Patent Number: 044255063
Section: summary

FIELD OF THE INVENTION The present invention is in the general area of charged particle beam optics and transport and particularly relates to achromatic beam deflection especially suitable for use in radiation treatment apparatus. BACKGROUND OF THE INVENTION Achromatic optical elements are essential in commercial and medical therapeutic irradiation systems because the primary attribute for such operations is the relatively high beam intensity and control thereof. A typical high beam current accelerator, such as the microwave linear accelerator, achieves the required beam intensities but the energy distribution is rather wide. In order to utilize the available beam it is therefore necessary to introduce optical lements which are relatively insensitive to the energy distribution of the beam. In particular it is desirable for x-ray apparatus to concentrate an intense beam onto a small beam spot on the x-ray target to obtain an x-ray source sufficiently small in relationship to the targeted irradiation region. Beam deflection systems in commercial irradiation and medical therapy applications are ordinarily subject to mechanical and geometrical constraints incident to the maneuverability of the apparatus, shielding and collimation of irradiation flux and as well as economic considerations in the construction of such apparatus. One achromatic beam deflection system of the prior art is described in U.S. Pat. No. 3,867,635 commonly assigned with the present invention. In this apparatus the beam traverses three uniform field sector magnets and two intermediate drift spaces, undergoing a 270.degree. deflection for incidence upon the x-ray target. The sector magnet poles are precisely specified in regard to the sector angles. The angles of incidence and egress of the beam with respect to each sector and a shunt of complex shape occupies the intermediate spaces as well as the entrance and exit regions of the deflector to assure required field free drift spaces. The mutual internal alignment of all components of the deflector is essential to achieve the performance of this prior art device as well as is the alignment of the assembled deflector with the accelerator beam. Another prior art system is known from U.S. Pat. No. 3,379,911 wherein 270.degree. deflection is accomplished in a uniform field to which there is introduced in the vicinity of the deflection midpoint (135.degree.) a gradient region, such that the magnetic field in this gradient region increases radially in the plane of deflection toward the outer portion of accepted trajectories. Thus, those trajectories characterized by a large radius of curvature (in the absence of a gradient) are subject to a somewhat more intense field than would be the trajectories for smaller radii of curvature. Proper adjustment of the gradient shim yields first order achromatic deflection through the desired angle. It is desirable in all of the described systems for the deflector to introduce no substantial momentum dispersion of the beam and to produce at the exit plane a faithful reproduction of conditions encountered at the entrance plane of the system. SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an especially simple first order achromatic deflection system in a charged particle irradiation apparatus. In one feature of the invention, a deflection magnet comprises a first uniform field region separated from a second uniform field region along a boundary, whereby particle trajectories traversing said first region are characterized by a large radius of curvature in said first region, a smaller radius of curvature in said second region, thence again traversing said first region with said large radius of curvature. In another another feature of the invention the ratio of fields in said first and second regions is a constant and is realized by first (wide) and second (narrow) gaps between stepped pole faces. In still another feature of the invention the boundary between said first and second regions is a straight line. In yet another feature of the invention, energy selection slits are disposed in the relatively narrow gap of said second field region whereby radiation from said slits is more effectively shielded by a greater mass of said magnetic pole-pieces in said second (narrow gap) field region. In still another feature of the invention, precise bending plane alignment of the deflection magnet with the axis of a particle accelerator is accomplished by a rotation of the magnet about an axis through the bending plane thereof without need for internal alignment of components of said magnet. In again another feature of the invention the magnitude of displacement of trajectories from the central orbit at the image plane of the magnet is equal to the displacement of the trajectory from the central orbit at the entrance plane of the magnet, whereby parallel rays at the entrance plane are rendered parallel at the exit plane. Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in conjunction with the accompanying drawings. In still yet another feature of the invention, a single quadrupole element is employed to cause a radial waist and a transverse waist in an achromatic charged particle beam deflection system to occur at a common target plane.