Inertial Confinement Fusion (“ICF”) is a process by which energy is produced by nuclear fusion reactions. The fuel pellet, generally called the target, is conventionally a spherical device which contains fuel for the fusion process. Various ways of driving and imploding the target have been utilized and considered (lasers, ion beams, etc.). These drivers transfer energy to the target which then implodes and ignites the fuel. If the fuel is sufficiently heated and compressed, a self-sustaining fusion reaction occurs, wherein the fuel self-heats and produces energy from the fusion reaction.
If the target is to be useful for energy production, it must output more energy than the amount of energy needed to drive the implosion. The amount of energy needed to drive the target may be quite high as very high temperatures and densities are required to initiate fusion reactions. Also, the amount of energy needed to drive the target must be physically and economically achievable.
The conventional approach to ICF target design is exemplified by the Department of Energy's program, NIF (National Ignition Facility). NIF target designs, as described in Lindl, “The Physics Basis for Ignition Using Indirect Drive Targets on the National Ignition Facility,” consists of a mostly plastic or beryllium ablator region which surrounds a cryogenic DT ice, and a central void which is filled with very low density DT gas. The target is then placed in a cylindrical hohlraum. The entire target assembly (hohlraum and target) is then placed in the target chamber, where a 192 beamline laser delivers up to 1.8 MJ of energy to the hohlraum. The hohlraum then converts the energy to x-rays which then ablate the ablator region, and by the reactive force, drives the DT shell inward.
However, NIF targets have, up to now, never ignited. The areal density (ρr) of the fuel and the temperature of the fuel has, to date, fallen short of the 0.3 g/cm2 and 10 keV they believe they require. It is not clear why the NIF targets have not achieved ignition, but it may be due to lower than expected shell velocities and greater than expected Raleigh-Taylor instability growth which may be due to low fall lines.
Although NIF targets utilize a DT shell, gold shells have been considered by K. S. Lackner, S. A. Colgate, N. L. Johnson, R. C. Kirkpatrick, R. Menikoff and A. G. Petschek “Equilibrium Ignition for ICF Capsules”, 11th International Workshop on Laser Interaction and related Plasma Phenomena, Monterey, Calif., Oct. 25-29, 1993. High Z shells have the effect of trapping the radiation in the DT core, thereby allowing the radiation field to reach its equilibrium blackbody spectrum and lowering ignition temperatures to about 2.5 keV. ICF has been hampered for the last many decades by stability and symmetry considerations. Lackner et al. proposed using high-Z shells and they realized the advantage of the low ignition temperature that would occur therein. However, the ICF community largely rejected high-Z shells on the grounds that the mix of the high-Z material with the fuel would tend to quench any of the reactivity of the fuel.
Unless otherwise indicated herein, the material described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
This has heretofore been unrecognized, by utilizing a fissionable shell, you favorably affect the stability properties of the implosions resulting in minimal mix because of suppressed Raleigh-Taylor instability. The result is an intrinsically stable implosion. By proper configuration of the high-Z shell's fissionable properties, and the timing, the 14 MeV neutrons provide sufficient energy deposition into the shell that it expands at the requisite rate during the implosion, you can get an intrinsically stable implosion.
Another aspect of this invention is how to reach high radiation temperatures with a high-Z shell. Radiation has not normally been proposed as the preferred output of an ICF target. At high enough radiation output temperatures (˜1 keV) where it becomes the dominant output or equivalently at high radiation yields (per unit mass), fusion reactor configurations become simpler or more effective because of the shift in the proportions of radiation, neutron, and debris energy output.
A third aspect of the invention is high energy yielding targets, or for a target of smaller total mass, we achieve the same net energy but a higher proportion as radiation.
A fourth aspect of this invention is the ability to achieve equivalent yields with lower drive energies.
A first embodiment 100 of this invention is shown in
It may be advantageous to make shell 104 of a material that can convert neutron energy to thermal energy. The shell 104 material is exemplified by 238U and 232Th, both of which have fission cross sections at 14 MeV that are substantial. Table 1 gives the approximate cross sections.
238U
232Th
For a class of simple targets 100 using single shell 104 and about 10−3 g of DT (mf=1 mg), the ϕΔrs of the shell 104 at the time when the fuel 102 ignites ranges from 5-10 g/cm2 depending on the shell to fuel mass ratio (ms/mf). Table 2 gives the probability of 14 MeV neutrons to cause fast fissions
and the local thermal energy thereby produced. Table 2 assumes 200 MeV local energy release with no further fissions due to the daughter neutrons. Definitions for column titles are as follows: pf is the probability of a neutron to cause a fast fission, ETOT(DT) is the total energy released by the DT, EDT Thermal is thermal energy released by the DT, ETOT Thermal is the combined thermal energy released by the DT and by fissions in the shell, ETOT is the total amount of energy released.
238U
232Th
Table 3 gives the local deposited energy to mass ratio where the mass includes allowance for an ablator mass (ma), hohlraum wall mass (mh), and shell mass (ms) for a typical target driven by a radiation field produced in a hohlraum.
238U
232Th
The above were computed for a configuration such as the embodiment 100 seen in
From Table 3, we see that substitution of fissionable materials in the shell may increase the energy available for radiation by up to a factor of roughly 10 for assembly 100. Also, if the ma/ms ratio is lowered to some 2:1, the yield per mass would then be further increased by a factor of 3.35 since
At an approximate heat capacity of 108 J/g, the case without fission would result in an output temperature of about 720 eV, the 238U case, about 6.1 keV, and the ma/ms=2 case with 238U, about 20 keV. In some embodiments the majority of the yield from the target may be in the form of radiation, for instance if the output temperature is above 1 keV and the thermal yield per mass (Yth/m) is greater than 108 J/g. When the fast neutron fluence becomes large enough, much of the shell material will be fissioned. A value,
characterizes this transition.
Using a computer simulation to calculate the thermal yield of a target utilizing 238U for the high Z shell material 104 in assembly 100 seen in
Another unexpected result of the substitution of fissionable material is control of the fall line of the target. The fall line parameter (γf) is defined as the radius at which the shell/fuel interface would have been ignoring effects of deceleration divided by the radius of the interface including the effects of deceleration at the time of stagnation of the shell/fuel interface (see
In some embodiments, like the one in
These advantages may lead to lower energy requirements for the drive mechanism (laser, ion beam, etc.) by reducing thermal losses to shell 104, increasing thermal yield from shell 104, and causing ignition earlier in time. Most ICF target designs have some amount of ignition margin built into their designs. Ignition margin can be defined in many ways, but overall ignition margin means robust ignition that is insensitive to noise levels in the drive and instability growth. The invention discussed here then allows that a target design with good ignition margin can be redesigned at lower drive energy while keep the ignition margin constant by the replacement of a non-fissionable shell with a fissionable shell. Lower drive energies are desirable as the driver in most ICF systems is the greatest portion of the cost.
Embodiments of this invention discussed in this application were designed using numerical simulations and hand calculations. This design process necessarily involves making approximations and assumptions. The description of the operation and characteristics of the embodiments presented above is intended to be prophetic, and to aid the reader in understanding the various considerations involved in designing embodiments, and is not to be interpreted as an exact description of how embodiments will perform, an exact description of how various modifications will change the characteristics of an embodiment, nor as the results of actual real-world experiments.
Additionally, the set of embodiments discussed in this application is intended to be exemplary only, and not an exhaustive list of all possible variants of the invention. Certain features discussed as part of separate embodiments may be combined into a single embodiment. Additionally, embodiments may make use of various features known in the art but not specified explicitly in this application.
Embodiments can be scaled-up and scaled-down in size, and relative proportions of components within embodiments can be changed as well. The range of values of any parameter (e.g. size, thickness, density, mass, etc.) of any component of an embodiment of this invention, or of entire embodiments, spanned by the exemplary embodiments in this application should not be construed as a limit on the maximum or minimum value of that parameter for other embodiments, unless specifically described as such.
This application claims the benefit of U.S. Provisional Application No. 62/517,402 filed on Jun. 9, 2017, which is incorporated herein by reference.
Number | Date | Country | |
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62517402 | Jun 2017 | US |