Patent Number: 
Section: description

Referring to FIG. 1, polymer dope is fed by positive displacement to an extruder 1 where it is forced through die orifice 2 by piston 11. Said piston is schematic only, and in practice it may consist of two gear pumps operating at differential speeds, or a diaphram which is displaced by pressurized gas. When manufacture is contemplated in space, the lack of gravity must be taken into consideration in designing the metering system. The polymer dope, selected to illustrate this process, consists of tritium substituted polystyrene dissolved in a suitable solvent. Many organic compounds have been reported as solvents for polystyrene and a related resin, poly a-methyl styrene. They include benzene, toluene, xylene, ethylbenzene, chlorobenzene, tertiary butyl benzene, isopropyl benzene, triphenyl methane, heptane, butyl acetate, methyl ethyl ketone, chloroform, carbon tetrachloride, tetrahydrofuran, carbon disulfide, and ethyl chloride. Preferred solvents should possess (1) good solvency, (2) low heat of vaporization, (3) ease of handling and (4) radiation resistance. Besides being readily soluble, tritium substituted polystyrene has a number of other advantages. It is resistant to radiation damage and it possesses a relatively low average atomic number (Z number). Tough, clear, cellophane-like films can be prepared from tritium substituted polystyrene with good chemical and physical properties. A particular form of this resin, isotactic polystyrene in which tritium is substituted for hydrogen, is of interest because of its predictable elevated melting point. Compressed gas 3 is metered at 12 into the center of the extrusion. An inert gas such as hydrogen, deterium, helium, nitrogen, carbon dioxide, air, fluorocarbon or any combination of these gases may be used. Subsequently, in another location, e.g. on earth, and shortly before use, the gas in the pellets can be exchanged for the required D-T fuel. Because of the permeability of plastic materials, the exchange of gases can be achieved by means of diffusion. The inert gas in each pellet will diffuse outward through the pellet wall as D-T diffuses into the pellet from the surrounding medium. A pressurized tank can be used for the above operation. By means of hyperbaric pressures the D-T charge can be increased over the amount in equilibrium at one atmosphere. The gas is introduced into the polymer dope through a hypodermic needle. The size of the bubbles is proportional to the orifice diameter. The needle must be perfectly centered in order to obtain symmetrical pellets. Because of the difficulty in centering the needle, it may be partly withdrawn from the tip of the extruder as shown in FIG. 4. Thus, a slight misalignment of the needle results in a smaller deviation as a percent of the extruder diameter. Since the flow of the dope is lamina;, the bubbles will move in a straight line. Because of the buoyancy of the bubbles, this scheme is best suited for operation in near zero gravity. Transducers 4 driven by driver 13 apply acoustical vibrations which have a frequency close to the natural frequency at which the extruding polymer rod tends to break up. This frequency can be estimated by the well-known Rayleigh equation or determined experimentally. The amplitude or strength of the vibration can be varied as needed. An additional effect helps to form drops from the extruded rod. A disruptive force is provided by the relative motion between the rod and surrounding gas. The relative flow rates can be altered by modifying the cross section of chamber 14 as shown in FIG. 4. As this drag effect becomes more important the contribution from the transducers can be reduced or eliminated. The preferred fuel for laser fusion is a combination of Deuterium and Tritium (D-T). An examination of rates for certain fusion reactions shows that D-T reactions occur with roughly 100 times the probability of its nearer competitor over the range of anticipated ion temperatures (0-10 Kev.). Thus the D-T fuel is the best to employ. The D-T fuel in the pellet is compressed to extremely high densities (103-104 times liquid density) by laser produced converging shock waves which also ignite a small portion of the compressed core. In the D-T thermonuclear reaction, alpha particles and neutrons deposit energy in the core, giving rise to a xe2x80x9cboot strapxe2x80x9d heating effect, and a propagating burn front. This burn front propagates through the core before the pellet has time to disintegrate, so that a significant fraction of the available fuel mass can burn. So-called D-T fuel is actually made up of a mixture of the molecules, D2, DT, and T2. Rather than using such a mixture, this invention envisions the likely use of only DT that has been spin-polarized or as high a proportion of this molecule as is practical. It has been reported that DT better retains nuclear polarization which would provide an assist in igniting the fuel. Unfortunately, DT slowly breaks up into D2 and T2 because of the radiation given off by tritium. Therefore, fuel pellets should be consumed as soon as possible after they are charged with fuel. The pellets are dried by removing the solvent as a portion of the gas surrounding the pellets in drying chamber 14 is recirculated by pump or blower 15 through an adsorbent 5. Activated carbon or silica gel has been found to be an effective adsorbent for many solvents. The rate of vaporization can be increased by applying heat to the pellets 9. The heat of vaporization of the solvent one way or another should be compensated for. While the pellets 7 are still in a fluid or plastic state, the surface tension tends to form spherical inner and outer surfaces. In the environment of near zero gravity this effect is greater because drag effects from the surrounding gas can be reduced. The application of acoustical vibration, in addition to breaking the extruded rods into pellets, helps to achieve concentric inner and outer surfaces. This vibration increases molecular motion within the walls of the pellet. The pressure of the gas external to the pellets 9 in chamber 14 is related to the pressure applied by piston 11 according to the following equation:   q  =      k    ⁢                  Δ        ⁢                  xe2x80x83                ⁢                  p          D                            μ        D             where q is the flow rate of the dope, k is a constant established by the geometry of the die, xcex94 PD is the pressure drop through the die, and xcexcD is the viscosity of the dope in the die. The injected gas material at 12 must have a slightly higher pressure than the dope contained in the extruder 1. The gas flow rate may be controlled by a pressure reducer and a needle valve. There are no fundamental restrictions on the absolute pressure in chamber 14 except as imposed by the design of the apparatus. As previously noted, however, because of contemplated operation in space, the apparatus should be kept as light as possible and therefore design pressures must be limited to a few atmospheres at most. The pellets, in a subsequent process, may be pressurized in a tank, whereby not only is DT exchanged for the injected gas, but the gas can be equalized at some elevated pressure. As the polymer rod leaves the die of the extruder it is observed to expand. This expansion will continue until the pressure of injected gas in the pellets equals the ambient pressure. Further expansion of the pellets can be achieved by heating them in a controlled or programmed manner as they are dried. Heat can be applied by means of infrared lamps located on the periphery of the drying chamber 14. As the temperature of the pellets is raised the vapor pressure of the solvent is increased. Using a modified form of Raoult""s Law, Dalton""s Law, and the Perfect Gas Law the equilibrium volume of the pellets can be estimated as follows:   V  =            n      ⁢              xe2x80x83            ⁢      RT              π      -      kP       where n is the moles of inert gas in each pellet, R is the gas law constant, T is absolute temperature, xcfx80 is the ambient pressure in the drying chamber, k is the relative vapor pressure and P is the vapor pressure of the solvent. As the pellet walls become more viscous due to the loss of solvent, the equilibrium volume will not be attained, but instead some intermediate value will be realized. As the pellets are expanded an important rheological effect is achieved in the plastic walls. This expansion of the wall of a pellet will tend to line up the polymer strands parallel to the wall. A similar effect will be achieved as when synthetic fibers are drawn to increase their strength. In the case of a pellet, the molecules become oriented in two dimensions resulting in what may be called xe2x80x9cbiaxial orientation.xe2x80x9d This change will contribute to significantly improved physical properties such as tensile strength and permeability. The pellets 9 are kept separated from each other and the surface of the container 14 until they have hardened or cured. Gas jets (not shown) may be used to keep the pellets 9 separated until they have hardened. Sonic vibrations are also useful in this application, inasmuch as, these exert small forces. A variation of the invention is that a final portion of the die orifice 2 may be rotated so that a spinning motion of the emerging bubbles will produce oblate spheroidal pellets which may be desirable in certain reactors. In space, the entire apparatus may be rotated to achieve this effect. As a corollary, rotation must be avoided in order to produce spherical pellets. The apparatus can be prevented from rotating about its axes by means of booster rockets or jets. The above described method of manufacturing microballoons by extrusion is not unique and can be replaced by other fabricating techniques. For example, Bayless in U.S. Pat. No. 4,107,071 describes a process for microcapsules whereby a core material is encapsulated with a polymeric resin. The encapsulation is achieved in an agitated system comprising two phases, one of which is the core material and the other is the vehicle for the polymer. Upon induction of phase separation, the polymer forms a sheath about the capsule core material. Only one further step is required to convert the coated capsules produced by the Bayless process to microballoons. Leaching, or vaporization or dissolution of the core material through the semi-permeable coating would result in hollow pellets suitable for charging with thermonuclear fuel. The Bayless process could be carried out in whole or in part in the near-zero gravity environment of space. As in the case of the extrusion process, pellets so produced would have improved symmetry. Already, in a noteworthy experiment, microscopic plastic beads have been produced in space that are not only more spherical than those manufactured on earth but also more uniform in size. (Science News, Aug. 10, 1985, pp. 92-93). Such beads could provide the core material for the Bayless process. It is known that tritium will substitute for hydrogen in organic materials. However, complete substitution by this method is unlikely in polymeric materials. Therefore the preferred method of preparing tritium substituted high polymers is by starting with monomers in which hydrogen has been completely replaced by tritium. Starting with tritium oxide the preparation of tritium substituted polyethylene is shown below:  Likewise tritium substituted polystyrene can be produced making use of the classical Reppe chemistry. In this instance tritium substituted acetylene is reacted with a catalyst to produce styrene and benzene both containing only tritium. The benzene compound can be burned in order to recover the tritium values. Tritium substituted polypropylene depends on the oxo reaction for its preparation. In this process an olefin (tritium substituted ethylene) is reacted with carbon monoxide and tritium to produce an aldehyde which is subsequently reduced to an alcohol. Dehydration of the alcohol produces tritium substituted propylene monomer. Monomer preparation by isotope exchange is carried out in the apparatus shown in FIG. 5. Tritium gas and a hydrogen containing monomer, in this case ethylene, are fed into reaction vessel 10. Agitator 11 assures the intimate mixing of the two reactants. In order to increase production, the reaction can be run at pressures above one atmosphere. A product sample is withdrawn from the vessel and passed through gas chromatograph column 13 to separate the sample into its component fractions. Hydrogen gas is introduced into the column via three-way valve 12 in order to elute the sample. Each of the fractions is identified by detector 14. Two three-way valves 15 and 16 divert the fractions to the proper lines. The valves are automatically operated by a controller (not shown) which receives a signal from the detector. Purified product, tritiated ethylene, is collected for subsequent polymerization. Reactants and partially substituted monomers are recycled to the reaction vessel, while tritium values are recovered from the hydrogen before the latter is discarded. Pellets produced by the invention may have varying dimensions. The outside diameter can range from 50 micrometers to 1 cm. and the wall thickness from 0.5 micrometer to 1 mm., however, these values are not meant to be limiting. The strength of the pellet, and thus the maximum pressure of the fuel gas, will depend on the tensile strength of the wall, the diameter of the pellet, and the wall thickness. Pellets produced in near-zero gravity of space can be expected to possess a non-concentricity of less than 5 percent and asphericity of under 3 percent. Those pellets, however, produced on earth cannot be expected to equal the ones produced in space with respect to symmetry or uniformity. Although it is possible to manufacture hollow spheres under normal gravitational conditions on earth, low or zero gravitational conditions help to reduce nonconcentricity and deformation. In the case of large gas-filled spheres, the gas bubble within the sphere has a tendency-to rise toward the top of the sphere in a gravitational environment. A zero-gravity environment helps to avoid deformations that can be produced by rapid gas movements past the sphere as it falls. This resistance tends to distort the shell , especially one which is relatively large, producing significantly reduced concentricity (FIG. 2, Dxe2x80x94D) and non-spericity of the shell (not shown). Low gravity of less than one tenth gravity at the surface of the earth may be considered zero gravity for the purposes of this process. A uniform wall thickness, FIG. 3, Axe2x80x94A, not only creates a higher energy yield, but also withstands higher pressures so that more fuel, usually DT, can be stored inside each sphere. Pellets, which are as described, may be subsequently coated with so-called pusher and/or ablator layers. These coatings are designed to increase the efficiency of the incident beams. Alternatively, the pellets may be used in a reactor that contains a gaseous atmosphere rather than being completely evaluated. For example, helium under low pressure would absorb some of the laser energy, but a controlled amount would reach the target. Being monoatomic, helium would absorb less laser energy than, for example, hydrogen would. Since the intensity of the beams is inversely proportional to the square of the reactor radius, most of the energy absorbed by the helium would be adjacent to the target. The helium in effect would function as an ablator. Fuel pellets made by the present invention possess unique properties which make them suited for use in devices employing thermonuclear fusion by inertial confinement. Such pellets, however, cannot be used for thermonuclear fusion by magnetic confinement because of the carbon impurities which would be introduced into the plasma. In fusion by magnetic confinement, feasibility studies have proposed the use of frozen deuterium/tritium pellets, the size of sand grains, which are injected at very high speed into the plasma. (Power, May 1982, p. 32) Experiments at MIT with the Alcator C Tokamak have confirmed the practicality of this concept. (Space Calendar, Nov. 21-27, 1983, p. 3). It will thus be seen that the objects set forth above among those. made apparent from the preceding description are efficiently attained, and, since certain changes may be made in carrying out the above processes and in the above described articles without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in the limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention hereinafter described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.