Patent Number: 044407149
Section: description

DETAILED DESCRIPTION OF THE INVENTION Refer to FIG. 1 which schematically discloses the basic concept of a variable pellet implosion site in a fusion reactor. FIG. 1 has the following features which are considered old art in ICF devices: a bottom blanket 1 and top blanket 2 wherein nuclear species are located for the breeding of useful isotopes (tritium and plutonium 239 being usual products); a plurality of radial blanket assemblies 3 wherein useful isotopes may also be generated; a group of fuel assemblies 4 wherein energy may be produced by nuclear fission as induced by neutrons released by the fusion process in the pellet; a pellet injection system 5; a plurality of beam sources 6, the preferred beam being laser light; means for heat removal shown here as a liquid sodium heat exchanger 7 having inlet 8 and outlet 9; a first wall 10, intended to encompass all interior surfaces of the reactor exposed to radiation directly from the pellet fusion burn. The pellet is injected into the reactor by the injection system 5 and by gravity and momentum travels along path A-A in FIG. 1. According to the prior art, the pellet is illuminated by the beams 19 upon reaching the central implosion site 11 which initiates a fusion reaction in the pellet. Radiation (not shown) from the pellet then emanates from site 11 outward striking first wall 10 and all blanket assemblies 1, 2, 3, and fuel assemblies 4. The new feature of this invention is the variation in axial position of the pellet implosion site 11. FIG. 1 shows a range B-B along path A-A which range is a locus of equally spaced points chosen to be implosion sites. The site variation causes the radiation deposited in reactor components to seem to have a line source. The time integrated radiation flux, defined as the fluence, is a line source fluence. Range B-B is limited in length such that no site is prohibitively close to the first wall 10. FIG. 1 shows representative implosion sites 12 and 13. The choice of an implosion site is accomplished by control of the timing of the firing of the laser such that the pellet is located at the desired site, and by control of the path of the laser light. The variation in implosion site along A-A may be accomplished in discrete steps taken between even subsequent pellet shots but may preferentially be achieved by changes made over a period of days or weeks. The description of the implosion site distribution in time and in space along B-B in order to achieve a line source fluence in the radial blanket assemblies 3, fuel assemblies 4, and first wall 10 may be preplanned or developed during reactor operation in response to radiation measurements. Applications may arise in which a non-uniform line source fluence is desired, which can be obtained by an appropriate frequency of implosions along range B-B. The axial variation of the implosion site along B-B matches the fluence to the use of cylindrical geometry in the first wall 10, fuel assemblies 4, and radial blanket assemblies 3. To provide a means of varying the point of implosion for ICF pellets, the optics of the laser system must permit focusing over a locus of points (B-B). Furthermore, the means of achieving this change in focal point should substnatially preserve the characteristics of symmetric illumination to avoid giving the pellet an asymmetric impulse during implosion. A first approach to achieving these conditions is simply to raise or lower the beam sources 6 of the laser beam system as illustrated in FIG. 2. FIG. 2 is a schematic diagram of an ICF reactor in which the beam sources 6 in FIG. 1 have been shown as having a system of optical mirrors 14. As shown in FIG. 2, the final two mirrors 14 in each of the beam sources 6 would be raised and lowered by an amount sufficient to change the locus of implosion sites over the desired distance. The coupling between the beam sources 6 and the remainder of the laser beam system would be accomplished by a sliding joint 15 in a periscope-like arrangement as shown in the detail of FIG. 3 for one of the beam sources 6. This approach would require a flexible penetration 17 for each of the beam sources 6 into the upper and lower access points of the reactor such that some degree of horizontal translation of the beam sources 6 is accommodated as they are raised and lowered. Position C in FIG. 2 is intended to correspond to a laser beam focus at implosion site 13 while position D corresponds to a laser beam focus at implosion site 12. A second arrangement to provide a linear adjustment or a variable locus of implosion sites is illustrated in FIG. 4. FIG. 4 is a schematic of an ICF reactor in which the beam sources 6 are not mobile as are the beam sources 6 in FIG. 2. This concept utilizes the techniques of adaptive optics (see Active Optical Devices and Applications-Volume 228, from the SPIE Proceedings, 1979) to change the curvature as well as the orientation of flexible mirrors 18 in each of the beam sources 6. This change is illustrated for one such flexible mirror 18 in FIG. 5 which shows the flexible mirror 18 in three different configurations with changes in both curvature and angle of inclination. The net effect is variation along range B-B of the focal point of the mirror 18 on line A-A. If done appropriately for all beam sources 6, the focal point of all beams 19 (of which 4 are shown in FIG. 4) will occur at the desired location for pellet implosion along A-A and within B-B. Variations in the focal length of the flexible mirrors 18 on the order of .+-.5% should be sufficient to effect the changes in implosion location of interest to this concept. There is a possibility that the effects of unequal focal lengths in the upper and lower laser beam sources 6 may give an asymmetric illumination and intensity and therefore an upward or downward impulse on the pellet during implosion. If this proves to be a problem, compensation for this asymmetry can be provided by introducing optical aberrations in the adaptive behavior of the optics to ensure symmetric illumination intensity. These aberrations would have the effect of slightly defocusing either the upper or lower beam sources 6 to appropriately adjust the symmetry of the illumination intensity. While in the above, two alternative methods to provide a locus of implosion sites is given, it is obvious that other means including optical, mechanical, and other techniques may be provided to accomplish the controlled aim of laser beams. The invention is not limited to laser fusion but can also be used in systems in which alternative fusion-initiating energy beams, such as electron beams, are employed. The reactors illustrated in FIGS. 1, 2, and 4 will of course have many components which are not included therein since there are not considered part of this invention. It is assumed, for example, that conventional systems will be used to time the firing of the beam sources 6 such that the pellet, in flight, is illuminated by the beam or beams at the proper implosion site. While in the foregoing a general invention has been described, it should be understood that various changes may be made without departing from the true spirit and scope of the invention. For example, the selection of the implosion site might be an automated decision based on a radiation fluence as continuously calculated by a computer using detected radiation flux levels. Therefore, the specification and drawings should be interpreted as illustrative rather than limiting.