阅读说明：本技术 用于减少非期望的涡流的系统和方法 (System and method for reducing undesired eddy currents ) 是由 N·拉思 于 2016-05-09 设计创作，主要内容包括：用于减少非期望的涡流的系统和方法。用以减小例如由FRC到约束室中的平移所感生的在导电结构中的非期望的涡流的振幅而同时使有益涡流不受影响的系统和方法。这是通过在等离子体平移到约束室中之前在相同导电结构中感生相反电流而实现的。(Systems and methods for reducing undesirable eddy currents. Systems and methods to reduce the amplitude of undesirable eddy currents in conductive structures, e.g., induced by the translation of an FRC into a confinement chamber, while leaving beneficial eddy currents unaffected. This is achieved by inducing opposing currents in the same conductive structure before the plasma is translated into the confinement chamber.)

1. A method for reducing undesired eddy currents induced in a conductive structure, the method comprising the steps of:

inducing a first set of eddy currents in the conductive structure before inducing a second set of eddy currents in the conductive structure, wherein the first set of eddy currents has a substantially equal and opposite sign distribution to the second set of eddy currents to substantially cancel the second set of eddy currents when induced in the conductive structure.

2. The method of claim 1, wherein the conductive structure is a wall of a plasma confinement vessel.

3. The method of claim 1, wherein the step of inducing eddy currents in the conductive structure comprises the step of

Ramping up and maintaining a constant current in the coil around the conductive structure until all eddy currents in the conductive structure have decayed, an

The current to the coil is interrupted to allow a first set of eddy currents to be excited in the conductive structure, conserving the magnetic flux through the structure.

4. The method of claim 1, further comprising the step of translating the plasma into the conductive structure, wherein the translating plasma injects a flux into the conductive structure that induces the second set of eddy currents in the wall of the vessel, reducing the amplitude of the eddy currents in the wall of the vessel back toward zero.

5. The method of claim 3, further comprising the step of translating the plasma into the conductive structure, wherein the translating plasma injects a flux into the conductive structure that induces the second set of eddy currents in the wall of the vessel, reducing the amplitude of the eddy currents in the wall of the vessel back toward zero.

6. The method of claim 1, wherein the step of inducing eddy currents in the conductive structure comprises the step of

Ramping up and maintaining a coil at a constant current around the conductive structure to generate a first set of eddy currents in the conductive structure, an

Translating a plasma into the conductive structure, wherein the translated plasma injects a flux into the conductive structure that induces a second set of eddy currents in the conductive structure that reduces the amplitude of the eddy currents in the conductive structure back toward zero.

7. A system for reducing undesired eddy currents induced in a vessel wall, the system comprising:

a container having a wall and an interior,

a plurality of coils disposed around the vessel, an

A control system coupled to the plurality of coils and configured to induce a first set of eddy currents in the wall of the vessel prior to inducing a second set of eddy currents in the wall of the vessel, wherein the first set of eddy currents has a distribution that is substantially equal and opposite in sign to a distribution of the second set of eddy currents to substantially cancel the second set of eddy currents when induced in the wall of the chamber.

8. The system of claim 7, wherein the control system is further configured to ramp up and hold the plurality of coils at a constant current until all eddy currents in the wall of the vessel have decayed, and then interrupt the current to the plurality of coils to allow a first set of eddy currents to be excited in the wall of the vessel conserving flux through the vessel.

9. The system of claim 8, further comprising a forming section attached to an end of the vessel, wherein the control system is further configured to translate the plasma from the forming section into the interior of the vessel, wherein the translating plasma injects a flux into the wall of the vessel that induces the second set of eddy currents in the wall of the vessel that reduces the amplitude of the eddy currents in the wall of the vessel back toward zero.

10. The system of claim 7, wherein the control system is further configured to ramp up and maintain the plurality of coils at a constant current to generate the first set of eddy currents in the conductive structure.

11. The system of claim 10, further comprising a forming section attached to an end of the vessel, wherein the control system is further configured to translate the plasma from the forming section into the interior of the vessel, wherein the translating plasma injects a flux into the wall of the vessel that induces the second set of eddy currents in the wall of the vessel that reduces the amplitude of the eddy currents in the wall of the vessel back toward zero.

12. A method for reducing undesired eddy currents induced in a conductive structure, the method comprising the steps of:

the method includes inducing a first set of eddy currents in a wall of a vessel having a wall and an interior prior to inducing the second set of eddy currents in the wall of the vessel, wherein the first set of eddy currents has a substantially equal and opposite sign distribution to the second set of eddy currents to substantially cancel the second set of eddy currents when induced in the conductive structure.

13. The method of claim 12, wherein the step of inducing eddy currents in the wall of the vessel comprises the step of

Ramping up and maintaining a constant current to a plurality of coils disposed around a wall of the vessel until all eddy currents in the conductive structure have decayed, an

Interrupting the current to the plurality of coils to allow a first set of eddy currents to be excited in the wall of the vessel conserving the magnetic flux through the wall of the vessel.

14. The method of claim 12, further comprising the step of translating the plasma into the vessel, wherein the translating plasma injects a flux into the wall of the vessel that induces the second set of eddy currents in the wall of the vessel that reduces the amplitude of eddy currents in the wall of the vessel back toward zero.

15. The method of claim 13, further comprising the step of translating the plasma into the vessel, wherein the translating plasma injects a flux into the wall of the vessel that induces the second set of eddy currents in the wall of the vessel that reduces the amplitude of eddy currents in the wall of the vessel back toward zero.

16. The method of claim 13, wherein the plasma is translated from opposing formation sections attached to opposing ends of the vessel.

17. The method of claim 16, wherein the FRC plasma is formed in opposing formation sections and translated into the vessel.

18. The method of claim 12, wherein the step of inducing eddy currents in the wall of the vessel comprises the step of

Ramping up and maintaining a plurality of coils disposed around a wall of the vessel at a constant current to generate a first set of eddy currents in the wall of the vessel, an

Translating the plasma into the vessel, wherein the translating plasma injects a flux into the wall of the vessel that induces a second set of eddy currents in the wall of the vessel that reduces the amplitude of the eddy currents in the wall of the vessel back towards zero.

19. The method of claim 18, wherein the plasma is translated from opposing formation sections attached to opposing ends of the vessel.

20. The method of claim 19, wherein the FRC plasma is formed in opposing formation sections and translated into the vessel.

Technical Field

The subject matter described herein relates generally to magnetic plasma confinement systems, and more particularly to systems and methods that facilitate cancellation of undesired eddy currents.

Background

The Field Reversed Configuration (FRC) belongs to the class of magnetic plasma confinement topologies known as compact plasma loops (CT). It shows a prominent earth poleA magnetic field of the outward direction and possessing a zero or small self-generated toroidal field (see m. Tuszewski, nuclear. Fusion 28, 2033 (1988)). Conventional methods of forming FRC use a field reversalθPinch technique, generating a hot high-density plasma (see a.l. Hoffman et al, nuclear Fusion 33, 27 (1993)). A variation of this is the translational trapping method, in which the plasma generated in the theta-pinch "source" is ejected more or less immediately outward from one end into the confinement chamber. The translated plasma cluster is then trapped between two strong mirrors at the ends of the chamber (see, e.g., h. Himura et al, phys. plasma 2, 191 (1995)).

Significant progress has been made over the past decade, and other FRC formation methods have been developed: spheromaks are incorporated with oppositely oriented helicities (see, e.g., y. Ono et al, nucl. Fusion 39, 2001 (1999)) and by driving a current with a Rotating Magnetic Field (RMF) that also provides additional stability (see, e.g., i.r. Jones, phys. plasma 6, 1950 (1999)). Recently, collision merging techniques proposed long ago (see e.g., d.r. Wells, phys. Fluids 9, 1010 (1966)) have been significantly further developed: two separate theta-pinches at opposite ends of the confinement chamber simultaneously generate and accelerate two plasma clusters toward each other at high velocity; they then collide and merge at the center of the confinement chamber to form a composite FRC. In the construction and successful operation of one of the largest FRC experiments to date, conventional collision merging methods have been shown to produce stable, long-lived, high-flux, high-temperature FRCs (see, e.g., m. Binderbauer et al, phys. rev. lett. 105, 045003 (2010)).

As the FRC translates into the confinement section, it induces eddy currents in any conductive (containment) structure within its vicinity (e.g., the vessel wall or conductive component in the vessel). These eddy currents affect the plasma state and decay (decay) over time, thereby contributing to the continuous evolution of the plasma and preventing any steady state until the eddy currents have decayed to a negligible amount. If the conductive structure is not axisymmetric (as is typically the case), the eddy currents break the axial symmetry of the FRC. In general, such translation-induced eddy currents are undesirable. Its initial excitation imposes constraints on the plasma shape and thus limits the ability of the conductive structure to provide passive stabilization of plasma instability, and its decay over time complicates plasma control by requiring continuous compensation even in the absence of plasma instability. Furthermore, any beneficial effects of the translation-induced eddy currents can also be provided by a suitable adjustment of the balancing magnetic field.

The translation-induced eddy currents were not the only eddy current types that occurred during the experiment. Plasma instabilities can excite eddy currents, which reduce the rate of growth of the instability and are therefore desirable. Eddy currents will also appear in response to the neutral beam current ramping up.

Plasma lifetime in other FRC experiments is typically limited to values significantly below the resistance time scale of the conductive walls, so that time-varying eddy currents do not pose any practical problem and have not received much attention.

One related technique to prevent the excitation of translation-induced eddy currents is to use an insulated axial "gap" in the vessel to prevent the excitation of axisymmetric eddy currents. The disadvantage of this method is that it requires structural changes to the conductive container and the eddy currents are not suppressed but the axisymmetric current is transformed into a 3-D current. This therefore exacerbates the deleterious effects from the 3-D field and also makes the wall unsuitable for passive stabilization of axisymmetric plasma instabilities.

The three-dimensional error field is often corrected by an error field correction coil that is not itself axially symmetric. At best, such coils can cancel as many harmonics as there are, but they tend to introduce new errors in the remaining harmonics and need to be able to follow any time variations of the error field during the experiment.

Accordingly, it is desirable to provide systems and methods that facilitate the reduction or elimination of undesirable eddy currents.

Disclosure of Invention

Embodiments provided herein are directed to systems and methods that facilitate a reduction in the amplitude of undesired eddy currents (wall currents), e.g., translation-induced eddy currents, such as those induced by the translation of an FRC plasma, while leaving beneficial eddy currents unaffected. The reduction in the amplitude of the undesired eddy currents is achieved by inducing opposing currents in the same structure prior to plasma translation, for example, using an active coil. If both the tangential and normal components of the total magnetic field on the surface separating the plasma from the conductive structure are measured, the field can be decomposed into a component generated by the plasma and a component generated by an external current (e.g., a balance coil current). By subtracting the known field from the external coil, a field due to eddy currents is left. The corresponding eddy current distribution can be reconstructed from the time evolution of the field. With the eddy current profile known, a similar profile with the opposite sign is induced using the active coil before the plasma is translated into the chamber. Calculating the necessary coil currents only requires knowledge of the geometry of the active coils and the passive structures. These two eddy current distributions overlap and cancel when the plasma is translated into the confinement chamber. The more accurately the eddy current distribution is reproduced, the more complete the cancellation.

The systems and methods described herein advantageously:

reducing time-varying external fields due to decaying eddy currents, which interfere with plasma control;

reducing the symmetry breaking effect of the non-axisymmetric wall; because both the pre-induced eddy currents and the translation-induced eddy currents have the same 3-D structure, the 3-D field is reduced without the need for non-axisymmetric coils; and is

Enabling the installation of a closely fitting, axisymmetric, structure in a container to increase passive stabilization of axisymmetric and non-axisymmetric instabilities.

Other systems, methods, features and advantages of the example embodiments will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description.

Drawings

The details of the example embodiments, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than truly or accurately.

FIG. 1 is a schematic view of a chamber or vessel having a forming tube attached to opposite ends and an axisymmetric coil disposed around the wall of the chamber for inducing eddy currents (wall currents) in the wall of the chamber.

Fig. 1A is a schematic diagram showing a control system coupled to an active coil system and a forming system.

Fig. 2 is a schematic view of the chamber and forming tube of fig. 1 with plasma present in the forming tube.

FIG. 3 is a schematic view of the chamber and forming tube of FIG. 1 after the plasma has translated into the chamber and illustrates the translation-induced eddy currents (translation-induced wall currents) formed in the walls of the chamber.

Fig. 4 is the chamber and forming tube of fig. 1 forming pre-induced eddy currents (pre-induced wall currents) in the wall of the chamber before the plasma is translated into the chamber.

Fig. 5 is the chamber and forming tube of fig. 1 after the plasma has translated into the chamber and shows pre-induced and translation-induced eddy currents (pre-induced and translation-induced wall currents) in the walls of the chamber.

Fig. 6 is the chamber and forming tube of fig. 1 after the plasma has translated into the chamber and shows that the translation-induced eddy currents in the wall of the chamber (translation-induced wall currents) are cancelled out by the pre-induced eddy currents in the wall of the chamber (pre-induced wall currents).

Fig. 7 is a graph showing simulated eddy current distributions (simulated wall current distributions) in the axisymmetric wall of the chamber for three (3) cases as follows: (1) no pre-induction, (2) pre-induction and (3) pre-induced and adjusted vacuum field.

It should be noted that elements of similar structure or function are generally represented by like reference numerals throughout the figures for illustrative purposes. It should also be noted that the figures are only intended to facilitate the description of the preferred embodiments.

Detailed Description

Each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide systems and methods that facilitate a reduction in the amplitude of undesired eddy currents (wall currents), such as translationally induced eddy currents, while leaving beneficial eddy currents unaffected. Representative examples of the embodiments described herein will now be described in further detail, with reference to the accompanying drawings, which examples utilize many of these additional features and teachings both separately and in combination. This detailed description is intended only to teach those skilled in the art further details for practicing preferred aspects of the teachings of the present disclosure, and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the teachings of the disclosure.

Furthermore, various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the teachings of the present disclosure. In addition, it is specifically noted that for the purposes of the original disclosure and for the purposes of limiting the claimed subject matter independent of the combination of features in the embodiments and/or the claims, it is intended that all features disclosed in this specification and/or the claims be disclosed separately and independently of each other. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure and for the purpose of limiting the claimed subject matter.

Embodiments provided herein are directed to systems and methods that facilitate a reduction in the amplitude of undesired eddy currents, e.g., translationally induced eddy currents, such as those induced by translating an FRC plasma, while leaving beneficial eddy currents unaffected. The eddy currents induced by translating the FRC plasma are not dependent on the prior field configuration or the presence of the prior current. Thus, if the current induced by the plasma translation is undesirable, it can be eliminated by generating an equal and opposite current pattern prior to the plasma translation.

In practice, this may be achieved with an axisymmetric active coil 20 disposed around the inside or outside of the container 10, as shown in fig. 1. A plasma (such as, for example, an FRC plasma) is formed in the vessel 10 and translated from forming tubes 12 and 14 disposed on opposite ends of the vessel 10 toward the mid-plane of the vessel 10. A detailed discussion of a system and method for forming and maintaining an FRC plasma is provided in published PCT application number WO 2015048092, which claims priority from U.S. provisional patent application number 61/881874 and U.S. provisional patent application number 62/001583, which are incorporated herein by reference as if fully set forth.

As shown in fig. 1A, the control system 100 is coupled to an active coil system 200 including an active coil 20, a power source, and the like and to a forming system including forming tubes 12 and 14, coils or belts, a power source, and the like.

Before the plasma is translated from the forming tubes 12 and 14, the coil 20 is ramped up and held at a constant current until all eddy currents in the wall of the vessel 10 have decayed. At this point, the current to the coil 20 is interrupted and the plasma formation sequence begins. Interruption of the current to the coil 20 will excite a specific eddy current profile in the wall of the vessel 10 to conserve flux through the vessel 10 until subsequent flux injection from the translating plasma reduces the eddy currents in the wall of the vessel 10 back towards zero. Alternatively, the coil 20 may be ramped up rapidly just prior to plasma translation. In this case, the rapid ramp up will produce the desired vortex distribution in the walls of the vessel 10, and the subsequent flux injection from the translating plasma will return the vortex to zero. After translation, the current in the coil 20 remains constant. This method may be used if the characteristic eddy current decay time of the wall 10 is sufficiently slow compared to the rate at which the coil 20 may ramp up. The offset can generally be increased by optimizing the geometry of the active coil, but even with a defined active coil geometry, the eddy current amplitude can be reduced.

In order to determine the current in the active coil that will maximize eddy current cancellation, the eddy current distribution induced by the plasma must be measured. This may be done by measuring at least two components of the magnetic field in the region between the conductive structure and the plasma. In case both components of the magnetic field are known, the magnetic field can then be separated into a component due to the plasma and a component due to the external current. This is easily seen in cylindrical geometry, i.e. for a given modulus m and phase, the magnetic index potential is determined by two amplitudes, one for r^mA proportional term, and another for r^-mThe proportional term. Two measurements with magnetic field at the same point in space allow to find two coefficients and use r^mThe proportional term very generally identifies the field from the plasma. In more complex geometries, the mathematical operations are not so simple, but the same procedure can be used. With the time evolution of both the internal and external magnetic fields known, the current distribution in the conductive structure can be calculated by least squares fitting to a finite element circuit model.

Fig. 2-6 illustrate the basic idea of reducing translation-induced eddy currents. The (white filled) plasma currents, (gray filled) plasma induced wall currents and (cross-hatched filled) pre-induced wall currents are shown in the figure in two stages, i.e. 1) before the translation and 2) after the translation. In fig. 2 and 3, no wall current is previously induced in the walls of vessel 10, so the net current in the walls after the plasma is translated from forming tubes 12 and 14 is a non-zero value. In fig. 4-6, some current has been previously induced in the wall of the vessel 10. After the plasma is translated from the forming tubes 12 and 14, the net current in the walls becomes zero.

The application of the proposed technique has been simulated using the LamyRidge-2 fluid simulation code to evaluate its effect on plasma formation and translation. Figure 7 shows the eddy current distribution in an axisymmetric wall two hundred microseconds (200 ms) after formation for three different cases:

1) in case 1 (-), no eddy current compensation was used, resulting in a plasma with a boundary radius of 39 cm and an elongation of 2.5.

2) In case 2 (-), the (exactly) opposite current pattern is applied on the wall before the formation starts. As expected, the amplitude of the eddy current at the end of the simulation decreased. The current is not exactly cancelled out because the presence of the pre-induced current causes an expansion of the plasma such that it reaches a radius of 46 cm with an elongation of 2.0.

3) In case 3 (- - - - - - - -), the current in the confinement coil is adjusted to compensate for the suppressed eddy currents, in addition to the pre-induced eddy currents in the chamber wall. In other words, the field generated by the confinement coil in case 3 at t-0 is now equal to the field generated by both the confinement coil and the eddy current in case 1 at t-200 us. This results in a plasma very similar to case 1 (radius 38 cm, elongation 2.5), but the eddy currents have been reduced by a factor of 10. This subsequent evolution of the plasma is therefore much less affected by wall vortices and is therefore easier to control and predict. Furthermore, the plasma dividing line radius can be directly controlled by adjusting the pre-induced wall current along with the confinement coil.

Other advantages

To stabilize the FRC position or shape, an axisymmetric, electrically conductive passive structure in the container may be used. If eddy currents are pre-induced in the passive structure in the vessel in the manner described above, the passive structure in the vessel can be installed without affecting the initial plasma shape and configuration. On the other hand, if the current is not pre-induced, ignoring most of the advantages of installing additional components in the vessel, the installation of passive structures in the vessel will reduce the FRC radius and thus reduce the coupling between the passive structures in the vessel and the plasma to approach the same coupling strength previously between the wall of the vessel and the plasma.

Similar problems apply to the control coil. In the case of a coil outside the vessel having insufficient plasma coupling to stabilize the plasma instability and used in the vessel, it is necessary to protect the coil in the vessel from the plasma, usually with an additional inner wall. If this eddy current in the coil wall in the vessel is not eliminated, it will reduce the plasma radius and the expected increase in coil-to-plasma coupling will be reduced. Thus, eliminating eddy currents increases coupling between the coil and the plasma, and thus reduces both current and voltage requirements on the control coil.

Due to the 3-D shape of the vessel, any induced wall currents will break the axial symmetry and potentially reduce confinement, excite instability, or otherwise reduce performance. An error field correction coil may be used to reduce a fixed number of specific harmonics, but is itself non-axisymmetric and therefore further amplifies the other sideband harmonics. In contrast, the elimination of eddy currents as described above requires only an axisymmetric coil, resulting in fewer side band harmonics, and does not require any current in the coil after the plasma has formed.

In summary, the proposed systems and methods provided herein increase the chance of stabilizing plasma instabilities; the efficiency of the plasma control system is increased by improving coupling to the wall, the amplitude of the symmetry breaking 3-D field is reduced, and the complexity of the real-time system is reduced. To a certain extent, all these advantages can also be achieved at very little cost by reusing existing coil systems. The best results can be achieved by taking eddy current cancellation into account for coil layout and design.

Example embodiments provided herein advantageously reduce time-varying external fields due to decaying eddy currents, which interfere with plasma control; reduces the symmetry breaking effect of the non-axisymmetric wall (reduces the 3-D field without the need for non-axisymmetric coils because both the pre-induced eddy currents and the translation-induced eddy currents have the same 3-D structure) and enables the installation of a closely-fitting, axisymmetric, in-vessel structure to increase passive stabilization of axisymmetric and non-axisymmetric instabilities.

However, the example embodiments provided herein are intended only as illustrative examples and are not limiting in any way.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. For example, the reader is to understand that the specific ordering and combination of process actions shown in the process flow diagrams described herein is merely illustrative unless otherwise stated, and that the invention can be performed using different or additional process actions, or different combinations or orderings of process actions. As another example, each feature of one embodiment may be mixed and matched with other features shown in other embodiments. Features and processes known to those of ordinary skill may be similarly incorporated as desired. Additionally and clearly, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.