lower portion of the vessel 22, including the lower portion of the separatrix 40 and the X-point 42 are schematically depicted in FIG. 3. The floor of the vessel 22 in the vicinity of the X-point 42 and separatrix 40 is referred to as the divertor floor. A ring electrode 56, toroidally symmetric with the major axis 24 of the tokamak, is placed at the entrance of the a gas plenum 54. The ring electrode 56 may include cooling channels 58, through which a suitable coolant, such as water, may be circulated. In a preferred embodiment, used with the DIII-D tokamak, located at General Atomics, San Diego, Calif., the ring electrode 56 has a 20 kA capability, and is covered with a graphite armor. A liquid helium (He) cryogenic pump may optionally be positioned within the plenum volume 54. Details associated with the design and construction of the divertor electrode 56 as shown in FIG. 3, and as implemented in the DIII-D tokamak at General Atomics, are documented by Smith, J., "Design of the DIII-D Advanced Divertor", Proceedings 13th IEEE Symposium on Fusion Engineering (Oct. 2-6, 1989), pp. 1315-18, incorporated herein by reference. It is sufficient for purposes of the present invention to note that the electrode 56 is electrically insulated laterally from the vacuum vessel 22 by two plasma-facing rings 63 and 64 made of boron nitride (BN). Boron nitride material from which such rings can be made is commercially available from various suppliers, such as Union Carbide Corporation. The lateral ring insulator 64 is placed at the entrance of the plenum 54 opposite the ring electrode 56. Additional insulators 60 and 62 are used to insulate the lower and outer surfaces of the ring electrode 56, respectively. The insulators 60 and 62 may be made from any suitable electrically insulative material suitable for high temperature use, such as mica and Al.sub.2 O.sub.3 pieces. It is noted that in FIG. 3, and other similar figures presented herein, only one divertor is illustrated, where the term "divertor" refers to the combination of the ring electrode 56, insulators 60, 62, 63 and 64, and entrance aperature to the plenum 54, or other ducting. The diverter illustrated in the figures is the "outer" divertor, it being located farthest from the primary axis 24, and nearest the outer wall of the vessel 22. It is to be understood that another divertor, termed the inner divertor, may also be used that is closest to the primary axis 24, and nearest the inner wall of the vessel 22. A power supply 66 applies electrical power (sometimes referred to as the "bias potential") between the electrode 56 and the conductive wall of the vacuum vessel 22. The inner vacuum vessel surfaces that interact with plasma are covered with graphite tiles, all of which are in electrical contact with the vessel 22. During operation, the ring electrode 56 is not heated, and does not reach temperatures at which thermionic emission is important. The coolant channels in the ring electrode help to maintain its temperature and prevent overheating, especially when the electrode 56 is also used as a divertor plate or target, as is the case for a preferred embodiment of the invention. In such preferred embodiment, used with the DIII-D tokamak, four toroidally distributed feed conductors are also used to connect the power source 66 to the ring electrode 56 to ensure that local magnetic errors are small, even at the 20 kA design maximum electrode current. As is known in the art, the separatrix strike position is controlled by the X-point location, which in turn is controlled by the current distribution in the lower poloidal field shaping coils. Advantageously, by controlling such current distribution in an appropriate manner, the outer strike position can be varied smoothly from the middle of the divertor floor to the upper surface of the ring electrode 56. Such feature permits the separatrix strike position to be swept across the divertor floor, thereby reducing the heat flux absorbed in the divertor floor by time averaging. Unfortunately, the gas static pressure in the plenum 54 is very geometry sensitive, i.e., it varies as a function of the position of sweeping X-point. To demonstrate this dependence, the gas static pressure was examined as a function of separatrix position by sweeping the X-point 42 slowly in the radial direction (over a time period of about 1600 msec) while maintaining otherwise unvarying plasma conditions. During this study, the potential of the electrode 56 was maintained at the same potential as the vessel structure 22, so as not to influence the plasma. The results of such a study, made using the DIII-D tokamak previously referenced, are illustrated in FIG. 4. As seen in FIG. 4, the plenum gas pressure increased as the separatrix neared the entrance aperture of the plenum 54. The maximum pressure occurred when the separatrix 40 was within about 1 cm of intercepting the lower inside corner of the electrode 56 (corresponding to a time of about 2500 msec on the horizontal axis of the pressure vs. time graph included in FIG. 4). The pressure then decreased rapidly with additional outward displacement. Pressures under the X-point and above the plenum remained low at all times. It is noted that plenum pressure at the optimum (highest pressure) separatrix location depends on several parameters, including plasma density and divertor surface condition. The pressure also depends on neutral beam heating power as shown in FIG. 5. The plenum gas pressures shown in FIG. 5 are for H-mode operation and are the maximum values measured during slow divertor sweeps like the one illustrated in FIG. 4. As seen in FIG. 5, pressures on the order of 10 mtorr are typically obtained, and such pressures are sufficient for practical high-throughput pumping in a tokamak, such as the DIII-D tokamak. Another factor influencing the plenum pressure is electrode bias. The effect of applied electrode bias is depicted in FIG. 6, where plenum pressure for Ohmic, L-mode and ELMing H-mode single-null discharges using the standard DIII-D tokamak toroidal magnetic field and plasma current directions is depicted. (Note that "ELMing" refers to the presence of Edge Localized instability Modes. ELMs are a common feature of H-mode operation and are described in the "ASDEX Team" reference, cited previously.) As seen in FIG. 6, negative electrode potential relative to the vacuum vessel increases the plenum gas pressure, while positive bias potential decreases the plenum gas pressure. Further, it has been observed by other measurements not illustrated in FIG. 6 that negative electrode potential decreases particle recycling at the inner divertor and the inner wall. Similarly, it is evident that positive bias potential increases particle recycling at the inner divertor and inner wall. These effects are qualitatively present, regardless of whether the separatrix strikes the electrode 56 or the vessel floor, so long as the separatrix 40 is close enough for the diverter plasma to interact with the biased electrode 56. As was the case for the data presented in FIG. 5, gas pressure remains low under the X-point at all times. The bias electrical power used for the experiments shown in FIG. 6 was approximately 1 MW. Some of this power is radiated, while the remainder appears at the divertor electrode as heat. Advantageously, electrode operation does not contribute to plasma impurities. The data presented in FIG. 6 was measured at the time of maximum plenum gas pressure during slow divertor sweeping as in FIG. 4 at two heating powers for medium density (2.3.about.2.5.times.10.sup.19 m.sup.-3) Ohmic and L-mode plasmas, all at B.sub.T =2.1 T, I.sub.p =1 MA, and standard field directions. As seen from the data in FIG. 6, the dependence on bias potential appears to be monotonic in all cases where data is available. Positive bias decreases the plenum pressure, and negative bias increases the pressure. Reversal of the toroidal field direction reverses the roles of positive and negative bias. It is noted that the observed changes to the particle recycling and plenum pressure are qualitatively consistent with the expected consequences of bias-induced drift velocity V.sub.E =E.times.B.sub.T /B.sub.T .sup.2 in the SOL as suggested by the early experiments of Strait, cited above. Thus, for example, as shown in FIG. 7, which is drawn for standard DIII-D field directions, the bias voltage establishes a poloidal electric field E.sub.P within the magnetic surfaces contacting the electrode 56. The resulting radial V.sub.E yields the observed recycling changes at the inner wall of the vessel 22. As also seen in FIG. 7, a radial electric field E.sub.r is established normal to the biased magnetic surfaces. When the separatrix 40 strikes below the electrode 56, the resulting E.sub.r .times.B.sub.T produces a poloidal flow in the SOL between the separatrix and the biased surface, directing plasma toward the inner divertor for positive bias potential, and toward the outer divertor for negative bias potential. This is again in qualitative agreement with the recycling and plenum pressure observations. When the separatrix strikes the electrode 56, the electric potential distribution becomes more complicated. Such potential distribution is shown qualitatively in FIG. 8. A large E.sub.r appears as a consequence of potential jumps across a thin boundary layer near the separatrix 40. This is so because just inside the separatrix, E.sub.P must be small, so the potential difference between large and small major radius SOL must appear across a boundary layer. A large E.sub.r also appears in the boundary layers separating biased magnetic surfaces from surfaces contacting insulators at one end and the vessel at the other, such as the boundary layer 72. Because the leakage of current across B is small, any surfaces contacting the vessel at one end and an insulator at the other remain close to vessel potential, FIG. 8 qualitatively depicts how the E.times.B.sub.T flow, which is along equipotential surfaces, drives plasma across the SOL, along paths 72; across the divertor separatrix along paths 74; across the X-point 42; and across the "arch-shaped" surfaces below the X-point, following paths 76. Then, the large E.sub.r in the boundary layer that grazes the lower inside corner of the electrode 56 drives the plasma rapidly along path 78 into the plenum entrance. This driving of the plasma thus allows the electrode/separatrix geometry shown in FIG. 8 to function as an efficent plasma pump. It is noted that although the plasma striking the electrode 56 before reaching the plenum entrance is neutralized, most of the neutral atoms are reionized near the neutralization site by the dense divertor plasma, and the reborn ions are thereafter subject to the same E.times.B drifts. Hence, all outer SOL ions, except those that get buried in the electrode and floor material, eventually reach the plenum entrance at the optimum position for pumping. This reionization and subsequent continued flow as plasma was absent from the early experiments of Strait, referenced previously. Advantageously, it has been determined experimentally that the above described process of E.times.B drift significantly reduces the sensitivity of the plenum pressure dependence on separatrix strike position, most notably when the separatrix strikes the electrode. Data demonstrating this feature is shown in FIG. 9, which shows a data plot comparing plenum gas pressure with neutral beam power during H-mode operation for three divertor conditions. The "+" marks are data for the separatrix diverting plasma into the entrance aperture with no bias applied to the divertor electrode 56 as in FIG. 5 (separatrix not striking the electrode, optimum positioning for maximum pressure). The "O" marks are data for the separatrix striking the electrode, still with no bias applied to the electrode 56. The " " marks are data for the separatrix striking the upper inside corner of the electrode, and with bias applied to the electrode 56. As seen from the data presented in FIG. 9, even when the separatrix strikes the upper inside corner of the electrode (which for the DIII-D where the data were taken is 7 cm above the top of the plenum entrance aperture), the plenum pressure with bias is almost as high as the pressure for the separatrix optimally positioned in the aperture without bias. Further, when the separatrix is in the high position, the pressures with bias are about four times higher than those without. Thus, the biased divertor acts as an E.times.B plasma pump, particularly when the separatrix strikes the electrode. The operation of the plasma pump of the present invention may be analyzed by considering a potential difference "V" applied across a plasma gap of width "w". The pump cross section is Lw, where "L" is the dimension in the magnetic field direction. The pump speed, S.sub.E, may then be expressed as EQU S.sub.E =Lwv.sub.E =LwE/B.sub.T =LV/B.sub.T, where EQU V=.intg.E.multidot.dw.apprxeq.wE. Note that the v.sub.r and v.sub.P pumping speeds are approximately the same. For the experiments described, L=2.pi.R.apprxeq.10 m; V/B.apprxeq.(200 V)/(2T)=100 m.sup.2 /s, so S.sub.E .apprxeq.1000 m.sup.3 /s. For comparison, the mechanical vacuum pumps presently installed on the DIII-D tokamak have a combined speed of only about 10 m.sup.3 /s. It is noted that the steady state plasma density in the DIII-D tokamak during operation in the H-mode is proportional to the current, and depends on little else. The density cannot be varied appreciably by normal operational procedures. However, modest controlled density changes were observed under conditions of strong divertor bias, as shown by the data presented in FIG. 10. FIG. 10 illustrates the applied bias potential, average density, and average temperature during H-mode operation as a function of time. As seen in FIG. 10, for normal field directions, negative bias reduced plasma density and positive bias increased plasma density. The density changes were achieved, even though the planned pumps have not yet been installed in the divertor plenum 52. The density changes were accompanied by reciprocal changes in the volume-averaged temperature (average of T.sub.e and T.sub.i) such that the total energy and energy confinement time remained nearly constant. Hence, as indicated above, it is seen that the present invention offers an excellent means for particle control in a tokamak, or similar plasma-confinement apparatus. Neutral gas pressures on the order of 10 mtorr are possible during operation in the H-mode when optimum separatrix position is maintained. By using the plasma pump of the present invention, the E.times.B drift also transports particles across the SOL and optimally into the plenum entrance when the separatrix strikes the electrode (which is far removed from the optimum positioning of the separatrix). Hence, through the selective application of a bias potential to the divertor electrode, the divertor acts as a geometry-insensitive high capacity pump to drive plasma into the plenum aperture. Such plasma pump can advantageously be used to reduce vacuum pumping requirements for steady state plasmas; to exhaust plasma from low density plasmas; to establish low collisionality, low density H-mode plasmas for current drive; and to make plasma exhaust insensitive to divertor geometry, especially to the variable geometry of swept divertors. While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.