Patent Number: 046577211
Section: description

With reference to FIG. 1, exposure of a spherical target by a laser beam utilizing an aspheric corrector plate is illustrated. The paraxial focus point is illustrated in the drawing and the marginal focus point is shown to the right of this. The ray traces in the vicinity of the target for relatively uniform illumination of the hemisphere are shown from a single f/1.0 lens together with an aspheric corrector plate. It will be noted that the beam coming from the left of the drawing has only one point P toward the center where it is clearly orthogonal to the surface. The largest angle A of incidence from the outside is 63.4.degree.. As previously pointed out, since the amount of energy actually absorbed decreases with the increased angle of incidence, the net effect is to cause a non-uniform distribution of absorbed energy. The main embodiment of the present invention is illustrated in FIG. 2 wherein two laser beams #1 and #2 of equal intensity are directed toward an apparatus from the left and from the right. The manner of accomplishing this is illustrated in FIG. 3 wherein a single laser beam meets a beam splitter 20 and divides to two angled mirrors 22 and 24. Each beam passes through a lens to focus the laser beam to a small spot. The beam from the left passes through lens 26; the beam from the right passes through lens 28. As shown best in FIG. 2, there are two reflective, concave ellipsoidal surfaces 34 and 36 facing each other. These surfaces or mirrors are formed in a body shown in cross-section and may be formed of glass with the ellipsoidal surfaces ground in or they may be formed of aluminum with the ellipsoidal surfaces polished to high reflectivity. Each ellipsoidal mirror is provided with respective central conical openings 38 and 40 which terminate in a small opening at the small end directly at the surface of each mirror. Each beam then passes through these openings and expands to reflect from the opposite and facing ellipsoidal mirror. These mirrors are each characterized by two focal points. Light image to one focus will be concentrated at the second focus after reflecting from the mirror surface. In the system shown in FIG. 2, one focus of each ellipsoidal mirror is placed coincident with the focus of the opposite lens at the points F.sub.1 and F.sub.2. The other two foci of each mirror are coincident at the center of the system between the mirrors where a pellet 42 will be located. After reflection from the mirrors, all the light is focused on to the pellet. One lens 26 and the ellipsoidal mirror combination illuminates one-half of one hemisphere of the pellet while the other lens 28 with a corresponding mirror combination illuminates the other hemisphere of the pellet. While the optical system shown in FIGS. 2 and 3 clearly illuminates all of the pellet, it can be shown that the illumination is not exactly uniform. The laser irradiance as seen from the pellet is plotted as a function of angle from the back of the spherical pellet as illustrated in FIG. 4. It will be seen that the curve assumes uniform laser energy across the beam coming into the lens and the sides of the pellet receive much more total laser energy than the front or back. Thus, this variance in uniformity of irradiance is apt to cause instabilities in the pellet implosion. Accordingly, a third optical component in the form of an aspheric corrector plate is added to each optical channel of FIG. 2 as illustrated in FIG. 5. One side of the system is shown with an aspheric corrector plate 44 and a similar plate would be placed on the other side. This plate is designed to redirect the laser rays to achieve uniform irradiance as illustrated by the diagrammatic ray lines. In addition, the pellet is moved away from the ellipsoidal focus by a fraction of its own diameter and at the same time the two mirrors 30 and 32 are separated by twice this fractional distance to achieve equal illumination patterns on both hemispheres. It will be appreciated that many different types of corrector plates can be designed to optimize various aspects of the complex hydrodynamic system involved with a laser driven fusion reaction. For example, the laser pulse, although normally very short in duration (on the order of 10.sup.-9 seconds), sees a different optical pattern in the vicinity of the pellet at different times during the laser pulse. More specifically, the laser energy early in the pulse ionizes part of the pellet producing a plasma around the pellet. This plasma expands in a cloud around the pellet and alters the optical characteristics seen by the later arriving laser energy. These later rays, although initially directed to a uniform pellet distribution, may be refracted toward other parts of the pellet causing an imbalance in absorbed energy. Thus, the exact design of the corrector plates is directed to optimizing the entire dynamic implosion event. In FIG. 5, a simplified example of a corrector plate design is shown. This design ignores any refraction in the plasma blow-off region and therefore optimizes pellet energy absorption only for the early parts of the laser pulse. The basic approach is a two-step process. First, the pellet is moved forward with respect to the elliptical focus as shown in FIG. 5. It will be appreciated that only one lens mirror combination is shown but in practice, both optical channels would be identical. Moving the pellet forward a distance TM as viewed in FIGS. 5 and 6 increases the energy density at the back where it is very weak as illustrated by the curve in FIG. 4. However, this pellet movement leaves an annular area around the side completely dark. The aspheric corrector plate 44 is, therefore, added as illustrated in FIG. 5. This plate does not alter the paraxial rays significantly, that is, the rays 50 which are on or near the optical axis. However, the plate does redirect the marginal rays 52 toward the sides of the target. To put it another way, in classical optical terms, this plate over-corrects for spherical aberration by directing the marginal rays behind the lens focal point FP. These rays, for a simple spherical lens, tend to converge in front of the paraxial lens focal point FP. This is known as spherical aberration. The exact form of the corrector plate is chosen mathematically to map equal energy areas of the incoming laser beam on to equal areas on the pellet. Ray trace in the vicinity of the pellet as a result of this optimization procedure is shown in FIG. 6. Only one quadrant is shown, but the same ray pattern is produced in all four quadrants of the pellet. It will be seen that the entire pellet is uniformly illuminated and the laser rays strike the target at nearly orthogonal incidence. The degree of aberration in the vicinity of the pellet (FIG. 6) for this case is given by ##EQU1## where u=the longitudinal aberration at the pellet focus, measured from the center of the pellet (see FIG. 6) t=the pellet radius PA1 k=(r/R).sup.2 PA1 r=the radius of a ray from the optical axis in the incoming laser beam PA1 R=the radius of the incoming laser beam PA1 F=effective f-number of the focusing lens PA1 F=(f/2 R) and PA1 f=focal length of the focusing lens Note that a different corrector plate is theoretically required for each pellet size, t. The aberration, u, is mathematically converted into a surface formula for the corrector plate by conventional ray tracing techniques.