Patent ID: 12262462

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG.1, a high-energy plasma system10may provide a pressure vessel12, for example, in the form of a sealed cylindrical shell of stainless steel or the like, extending along an axis14for receipt of a reaction gas, such as deuterium or tritium, through valve inlet assembly13from a pressure tank or the like (not shown).

First and second electromagnetic coils16aand16may be positioned within the pressure vessel12near the opposed ends of the pressure vessel12to define a containment volume17therebetween having a magnetic containment field15. The electromagnetic coils16are oriented and separated to form a Helmholtz pair aligned along axis14for establishing an axial B0field therebetween. In one embodiment, the electromagnetic coils16may be pancake coils providing spirals about axis14powered by an external, controllable DC power supply18of the type understood in the art.

Positioned between the electromagnetic coils16but proximate to one electromagnetic coil16bis a radiofrequency antenna19(shown in simplified form), for example, providing a circularly polarized radio field extending along axis14when driven by a radiofrequency generator20. As is understood in the art, the polarized radio field provides an electrical vector21perpendicular to axis14rotating thereabout. Further discussion of loop antennas suitable for this purpose are found in T. H. Stix, “Fast Wave Heating of a Two-Component Plasma,” Nuclear Fusion 15, 737 (1975) and R. W. Harvey, M. G, McCoy, G. D. Kerbel, and S. C. Chiu, “ICRF Fusion Reactivity Enhancements in Tokamaks,” Nuclear Fusion 26, 43 (1986) hereby incorporated by reference.

A treatment volume22may be located radially outside the pressure vessel12, for example, in the form of a concentric outer cylindrical tank which may be filled with, for example, an aqueous material for transmutation by high-energy neutrons such as precursors to medical isotopes99Mo (molybdenum 99),131I (iodine 131),133Xe (xenon 133) and177Lu (lutetium 177) or which may support racks holding spent nuclear fuel rods being rejuvenated through transmutation by high-energy neutrons.

A neutral beam generator26is positioned to inject a beam28of neutral particles29(that is non-ionized particles having zero net charge) at a pitch angle θ into the containment volume17. The pitch angle θ is defined as an acute angle between an angle of the beam28and the axis14. The neutral particles29, for example, are atoms of deuterium or tritium introduced through a gas line24and ionized by a local plasma (not shown). These ions are accelerated in an accelerator chamber27having a successive set of electrically charged plates as is generally understood in the art. The ions then pass through a neutralizing gas cell31to produce neutral particles29by a charge exchange process to produce the neutral particles29of the beam28.

Referring now also toFIG.2, the magnetic flux lines30generated by the coils16will produce a “bottle” shape expanding radially from the axis14at a midpoint between the coils16and contracting radially at the location of the coils16. As is generally understood in the art, this configuration produces a mirror containment volume where randomly distributed “thermal” plasma ions of sufficient pitch angle32spiral around flux lines30between regions defined by turning points34.

These thermal plasma ions can be established in a variety of ways for example by using the radiofrequency antenna19(albeit at a low efficiency) or a separate heating system using high-frequency microwaves producing electron cyclotron resonance heating, as is understood in the art

At the regions of the turning points34, the thermal plasma ions32reverse direction caused by increasing axial components of the magnetic Lorentz force produced by the convergence of the flux lines30. The frequency35of the spiraling about the flux lines30is termed the “cyclotron frequency” and is a function of the strength of the magnetic field37along axis14, and for this reason the cyclotron frequency35generally increases toward the electromagnetic coils16. For ions of equal mass and charge, the cyclotron frequencies will be nominally identical at a given location along the axis14, independent of the velocities or energies of the ions; however, ions32of equal mass having different pitch angles will normally have different turning points34.

The velocity and hence the energy of the neutral particles29of the neutral beam28and the pitch angle θ of the neutral beam28are set so the majority, for example, greater than 50 percent, of the particles of the neutral beam28will be ionized into plasma ions36within the containment volume17before exiting the containment field. These plasma ions36at the same pitch angle, now having electrical charge, are captured by the magnetic flux lines30to increase the plasma density.

In order to promote this entrapment of the majority of the neutral particles29of the neutral beam28, the energy of the neutral beam28is limited to provide sufficient time-of-flight for the neutral particles29to be ionized. Generally, the desirable energy of the neutral beam28for ionization will be well below the kinetic energy required for substantial fusion, and typically less than 100 thousand electron volts or preferably less than 50,000 electron volts and more typically on the order of 15-25 keV. This is in contrast to prior art approaches which require neutral particles29with energies exceeding the energy necessary to promote fusion between the plasma ions36and typically having energies more than one million electron volts for D-D fusion, By limiting the energy of the neutral beam28, a trade-off may be affected in common neutron beam generators26to produce a higher flux density of neutral particles29, also increasing the plasma density.

Referring still toFIGS.1and2, the pitch angle θ of the neutral beam28is selected to provide predetermined turning points34′ along axis14for the resulting plasma ions36and thus to provide a corresponding predetermined cyclotron frequency35of the plasma ions36at the turning points34′. This cyclotron frequency is used to set the frequency of the radiofrequency generator20as will be discussed below.

In addition, the antenna19is placed proximate to one of the turnaround points34′ to provide a maximum field strength in that region.

Finally, within the energy levels for the neutral beam28that provide the desired capture of the neutral particles29within the containment volume17, the energy of the neutral beam28is set to be as high as possible so that the radius of orbit of the of the plasma ions36produced by the neutral beam28(gyro-orbit52) is higher than the average distribution gyro-orbit52of “thermal ions”32, being ions not immediately derived from the neutral beam28.

While the inventors do not wish to be bound by a particular theory, the above-described: (a) setting of the cyclotron frequency of the radiofrequency generator20to a harmonic of the cyclotron frequency of the plasma ions36at the turning point34′, (b) boosting of the energy of the plasma ions36above the average distribution of the thermal plasma ions32, and (c) maximizing the electrical field strength at the turning point34′, all work together to allow the radiofrequency generator20to preferentially boost the energy of the plasma ions36from the neutral beam28tree from the damping effect of thermal plasma ions32.

In this regard, the setting of the radiofrequency generator20(per (a)) provides preferential coupling to the plasma ions36having a matching (e.g., a harmonically related) cyclotron frequency35, in contrast to thermal plasma ions32having a range of different Doppler-shifted cyclotron frequencies and less effective coupling. The coupling may be proportional to the square of the Bessel function Bn-1(k⊥*ν⊥/ωci) where:n is the resonant cyclotron harmonic number of the injected wave,k⊥the perpendicular wave number, andωciis the cyclotron frequency of the resonance ions.

The quantity k⊥/ωcimay be ˜vA, the Alfven velocity of the ions (cf. T. H. Stix, “Fast Wave Heating of a Two-Component Plasma,” Nuclear Fusion 15, 737 (1975)). Given the dependence of the Bessel function on ν⊥, the coupling is proportional to powers of the perpendicular velocity of the ions, and can be adjusted to preferentially damp on hot tail ions from the neutral beam and on those diffused to higher energy by the radiofrequency waves.

Further, by setting the frequency of the radiofrequency generator20according to the cyclotron frequency35at the turning point34′, the influence of the electrical field from the radiofrequency generator20on the plasma ions36is increased because of the prolonged dwell time50of the plasma ions36at the turning point34′ during their lowest axial velocity as they turn around. This is in contrast, for example, to thermal plasma ions32which move quickly through this zone to further turning points34or which do not reach as far as the turning point34′.

As noted above, by boosting the energy of the plasma ions36above the distribution of thermal plasma ions32(per (b)) and by setting the RF generator20to an RF frequency which is a high harmonic of the cyclotron frequency35of the plasma ions36, higher energy plasma ions36having a higher radius of gyro-orbit52preferentially absorb power over the thermal plasma ions32having a lower gyro-orbit52. In some embodiments, the RF frequency may be set to a range from 20 to 100 megahertz and/or to a harmonic n greater than n=2 and preferably n=4.

Generally, the higher harmonics boost the relationship between energy absorption and gyro-orbit52according to increasing Bessel function numbers associated with those harmonics, Specifically, energy absorption will be proportional to Jn-1(k⊥ρ) where: Jn-1is the Bessel coefficient for a given harmonic n, ρ is the radius of the particle's gyro-orbit52about the magnetic flux lines30which increases with energy by

ρ=√2⁢mEeB
and k⊥is a wave number of the plasma ions36being a property of the wave within the plasma and the polarization of the antenna19launching the wave.

It will be appreciated that this effective preferential absorption of energy by the plasma ions36will be self-reinforcing as energy is absorbed and the gyro-orbit of the plasma ions36is increased.

Finally, by placing a highest field strength of the antenna19near the turning point34′, the plasma ions36are preferentially influenced.

Generally, the magnetic containment field15will tend to lose some plasma ions32having low pitch angles through its ends. These particles are said to be in the “loss cone.” By boosting the population of the plasma ions36having a known pitch angle θ outside of the loss cone, increased plasma densities can be obtained.

While the cyclotron frequency of the plasma ions36near the turning point34′, and hence the desired setting of the frequency of the radiofrequency generator20, is primarily a function of the vacuum magnetic field strength37, it will shift slightly as a function of increasing plasma density/pressure. Accordingly, the invention contemplates that either or both of the DC power supply18or the RF frequency generator20may be adjusted during operation to maintain the above relationships which boost energy transfer to the plasma ions36. In particular, this adjustment may be made via a closed-loop feedback control using a sensor56detecting plasma pressure, for example, using a diamagnetic loop, which will measure the decrease in magnetic field due to increased plasma pressure to ensure a matching of the excitation frequency of the RF generator20with the actual and dynamic cyclotron frequency35at the turnaround point34′. To the extent that the cyclotron frequency is dictated by the total field (vacuum field from coil plus plasma diamagnetism); the invention also contemplates that no frequency change may be required but the location of the turning point will move closer to the electromagnetic mirror coil.

Referring now toFIG.3, this benefit of the present invention in providing high plasma densities makes it useful as part of a system where two high-energy plasma systems10may act as “plugs” to trap high-energy plasma ions in a larger scale neutron generator60for the purpose of transmutation (as discussed above) or fusion power generation. Such a design, for example, may make use of a tandem mirror scheme, for example, described at G. Dimov, V, Zakaidakov, and M. Kishinevski, Fiz. Plazmy 2 597 (1976), [Sov. J. Plasma], Phys 2, 326 (1976)] and T. K. Fowler and B. G. Logan, Comments on Plasma Physics and Controlled Fusion 2, 167 (1977) and hereby incorporated by reference.

More specifically, in such a tandem mirror neutron generator60, first and second high-energy plasma systems10aand10bare placed in opposition along axis14flanking a generator volume62, Generally, the high-energy plasma systems10will have an axial length on the order of 2 meter whereas the generating volume62will be much larger, for example, on the order of 50 meters or more.

The electromagnetic coils16of both of the high-energy plasma systems10aand10bare axially aligned to provide a same direction of polarization of the magnetic field along the common axis14. As such, the flux lines30of the first high-energy plasma system10amay continue through the volume62to the second high-energy plasma system10b. Within the volume62, the flux lines30are focused by an axially-extending solenoid coil66circling the axis14around the volume62.

For this purpose, the electromagnetic coils16may be superconducting magnets for example per D. Whyte, J. Minervini, B. LaBombard, E. Marmar, L. Bromberg, and M. Greenwald, “Smaller and sooner: Exploiting high magnetic fields from new superconductors for a more attractive fusion energy development path,” Journal of Fusion Energy, 35, 41 (2016) also hereby incorporated by reference.

A subset of thermal plasma ions32, having a uniform distribution of pitch angles and having been boosted to higher energies by kinetic transfer from the plasma ions36, may escape from the high-energy plasma systems10into the volume62containing a reactant gas, for example, deuterium or tritium, to promote fusion and the emission of neutrons64from the volume62. The high pressure of the high-energy plasma systems10blocks the escape of high-energy plasma ions from the volume62to maintain the high densities for significant fusion.

The volume62may be surrounded by a contained volume22which may include a heat exchanger liquid68, for example, for receiving, through one or more heat exchangers, a working fluid70of a thermodynamic engine such as a turbine or the like, for example, for the generation of electrical power. Alternatively, the contained volume22may be used for the transmutation of materials to generate medical isotopes or to rejuvenate spent nuclear fuel as discussed above.

The present application incorporates disclosure of US patent application 2019/0326029 entitled: Apparatus and Method for Generating Medical Isotopes, and US application 2013/0142296 entitled: Apparatus and method for generating medical isotopes which describe additional techniques for managing isotope transmutation including the use of neutron multiplier generators and other construction details and mechanisms for producing a neutral beam discussed above.

Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.

When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising” “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.