INCREASING ENERGY GAIN IN MAGNETICALLY CONFINED PLASMAS BY INCREASING THE EDGE TEMPERATURE: THE SUPER-XT DIVERTOR

Abstract

A toroidally confined plasma vessel with a substantially symmetric magnetically confined plasma region where a plurality of magnetic field coils are configured to provide at least one X-point, and to guide plasma particles from the magnetically confined region to the divertor target; and wherein the total magnetic field strength (comprising all components of the magnetic field) at the divertor target is lower than the total magnetic field strength (comprising all components of the magnetic field) of a position in the SOL between the divertor target and X-point on the last closed flux surface that is nearest to it. When the mean free path of the neutrals is longer than the width of the SOL, one can separate the two critical functions: a) withstanding high-heat flux, and b) pumping of plasma particles to maintain a low density.

Claims

1. A toroidally confined plasma vessel comprising: a toroidal plasma chamber; a magnetically confined plasma region where particles traveling along magnetic fields substantially never strike a wall; where the magnetically confined plasma region is substantially symmetric by rotation around a central axis; a plurality of magnetic field coils; a divertor assembly with a divertor target; wherein a plurality of magnetic field coils are configured to provide at least one X-point, and guide plasma particles from the magnetically confined region to the divertor target; wherein the divertor target has a cover, wherein a side of the cover substantially facing the divertor target comprises a material that is liquid on at least some of the surface of the side of the cover for at least some of the time that the cover is in the toroidally confined plasma vessel; wherein the total magnetic field strength (comprising all components of the magnetic field) at the divertor target is lower than the total magnetic field strength (comprising all components of the magnetic field) of a position between the divertor target and X-point on the last closed flux surface that is nearest to it; whereby at least one of: the radiation from the magnetically confined plasma does not increase in time until a 40 percent drop in the fusion rate in the magnetically confined plasma or until a 40 percent drop in the highest plasma temperature in the magnetically confined plasma, the power radiated from the magnetically confined plasma by photons does not exceed 70% of the heating power (where the heating power is the sum of externally applied heating plus the heating that arises from the nuclear reactions in the magnetically confined plasma), the effective Z is below 3 (where the effective Z is defined as the ratio where the numerator is the sum over all ions in the magnetically confined region times the square of the charge state of the ion and the denominator is the total number of electrons in the magnetically confined plasma), and the sum of the electric charges of fusion fuel ions in the magnetically confined plasma is greater than 0.6 times the sum of the electric charges on all the electrons in the magnetically confined plasma.

2. The toroidally confined plasma vessel of claim 1, wherein the divertor target has a shield that substantially blocks lines of sight from the divertor target to the magnetically confined plasma region and to important components that sustain operation of the device.

3. The toroidally confined plasma vessel of claim 2, wherein the shield that substantially blocks lines of sight from the divertor target to the magnetically confined region and to important components that sustain the operation of the device is covered by liquid on the side substantially facing the divertor target for at least some of the time that the cover is in the toroidally confined plasma vessel.

4. The toroidally confined plasma vessel of claim 1, wherein the component of the poloidal magnetic field, which is the component of the magnetic field in the plane perpendicular to the direction of rotation of the central axis, has a magnitude at the divertor target that is larger than one third of the maximum value of the poloidal magnetic field on the boundary of the magnetically confined plasma region.

5. The toroidally confined plasma vessel of claim 1, wherein material that absorbs and slows down neutrons is located substantially in between the magnetically confined plasma region and the divertor target.

6. The toroidally confined plasma vessel of claim 1, wherein the divertor target surface comprises a material that is liquid over at least some of the surface at least some of the time.

7. The toroidally confined plasma vessel of claim 1, further comprising a pumping duct extending from a position near the divertor target to a pumping means to pump out helium, hydrogen isotopes, other gasses, or any combination of these, and where a distance from the divertor target to the pumping means is less than one half of the distance from the X-point to said pumping means.

8. The toroidally confined plasma vessel of claim 1, wherein at least one of: the electron temperature is above 200 eV at the boundary of the magnetically confined region or the temperature of electrons immediately adjacent to the divertor target is above 25 eV, and the ratio of the plasma electron density at the last closed flux surface to the line averaged electron density for a chord passing near the center of the magnetically confined plasma is less than 0.2.

9. The toroidally confined plasma vessel of claim 1, wherein at least one of: the electron temperature is above 1000 eV at the boundary of the magnetically confined region, the temperature of electrons immediately adjacent to the divertor target is above 100 eV, and wherein the ratio of the plasma electron density at the last closed flux surface to the line averaged electron density for a chord passing near the center of the magnetically confined plasma is less than 0.15.

10. The toroidally confined plasma vessel of claim 1, wherein at least one of: the radiation from the magnetically confined plasma does not increase in time until a 20 percent drop in the fusion rate in the magnetically confined plasma or until a 20 percent drop in the highest plasma temperature in the magnetically confined plasma, the power radiated from the magnetically confined plasma by photons does not exceed 50% of the heating power (where the heating power is the sum of externally applied heating plus the heating that arises from the nuclear reactions in the magnetically confined plasma), the effective Z is below 2.5 (where the effective Z is defined as the ratio where the numerator is the sum over all ions in the magnetically confined region times the square of the charge state of the ion and the denominator is the total number of electrons in the magnetically confined plasma), and the sum of the electric charges of fusion fuel ions in the magnetically confined plasma is greater than 0.75 times the sum of the electric charges on all the electrons in the magnetically confined plasma.

11. A toroidally confined plasma vessel comprising: a toroidal plasma chamber; a magnetically confined plasma region where particles traveling along magnetic fields substantially never strike a wall; wherein the magnetically confined plasma region is substantially symmetric by rotation around a central axis; a plurality of magnetic field coils; a divertor assembly with a divertor target; wherein a plurality of magnetic field coils is configured to provide at least one X-point and guide plasma particles from the magnetically confined region to the divertor target; wherein at least one of: the electron temperature is above 200 eV at the boundary of the magnetically confined region, the temperature of electrons immediately adjacent to the divertor target is above 25 eV, and the ratio of the plasma electron density at the last closed flux surface to the line averaged electron density for a chord passing near the center of the magnetically confined plasma is less than 0.2; wherein the divertor target, on the surface facing the plasma, comprises a material that is liquid at least some of the time and whose composition is less than 50% lithium by atomic fraction; wherein the total magnetic field strength (comprising all components of the magnetic field) at the divertor target is lower than the total magnetic field strength (comprising all components of the magnetic field) of a position in the SOL between the divertor target and X-point on the last closed flux surface that is nearest to it; whereby at least one of: the radiation from the magnetically confined plasma does not increase in time until a 40 percent drop in the fusion rate in the magnetically confined plasma or until a 40 percent drop in the highest plasma temperature in the magnetically confined plasma, the power radiated from the magnetically confined plasma by photons does not exceed 70% of the heating power (where the heating power is the sum of externally applied heating plus the heating that arises from the nuclear reactions in the magnetically confined plasma), the effective Z is below 3 (where the effective Z is defined as the ratio where the numerator is the sum over all ions in the magnetically confined region times the square of the charge state of the ion and the denominator is the total number of electrons in the magnetically confined plasma), and the sum of the electric charges of fusion fuel ions in the magnetically confined plasma is greater than 0.6 times the sum of the electric charges on all the electrons in the magnetically confined plasma.

12. The toroidally confined plasma vessel of claim 11, wherein the divertor target comprises a cover, wherein a side of the cover substantially facing the divertor target comprises a material that is liquid on at least some of the surface of the side of the cover.

13. The toroidally confined plasma vessel of claim 11, wherein material that absorbs and slows down neutrons is located substantially in between the magnetically confined plasma region and the divertor target.

14. The toroidally confined plasma vessel of claim 11, further comprising a pumping duct extending from a position near the divertor target to a pumping means to pump out helium or hydrogen isotopes, other gasses, or any combination of these, and where a distance from the divertor target to the pumping means is less than one half of the distance from the X-point to said pumping means.

15. The toroidally confined plasma vessel of claim 11, wherein the component of the poloidal magnetic field, which is the component of the magnetic field in the plane perpendicular to the direction of rotation of the central axis, has a magnitude at the divertor target that is larger than one third of the maximum poloidal magnetic field around the boundary of the magnetically confined plasma region.

16. The toroidally confined plasma vessel of claim 11, wherein a shield that substantially blocks lines of sight from the divertor target to the magnetically confined region and to important components that sustain the operation of the device is covered by liquid on a side of the shield substantially facing the divertor target.

17. The toroidally confined plasma vessel of claim 11, wherein at least one of: the electron temperature is above 1000 eV at the boundary of the magnetically confined region, the temperature of electrons immediately adjacent to the divertor target is above 50 eV, and the ratio of the plasma electron density at the last closed flux surface to the line averaged electron density for a chord passing near the center of the magnetically confined plasma is less than 0.15.

18. The toroidally confined plasma vessel of claim 11, wherein at least one of: the radiation from the magnetically confined plasma does not increase in time until a 20 percent drop in the fusion rate in the magnetically confined plasma or until a 20 percent drop in the highest plasma temperature in the magnetically confined plasma, the power radiated from the magnetically confined plasma by photons does not exceed 50% of the heating power (where the heating power is the sum of externally applied heating plus the heating that arises from the nuclear reactions in the magnetically confined plasma), the effective Z is below 2.5 (where the effective Z is defined as the ratio where the numerator is the sum over all ions in the magnetically confined region times the square of the charge state of the ion and the denominator is the total number of electrons in the magnetically confined plasma), and the sum of the electric charges of fusion fuel ions in the magnetically confined plasma is greater than 0.75 times the sum of the electric charges on all the electrons in the magnetically confined plasma.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0138] FIG. 1A gives an example from the prior art and also gives a graphical example of the terminology used in the descriptions. This terminology is standard within the art. The surfaces that are tangential to the magnetic field are flux surfaces. The plasma particles travel mainly within these surfaces and move relatively slowly across them. The geometry is approximately a torus, and so, there is an approximate axis of revolution. The distance of a point from this axis is referred to as the major radius of the point. A plane such as is shown in FIG. 1A, in which the axis of revolution lies, is referred to as a poloidal plane. The magnetic field component that lies in this plane is called the poloidal magnetic field. The Last Closed Flux Surface (LCFS) 102 has one or more X-points 120 on it where the poloidal magnetic field vanishes. It is well known in the art that the flux surfaces at an X-point form an “X”, as shown. The region inside of the LCFS 101 is the Magnetically Confined plasma, or MC plasma. It is also well known in the art that a high-temperature SOL plasma has most of the heat and particles substantially bounded between two flux surfaces, one being the extension of the LCFS 130, called the separatrix within the art, and the other being the bounding SOL surface 103.

[0139] The SOL plasma strikes a material surface at the strike point 104 and 140, and the material surface is called the divertor target 105 and 150, which is also shown. Each X-point has two divertor targets associated with it, one of which is further from the axis of rotation 105, called the outboard divertor, and most of the particles and energy flow to this target. The other divertor is referred to as the inboard divertor target 150.

[0140] In FIG. 1A, the X-point is at the bottom of the plasma; this configuration is called, in the art, a “lower null”. A configuration where the divertor is near an X-point at the top of the plasma is called an “upper null”.

[0141] In FIG. 1B is another geometry from the prior art, called a Super-X divertor. Shown is an actual geometry for a device that has been constructed in the United Kingdom called MAST-U. It shows the MC plasma 101, the LCFS 102, the lower X-point 120, the bounding surface of most of the SOL 103, the strike point 104 and the divertor target 105. The divertor target and the SOL near it are contained in a divertor chamber 190.

[0142] FIG. 2 is of the prior art, and is a close-up of a conventional outboard divertor region, for a “lower null” configuration, which shows how neutral gas is pumped. It shows the MC plasma 101, the LCFS 102, the lower X-point 120, the bounding surface of most of the SOL 103, the strike point 104 and the divertor target 105. Walls of the region containing the plasma 160 are shown. Neutral gas particles arise in the region of the divertor target and must be removed by being pumped out. This is accomplished by having a pumping duct 170 with an entrance 172 in the divertor region and at the end of which is a pumping means 171 that pumps out neutral particles. Neutrons generated in the MC plasma 101 are absorbed by neutron absorbing material 110 that generally surrounds the MC plasma 101, but the entrance to the divertor region does not have significant neutron absorbing material between it and the MC plasma.

[0143] FIG. 2B is of the prior art, and has a pumping duct arrangement similar to the ITER device, that differs from the one in FIG. 2. It shows the MC plasma 101, the LCFS 102, the lower X-point 120, the bounding surface of most of the SOL 103 and the divertor target 105. Walls of the region containing the plasma 160 are shown. Neutral gas particles arise in the region of the divertor target and must be removed by being pumped out. This is accomplished by having a pumping duct 170 with an entrance 172 in the divertor region and at the end of which is a pumping means 171 that pumps out neutral particles. Neutrons generated in the MC plasma 101 are absorbed by neutron absorbing material 110 that generally surrounds the MC plasma 101, but the entrance to the divertor region does not have significant neutron absorbing material between it and the MC plasma.

[0144] FIG. 3 gives an example of one embodiment of the invention disclosed here, when there is a lower null. There are numerous differences from the prior art that are explained in the detailed description section below. The figure shows the MC plasma 101, the LCFS 102, the bounding SOL surface 103, the divertor target 105, the pumping duct 170, the entrance to the pumping duct 172 and the pumping means 171. New aspects include a material on the divertor target facing the SOL that is liquid at least some of the time 106, a cover 107, material facing the SOL on top of the cover that is liquid at least some of the time 108, the shield 109, and material facing the SOL on top of the shield that is liquid at least some of the time 111. Also shown is neutron absorbing material 110 that is between the MC plasma and the both the divertor target 105 and the entrance of the pumping duct 172.

[0145] In other embodiments of the invention, there would be an X-point at the top of the upper half. This case is an example of an “upper null”, shown in FIG. 4. The figure shows the MC plasma 101, the LCFS 102, the bounding SOL surface 103, the divertor target 105, the pumping duct 170, the entrance to the pumping duct 172 and the pumping means 171. New aspects include a material on the divertor target facing the SOL that is liquid at least some of the time 106, a cover 107, material facing the SOL on top of the cover that is liquid at least some of the time 108, the shield 109, and material facing the SOL on top of the shield that is liquid at least some of the time 111. Also shown is neutron absorbing material 110 that is between the MC plasma and the both the divertor target 105 and the entrance of the pumping duct 172.

[0146] Some magnetic geometries have two X-points on (or very near to) the LCFS, where one is near the top and one is near the bottom. This is called a “double null” in the art. In this circumstance, an exemplary embodiment of the invention could have the lower divertor region such as FIG. 3, and an upper divertor region such as FIG. 4.

[0147] These figures are for magnetic configurations of the MC plasma that might be a tokamak, spherical tokamak, a toroidal pinch or a Reversed Field Pinch (RFP). The invention can also apply to other magnetic configurations such as a Field Reversed Configuration (FRC) or a spheromak, as well as others. An example of aspects of the invention applied to an FRC is seen in FIG. 5. The magnetic field configuration is taken from U.S. Pat. No. 11,337,294 (specifically FIG. 2 of that patent). Aspects of that figure that are extraneous to the considerations related to this patent have been omitted for clarity. FIG. 5 shows the MC plasma 101, the LCFS 102, the SOL boundary 103, the divertor target 105 and a cover 107. New aspects include a material on the divertor target facing the SOL that is liquid at least some of the time 106, material facing the SOL on top of the cover that is liquid at least some of the time 108. Also shown is neutron absorbing material 110 that is between the MC plasma and the entrance of the pumping duct 172.

DETAILED DESCRIPTION

[0148] Some embodiments of the invention are for magnetic geometries that are a lower single null. Other embodiments are for magnetic geometries with an upper single null. Yet other embodiments of this invention employ a double null geometry.

[0149] Note that in the specific case of double null geometries, the invention differs in numerous respects from the US Patent application by Buxton et. al. that also applies to double null geometries (US 20210265068A1, Pub. No.: US2021/0265068A1). The essence of our invention, and its operation, is quite different from Buxton et. al. The purpose of the present invention is to find a solution to the SOL Issues described above. The purpose of the Buxton invention is to find a way to apply a liquid metal divertor target to a double null geometry, and, as is implicitly implied by the claims, to use gravity to assist in the flow of that liquid (since the flow the inlet must be above the outlet in Buxton). Most of the SOL Issues that motivate the present invention are not even mentioned in the Buxton patent. Thus, many of the aspects of this invention, which are for the purpose of solving the SOL Issues, are not present in the Buxton invention. And solving the SOL Issues does not require a double null geometry, which is the essence of the Buxton patent. Furthermore, using gravity to assist in the flow of a liquid metal is not in any way crucial for our invention. For example, the heat can be removed by conduction through a liquid metal to a heat sink behind it, without using gravitationally assisted flow at all. So as can be readily appreciated, this invention is very different from Buxton.

[0150] Some embodiments of the invention apply to cases of a single lower null and cases of a single upper null. As another example, our invention addresses the issues of sputtering, erosion, and helium, dwells upon these at length in the descriptions, and these issues motivate many of the claims. But in Buxton, the terms sputtering, erosion and helium appear only once each in parenthetical comments, and certainly do not motivate any of the clams. Furthermore, note that in Buxton et. al., the liquid flow has an inlet above where the outlet is. But our invention can have a divertor target that can be substantially horizontal to promote a smooth surface. Our invention only requires a slow flow of liquid on the divertor target in order to replenish eroded material, and this can readily be provided without gravitational assistance, for example, by using capillary action, or by electromagnetic means, or by other means, so that a flow inlet above the outlet is not required for our invention.

[0151] Buxton is largely immaterial to the operation of our invention. The essence of this invention is quite different from Buxton et. al, and the claims of our invention apply to very many cases where Buxton does not apply. Nonetheless, it is possible that some embodiments of our invention could be double null geometries with the liquid flow inlet above the outlet.

[0152] There are several key insights that motivate our invention. Firstly, the goal is to produce a high-temperature, low-density SOL in a practical way, that is, so that all the disqualifying SOL Issues are resolved. This will then lead to a higher energy confinement time, higher fusion gain, and fusion gain in smaller devices.

[0153] The following insights, not in the previous art, show that there are regimes where certain physical dynamics in the SOL can operate to advantage. The various aspects of the invention cause the favorable physical dynamics to be actually manifest, and hence, allow a MC plasma with high-energy confinement by avoiding the SOL Issues. Other additional aspects are also helpful to allowing the SOL Issues to be further resolved.

[0154] Two key physical dynamics, that the aspects of the invention will cause to operate to advantage, are described below. These dynamics motivate some of the key features of this invention, so we discuss them below. We distinguish these two by referring to them as dynamics number one and dynamics number two.

[0155] 1) The following section describes dynamics number one and its ramifications.

[0156] Our analysis shows that by placing the divertor target in a region of lower magnetic field strength B, a strong electrostatic potential arises that has the effect of preventing impurities generated at the divertor target from reaching the MC plasma. The electrostatic potential, in effect, shields the MC plasma from the damaging impurities that are generated near the divertor target. These impurities would otherwise become ionized and travel along magnetic field lines to reach the MC plasma. This dynamic has not been used to advantage in the art, to the authors knowledge, until this disclosure.

[0157] Note that this dynamic only occurs in an SOL with a high-temperature, low-density SOL, which is the regime of interest in this invention. Specifically, this electrostatic potential is strong when the mean free path for Coulomb collisions is longer than characteristic distance traveled by a particle going along a magnetic field line from the divertor target to the MC plasma. Such a long mean free path arises in an SOL with high-temperature and low density. This electrostatic potential is far weaker in the conventional operating regime of a divertor, which is the opposite: the low temperature and high-density of such regimes (as in MAST-U and many other cases within the art) results in a short mean free path.

[0158] It appears to be true that, since the fusion community has concentrated its attention primarily on the regime of short Coulomb collisional mean free path, the ability of this electrostatic potential, in the long mean free path regime, to help prevent contamination of the MC plasma, has not been recognized until this disclosure. The patent arranges aspects of the invention to use this dynamic advantageously.

[0159] The following explains how this is done. In the regime of interest to this invention, the magnitude of the potential difference along a magnetic field line between point 1 and point 2 is very roughly of a magnitude ˜(T.sub.e/e) ln(B.sub.1/B.sub.2), where T.sub.e is the electron SOL temperature, e is the charge on the electron, B.sub.1 is the total magnetic field strength at point 1 and B.sub.2 is the total magnetic field strength at point 2, and ln is the natural logarithm.

[0160] A potential with the magnitude of T.sub.e will have a very large impact on the path of an impurity in this regime. Impurities in the SOL plasma are generated by sputtering or evaporation or recycling, and in all these cases the energy of the impurity is in the range of several eV or less. This invention applies to SOL where T.sub.e is about 200 eV or more, which is far greater than the energy of the impurities. In this case, the potential can prevent impurities from reaching the MC plasma. Impurities are positively charged, so the sign of the electrostatic potential will do just that when the divertor target is in region where B is relatively small in comparison to other positions between the divertor target and the MC plasma.

[0161] Also, if the Coulomb collisional mean free path is long, the electrostatic potential reflects impurities back to the divertor on a time scale much shorter than the time for the impurity to be heated by the SOL plasma to a high enough energy so that the impurity can overcome the electrostatic potential. Such heating is only strong enough in the regime of short mean free path, which is the opposite of the one considered in this invention.

[0162] To summarize the preceding few paragraphs: in the regime of density and temperature for this embodiment of the invention, the MC plasma is insulated from impurities generated in the SOL at the divertor target, if the target is in a region of relatively low total magnetic field strength. This is of crucial importance to avoid contamination of the MC plasma, since as we have described before, experiments and neoclassical transport say that impurities in the SOL can be strongly concentrated in the MC plasma.

[0163] Hence, an aspect of the invention is that the magnetic field strength at the divertor target is less than the strength as some position in the SOL between the target and the X-point of the MC plasma.

[0164] The impurities for which this desirable dynamic applies include impurities generated by sputtering, by evaporation, and by recycling, and including, but are not limited to, elements of the materials that face the plasma, and helium, and impurities arising from small leaks of air or other gasses into the chamber of the plasma.

[0165] As the preceding paragraphs make clear, one of the essential goals of this invention is to prevent unacceptable impurity contamination of the MC plasma. Impurities have several serious negative affects upon the plasma, which are known in the prior art. An appropriate way to specify the purity of the MC plasma is to specify that those negative effects are small. So we describe the purity of the plasma in terms that the following four deleterious effects from impurities must be small. 1) In experiments, one important way that excessive contamination is manifest is that the radiation from the plasma grows in time until it becomes excessive and there is a collapse of the MC plasma temperature or fusion rate. So we specify the condition of low impurities in the claims the same way: the radiation from the magnetically confined plasma is not increasing in time until a specified percentage drop in the plasma temperature in the magnetically confined region or a drop in the fusion rate. A percentage drop that exceeds this would significantly impair the operation of the device. 2) Another important way that impurities degrade the MC plasma is by producing radiation power losses from it that become a large fraction of the heating power, so that the heating power becomes inadequate to sustain sufficiently high temperature. We describe this in the claims by saying that the power radiated from the MC plasma by photons must not exceed a numerical fraction of the heating power, where the heating power is the sum of externally applied heating plus the heating that arises from the nuclear reactions in the MC plasma. If the radiation exceeds this fraction the operation of the device would be significantly impaired. 3) Another way that impurities can degrade the plasma is by increasing its electrical resistance so that it is difficult to drive necessary current. It is known in the prior art that the electrical resistance is closely related to the effective Z. Hence the effective Z must stay below a numerically specified value, above which operation of the MC plasma would be significantly impaired. The effective Z used here is an average over the MC plasma, and is defined as the ratio where the numerator is the sum of N.sub.s times Z.sub.s.sup.2, where N.sub.s is the number of ions of species sin the MC plasma, and Z.sub.s.sup.2 is square of the charge state of each plasma species s (the charge state is the number of electrons lost from the atom), and the denominator is the sum of the number of electrons N.sub.e in the MC plasma. 4) A further way that impurities can degrade operation is by diluting the fusion fuel so that the fusion reaction rate is reduced. Within the art, the dilution of the fusion fuel is often specified in relative terms as the number of fuel ions compared to the number of electrons. In a perfectly pure plasma, the charge on all of the fuel ions would equal to the charge on all the electrons. Impurities reduce the fuel ions and the total charge they carry. So we specify a low dilution of an MC plasma by saying that the sum of the electric charges of each fusion fuel ion is greater than a specified fraction of the sum of the electric charge on all the electrons. Dilution greater than this would significantly impair the operation of the device.

[0166] We now turn to the subject of how the aspects that lead to the electrostatic potential also make it easier to attain other benefits for solving the SOL Issues, so that they work advantageously together.

[0167] The magnetic field strength in tokamaks, in the SOL, usually decreases with increasing major radius. Hence, in order to apply the dynamic above advantageously, it is often the case that the divertor target 105 must be at a larger distance from the MC plasma 101. See FIG. 3 and FIG. 4. And note that the entrance to the pumping duct 172 must be fairly close to the divertor target 105. This brings another advantage, applicable to helium pumping: the entrance to the pumping duct 172 is considerably further from the MC plasma, and hence closer to the pumps 171. Thus, the duct to the pump 170 can be shorter, improving the transport of helium through the ducts to the pumps for its removal. This addresses a substantial problem, described before, in a regime where the SOL density is low: helium transport through ducts is relatively poor.

[0168] Another aspect of the geometry mentioned above can further be advantageous for helium pumping: if the divertor target 105 is located some distance away from the MC plasma, it is possible to put the target in a region of low neutron flux, because there is sufficient space to place neutron absorbing materials 110 between the divertor target 105 and the MC plasma 101 that generates neutrons. Thus, less neutrons enter the pumping duct entrance 172. So it is possible to have a pumping duct 170 that has a larger cross-sectional area, without an unacceptable number of neutrons being transmitted through the duct to overheat cryogenic regions of the pumps 172, or to degrade the vacuum pumps themselves. This further improves helium transport through the ducts. Divertor configurations that are standard in the art, as in FIG. 2, have the entrance to the duct 172 in a much higher neutron flux, and so the ducts cannot have too large an area to allow excessive neutrons to travel through the ducts.

[0169] So summarizing the paragraphs above, a number of important advantageous aspects can work together in the invention that is disclosed here.

[0170] Regarding the advantage of a shorter duct described above, in the claims, we refer to this by saying that the distance between the divertor target and the pump is shorter than half of the distance between the X-point and the pump.

[0171] This concludes the section that describes the dynamics number one, as defined above, and all the ways that it can contribute advantageously to the operation of the device.

[0172] 2) The following paragraphs describe dynamics number two, as defined above, that the claims will cause to be advantageous to the operation of the device:

[0173] For parameters of an energy gain device, there are magnetic field geometries, together with parameters of SOL temperature and density, that have another important property: neutral atoms and molecules have a relatively long mean free path in the SOL, so that many of them pass through the SOL without being ionized. Specifically, when this applies in the divertor region, then many of the neutrals generated at the divertor target tend to go through the SOL and remain as neutral particles. These neutrals include both recycled hydrogen, helium and sputtered impurities. This is highly beneficial for reasons explained below. So an aspect of this device is to cause the width of the SOL to be less than the mean free path of neutrals generated at the divertor plate, so that many of the neutrals pass through the SOL.

[0174] One reason that this is beneficial is this: in order to achieve a low-density SOL, it is not necessary to place a chemical absorbing material like lithium at the divertor target 105, where heat fluxes are very high. Instead, such absorbing material can be placed at a different location than the divertor target, where the heat flux is far lower. For example, in some embodiments, the cover 107 can have a liquid lithium surface facing the SOL 108. Or in other embodiments, the shield 109 can have a liquid lithium surface facing the SOL 111. Or in other embodiments, the neutrals could enter the duct at 172, travel through the duct 170, and be removed by the pumps 171. In fact, in some embodiments, there can be no chemical absorbing material at all, and other reliable pumping means that are known in the art can remove the neutrals, such as diffusion pumps or cryopumps.

[0175] Recall that a central feature of the previous art for obtaining a low-density, high-temperature SOL was to use lithium at the divertor target. And recall that this leads to major problems in the previous art: because of the high-heat fluxes at the divertor target, it is very difficult to maintain a surface temperature that is low enough to be acceptable for lithium (which some estimate as 400-450 degrees C. and others estimate as 300-380 C). These problems can be avoided in the present invention, because lithium need not be at the divertor target, as described in the paragraph above.

[0176] In summary, an SOL that is not too wide allows important advantages: we can separate two critical functions a) withstanding high-heat flux, and b) pumping of plasma particles to maintain a low density. Splitting these functions allows one to use appropriate materials and means for the optimal effectiveness of each of these crucial functions, rather than having to use lithium for both. One can use a material for the divertor target other than lithium, that can withstand higher temperatures. This is a major benefit since there is no material or means which is optimal at both functions simultaneously for conditions of a practical fusion device. And, to reiterate, this separation of functions becomes possible when width of the SOL is not too much longer than the mean free path of the neutrals.

[0177] An additional important advantage of an SOL that is not much larger than the mean free path is the following: it becomes possible to avoid self-sputtering avalanches, even for materials whose self-sputtering coefficient is greater than one at the high-SOL temperature. This allows a much wider class of materials to be considered for the divertor plate, some of which, have a much more favorable operating temperature range, or other advantages. Recall what this avalanche is: ionized impurities are accelerated to high-energy in the SOL, and when they impact the plate, they create more than one neutral impurity atom. Evidently, this can lead to exponential growth: the avalanche. However, in the present geometry, many of the neutral sputtered atoms pass through the SOL without being ionized. Hence, they will not be accelerated to an energy where they cause high-self-sputtering. Thus, self-sputtering avalanches are avoided because many sputtered impurities avoid the crucial step of being ionized in the SOL and hence being accelerated to high-energy.

[0178] A further additional important advantage of operating in a regime where the SOL width is less than the mean free path is that it becomes much easier to pump the helium ash. To effectively pump helium, the pressure of neutral helium must be high-enough in the divertor region to create a high-enough pressure in the pump region. Some of this neutral helium will be ionized in the SOL, and tends to contribute to plasma contamination. By having the SOL be thin, the amount of such ionization is lower. This implies that a higher density of neutral He can be present in the divertor region, which will make it easier to pump the helium in the pump region.

[0179] To summarize the paragraphs above, several benefits accrue to having the SOL width not be much longer than the particle mean free path. A narrow SOL near the diverter target, is obviously beneficial for this. Importantly, this condition is not usually the case in the conventional art: the plasma SOL width near the divertor target is, generally, quite wide. This is because in the conventional art, the divertor target is usually not far from the MC plasma X-point, and the SOL is very wide in this region.

[0180] The present invention employs a configuration of magnetic fields other than the usual one in the prior art in order to have this characteristic. To see how this is done, one first derives a criterion for a relatively narrow SOL width using methods well known in the art.

[0181] Particles in the SOL follow the magnetic fields, especially at high-temperature and low density. So the width w of the SOL follows the width of the region between flux surfaces that bound it. This implies w varies as 1/(B_pol_R), where R is the major radius of the position and B_pol is the poloidal component of the magnetic field, that is, the component of the magnetic field in the plane that is perpendicular to the direction of revolution around the axis.

[0182] A conventional divertor geometry has the divertor target 105 considerably closer to the vicinity of a plasma X-point 120 (See FIG. 2) in comparison with embodiments of this invention (FIG. 3 and FIG. 4). The X-point is the position on the LCFS where the poloidal magnetic field vanishes. For the conventional geometry, where each divertor target is in the vicinity of an X-point, the B_pol is rather small at the divertor target. Hence, such a divertor would have a large SOL width, and would not be able to access the advantages of a low SOL width.

[0183] To avoid the large SOL width of the conventional divertor geometry, at least one divertor target must be placed at a different position from the conventional geometry. In the art, it is known that each X-point has two divertor targets associated with it, one with a larger major radius R than the other. Most of the heat and particles flow to the outer divertor target with larger R, called the outer divertor target.

[0184] So we give a specification that the poloidal magnetic field at the outer divertor target, such that is must not be too small. The specification is as follows, and, is based upon the B_pol at the outer divertor target: for this invention, at the position of at least one outer divertor target, the poloidal magnetic field must be at least as large as a factor times the largest value, anywhere on the LCFS of the MC plasma, of the poloidal magnetic field. A good value for that factor is ⅓.

[0185] The requirement that the divertor target is located away from the X-point also has the consequence that it is easier to locate the target in a region that accesses dynamic number one described above, namely, that the divertor target is in a region of low total magnetic field strength, since: if the target were close to the X-point, it would be difficult to locate it in a region where the total magnetic field strength at the target was different from the value at the X-point or any other point in between. Hence, requirements for dynamics one and dynamics two work together advantageously to a significant extent.

[0186] In addition, locating the divertor target at a position away from the LCFS X-point, there is space to put neutron absorbing materials between the MC plasma and the divertor target, so that there is a lower neutron flux at the divertor target. This has the same benefits of a low neutron flux as in section 1) above. This is another way that aspects of this invention work together advantageously.

[0187] Some additional key aspects of the invention assist in allowing all the SOL Issues to be solved simultaneously:

[0188] 3) If the SOL is narrow near the divertor target, as described above, this tends to concentrate heat, which makes solving Issue #1 more difficult. It is well known in the art that to spread out the heat flux, one can make the magnetic field nearly tangential to the plate. It is recognized in the art that this spreads out the heat by an amount that is inversely proportional to sin(theta), where theta is the angle of the magnetic field with the plate. It is also well known in the art that unevenness in the target plate limits how small the angle can be. Hence, unevenness in the plate limits the degree to which the heat flux can be reduced. Such unevenness arises in mechanical designs for conventional high-heat flux divertors, for example, on ITER. These designs have divertor surfaces consisting of many so called “monoblocks” that are not perfectly aligned. For ITER, due to this unevenness, the minimum allowed angle theta is roughly 2 to 3 degrees. Improved mechanical designs, such as those suggested for the Fusion Nuclear Facility by the General Atomic company, allow values of theta which are lower: roughly one degree. This angle would give lower heat flux. However, even this low of an angle might not be enough for needed reduction in heat flux.

[0189] So, it is desirable to have a divertor plate that avoids unevenness, that is, it is smoother, so that the heat can be spread even more.

[0190] For this, the present invention uses the elementary fact that liquid surfaces are smooth, as long as they do not flow too quickly and become turbulent. Hence, the claims include liquid surfaces 106 for the divertor target 105, in FIG. 3, FIG. 4 and FIG. 5. This helps solve SOL issue #1. A smooth liquid surface arises for a nearly horizontal plate of very slowly moving liquid. It is also known in the art that magnetic fields make liquid metal flows much more laminar, which can also enhance smoothness. Sufficiently slowly flowing liquid metals in a magnetic field can maintain high-smoothness, even for some degree or departure from purely horizontal. Hence, in some embodiments, this invention can have flowing liquid surfaces on the divertor target.

[0191] As is known in the art, Capillary Pore Systems (hereafter called CPS), if made of a fine enough mesh, can also have very smooth liquid surfaces. Hence, in other embodiments, the present invention can also use liquid surfaces in this manner, that is, with liquid in a CPS.

[0192] So in summary, this invention uses liquid surfaces that have the advantage of allowing a smoother surface to spread heat out more. This works advantageously with another advantage of liquids that was mentioned previously: liquids allow replenishment of the surfaces that are eroded quickly, such as the divertor target.

[0193] Another way to make a smooth surface is to have a molten metal that covers the divertor target and slowly cools to solidify. In other words, the divertor target does not need to always be a liquid. It might only be a liquid for some of the time, to allow the creation of a solidified surface that is very smooth.

[0194] In some embodiments, heat that strikes the divertor target could be removed by cooling from behind the surface of the divertor target facing the SOL. This is well known in the art for solid divertor targets. In other embodiments, if a practical means of creating rapidly flowing liquid metal to remove heat in a divertor becomes available, that might also be used for this surface. In yet other embodiments, a combination of these two methods could be used.

[0195] 4) Many of the sputtered or evaporated impurities are entirely neutral, and travel along substantially straight lines (they do not respond to the magnetic field). These impurities originate mainly at the divertor target 105, or from material that covers the divertor target 106. These impurities could get into the MC plasma 101 and contaminate it. Hence, an additional aspect of the invention is to avoid lines of sight from the divertor target to the MC plasma, by employing a material surface to block them. We will call this a shield 109 which can be seen in FIG. 3 and FIG. 4.

[0196] In some embodiments, such as FIG. 3, the shield 109 is an extension of the cover 107. The cover already blocks many lines of sight from the divertor target to the MC plasma, and the shield blocks additional lines of sight from the divertor target to the MC plasma. In other embodiments, such as FIG. 4, the cover 107 does not block lines of sight to the MC plasma, and the shield 109 does this.

[0197] By blocking sputtered and evaporated material, the cover 107 and shield 109 also avoids the accumulation of that redeposited material in undesired places around the plasma chamber. Accumulation of material on walls around the MC plasma, or other locations where it would cause problems is, thereby, prevented.

[0198] The cover must also have an opening which allows particles to escape and be pumped out. This opening is the entrance to the duct 172 and is connected to the duct 170 which has pumping means 171 at the end. These particles that must be pumped may include helium, and possibly also hydrogenic particles, and possibly other particles as well.

[0199] 5) The cover 107 and shield 109 described in 4), in addition to blocking sputtered and evaporated material from the MC plasma, can have other functions as well. These can be realized by having a further aspect of the invention be that a liquid surface 108 on the cover 107 faces the SOL, and a liquid surface 111 on the shield 109 faces the SOL. The material facing the SOL must be a liquid at least some of the time, and over at least some of the surface. The liquid could have several benefits, which we now describe.

[0200] One benefit is obtained if the liquid is lithium, so that it would chemically bind with recycled hydrogen. This could produce the low-density, high-temperature SOL. Since neither the cover nor the shield is subject to the enormous heat fluxes at the divertor target, it is far easier to keep the lithium in a temperature range where the chemical binding is very strong, and where evaporation and temperature dependent sputtering is low. Hence placing the liquid lithium surface 108 or 111 at a location other than the divertor target, therefore, has important advantages.

[0201] There are other benefits that the liquid can confer. These functions could be obtained by liquids such as molten metals, including, but not limited to, lithium tin, gallium, lead, and alloys containing these. To see the function of these liquids, realize that the cover and shield would be subject to accumulation of redeposited sputtered material from the divertor target. Consequently, they could accumulate a problematic thickness of this redeposited material over time. This liquid 108 and 111 would allow accumulated material to be removed, for example by pumping the liquid through pipes, without having to replace any solid part of the divertor. The flow rates required for this are rather low. As mentioned before, replacement of such solid parts is typically very time consuming, and is estimated to take on the order of months. Removal of material through pipes is much faster and easier and does not require any maintenance shut-down.

[0202] An additional benefit that the liquid 108 and 111 can confer is that the liquid can be replenished easily, even by a slow liquid flow. This is important because there can be considerable erosion when a high temperature SOL is nearby, since there will be some high energy neutrals that arise by charge exchange of recycled neutrals with the SOL plasma. The high energy neutrals will strike and erode the material of the cover 107 and the shield 109. By having liquid 108 on the surface of the cover 107 or liquid 111 on the surface of the shield 109, this eroded material could be replenished easily.

[0203] Also, transient events in the MC plasma can cause erosion because of high energy fluxes to be deposited on the divertor target, cover and shield. By having these components covered with liquid, that material can be replenished by a slow liquid flow.

[0204] The liquid surface might be continuously flowing. Then the redeposited material would dissolve in the liquid and be removed continuously. Or the surface could be solid most of the time, and liquid would flow only for relatively short periods to periodically dissolve and remove the accumulated material.

[0205] 6) Aspect 3 above indicated the advantages of having a divertor target that is liquid. However, a solid target is still possible. Due to erosion, however, the lifetime of such a target would be limited. Hence, the divertor target might be solid most of the time, but would only be liquid to allow eroded material to be replaced and to make the surface smooth flat again when the liquid solidifies. This could happen during brief shutdowns of the MC plasma operation. The replenishing material could be introduced, for example, through pipes, while keeping the divertor target in place.

[0206] This technique, allowing a solid divertor target surface to be replenished and made smooth without removing it, works much faster than removing the solid target; the latter is estimated to take months.

[0207] 7) The physical arrangement described by all the aspects above lends itself to placing neutron shielding around the diverter region. To see this, note that by being in a region with larger magnetic field strength than at the X-point, the divertor target must be located a significant distance away. It is also true that the requirement on the poloidal magnetic field described above tends to place the divertor target away from the plasma X-point. Either of these considerations implies a location for the divertor target 105 where there is space between the divertor target and the MC plasma 101, and neutron absorbing material 110 can be placed in that space. See FIG. 3 and FIG. 4.

[0208] For a deuterium-tritium reactor, the neutron shield could be the tritium breeding blanket that contains the neutron absorber Lib. Other neutron shielding means can also be used, as well, either in conjunction with a lithium breeding blanket or instead of it, using materials including, but not limited to, those that contain B.sup.10. Such materials might also partially surround the divertor region, in addition to being between it and the MC plasma.

[0209] Also, the neutron absorber 110 will reduce the neutron flux from the MC plasma 101 to the divertor target 105. See FIG. 3 and FIG. 4. Several major benefits for the divertor target can result.

[0210] One of these is that materials with very high-thermal conductivity can be used, which remove heat more efficiently, but which are rapidly degraded by neutrons. Some examples of such material that are known in the art include, but are not limited to, copper and copper alloys, forms of carbon, graphite, and carbon composites, and forms of silicon carbide and silicon carbide composites. These have high-thermal conductivity and other desirable properties but degrade rapidly in a flux of fusion neutrons. But because the divertor target in this invention can have substantial neutron shielding, the use of these, and other, materials becomes possible. These materials can remove high-heat flux more efficiently and can have other advantages such as corrosion resistance.

[0211] The low neutron flux might also make it possible to use better coolants in the divertor region. As mentioned above, a water coolant, which is used in ITER because of its efficiency, would present a serious risk of accidental lithium fire if lithium is present in the divertor region. If neutron bombardment is small, various liquids with low melting points becomes available, that are not dangerous in contact with lithium. These liquids would be degraded by a large flux of neutrons. Such liquids include, but are not limited to, silicon-based liquids, mineral oils, synthetic oils, some molten salts, or other liquids. These liquids have desirable properties, but either degrade quickly, or, produce undesirable radioactive byproducts, when exposed to neutrons. So by putting the divertor target in a low neutron flux region, the use of these coolants becomes possible.

[0212] Because of these aspects in 7) and possibly 3), it might become easier to employ a lithium divertor target, despite the low surface temperature limit.

[0213] 8) The intent of the present invention is to create an SOL where the average electron energy is higher than is possible with conventional means. So, we include this as a separate claim.

[0214] In some embodiments, the temperature of the electrons immediately adjacent to the divertor target must exceed 25 electron Volts. Temperatures of this range would cause unacceptable erosion in a solid target for operation at high duty cycles. We note that much higher temperatures are possible, but this SOL temperature in the divertor region is already inaccessible to the standard art for fusion relevant conditions and operation for long times. In other embodiments, the temperature of the LCFS next to the main plasma is 200 eV or higher. So, we describe this requirement as an electron temperature above 200 eV on the last closed flux surface of the MC plasma or a temperature of electrons of 25 eV immediately adjacent to the divertor target.

[0215] We note that it is sometimes technically difficult to precisely measure the SOL electron temperature. It is easier to measure the SOL temperature near the LCFS next to the MC plasma. This is another reason to describe this requirement as an electron temperature of at the LCFS near the MC plasma, in addition to as an SOL temperature immediately adjacent to the divertor target.

[0216] In other embodiments, density of the MC plasma at the LCFS is much less than the line average density of the MC plasma. This is also desirable for a high energy confinement. We describe this in terms of quantities that are easiest to measure, as a ratio of the electron density on the last close closed flux surface to the line averaged electron density of the MC plasma of 0.2 or less.

[0217] Before proceeding to the claims, we mention two important points. 1) It is hypothetically conceivable that some future Flowing Liquid Surface Means becomes available, that could resolve the surface heat flux issue. We will refer to this hypothetical future technology as an FLSM. It is important to realize that even if an FLSM is somehow accomplished, it does not solve all the five SOL Issues, but only, perhaps, Issue #1 and Issue #3. Hence the present invention would still be needed for a working fusion reactor or neutron source for transmutation. Aspects and claims of the present invention would still be necessary. The hypothetical FLSM would simply be operative at the divertor target, as one specific example, among many possibilities, of what is termed in the claims as a divertor target where some of the surface is covered by liquid at least some of the time.

[0218] 2) Commercially, applications of MC plasma may be classified in two types with different goals. The first type has a high duty cycle, that is, the MC device operates for a substantial fraction of the time. Devices to produce useful energy or neutron sources for transmutation are of the high duty cycle type. The second type of device has low duty cycle. These can operate in the pulse mode—pulses lasting seconds or minutes, followed by long periods with no operation. Devices with low duty cycle are often research devices that are prototypes to develop devices of the first type. Examples of commercial devices with short duty cycle are SPARC, soon to be built by the company Commonwealth Fusion Systems, and ST40, presently being operated by Tokamak Energy Ltd. Government operated research devices such as ITER are also of the second type.

[0219] For short duty cycle devices, target erosion (Issue #3) and redeposition (Issue #4) may not be so serious, because these problems accumulate over days of continuous divertor target operation. However, the issue of heat flux (Issue #1) and impurity accumulation (Issue #2) are still critical. And depending upon the research device, helium exhaust (Issue #5) may or may not be important for a research device. It is definitely important for ITER, but it is not for ST40, and it is probably somewhat important, but not crucially important, for SPARC. Furthermore, for short duty cycle devices, some produce a substantial flux of neutrons (e.g. ITER and SPARC) that can impact the pumping means, and some do not (e.g. ST40). Recall from the discussions above that neutrons can be highly problematic for the pumping means, so that it is very beneficial to put neutron absorbing materials between the source of the neutrons and the entrance to the pumping duct. These is not important for research devices that do not produce neutrons.

[0220] We can apply the present invention to devices with both low duty cycle and high duty cycle. The latter must have more aspects than the former, in order to better resolve Issue #3 and Issue #4. And Issue #5, while crucial for a high duty cycle device, is not important for some research devices.

[0221] The claims below are organized to reflect this, with some dependent claims that are important for devices of type 1) but not of type 2).