Patent Publication Number: US-2010119025-A1

Title: Replaceable fusion neutron source

Description:
ACKNOWLEDGEMENT 
     This invention was made with U.S. government support under Grant Nos. DE-FG02-04ER54742 and DE-FG02-04ER54754 awarded by the United States Department of Energy. The U.S. government has certain rights in the invention. 
    
    
     BACKGROUND OF INVENTION 
     Global warming is a pressing, potentially disastrous problem for humanity. This has created a need for energy sources that do not emit greenhouse gasses, and that could supplant a substantial fraction of carbon-based energy supply on a relatively short time scale. Nuclear (fission) power that utilizes existing technology to provide the required amount of energy in a reasonable period of time, has been increasingly advocated as one strategy to combat global warming. 
     While renewable energy sources are also advocated, their current state of development and intermittent nature limit the fraction of the energy mix they can supply. At present, power from nuclear fission offers the best hope for replacing a significant portion of coal, oil and gas fired power plants—the largest single group of contributors to greenhouse gasses. Nuclear (fission) power, has challenges that have stunted its growth and acceptability. One of the largest of these challenges is safe and efficient disposal of nuclear wastes. 
     One of the more challenging elements of nuclear fission wastes are transuranics (TRUs). Many serious objections to nuclear waste disposal sites such as the Yucca Mountain Project relate to the release of very long-lived (isotopes having a half-life of over 100,000 years) transuranics to the biosphere several hundreds of thousands of years in the future. Some waste disposal can be carried out in a relatively inexpensive thermal spectrum reactor. Thermal neutrons do reduce the total amount of nuclear waste, but they do not affect an important minority comprising many of the long-lived transuranics. These elements are highly problematic for geologic disposal. 
     The destruction of long-lived transuranics requires a different, more expensive methodology. Within pure fission, a second reduction takes place in a fast fission reactor i.e., a fission reactor with a fast neutron spectrum. However there remains drawbacks to using fast-fission reactors, including requiring a sufficient quantity of easily fissile materials to sustain a chain reaction. In addition, fast fission reactors, if used for destroying long-lived transuranics, have stability restrictions. 
     While nuclear fission alone may not be capable of cheaply overcoming the aforementioned nuclear waste challenges, nuclear fission in combination with nuclear fusion may provide a better solution to this challenge. Nuclear fusion is an energy source derived from nuclear combinations of light elements into heavier elements resulting in a release of energy. In fusion, two light nuclei (such as deuterium and tritium) combine into one new nucleus (such as helium) and release enormous energy and another particle (such as a neutron in the case of the fusion of deuterium and tritium) in the process. Nuclear fusion is more neutron-rich energy source than fission. While fusion is a spectacularly successful energy source for the sun and the stars, the practicalities of harnessing fusion on Earth are technically challenging, given that to sustain fusion, a plasma (a gas consisting of charged ions and electrons), or an ionized gas, has to be confined and heated to millions of degrees Celsius in a fusion reactor for a sufficient period of time to enable the fusion reaction to occur. The science behind fusion is well advanced, rooted in more than 100 years of nuclear physics and electromagnetic and kinetic theory, yet current engineering constraints make the practical use of nuclear fusion very challenging. One approach to fusion reactors uses a powerful magnetic field to confine plasma, thereby releasing fusion energy in a controlled manner. To date, the most successful approach for achieving controlled fusion is in a donut-shape or toroidal-shape magnetic configuration called a tokamak. While a tokamak can, in principle, be used as a source of the fast neutrons needed in the second step of the two-step process mentioned above, the current art of fusion reactors limits tokamaks to power densities that are far too low (by factors of 5 or more) for this purpose. 
     With current tokomak technology, the confinement of plasma to produce nuclear fusion reactions can be accomplished with a magnetic field (i.e., a magnetic bottle) created inside a vacuum chamber of a fusion reactor. Since the plasma is ionized, plasma particles tend to gyrate in small orbits around magnetic field lines, i.e., they essentially stick to the magnetic field lines, while flowing quite freely along the field lines. This can be used to “suspend” bulk plasma in the vacuum chamber by using a properly designed magnetic field configuration, which is sometimes called a magnetic bottle. The plasma can be magnetically contained within the chamber by creating a set of nested toroidal magnetic surfaces by driving an electric current in the plasma, and by the placement of current-carrying coils or conductors adjacent to the plasma. Since magnetic field lines on these magnetic surfaces do not touch any material objects such as walls of the vacuum chamber, the very hot plasma can ideally remain suspended in the magnetic bottle, i.e., in the volume containing closed magnetic surfaces, for a long time, without the particles coming into contact with the walls. However, in reality, particles and energy very slowly escape magnetic confinement in a direction perpendicular to the magnetic surfaces as a result of particle collisions with one another or turbulence in the plasma. Decreasing this slow plasma loss, so that the particles and energy of the plasma are better confined, has been a fundamental focus of plasma confinement research. 
     The boundary of the magnetic bottle containing closed magnetic surfaces, i.e., the “core plasma”, is defined by either material objects called limiters (e.g.,  610  with reference to  FIG. 6 ), or by a toroidal magnetic surface called a separatrix (e.g.,  630  with reference to  FIG. 6 ), outside of which the magnetic field lines are “open”, i.e., they terminate on material objects called divertor targets (e.g.,  620  with reference to  FIG. 6 ). The particles and energy slowly escaping the core plasma mainly fall on small areas of either limiter or divertor targets and generate impurities. Since limiters are right at the plasma boundary, while divertor targets can be placed farther away, core plasma can be better isolated from such impurities by using divertors. Since the invention of divertors, the preferred mode of plasma operation has been to have a separatrix and a divertor, since such operation has been found to enable a mode of operation called the H-mode, where the plasma particles and energy in the core are better confined. 
     Since particles flow very fast along magnetic lines but very slow across them, any particles and energy that escape across the separatrix reach divertor targets quickly along open field lines before moving much across them. This creates a necessarily narrow “scrape-off layer” with a high “scrape off flux” of particles and energy that falls on narrow areas of the divertor plates. The maximum “scrape off flux” that a divertor can handle limits the highest power density that can be sustained in a magnetic bottle. 
     High “scrape off flux” creates a multitude of challenges. In addition to heat and particle fluxes, the divertor plates also have to withstand large fluxes of neutrons created in fusion. These neutrons cause a degradation of many important material properties, making it extremely difficult for a divertor plate to handle both the high heat fluxes and neutron fluxes without having to be replaced frequently. Periodically replacing the damaged components is very time consuming and requires the fusion reaction to be shut off. Further, trying to reduce the “scrape off flux” by injecting impurities to radiate energy before it reaches divertor plates is not workable because the density of power coming out of the plasma becomes so high that it seriously degrades the plasma confinement, which results in a serious reduction of the fusion reaction rate in the core plasma. Neutron flux also degrades components of    
     To lower neutron and heat fluxes on a divertor and thus mitigate the damage to a divertor component, a reactor could simply be made larger to decrease the density of power within a device. However, this approach significantly increases the reactor cost, and hence the cost of any energy produced with it, to levels that are economically non-competitive with other methods for the generation of power or neutrons. 
     A high level of “scrape off flux” is a critical roadblock for many fusion applications, including fusion-fission hybrid applications. For example, for fusion reactors of sizes that can make them economically competitive with other methods of energy production, the high “scrape off flux” is intolerable for divertor designs based on current art. One way of handling challenges presented by high scrape off flux and enabling compact high-power density fusion neutron sources is described in U.S. patent application Ser. No. 12/197,736 to Kotschenreuther, et al, filed Aug. 25, 2008, fully incorporated herein by reference and made a part hereof. Furthermore, a system and way of reducing nuclear waste and providing an improved nuclear fuel cycle through the use of an improved hybrid fusion-fission reactor is described in U.S. patent application Ser. No. 12/249,303 to Kotschenreuther, et al, filed Oct. 10, 2008, fully incorporated herein by reference and made a part hereof. 
     Therefore, there remains a need for design improvements that simplify and speed up the component replacement of a hybrid nuclear fusion-fission reactor and to minimize offline time and mitigate deterioration of reactor components caused by neutron exposure. These needs and other needs, some of which are mentioned above, are satisfied by embodiments described herein. 
     SUMMARY OF INVENTION 
     Disclosed herein are embodiments for containing plasma or fusion plasma, a replaceable fusion neutron source, and a tokamak, optionally comprising a magnetically confined plasma, wherein a layer of fissionable materials is substantially adjacent to at least a portion of said fusion neutron source. Also disclosed are methods and nuclear fuel cycles for fissioning said fissionable materials using disclosed embodiments. The various embodiments described herein can be useful in applications that desire a reduction in fissionable materials. 
     In one aspect, a reactor is disclosed. One embodiment of the reactor comprises a replaceable fusion core. The replaceable fusion core further comprises a first chamber enclosed by walls about a central axis. In one embodiment, the first chamber has an outer radius of about four meters or less relative to the central axis. The first chamber encloses a high power density neutron source. The replaceable fusion core can be substantially surrounded by fissionable materials such as fuels rods or nuclear waste materials. In one embodiment, the reactor further comprises a second chamber enclosing one or more layers of fissionable materials substantially adjacent to at least a portion of the replaceable fusion core. The second chamber can also contain neutron-absorbing and neutron-reflecting materials. Neutrons provided to the fissionable materials from the high power density neutron source increase nuclear fission reactions in the fissionable materials. 
     In another aspect, a method of placing a replaceable fusion core in a reactor is disclosed. One embodiment of the method comprises providing a replaceable fusion core comprised of a first chamber enclosed by walls about a central axis. A high power density neutron source is contained within the first chamber. The replaceable fusion core is placed within a second chamber. The second chamber also contains fissionable materials substantially adjacent to at least a portion of the replaceable fusion core. Neutron-absorbing and neutron-reflecting materials can also be placed in the second chamber so that neutrons from the high power density neutron source increase nuclear fission reactions in the fissionable materials. 
     In one embodiment, the above-described method can include removing the replaceable fusion core from the second chamber and placing a second replaceable fusion core within the second chamber to replace the removed replaceable fusion core. 
     Additional advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice. Other advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying figures, not necessarily drawn to scale, which are incorporated in and constitute a part of this specification, illustrate several embodiments and together with the description serve to explain the principles of the invention, and in which: 
         FIG. 1  shows a cross-sectional view of a disclosed embodiment; 
         FIGS. 2A and 2B  show three dimensional views of a disclosed embodiment; 
         FIG. 2C  illustrates an embodiment of a method of placing a replaceable fusion core in a reactor; 
         FIG. 3  shows a cross-sectional view of a disclosed embodiment generated by CORSICA™; 
         FIG. 4  shows a vessel around a central axis; 
         FIGS. 5A-5E  show flow charts for methods for transmutating fissionable materials using disclosed embodiments; 
         FIG. 6  shows a prior art magnetic confinement configuration comprising a limiter and a divertor; 
         FIG. 7  shows a prior art magnetic confinement configuration comprising an X divertor, as described in Kotschenreuther et al. “On heat loading, novel divertors, and fusion reactors,” Phys. Plasmas 14, 72502/1-25 (2006); 
         FIG. 8  shows a modified schematic of a tokamak comprising an embodiment of a disclosed divertor; 
         FIG. 9A  shows an upper region of CORSICA™ equilibrium for an exemplary embodiment; 
         FIG. 9B  shows an upper region of CORSICA™ equilibrium for an exemplary embodiment, wherein the divertor coil is split into two distinct divertor coils; 
         FIG. 9C  shows an upper region of CORSICA™ equilibrium for an exemplary embodiment, wherein the divertor coil is split into four distinct divertor coils; 
         FIG. 10  shows an exemplary diagram of a Fusion Development Facility (FDF) based embodiment for a disclosed FDF based reactor; 
         FIG. 11  shows an upper region of CORSICA™ equilibrium for an exemplary embodiment for a Component Test Facility (CTF) with Cu coils; 
         FIG. 12  shows an upper region of CORSICA™ equilibrium for an exemplary embodiment for a Slim-CS, a reduced size central solenoid (CS) based reactor with superconducting coils; 
         FIG. 13  shows upper region of CORSICA™ equilibrium for an exemplary embodiment for an ARIES (Advanced Reactor Innovation and Evaluation Study) based reactor (using modular coils that fit inside the extractable sections bounded by the dotted line); 
         FIGS. 14A &amp; 14B  shows (a) a diagram of National High-power Advanced Torus Experiment (NHTX) based embodiment and (b) CORSICA™ equilibrium for a disclosed NHTX based reactor; 
         FIG. 15A  shows a standard NHTX configuration (prior art); 
         FIG. 15B  shows a SOLPS (Scrape-off Layer Plasma Simulation) calculation for an NHTX based reactor comprising an embodiment of a disclosed divertor configuration; 
         FIG. 15C  shows upper region of CORSICA™ equilibrium for a disclosed NHTX based embodiment; 
         FIG. 16  shows a cross-section plot of ITER (International Thermonuclear Experimental Reactor) plasma size compared to high power density plasma sizes achievable using embodiments described herein; and 
         FIG. 17  is a plot showing the reduced effect of plasma motion on location of divertor strike-point for a disclosed divertor as compared to the greater effect of the same plasma motion on plasma X point. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The devices, systems and methods described herein may be understood more readily by reference to the following detailed description and the examples included therein and to the figures and their previous and following description. 
     Before the present systems, articles, devices, and/or methods are disclosed and described, it is to be understood that this invention is not limited to specific systems, specific devices, or to particular methodology, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. 
     The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the embodiments of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof. 
     Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, example methods and materials are now described. 
     Throughout this application, various publications are referenced. Unless otherwise noted, the disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this pertains. The references disclosed are also individually and specifically incorporated by reference herein for the material contained in them that is discussed in the sentence in which the reference is relied upon. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein may be different from the actual publication dates, which may need to be independently confirmed. 
     As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a divertor plate,” “a reactor,” or “a particle” includes combinations of two or more such divertor plates, reactors, or particles, and the like. 
     Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. It is also understood that when a value is disclosed that “less than or equal to” the value, “greater than or equal to the value” and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value “10” is disclosed the “less than or equal to 10” as well as “greater than or equal to 10” is also disclosed. It is also understood that throughout the application, data is provided in a number of different formats and that this data represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point “10” and a particular data point 15 are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed. 
     As used herein, the terms “optional” or “optionally” means that the subsequently described aspect may or may be present or that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. For example, a disclosed embodiment can optionally comprise a fusion plasma, i.e., a fusion plasma can or cannot be present. 
     “Exemplary,” where used herein, means “an example of” and is not intended to convey a preferred or ideal embodiment. Further, the phrase “such as” as used herein is not intended to be restrictive in any sense, but is merely explanatory and is used to indicate that the recited items are just examples of what is covered by that provision. 
     The term “replaceable,” as used herein, refers to the ability to substitute one component for another component. In one aspect, a “replaceable component” refers to a component that can be replaced without having to move, destroy, or otherwise significantly modify other major components. While a replacement can, in some aspects, require minor electrical, hydraulic, or mechanical adjustments, such adjustments are not considered major modifications to a device. It should be appreciated that, unlike what is known in the art, the replaceability of a disclosed fusion neutron source can be conveniently facilitated by its modular characteristics, including, for example, small size, shape, and high power density. 
     Disclosed are the components to be used to prepare the compositions as well as the compositions themselves to be used within the methods disclosed herein. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutation of these compounds may not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a particular compound is disclosed and discussed and a number of modifications that can be made to a number of molecules including the compounds are discussed, specifically contemplated is each and every combination and permutation of the compound and the modifications that are possible unless specifically indicated to the contrary. Thus, if a class of components A, B, and C are disclosed as well as a class of components D, E, and F and an example of a combination component, A-D is disclosed, then even if each is not individually recited each is individually and collectively contemplated meaning combinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considered disclosed. Likewise, any subset or combination of these is also disclosed. Thus, for example, the sub-group of A-E, B-F, and C-E would be considered disclosed. This concept applies to all aspects of this application including, but not limited to, steps in methods of making and using the compositions. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the methods. 
     It is understood that the compositions disclosed herein have certain functions. Disclosed herein are certain structural requirements for performing the disclosed functions, and it is understood that there are a variety of structures that can perform the same function that are related to the disclosed structures, and that these structures will typically achieve the same result. 
     Disclosed are vessels for containing plasma or fusion plasma, fusion neutron sources, and tokamaks, wherein a reactive plasma can optionally be present therein; and wherein a layer of fissionable materials is substantially adjacent to at least a portion of said plasma or a chamber for confining said plasma. Also disclosed are methods of fissioning fissionable material using a disclosed embodiment, wherein a reactive plasma is present. Also disclosed are nuclear fuel cycles for fissioning fissionable material using a disclosed method. 
     As an example, a disclosed embodiment can have a general configuration as shown in  FIG. 1  and  FIGS. 2A and 2B .  FIG. 1  is a cross-sectional view of one half of a disclosed reactor  100 . As shown in  FIG. 1 , a disclosed embodiment can comprise a toroidal chamber substantially enclosed by walls  170  about a central axis  250 . The chamber walls have an inner radius  240  that is closest to the central axis  250  and an outer radius  230  that is farthest from the central axis  250 . The toroidal chamber can optionally comprise a core plasma  160  that, when present, can be contained within said the toroidal chamber by closed magnetic surfaces  180  and open magnetic field lines  260  relative to the core plasma. The core plasma can produce fast (about 14 million electron volts) neutrons via fusion reactions, which, since uncharged, can travel away from the core plasma on a given trajectory. The neutrons, when present, can bombard a layer of fissionable materials  150  substantially adjacent to at least a portion of said core plasma  160 . To insulate the reactor from neutrons, portions of the reactor can comprise Pb sections. In addition, a Pb sheath  110  can substantially surround the layer of fissionable materials  150 . The open  260  and closed  180  magnetic field lines can be created by a current induced by current-carrying conductors, including, without limitation, toroidal field (TF) coils  280  and  220  as well as poloidal field (PF) coils  120 ,  140 ,  190 , and  210 . A main boundary or separatrix  270  can exist between open  260  and closed  180  magnetic field lines, i.e., the boundary between opened and closed magnetic drift trajectory. Particles, heat, and/or energy that cross the closed magnetic surfaces  180  (i.e., cross-field flux) can be directed to one or more divertor plates  130  and  200  by the open magnetic field lines  260 . 
     In one embodiment, a fusion core  290  can be placed, or replaced, as a unit within a fission reactor to form a hybrid fusion-fission reactor  100 . Similarly, the fusion core  290  can be removed from a reactor  100  for replacement, maintenance, or any other purpose. In one aspect, the replaceable fusion core  290  can be referred to as a battery. Auxiliary connections, such as power, coolant, vacuum, etc. can be severed or otherwise disconnected and re-connected during the removal or replacement processes. 
     The fusion core  290  facilitates rapid replacement of components of the reactor in order to maintain a high availability factor for the reactor  100 . In situ maintenance of highly radioactive components inside a reactor can be difficult, dangerous and time consuming. By replacing the fusion core  290  as a unit, many of these challenges can be alleviated. For example, components of a fusion neutron source that are exposed to the core plasma will eventually require replacement. Generally, it is anticipated that such replacement will occur about once every two years. For example, the total neutron dose from 1-2 MW-year/m 2  neutron flux reaches about 20 dpa in about one to two years. Survival and dependability of steel components beyond this damage level is currently being researched. It can be estimated that such replacement could take approximately 4-6 months in a conventional hybrid fusion-fission reactor, whereas it is anticipated that could take only about one month if the hybrid fusion-fission reactor utilized an embodiment of the disclosed fusion core  290 . 
     In one aspect, the fusion core  290  is cylindrical and fits within a surrounding annular blanket of fissionable materials, though other shapes are contemplated within the scope of various embodiments. The fissionable materials can be waste nuclear materials or nuclear fuel rods. The fusion core  290  can be raised or lowered using cranes or platforms. In one embodiment, the fusion core  290  can be placed inside fissionable materials with straight coolant paths and long, straight fuel rods (and control rods, if needed), as used in most current fission reactor designs. 
     Furthermore, in one embodiment the fissionable materials are placed outside the TF coil field. This arrangement leaves only smaller and nearly vertical magnetic fields in the fissionable materials. These remnant magnetic fields almost follow the coolant flow pattern, thus minimizing the magnetohydrodynamic pressure drop in the coolant flow, which otherwise would make it challenging to push the coolant through. The use of liquid metals (such as sodium or lead alloys) as coolants further increases the challenges associated with magnetohydrodynamic pressure drop. 
     In one embodiment, a reactor  100  is described. The embodiment comprises a replaceable fusion core  290 . The replaceable fusion core  290  further comprises a first chamber  222  enclosed by walls  224  about a central axis  226 . In one embodiment, the first chamber  222  has an outer radius of four meters or less relative to the central axis  226 . The first chamber  222  encloses a high power density neutron source. Further comprising the reactor  100  is a second chamber  228  enclosing one or more layers of fissionable materials  150  substantially adjacent to at least a portion of the replaceable fusion core  290 . In one aspect, the second chamber  228  also encloses neutron-absorbing and neutron-reflecting materials. In the reactor  100 , neutrons are provided to the fissionable materials  150  from said high power density neutron source thereby increasing nuclear fission reactions in the fissionable materials  150 . 
       FIG. 2C  illustrates an embodiment of a method of placing a replaceable fusion core  290  in a reactor  100 . The method comprises step  230  of providing a replaceable fusion core  290  comprised of a first chamber  222  enclosed by walls  224  about a central axis  226 , wherein a high power density neutron source is contained within the first chamber  222 . At step  232 , the replaceable fusion core is placed within a second chamber  228 . The second chamber  228  also contains fissionable materials  150  substantially adjacent to at least a portion of the replaceable fusion core  290 . In one aspect, neutron-absorbing and neutron-reflecting materials can also be placed in the second chamber  228  so that neutrons from the high power density neutron source increase nuclear fission reactions in the fissionable materials  150 . Furthermore, in one embodiment the process of  FIG. 2C  can further comprise the steps of removing the replaceable fusion core from the second chamber, and placing a second replaceable fusion core within the second chamber to replace the removed replaceable fusion core. 
     In one aspect, a core plasma can be a fusion plasma that emits neutrons from the fusion plasma such that one or more nuclear fission reactions occur in the layer of fissionable materials. In one aspect, such a reaction of the fissionable materials can transmute said fissionable materials to materials that are more stable relative to the fissionable materials or materials having a shorter radioactive half-life than the fissionable materials. 
     In a further aspect, a disclosed embodiment comprising a layer a fissionable materials can comprise nuclear waste within the layer of fissionable materials. In general, nuclear waste can be any waste capable of undergoing fission. In one aspect, nuclear waste can be radioactive. In a further aspect, nuclear waste can be nuclear reactor waste that would otherwise be stored in a nuclear repository, e.g., Yucca Mountain. It should be appreciated that storing nuclear waste in geological repositories is estimated to cost approximately $96 billion; thus, in one aspect, a disclosed embodiment can mitigate the cost of geological repositories by reducing the amount of nuclear waste. 
     In one aspect, nuclear waste originating from nuclear reactors can be channeled through one or more light water reactors (LWRs) or other reactors before being placed in a disclosed embodiment. Thus, in one aspect, nuclear waste can be partially transmuted before being used with a disclosed embodiment. As used herein, “transmuted” is intended to refer to a process in which one chemical element or isotope is converted into another chemical element or isotope through a nuclear reaction. 
     In one aspect, the layer of fissionable materials can comprise transuranic elements. Transuranic elements, also known as transuranium elements, are elements that have an atomic number higher than 92. Examples of transuranic elements include neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No), lawrencium (Lr), rutherfordium (Rf), dubnium (Db), seaborgium (Sg), bohrium (Bh), hassium (Hs), meitnerium (Mt), darmstadtium (Ds), roentgenium (Rg), ununbium, ununtrium, ununquadium, ununpentium, ununhexium, ununoctium. These elements can be referred to as Hard-to-Fission TRU nuclear waste, and the remaining fissionable elements can be referred to as Easy-to-Fission elements. 
     In a further aspect, a disclosed fusion neutron source can be used to decrease radio-toxicity levels of fissionable materials, e.g., transuranic elements. “Radio-toxicity,” as used herein, means potential toxicity following absorption of a radio-active substance into a living subject. In general, elements with relatively short half-lives can be radio-toxic, such as for example, nuclear materials including transuranic elements. Radioactive decay results in a reduction of summed rest mass, which is converted to energy (the disintegration energy) according to the formula E=mc 2 . The energy can, in some aspects, be radio-toxic. 
     In one aspect, wherein a disclosed embodiment comprises a layer of fissionable materials, the disclosed embodiment can be used to reduce the amount of fissionable materials, e.g., transuranic elements, in the layer of fissionable materials. In one aspect, a disclosed fusion neutron can be used to reduce the amount of nuclear waste products in the layer of fissionable materials. Thus, in one aspect, the fusion neutron source can be used to reduce the radio-toxicity level of said nuclear waste products. A nuclear fission reaction of said fissionable materials can transmute said fissionable materials to materials that are more stable relative to the fissionable materials or materials having a shorter radioactive half-life than the fissionable materials. 
     A disclosed embodiment can have a magnetic geometry and coil and divertor configuration, for example, as shown in  FIG. 3 , which is a cross-sectional view of a section of a toroidal reactor generated by a CORSICA™ computer program. CORISICA™ is software developed by The Lawrence Livermore National Laboratory, Livermore, Calif., for simulating physics processes in a magnetic fusion reactor. In this embodiment, plasma  310  can be primarily confined by closed magnetic surfaces  340 , wherein a scrape off layer (SOL)  300  exists beyond said closed magnetic surfaces. The closed magnetic surfaces  340  (i.e., the toroidal field) about the plasma  310  are caused by a current induced in the plasma  310  by a toroidal field (TF) coil or conductor (not shown) that goes substantially through the center of the toroid, thereby inducing the current in the plasma  310  by a transformer action, as known in the art. The SOL  300  can comprise open magnetic field lines (relative to the closed magnetic surfaces  340  of the fusion plasma). A vacuum chamber  345  can be substantially enclosed by walls  350 . Additional magnetic field lines  370  can exist outside said vacuum chamber. Coils  320  or current carrying conductors in or adjacent to the walls  350  can be used to produce magnetic fields (i.e., poloidal fields (PF)) that cause the open magnetic field lines. Said coils  320  or current-carrying conductors can shape and/or control magnetic field lines if there is a need to shape and/or control said lines, and create the open magnetic field lines for diverting cross-field flux (or scrape-off flux), i.e., particles that migrate from the fusion plasma  310  across the closed field lines  340  to the open magnetic field lines. Scrape-off flux can be diverted by the open magnetic field lines to a divertor plate  330 , which as shown in  FIG. 3  and can optionally be shielded from neutrons emitted from the fusion plasma  310 . Because the divertor plate  330  is at a radial distance (straight line distance) from the fusion plasma  310  and at a magnetic distance (distance along a magnetic field line from the fusion plasma to the divertor plate) that is greater than other fusion reactors found in the art, the open magnetic field lines can be spread further at the divertor plate, thereby mitigating heat concentration on the divertor plate  330 , and allowing radiant cooling of the particle from the time it leaves the fusion until it arrives at the divertor plate  330 . In this embodiment, a layer of fissionable materials (not shown) can be substantially adjacent to at least a portion of said plasma  310  and/or said vacuum chamber  345  for confining said plasma. Various modifications of this embodiment can be made, as will be apparent from the present disclosure. 
     As used herein, a “vessel for containing plasma” can be any vessel compatible with fusion, and is not necessarily limited to known vessel designs. A vessel for containing plasma can be a fusion neutron source, if a reactive plasma is present. A vessel for containing plasma can also be a tokamak. It is understood that any disclosed component or embodiment can be used with any disclosed vessel for containing plasma, fusion plasma, fusion neutron source, or tokamak, or method of exhausting heat therefrom, unless the context clearly dictates otherwise. 
     In one aspect, a disclosed embodiment can comprise a toroidal chamber substantially enclosed by walls about a central axis, wherein said toroidal chamber has an inner radius and an outer radius relative to the central axis; a divertor plate for receiving exhaust heat from a fusion plasma substantially contained within the toroidal chamber by magnetic fields, said divertor plate having a divertor radius relative to the central axis and said divertor radius at least greater than or equal to the inner radius of the toroidal chamber. A layer of fissionable materials can be substantially adjacent to the fusion plasma. 
     As used herein, “central axis” refers to an axis lying within a plane and passing through the centroid of a disclosed embodiment. A portion of a vessel, for example, surrounding a central axis is shown in  FIG. 4 . A portion of a vessel  410  surrounds a central axis  420 . A point in space extending outward and substantially perpendicular to said central axis has a radius relative to said central axis. For example, said vessel can have an inner radius  430  closest to said central axis  420  and an outer radius  440  farthest from said central axis  420 . In one aspect, said inner and said outer radius can be defined as a point extending from an imaginary line substantially perpendicular to said central axis  420  and positioned along the same x-y-z plane as the diameter of said vessel. 
     A disclosed chamber can be any shape compatible for confining fusion plasma. In some aspects, at least a portion of the disclosed chamber can be toroidal. By “toroidal,” it is meant that a rotation around a point on a central axis would be a toroidal rotation. Thus, in one aspect, a disclosed chamber is not necessarily toroidal as a whole, but rather a point within or on said chamber can produce, when rotated around a central axis, a toroidal shape. 
     In one aspect, a disclosed vessel can comprise any material known to be compatible with fusion reactors. Non-limiting examples include metals (e.g., tungsten and steel), metal alloys, composites, including carbon composites, combinations thereof, and the like. 
     In one aspect, a disclosed embodiment comprises an improved divertor. As used herein, the “divertor” is meant to refer to all aspects within an embodiment that divert heat, energy, and/or particles from the core plasma to a desired location away from the core plasma. Examples of aspects of a divertor include, but are not limited to, the scrape-off layer, open magnetic field lines containing scrape-off flux therein, one or more divertor plates (or divertor targets), and one or more separatrices. 
     In one aspect, said divertor plate can comprise any material suited for use with a fusion reactor. Known existing divertor compositions can be used, such as, for example, tungsten or tungsten composite on a Cu or carbon composite. Other materials that can used include steel alloys on a high thermal conductivity substrate. 
     In a further aspect, a divertor plate can have a divertor radius relative to the central axis and said divertor radius can be located at a position relative to another component or point within a disclosed embodiment. As one skilled in the art will appreciate, the ratio of the divertor radius relative to other components, e.g., the plasma or the chamber wall, etc., is intended to encompass any appropriate individual radius, and thus any actual divertor radius disclosed is meant to be purely exemplary, and as such, non-limiting. 
     As used herein, and represented by R div , the term “divertor radius” is meant to refer to the farthest radial distance of the divertor plate from the central axis. 
     In one aspect, a divertor plate can have a divertor radius greater than or equal to about the outer radius of the toroidal chamber. In a further aspect, a divertor plate can have a divertor radius less than or equal to about the outer radius of the toroidal chamber. In a still further aspect, a divertor plate can have a divertor radius greater than or equal to about the inner radius of the toroidal chamber. 
     In one aspect, the ratio of the divertor radius, R div , to the outer radius of the toroidal chamber, R c , can be from about 0.2 to about 10, or from about 0.5 to about 8, or from about 1 to about 6, or from about 1 to about 5, or from about 1 to about 3, or from about 1 to about 2, of from about 1 to about 1.5. 
     In general, it is contemplated that any sized embodiment can be used. But, for example, said divertor plate can have a radius of about 0.2 m, 0.5 m, 1 m, 1.5 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, or about 10 m. In a further aspect, a divertor radius can be about 1.9 m, 3.3 m, 4 m, 7.3 m, or 7.5 m. 
     In one aspect, a divertor plate can have a divertor radius relative to an X point on a separatrix. As used herein, the term “separatrix” refers to the boundary between open and closed magnetic surfaces, and an X point refers to a point on the separatrix where the poloidal magnetic field is zero. In one aspect, multiple X points exist in a disclosed embodiment, and main plasma X point refers to an X point adjacent to the said core plasma. For example, referring back to  FIG. 3 , the main X point is shown as  360 . The radius of a main X point generally depends on the configuration of the magnetic field lines. In one aspect, a divertor plate can have a major radius that is greater than or equal to the radius of the main X point. 
     In one aspect, the ratio of the divertor plate radius to the X point radius, R div /R X  can be from about 1 to about 5, or from about 1 to about 4, or from about 1 to about 3.5, or from about 1.5 to about 3.5. For example, a disclosed divertor plate and a disclosed separatrix can have radii as listed in Table 1, along with the corresponding ratio. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Examples of R div  and R X . 
               
            
           
           
               
               
               
            
               
                 R div  (m) 
                 R X  (m) 
                 R div /R X   
               
               
                   
               
               
                 3.25 
                 1.75 
                 1.9 
               
               
                 7.25 
                 4.50 
                 1.6 
               
               
                 7.50 
                 4.25 
                 1.8 
               
               
                 4.00 
                 1.50 
                 2.7 
               
               
                 3.25 
                 1.75 
                 1.9 
               
               
                 1.90 
                 0.60 
                 3.2 
               
               
                 1.95 
                 0.70 
                 2.8 
               
               
                 4.00 
                 2.20 
                 1.8 
               
               
                   
               
            
           
         
       
     
     In yet a further aspect, a divertor plate can have a divertor radius relative to the major plasma radius, defined as the distance from said central axis to said plasma center. For example, the ratio of the divertor radius to the major plasma radius (R), R div /R, can be from about 0.5 to about 10, or from about 1 to about 8, or from about 1 to about 6, or from about 1 to about 5, or from about 2 to about 5, including, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. As a specific non-limiting example, if a plasma major radius is 1 m, and a divertor radius is 2 m, then R div /R=2. 
     In one aspect, said divertor plate can be at least partially shielded from neutrons emitted from the core plasma. In a further aspect, said chamber walls at least partially shield the divertor plate from neutrons emitted from said core plasma, as shown, for example, in  FIG. 3 . 
     The neutron flux, defined as a measure of the intensity of neutron radiation in neutrons/cm 2 -sec. Neutron flux is the number of neutrons passing through 1 square centimeter of a given target in 1 second. Using embodiments of a divertor plate described herein, calculations show a decrease in neutron flux by a factor of over 10 as compared to other divertor plate designs. 
     Additional divertor plates, not corresponding to the radii disclosed herein, can also be used in combination with a disclosed divertor plate. Specifically, known reactor designs can comprise divertor plates, wherein the divertor radius is less than the outer radius of a chamber, a plasma major radius, a separatrix, or another component or point within a vessel for containing fusion plasma. These known designs, in some aspects, can simply be augmented with an additional disclosed divertor design. Examples of such divertors include the standard divertor, as discussed herein, and the X divertor, as discussed in Kotschenreuther et al. “On heat loading, novel divertors, and fusion reactors,” Phys. Plasmas 14, 72502/1-25 (2006), which is hereby incorporated into this specification by reference in its entirety (hereinafter Kotschenreuther). An exemplary embodiment of an X divertor is shown in  FIG. 8 , wherein four poloidal field coils placed substantially adjacent to divertor plates expand the magnetic flux near the divertor plates so that the heat and plasma particle fluxes flowing from the core plasma into the SOL fall on larger areas of the divertor plates. 
     Referring to  FIG. 3  and  FIG. 4 , in one aspect, a disclosed embodiment comprises a toroidal chamber  410  about a central axis  420 . A major radius of any point denotes its perpendicular distance from the central axis  420 . Directions perpendicular to the central axis  420  are radial, and directions in any plane containing the central axis  420  are poloidal. A toroidal core plasma  310  is substantially confined within the toroidal chamber  145  by closed magnetic surfaces  340  that stay substantially on closed toroidal magnetic surfaces. The toroidal core plasma  340  is substantially enclosed by a region of open magnetic field lines  300  that intersect one or more divertor plates  330  (this region can be referred to as the SOL (i.e., Scrape-Off Layer)). A magnetic surface known as a separatrix separates the core plasma and the SOL and intersects the divertor plates  330 . Particles and energy that flow from the core plasma  340  across the separatrix into the SOL are directed along the open magnetic field lines  300  to the divertor plates  330 . Both the closed magnetic surfaces  340  in the core plasma  310  and the open magnetic field lines  300  in the SOL are created by a current in the toroidal core plasma  310  and by currents in conductors  320  substantially adjacent to the toroidal chamber  145 . The core plasma  310  and the SOL regions together are substantially enclosed by walls  350 . An equatorial plane, which is perpendicular to the central axis  420 , and which passes through a point at the largest major radius in the core plasma  340 , divides the toroidal chamber  145  into upper and lower regions. When only the upper region is shown, as in  FIGS. 3 and 4 , the lower region is substantially a mirror image of the upper region in the equatorial plane. A major radius of any point is that point&#39;s perpendicular distance from the central axis. The major radii of points in the core plasma  340  that are farthest (or closest) from the central axis  420  are the outer plasma major radius (or inner plasma major radius). Half of the sum of the outer and inner plasma major radii is the plasma major radius, and half of the difference between the outer and inner plasma major radii is the plasma minor radius. A point in the upper (or the lower) region of the core plasma  340  farthest from the equatorial plane is the upper (or the lower) peak point. The largest major radius of points of intersection between the separatrix and the divertor plates  330  are the outboard divertor major radius and the corresponding divertor plate is the outboard divertor plate  330 . A length along an open magnetic field line from a point approximately one-half centimeter outside the separatrix in the equatorial plane to the outboard divertor plate  330  is the SOL length, also known as the magnetic connection length. 
     A layer of fissionable material can be substantially adjacent to the core plasma  310 , when present, and/or the toroidal chamber  410 . An equatorial plane, which can be perpendicular to the central axis  420 , and which passes through a point on the largest major radius line in the core plasma  310 , divides the toroidal chamber  145  into upper and lower regions. The major radii of points in the core plasma  310  that are farthest (or closest) from the central axis  420  are the outer plasma major radius (or inner plasma major radius). Half of the sum of the outer and inner plasma major radii is the plasma major radius, and half of the difference between the outer and inner plasma major radii is the plasma minor radius. A point in the upper (or the lower) region of the core plasma  310  farthest from the equatorial plane is the upper (or the lower) peak point. The largest major radius of points of intersection between the separatrix and the divertor plates  330  are the outboard divertor major radius and the corresponding divertor plate is the outboard divertor plate  330 . A length along an open magnetic field line from a point approximately one-half centimeter outside the separatrix in the equatorial plane to the outboard divertor plate  330  is the SOL length. 
     A stagnation point is defined as any point where a poloidal component of the magnetic field is zero. In one aspect, the separatrix contains at least one stagnation point whose perpendicular distance from the equatorial plane is greater than the plasma minor radius, and, for at least one divertor plate  330 , the outboard divertor major radius is greater than or equal to the sum of the plasma minor radius and the major radius of the peak point closest to the corresponding divertor plate  330  In one aspect, this divertor plate  330  can be referred to as a Super-X Divertor or a Super X Divertor (SXD). 
     In one aspect, current-carrying conductors or coils substantially adjacent to the toroidal chamber expand a distance between said open magnetic field lines at the divertor plate relative to a distance between the open magnetic field lines at an outer radius of the toroidal chamber such that heat transferred to said divertor plate by said particles striking the divertor plate is distributed over an expanded area of the divertor plate. The current carrying conductors  320  substantially adjacent to the toroidal chamber  145  can create a magnetic flux expansion in the SOL, i.e., decrease the poloidal component of the magnetic field in the SOL. Therefore, energy and particles transferred to the divertor plate  330  can be distributed over an expanded area of the divertor plate  330 , thus decreasing the average and peak fluxes of energy and particles incident on the divertor plate  330 , and the SOL length can be optionally increased. In one aspect, the SOL length is greater than twice the SOL length for an instance in which the divertor plate is located at the corresponding stagnation point and in a plane perpendicular to the central axis. In a further aspect, the SOL length to the divertor plate is long enough so that electrons coming from the core plasma cool to a temperature of less than about 40 electron volts (eV) of energy before reaching said divertor plate. 
     In yet a further aspect, the low plasma temperature near the divertor plate  330  allows an increase in radiation of energy from the plasma near the divertor plate  330 . In a still further aspect, the SOL lengths to the divertor plate  330  are long enough to maintain a detached plasma, i.e., maintain a stable zone of plasma at a temperature less than about 5 eV between the divertor plate  330  and the plasma. 
     In one aspect, the pumping ability (i.e., the pumping of helium ash from fusion reactions) can be enhanced by embodiments of the divertor plate as described herein because the major radius of the divertor plate is larger than the major radius of the nearest peak point by an amount grater than the plasma major radius. While not wishing to be bound by theory, this enhancement can result in a) an increase in the neutral pressure near the divertor plate, b) decreased pumping channel lengths from the divertor to pumps, and/or c) increased maximum area of the pumping ducts due to the larger major radius of a disclosed divertor. 
     Because of the larger major radius of embodiments of the divertor plates as described herein, a liquid metal such as, for example, lithium, can be present or flowing on a disclosed divertor, and can, in some aspect, be used efficiently on the divertor plates because the lower magnetic field at the larger major radius reduces the magnetohydrodynamic effects on the liquid metal. 
     In one aspect, the purity of the core plasma can be increased by embodiments of the divertor plate described herein. Without wishing to be bound by theory, this can result from a) a reduction in sputtering from the divertor plate due to lower plasma temperature, b) an increase in plasma density near the plate that can reduce the amount of sputtered material reaching the core plasma, and/or c) the increased length of a disclosed divertor as compared to standard divertors, which results in any sputtering occurring further from the core plasma and sputtering at the divertor plate can be shielded from the core plasma by the walls of the toroidal chamber or the longer SOL distance between the divertor plate and the core plasma. 
     It should be appreciated that in a further aspect, the longer line length of the SOL in the divertor can enable one or more of the following improvements as compared to devices with standard divertors: a) allowing lower plasma temperature near the divertor plates, b) allowing higher plasma and neutral densities near the divertor plates, c) enhanced spreading of heat by either plasma-generated or externally driven turbulence in the SOL, without also significantly increasing the turbulence in the core plasma, and/or d) sweeping the regions of highest heat or particle flux on the SXD plates at a rate fast enough so that the resulting spatial and temporal redistribution of the heat flux reduces the peak temperature of the divertor plate. 
     In one aspect, the use of embodiments of the divertor plate described herein allows power density in the core plasma to be substantially higher than known toroidal plasma devices. In a further aspect, the fusion power density in the core plasma is substantially higher than known toroidal plasma devices. For example, if power density is defined as the quotient of the core heating power in megawatts and the plasma major radius (described in more detail herein) in meters, then embodiments described herein can produce a power density of about five megawatts per meter or greater. Of course, lower power densities are also contemplated within the scope of the described embodiments. This high power density can result in a core plasma of sufficient heat and density to produce a large number of neutrons from fusion reactions of plasma particles. 
     It will be apparent that the various disclosed radii for components within a disclosed embodiment can be determined by a physical measurement of a working embodiment. Or, in the alternative, a disclosed radius can be determined through a model, such as, for example, a model generated by CORSICA™. Thus, in one aspect, a physical embodiment can be deduced to a model, and the various parameters can be determined by the model. 
     In one aspect, a disclosed embodiment comprises plasma or fusion plasma that is substantially magnetically contained within a vessel for containing the plasma, a fusion neutron source, or a tokamak, by closed magnetic surfaces and open magnetic field lines relative to the fusion plasma. A disclosed core plasma can have a major radius and a minor radius. The major radius of the plasma can be the radius of the plasma as a whole (from the central axis to the center of the plasma). The minor radius can be the radius of the plasma itself, i.e., a distance extending from the center of the plasma to the perimeter of said plasma. 
     The fuel to be used as plasma can, at least in principle, comprise combinations of most of the nuclear isotopes near the lower end of the periodic table. Examples of such include, without limitation, boron, lithium, helium, and hydrogen, and isotopes thereof (e.g.,  2 H, or deuterium). Non-limiting reactions of deuterium and helium, for example, which can occur within nuclear fusion plasma are listed below. 
         D+D→p+T  (tritium)+˜3 MeV, wherein p is a proton. 
         D+D→n+   3 He+˜4 MeV, wherein n is a neutron. 
         D+T→n+   4 He+˜17 MeV. 
         D+   3 He→ p+   4 He+˜18 MeV. 
     Any known means of heating a fuel to create said fusion plasma, and heating said fusion plasma to the temperatures required for fusion to occur can be used in combination with the disclosed embodiments, including the disclosed methods. Plasmas can be generated in various ways including DC discharge, radio frequency (RF) discharge, microwave discharge, laser discharge, or combinations thereof, among others. Plasmas can be generated and heated, for example, by ohmic heating, wherein plasma is heated by passing an electrical current thought it. Another example is magnetic compression, whereby the plasma is either heated adiabatically by compressing it though an increase in the strength of the confining field, or it is shock heated by a rapidly rising magnetic field, or a combination thereof. Yet another example is neutral beam heating, wherein intense beams of energetic neutral atoms can be focused and directed at the plasma from neutral beam sources located outside the confinement region. 
     Combinations of the aforementioned heating protocols can be used, as well other methods of heating. For example, neutral beam heating can be used to augment ohmic heating in a magnetic confinement device, such as a tokamak. Other methods of heating include, without limitation, heating by RF, microwave, and laser. 
     Any appropriately shaped plasma of any size compatible with a disclosed embodiment can be used. A discussion of plasma shapes can be found in “ITER,” special issue of Nucl. Fusion 47 (2007), which is hereby incorporated by reference into this specification in its entirety. The shape of fusion plasma, in one aspect, can determine the desire of a particular shape of a vessel for containing said fusion plasma. 
     Various factors can determine a desired plasma size, one of which is the containment time, which is Δt=r 2 /D, wherein r is a minimum plasma dimension and D is a diffusion coefficient. The classical value of the diffusion coefficient is D c =α i   2 /τ ie , wherein α i  is the ion gyroradius and τ ie  is the ion-electron collision time. Diffusion according to the classical diffusion coefficient is called classical transport. 
     The Bohm diffusion coefficient, attributed to short-wavelength instabilities, is D B =( 1/16)α i   2 Ω i  wherein Ω i  is the ion gyrofrequency. Diffusion according to this relationship is called anomalous transport. The Bohm diffusion coefficient for plasma, in some aspects, can determine how large plasma can be in a fusion reactors, vis-à-vis a desire that the containment time for a given amount of plasma be longer than the time for the plasma to have nuclear fusion reactions. On the contrary, reactor designs have been proffered wherein a classical transport phenomenon is, at least in theory, possible. Thus, in one aspect, one or more disclosed embodiments can be compatible with plasma comprising anomalous transport and/or classical transport. 
     During magnetic confinement of plasma, ionized particles can be constrained to remain within a defined region by specifically shaped magnetic fields. Such a confinement can be thought of as a nonmaterial furnace liner that can insulate hot plasma from the chamber walls. 
     In one embodiment, a magnetic field can be created to form a torus or a doughnut-shaped figure within which magnetic field lines form nested closed surfaces. Thus, in this geometry, plasma particles are permitted to stray only by crossing magnetic surfaces. In theory, this diffusion is a very slow process, the time for which has been predicted to vary as the square of the plasma minor radius, although much faster cross-diffusion patterns have been observed in experiment. 
     To direct anomalous and/or classical cross-magnetic field particle transport away from the plasma, particles from the fusion plasma that cross said separatrix can be directed to a plasma-wetted area on said divertor plate by said open magnetic field lines in said scrape off layer outside said separatrix. 
     In a further aspect, a disclosed embodiment can provide at least one divertor plate wherein the plasma-wetted area, A w , on at least one divertor plate is increased beyond currently known fusion neutron source designs. Without wishing to be bound by theory, in an embodiment comprising one or more divertor plates, A w , on the divertor plate can be bound via the equation Divergence of B=0, to be 
     
       
         
           
             
               
                 A 
                 w 
               
               = 
               
                 
                   
                     
                       B 
                       
                         p 
                         , 
                         sol 
                       
                     
                     
                       B 
                       div 
                     
                   
                    
                   
                     
                       A 
                       sol 
                     
                     
                       sin 
                        
                       
                         ( 
                         θ 
                         ) 
                       
                     
                   
                 
                 ≈ 
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               B 
                               p 
                             
                           
                         
                         
                           
                             
                               B 
                               t 
                             
                           
                         
                       
                       ] 
                     
                     sol 
                   
                    
                   
                     
                       R 
                       div 
                     
                     
                       R 
                       sol 
                     
                   
                    
                   
                     
                       A 
                       sol 
                     
                     
                       sin 
                        
                       
                         ( 
                         θ 
                         ) 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     wherein R sol , W sol , and A sol =2πR sol W sol  are the radius, width, and area of the scrape-off layer (SOL) at the (outer or inner) midplane for the corresponding divertor plates, wherein   is the angle between the divertor plate and the total magnetic field, B div , and the subscripts p(t) denote the poloidal (toroidal) directions. For a given W sol  and B p /B t  at the midplane, A w  can be increased, in one aspect, by reducing  . However, it is apparent that engineering constraints can, in some aspects, place a limit of about 1 degree on the minimum  , as determined, for example, in the ITER design, outlined in “ITER,” special issue of Nucl. Fusion 47 (2007), which is hereby incorporated by reference into this specification in its entirety. However, some disclosed designs comprise a divertor plate with a   of less than about 1 degree (e.g., 0.9°). 
     In one aspect, a disclosed embodiment can comprise an increase in R div , the divertor radius (with respect to the central axis) to affect an increase in A w . It should be appreciated that increasing R div , in one aspect, increases the distance between the divertor plate and the current in the plasma, which can make the divertor less sensitive than a standard divertor to plasma fluctuations. For example, as shown in  FIG. 17 , by changing the plasma pressure (or current) by ±5% (while holding coil currents and flux through the wall fixed to simulate sudden changes), this moves the outer strike points on the disclosed divertor plate by only about ±0.05 cm (see curve labeled dSXD in  FIG. 17 ) which is much smaller than about ±2.5 cm motion produced in a standard divertor (see curve labeled dX in  FIG. 17 ), Such small motions are small fractions of the widths of an exemplary plasma-wetted area (about 20 cm). 
     In one aspect, particles from said fusion plasma can travel a magnetic distance along open magnetic field lines from the fusion plasma to the divertor plate that is greater than a radial distance from the fusion plasma to the divertor plate. In a further aspect, the particles cool while traveling the magnetic distance along the open magnetic field lines to the divertor plate. 
     It is apparent that an increase in R div /R sol  can increase the magnetic connection length, L, of a scrape off flux particle by increasing the poloidal field all along the divertor leg at R. In one aspect, an extended L can increase the maximum allowed power (P sol ) in the scrape-off layer (SOL). The maximum divertor radiation fraction and the cross-field diffusion can both be enhanced. The longer L in a disclosed divertor can restore the capacity for substantial radiation even at high q II  (heat transferred per unit mass), increasing P sol  relative to a standard divertor by a factor of about 2. The longer line lengths can lower the plasma temperature at the plate at relevant high upstream q II . These results can be obtained, for example, by 1D-code, using CORSICA™, for example, as described in Kotschenreuther. As the plasma particles flow to the divertor along the extended field lines, cross-field diffusion effectively widens the SOL, resulting in a larger plasma footprint on the divertor plate. In one aspect, for example, an increase in SOL width by about 1.7 relative to a standard divertor can be expected. 
     A disclosed embodiment can provide for improvements in the capability of a fusion neutron source, vessel for containing fusion plasma, or tokamak to manage the problem of heat exhaust. The heat exhaust that occurs during the operation of a nuclear fusion reactor can be related to the heating power, P h =auxiliary heating power, P aux  plus about 20% of the fusion power, P f . For example, two of largest current tokamaks, the joint European torus (JET) in the European Union, with a major radius R=3 m, and the JT-60 tokamak in Japan, with R=3.4 m, each have a P h =120 MW, which is less than the P f  of about 400-500 MW. ITER (France), a joint international research and development project that aims to demonstrate the scientific and technical feasibility of fusion power, by contrast, is designed for a P h ˜400-720 MW, with P f ˜864803 2000-3600 MW. A measure of the severity of the heat flux problem can be estimated, in some aspects, as P h /R, wherein R is the plasma major radius. 
     Kotschenreuther et al. in “On heat loading, novel divertors, and fusion reactors,” Phys. Plasmas 14, 72502/1-25 (2006), which is hereby incorporated by reference in its entirety, discusses the severity of the heat flux problem in detail. Specific reference is made to Table 1 of Kotschenreuther and the discussion of the data presented therein, as it applies to the present context, wherein various P h /R values for known reactors, including future reactors, are listed. 
     In one aspect, a disclosed embodiment can be a tokamak. As used herein, the term “tokamak” refers to a magnetic device for confining plasma. While tokamaks generally comprise a toroidal shaped magnetic field which is substantially axisymmetric, i.e., approximately invariant under toroidal rotations about a central axis, a “tokamak,” as disclosed herein, is not limited to an axisymmetric toroidal shape. Other toroidal designs and shapes, both known and unknown, will likely be compatible with the various embodiments disclosed herein. Known toroidal alternatives to the traditional tokamak reactor are stellarators, spherical toroids (i.e., a cored apple shaped tokamak), reverse-field pinch reactors, and spheromaks. 
     In one aspect, a tokamak can further comprise a layer of fissionable materials substantially adjacent to a chamber for confining core plasma. In addition, a sheath of neutron reflecting material (e.g., Pb) can substantially surround the tokamak, or at least the inner chamber of the tokamak. 
     It should be appreciated that, in various embodiments, the geometrical configurations of the divertor plate as described herein can be accommodated by most, if not all, known tokamak designs, including predicted future tokamak designs. As an example, a divertor plate can fit inside toroidal field coils in corners or sections that often go unused, and any toroidal field ripple (unwanted curving of magnetic field lines) arising at the divertor plates can be handled by slight shaping of the magnetic field lines using, for example, an induced current. 
     In one aspect, a disclosed embodiment can be a Tokamak based High Power Density (HPD) Device. High power density of a disclosed device can be attained, for example, by reducing the size of the device, thereby increasing the power density. In one aspect, a disclosed high power density embodiment can have a major radius R of from about 1 m to about 5 m, or from about 1 m to about 4 m, or from about 1 m to about 3 m. Parameters for an exemplary high power density device are listed in Table 2. With reference to Table 1, an exemplary device can have a major radius of about 2.2 m, with an aspect ratio of about 2.5, wherein the aspect ratio is defined as the major/minor dimensions of the plasma torus at the horizontal equatorial plane (plasma major radius/plasma minor radius=aspect ratio). 
     Angular brackets such as &lt; &gt; denote average value of a parameter averaged over the core plasma volume. For example, &lt;n&gt; denotes the average density of particles in the core plasma. 
     Elongation of the plasma confined in a disclosed embodiment of a Tokamak based High Power Density (HPD) Device can be from about 1.5 to about 4, or from about 2 to about 3. Elongation measures the vertical height of the plasma minor cross section compared to the horizontal minor cross section. This parameter is typically measured at the separatrix (i.e., the magnetic surface dividing the closed plasma nested flux surfaces from the open ones that intersect the material walls) as well as at 95% of the flux at the separatrix (it can be zero at the plasma centre), which gives a good measure of the useful part of the plasma—the last 5% is affected somewhat by particles which are sometimes outside the separatrix and sometimes inside. With reference to Table 1, an exemplary high power density device can have an elongation of about 2.4 to about 2.7. 
     A disclosed embodiment of a Tokamak based High Power Density (HPD) Device can have a toroidal plasma current (I p ) of from about 10 to about 20 MA, or from about 10 to about 15 MA. It will be apparent that I p  can change during the operation of an embodiment. With reference to Table 2, for example, I p  for an exemplary embodiment can be from about 12 to about 14 MA. A disclosed HPD device can have a self-generated confinement magnetic field (bootstrap current fraction) of about 30 to about 90%, or from about 30 to about 80%. An exemplary device, for example, can have a bootstrap fraction of from about 40 to about 70% (Table 2). The current drive power in such a device, can be, for example, from about 20 to about 90 MW (e.g., from about 25 to about 60 MW, see Table 2). Although not wishing to be bound by theory, in one aspect, additional power for D-D fusion and/or Ion Cyclotron Resonance Heating (ICRH) can be from about 20 to about 50 MW. For example, power for these processes can be about 40 MW (Table 2). 
     If a Cu coil (e.g., a coil with about 60% Cu) is used for an HPD device, coil related dissipation can be about 160 MW for an exemplary device. The CD electric input to provide power to these coils can be, for example, from about 50 to about 120 MW. It is thought that the B T  at an exemplary Cu coil would be about 7 T (Table 2). 
     The I p  and other induced currents, if present, can create a magnetic flux density at the plasma center, B T , of from about 2 T (Tesla) to about 10 T, or from about 2 T to about 5 T. For example, a disclosed HPD device can have a magnetic flux density at the plasma center of about 4.2 T (Table 2). The volume averaged temperature &lt;T&gt; can be from about 10 to about 20 keV, or from about 10 to about 18 keV. For example, an HPD device can have a volume averaged temperature &lt;T&gt; of about 15 keV (Table 2). 
     The normalized β (β N ) in a disclosed HPD device can be from about 2 to about 8, or from about 2 to about 5. An exemplary device, as listed in Table, can have a β N  of about 3-4.5. Normalized β (β N ), as used herein, is plasma beta times a·B/I (a=minor radius, B=toroidal magnetic field on central axis, and I=plasma current). Plasma beta is the ratio of plasma pressure (the sum of the product of density and temperature over all the plasma particles) divided by the magnetic pressure (B 2 /2μ 0 )−a volume-integrated parameter which measures how good the magnetic field is at confining the plasma, and is typically a few % (percent). 
     Peaking value of a parameter is the ratio of its maximum value to its volume averaged value in the core plasma. 
     A disclosed HPD device can have a fusion power of up to 500 MW, or from about 0 MW to about 500 MW. An exemplary device, as listed in Table 2, can have a fusion power of up to about 400 MW, or from about 0 MW to about 400 MW. Fusion power, as used herein, is the total power generated by the fusion reactions in the plasma (i.e., not taking account of any energy multiplication that can take place by reactions in the surrounding structure). Other power parameters include Alpha-particle power, which is the part of the fusion power carried by the fused nuclei. Alpha power plus external heating power minus radiated power is the net heating power to the plasma. For a plasma generating a fusion power of up to 500 MW, an exemplary device can have a neutron wall load of from about 2 to about 3 MW/m 2  (Table 2). Impurities in the plasma, depending on the composition, can, in one aspect, comprise He (e.g., 10% He) and/or Ar (e.g., 0.25% Ar). 
     With reference to Table 2, a disclosed HPD device can have a H 89P , wherein H 89P  is the energy confinement improvement factor compared with the ITER89-P, of from about 2.6 to about 2 (for DIII-D reactions). It will be apparent that such a device can have a Q value, defined as the fusion power/input power of about 0.1 to about 1.9. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Parameters for exemplary Tokamak High Power Density Device 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 R major 
                 2.2 
               
               
                   
                 Aspect ratio 
                 2.5 
               
               
                   
                 Elongation 
                 2.4-2.7 
               
               
                   
                 I p   
                 12-14 MA 
               
               
                   
                 B T  (plasma center) 
                 4.2 T 
               
               
                   
                 &lt;n&gt; 
                 1.6 × 10 20   
               
               
                   
                 &lt;T&gt; 
                 15 kev 
               
               
                   
                 β N   
                 3-4.5 (DIII-D) 
               
               
                   
                 Peaking p(0)/&lt;p&gt;, n(0)/&lt;n&gt; 
                 2-2.5, 0-1.6 
               
               
                   
                 Fusion Power 
                 Up to 400 MW 
               
               
                   
                 Bootstrap fraction 
                 40%-70% (DIII-D) 
               
               
                   
                 Current Drive power 
                 25-60 MW 
               
               
                   
                 Other power for DD (ICRH?) 
                 40 MW 
               
               
                   
                 H 89P  factor 
                 2.6-2 (DIII-D) 
               
               
                   
                 CD η (scaled from reactor 
                 .15 
               
               
                   
                 studies as n/R) 
               
               
                   
                 Impurities 
                 10% He .25% Ar 
               
               
                   
                 Fusion Power 
                 300-400 MW 
               
               
                   
                 Coil related dissipation 
                 160 MW 
               
               
                   
                 CD electric input 
                 50-120 MW 
               
               
                   
                 B T  at Copper TF coil 
                 7 T 
               
               
                   
                 Cu fraction in coil 
                 60% 
               
               
                   
                 Current Drive wall plug- 
                 50% 
               
               
                   
                 plasma efficiency 
               
               
                   
                 Neutron Wall load 
                 2-3 MW/m 2   
               
               
                   
                 Q XT   
                 1.-1.9 
               
               
                   
                   
               
            
           
         
       
     
     It is understood that the disclosed tokamaks can be used in combination with the disclosed components (e.g., divertor plates, etc.), methods, devices, and systems. 
     Also disclosed are methods of transmutating fissionable materials (e.g., nuclear waste). In one aspect, as shown in the partial flowchart of  FIG. 5A , a method of transmutating fissionable materials using a disclosed embodiment comprises the steps of: creating a fusion plasma in a toroidal chamber about a central axis, said toroidal chamber substantially enclosed by walls and having an inner radius and an outer radius, wherein the fusion plasma is substantially contained within said toroidal chamber by closed and open magnetic field lines relative to the fusion plasma and created by a current induced in said fusion plasma and by current-carrying conductors substantially adjacent to said toroidal chamber; and directing particles from the fusion plasma that cross said closed magnetic surfaces to said open magnetic field lines to a divertor plate having a divertor radius relative to the central axis that is greater than or equal to the outer radius of the toroidal chamber, said particles directed to said divertor plate by said open magnetic field lines; and providing neutrons to a layer of nuclear waste fissionable materials substantially adjacent to at least a portion of said core plasma from said core plasma such that said nuclear waste fissionable materials undergo a nuclear fission reaction. 
     Another aspect of transmutating fissionable materials using a disclosed embodiment as shown in the exemplary partial flowchart of  FIG. 5B  comprises the steps of: creating a fusion plasma in a toroidal chamber about a central axis, said toroidal chamber substantially enclosed by walls and having an inner radius and an outer radius, wherein the fusion plasma is substantially contained within said toroidal chamber by closed and open magnetic field lines relative to the fusion plasma and created by a current induced in said fusion plasma and by current-carrying conductors substantially adjacent to said toroidal chamber; and directing particles from the fusion plasma that cross said closed magnetic surfaces to said open magnetic field lines to a divertor plate, wherein the divertor plate is at least partially shielded from neutrons emitted from the fusion plasma; and providing neutrons to a layer of nuclear waste fissionable materials substantially adjacent to at least a portion of said core plasma from said core plasma such that said nuclear waste fissionable materials undergo a nuclear fission reaction. 
     Another aspect of transmutating fissionable materials using a disclosed embodiment is described in  FIG. 5C . In this process, a toroidal core plasma is created in a toroidal chamber about a central axis. The toroidal core plasma is substantially confined within the toroidal chamber by magnetic field lines that stay substantially on closed toroidal magnetic surfaces. The magnetic field lines are created by currents in the core plasma and in current-carrying conductors substantially adjacent to the toroidal chamber. The toroidal core plasma is substantially enclosed by a region of open magnetic field lines that intersect one or more divertor plates. Particles are directed from the toroidal core plasma that cross the closed magnetic surfaces to the open magnetic field lines to the one or more divertor plates. At least one of the one or more divertor plates is placed at an outboard divertor major radius that is greater than or equal to a sum of a plasma minor radius and a major radius of the peak point closest to the corresponding divertor plate. Neutrons are provided to a layer of nuclear waste fissionable materials substantially adjacent to at least a portion of said core plasma from said core plasma such that said nuclear waste fissionable materials undergo a nuclear fission reaction. 
       FIG. 5D  illustrates a two-step method (a nuclear fuel cycle) for the transmutation of transuranic waste. The first step lies in carrying out some amount of waste disposal in relatively inexpensive thermal spectrum reactors. Thermal neutrons do reduce the total amount of transuranic material, but they do not substantially affect an important minority of transuranic materials comprising many of the long-lived transuranics. These elements are highly problematic for geologic disposal. The second step, designed specifically for the destruction of these problematic long-lived transuranics, utilizes a fusion neutron source to provide fast neutrons for fissioning the transuranics. In one aspect, the fusion neutron source is a high power density neutron source with a total power of about 0.1 megawatts per meter squared per second, or higher, of neutrons crossing a surface of the high power density neutron source. In another aspect, the first step (involving a thermal spectrum reactor) can be carried out two or more times before the second step (use of a fusion neutron source) is performed. 
       FIG. 5E  illustrates an embodiment of a method of reducing nuclear waste. The described embodiment comprises step  502 , providing a first chamber enclosed by walls about a central axis. The first chamber has an outer radius of about four meters or less relative to the central axis. At step  504 , a high power density neutron source is contained within the first chamber. In one aspect, the high power density source is a compact fusion neutron source containing a core plasma and comprised of at least one divertor plate that has an outboard divertor major radius that is greater than a sum of a fusion plasma minor radius and a major radius of a peak point closest to the corresponding divertor plate. In one aspect, the compact fusion neutron source has a ratio of total heating power to a core plasma major radius of about 5 megawatts/meter or higher. Further, in one aspect the high power density neutron source is a tokamak with a core plasma major radius of about three meters or smaller. The high power density neutron source has a total power of about 0.1 megawatts per meter squared per second, or higher, of neutrons crossing a surface of the high power density neutron source. At step  506 , one or more layers of fissionable materials are placed in a second chamber that is substantially adjacent to at least a portion of the first chamber. Neutron-absorbing and neutron-reflecting materials are also placed in the second chamber so that at step  508  neutrons from the high power density neutron source increase nuclear fission reactions in the fissionable materials. 
     In one aspect, at least a portion of the fissionable materials comprise transuranic (TRU) elements. 
     In another aspect, at least a portion of the fissionable materials comprise Hard-to-Fission TRU nuclear waste that remains after a pre-burn comprising an extra burn cycle in a thermal-spectrum reactor is used to transmute Easy-to-Fission elements such as PU 239  in nuclear reactor waste. Such a nuclear cycle involving pre-burn is described in relation to  FIG. 5D . 
     In yet another aspect, at least a portion of the fissionable materials comprise Hard-to-Fission TRU nuclear waste that remain after a Pre-Burn comprising an extra burn cycle in a thermal-spectrum reactor reduces original nuclear waste to Hard-to-Fission TRU waste whose weight is about 25% or less compared to the weight of the original nuclear waste. 
     In another aspect, at least a portion of the fissionable materials comprise Hard-to-Fission TRU nuclear waste which makes low-grade reactor fuel. The low-grade nuclear fuel is unsuitable as a fuel for thermal-spectrum reactor such as a Light Water Reactor (LWR), or stable operation of a fast-spectrum fission reactor. 
     The neutrons as provided from said high power density neutron source can reduce an amount of the fissionable materials. The neutrons from said high power density neutron source can increase the rate of nuclear fission reactions in the fissionable materials to transmute the fissionable materials to materials that are more stable relative to the fissionable materials or to materials having a shorter radioactive half-life than the original fissionable materials. The neutrons from the high power density neutron source can also be used to decrease radio-toxicity levels of the fissionable materials. 
     In one aspect, the fissionable materials have a first radio-toxicity level and neutrons from the compact fusion neutron source increase the rate of nuclear fission reactions of the fissionable materials thereby transmuting the fissionable materials to materials having a second radio-toxicity level. Generally, the second radio-toxicity level will be less than the first radio-toxicity level. 
     It is understood that the disclosed methods can be used in combination with any aspect of any disclosed embodiment, including vessels for containing fusion plasma, fusion neutron sources, and tokamaks. Thus, for example, a method of exhausting heat comprising a disclosed step can be applied to a vessel for containing fusion plasma, a fusion neutron source, or a tokamak. 
     EXAMPLES 
     The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric. 
     1. Modified Design of Steady State Superconducting Tokamak 
       FIG. 8 , modified from Bora et al., Brazilian Journal of Physics Vol. 32, no. 1, pg. 193-216, March 2002, the contents of which are incorporated herein by reference, displays an exemplary modified design of a Steady State Superconduction Tokamak (SST). Various parameters for the SST embodiment are listed in Table 3. An SST device can comprise a toroidal chamber, wherein at least a portion of the toroidal chamber comprises graphited-bolted tiles. Stabilizer materials can also be used with such a device and can comprise, for example, a Cu alloy (e.g., a Cu—Zr alloy). An exemplary SST design can have a plasma major radius, R, defined as the distance from the central axis to the center of the plasma, of about 1.1 m, and a plasma minor radius, α, defined as the distance from the center of the plasma to the perimeter of the plasma where the plasma is thickest, of about 0.2 m. The plasma current, I p , as defined hereinabove, can be about 220 kA, with a Toroidal Field begin defined by a magnetic flux density at the plasma center, B T , of about 3 Tesla. Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     The plasma for such an SST design can have an elongation of ≦about 1.9, and a triangularity of ≦about 0.8, wherein triangularity refers to a measure of the degree of distortion towards a D-shaped plasma minor cross section from an elliptic shaped plasma cross section. A fuel for a plasma confined within an SST device can, for example, comprise hydrogen gas. The plasma can be created and/or heated by ohmic heating, discussed hereinabove. Additional current that can be used during the course of an operation of an SST device include LHCD, or Lower Hybrid Current Drive, which can be current originating from quasi-static electric waves propagated in magnetically confined plasmas. The ohmic heating plus the LHCD can be, for example, 1 MW at 3.7 GHz. Ion Cyclotron Resonance Heating (ICRH) and Neutral Beam Injection Heating (NBI) can each be about 1 MW, wherein the sum of each is about 2 MW. 
     An exemplary SST device can have a divertor configuration as defined herein, wherein the divertor plate is positioned relative to a component or aspect of a device. A divertor configuration can be a double null (DN configuration). Such a divertor system can be compatible, for example, with an average heat load of about 0.5 MW/m 2 , with a peak heat load of about 1 MW/m 2 . 
     For a pulsed experiment, a discharge duration (i.e., the amount of time external current is applied to the device per pulse) can be, for example, about 1000 seconds. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Parameters for modified SST design 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Major Radius, R 
                 1.1 m 
               
               
                   
                 Minor Radius, a 
                 0.2 m 
               
               
                   
                 Plasma Current I p   
                 220 kA 
               
               
                   
                 Toroidal Field, B T   
                 3 Tesla 
               
               
                   
                 Elongation 
                 ≦1.9 
               
               
                   
                 Triangularity 
                 ≦0.8 
               
               
                   
                 Discharge duration 
                 1000 seconds 
               
               
                   
                 Fuel Gas 
                 Hydrogen 
               
               
                   
                 Divertor Configuration 
                 DN 
               
               
                   
                 Divertor Heat Load 
                 0.5 MW/m 2  (average); 
               
               
                   
                   
                 1 MW/m 2  (peak) 
               
               
                   
                 First Wall Material 
                 Graphited-bolted tiles 
               
               
                   
                 Stabilizer Material 
                 Cu—Zr alloy 
               
               
                   
                 Number of SC TF Coils 
                 16 
               
               
                   
                 Number of SC PF Coils 
                  9 
               
               
                   
                 Number of SC PF Coils 
                  6 
               
               
                   
                 Current Drive 
                 Ohmic + LHCD (1 MW @ 3.7 GHz) 
               
               
                   
                 Heating 
                 ICRH(1 MW) NBI (1 MW) = 2 MW 
               
               
                   
                   
               
            
           
         
       
     
     2. Divertor Designs Comprising Extended Single and Split Divertor Coils 
     CORSICA™ equilibrium for an exemplary design, are shown in  FIG. 9A . With reference to  FIG. 9A , an exemplary design can comprise one extra poloidal field (PF) coil or current-carrying conductor  710  which can be shielded in a toroidal field (TF) corner (i.e., a section near the toroidal field coils wherein neutron flux is substantially lower than a non-shielded section of the device). Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     Various parameters for this device are listed in Table 4. The listed B Angle in Table 4 is θ, or the angle between the divertor plate  715  and the total magnetic field, B div . The B Length, is the magnetic distance, or the magnetic line length, as discussed hereinabove. R div  is the divertor radius. Max area is the plasma wetted area on the divertor plate, as discussed hereinabove. The volume averaged temperature is represented by T in units of eV. The values for T listed in Table for are in reference to peak operation volume average temperatures. The results from Scrape-off layer plasma simulation calculations (SOLPS) are also presented. 
     With reference to Table 4 and  FIG. 9A , various parameters for this embodiment are as follows: R div =4.01 m, 1° Wet Area=5.6 m 2 , B Length=61.8 m, B Length gain=4.0, MA-m ratio=1.62. As shown in  FIG. 9A , both the standard divertor (SD) (R div =2.3 m) and the X divertor (XD) (R div =2.5 m) (see Kotschenreuther) have a smaller R div  than the disclosed divertor plate  715  (SXD). For comparative examples, Table 4 lists various parameters for the three aforementioned divertor designs, including a presently disclosed design. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Parameters for standard divertor (SD), X divertor (XD), and an 
               
               
                 embodiment of a disclosed divertor (SXD) for a reactor design. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Div 
                 B Angle 
                 B Length 
                 R div   
                 Max Area 
                 T eV 
                 SOLPS 
               
               
                 Plate 
                 Degrees 
                 [m] 
                 [m] 
                 m 2  (at 1°) 
                 at Peak 
                 MW/m 2   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 SD 
                 1.28 
                 27.4 
                 2.34 
                 3.27 
                 150 
                 58 
               
               
                 XD 
                 0.93 
                 39.7 
                 2.51 
                 3.51 
                 150 
                 28 
               
               
                 SXD 
                 1.2 
                 61.6 
                 4.01 
                 5.61 
                 10 
                 18 
               
               
                   
               
               
                 For 5 mm wSOL at z = 0 
               
            
           
         
       
     
     CORSICA™ equilibrium for yet another exemplary design are shown in  FIG. 9B , wherein a design comprises a divertor plate with two additional PF coils ( 720  and  730 ). In this example, more flux expansion and greater line length can be achieved by splitting a single divertor coil into two separate divertor coils. Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     Various parameters for this device are listed in Table 5. The listed B Angle in Table 5 is θ, or the angle between the divertor plate  740  and the total magnetic field, B div . The B Length, is the magnetic distance, or the magnetic line length, as discussed hereinabove. R div  is the divertor radius. Max area is the plasma wetted area on the divertor plate, as discussed hereinabove. The volume averaged temperature is represented by T in units of eV. The values for T listed in Table for are in reference to peak operation volume average temperatures. The results from Scrape-off layer plasma simulation calculations (SOLPS) are also presented. 
     With reference to Table 5 and  FIG. 9B , the parameters for this design are as follows: R div =4.04 m  740 , 1° Wet area=5.73 m 2 , B Length=66.6 m, B Length gain=4.24, MA-m ratio=1.89. Table 5 show parameters for this exemplary split design, in comparison with a standard divertor (SD) and an X divertor (XD) (see Kotschenreuther). 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Parameters for standard divertor (SD), X divertor (XD), and an 
               
               
                 embodiment of a disclosed divertor 740 (SXD) for a reactor design 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Div 
                 B Angle 
                 B Length 
                 R div   
                 Max Area 
                 T eV at 
                 SOLPS 
               
               
                 Plate 
                 Degrees 
                 [m] 
                 [m] 
                 m 2  (at 1°) 
                 Peak 
                 MW/m 2   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 SD 
                 1.14 
                 28.0 
                 2.33 
                 3.30 
                 150 
                 58 
               
               
                 XD 
                 1.07 
                 42.0 
                 2.51 
                 3.56 
                 150 
                 28 
               
               
                 SXD 
                 1.00 
                 66.6 
                 4.04 
                 5.73 
                 &lt;8 
                 &lt;18 
               
               
                   
               
               
                 For 5 mm wSOL at z = 0 
               
            
           
         
       
     
     CORSICA™ equilibrium for another exemplary design are shown in  FIG. 9C , wherein there are four extra PF coils  810 ,  820 ,  830 , and  840  (wherein 1 coil is split into 4 coils). Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     Various parameters for this device are listed in Table 4. The listed B Angle in Table 4 is θ, or the angle between the divertor plate  850  and the total magnetic field, B div . The B Length, is the magnetic distance, or the magnetic line length, as discussed hereinabove. R div  is the divertor radius. Max area is the plasma wetted area on the divertor plate, as discussed hereinabove. The volume averaged temperature is represented by T in units of eV. The values for T listed in Table for are in reference to peak operation volume average temperatures. The results from Scrape-off layer plasma simulation calculations (SOLPS) are also presented. 
     With reference to Table 6 and  FIG. 9C , the parameters for this design are as follows: R div =3.95 m  850 , 1° Wet area=5.57 m 2 , B Length=73.6, B Length gain=4.69, MA-m ratio=1.72. It is also apparent that more B length can be obtained by changing coil locations. It will be apparent that the location of the PF coils can direct and/or shape the SOL to the divertor plate, and thereby expand or reduce the particle flux (heat flux) coming from the SOL. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Parameters for standard divertor, X divertor, and a disclosed 
               
               
                 divertor (split into four divertors) for a reactor design 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Div 
                 B Angle 
                 B Length 
                 R div   
                 Max Area 
                 T eV at 
                 SOLPS 
               
               
                 Plate 
                 Degrees 
                 [m] 
                 [m] 
                 m 2  (at 1°) 
                 Peak 
                 MW/m 2   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 SD 
                 1.18 
                 27.8 
                 2.34 
                 3.30 
                 150 
                 58 
               
               
                 XD 
                 0.92 
                 40.3 
                 2.51 
                 3.54 
                 150 
                 28 
               
               
                 SXD 
                 1.0 
                 73.6 
                 3.95 
                 5.57 
                 &lt;5 
                 &lt;18 
               
               
                   
               
               
                 For 5 mm wSOL at z = 0 
               
            
           
         
       
     
       FIG. 10  shows, for example, a cross section of an exemplary fusion reactor  855  with a vertical height of about 7.15 m ( 1030 ) comprising components that can be used in a disclosed embodiment. Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     In this example, ohmic heating coils (OHCs)  945  are used to produce and/or heat the confined plasma, with a major plasma radius  920  of about 2.49 m, and with minor plasma radius of about 1.42 m. Extending from the central axis with a radius of about 1.78 m ( 930 ), is a blanket (i.e., the chamber walls)  940  that substantially encloses the plasma. The blanket shown is about 0.5 m thick. 
     The toroidal field (TF) center post  860  lies adjacent to the central axis, with a radius of about 1.2 m ( 1000 ), which is in physical communication with a TF wedge  880 , the farthest radius of which extends about 4.35 m ( 1020 ) connected to TF outer verticals  890 , the farthest radius of which extends about 5.72 m ( 1010 ). Exemplary poloidal field (PF) coils,  870 ,  900 , and  910  inside the perimeter of the toroidal field, are positioned substantially adjacent to the fusion plasma. The distance  1040  between the two outermost (i.e., farthest away from the central axis) PF coils is about 1.0 m. 
     In this embodiment, a disclosed divertor plate  895  is shown substantially adjacent to a poloidal field coil  900 . In the exemplary fusion reactor of  FIG. 10 , a standard divertor plate (SD)  950 , as is known in the art, is shown in comparison to a disclosed divertor (SXD)  895 . A standard divertor plate  950  configuration as shown in  FIG. 10  can be used in combination with a disclosed divertor plate  895  configuration. It should be noted that the dimensions shown in  FIG. 10  are exemplary in nature and variance of the dimensions or design of the fusion reactor is contemplated to be within the scope of various embodiments of the invention. 
     3. Modified Design of Future Machines 
     Using CORSICA™ (J. A. Crotinger, L. L. LoDestro, L. D. Pearlstein, A. Tarditi, T. A. Casper, E. B. Hooper, LLNL Report UCRLID-126284, 1997 available from NTIS PB2005-102154), MHD (magnetohydrodynamic) equilibrium can be generated for various future machine types, as presented herein. The results of a calculation for a Cu high power density reactor are shown in  FIG. 11 . The results of a calculation for a superconducting (SC) SLIM-CS reactor with small radial build for TF (assuming remote handling ability) are shown in  FIG. 12 . The results of a calculation for an ARIES-AT reactor (also SC) with radially large TF coils are shown in  FIG. 13 . For the ARIES design, it is apparent that an embodiment of a disclosed divertor design can be used wherein poloidal field (PF) coils are outside toroidal field (TF) coils. The design shown in  FIG. 13 , however, uses modular SC (superconducting) divertor coils that fit inside unused volume in the reactor, thereby enabling larger radial divertor extension. For the configurations in  FIGS. 11 ,  10 , and  11 , the gains in R div /R sol  are 2, 1.7, and 2, respectively, while the line length goes up (over a standard divertor, discussed in more detail in Kotschenreuther) by factors of 5, 3, and 4, respectively. Such a device can comprise a layer of fissionable materials substantially adjacent to the toroidal chamber. 
     It should be appreciated that, through experimentation with CORSICA™ equilibrium, a wide variety of plasma shapes (aspect ratios, elongations, triangularities, as defined hereinabove, etc.) can be accommodated with a disclosed embodiment. In some aspects, it is possible to modify the design of an existing or future reactor from a standard divertor design, to a disclosed divertor design with a small change in the number of coils and net applied power, while keeping the core geometry substantially unaffected. Thus, in one aspect, a disclosed divertor design can be applied to a known reactor configuration. 
     While aspects of the present invention can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present invention can be described and claimed in any statutory class. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification. 
     Although several aspects of the present invention have been disclosed in the specification, it is understood by those skilled in the art that many modifications and other aspects of the invention will come to mind to which the invention pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the invention is not limited to the specific aspects disclosed hereinabove, and that many modifications and other aspects are intended to be included within the scope of the appended claims. Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described invention.