Abstract: Magnetic field-generating apparatus for an installation for accelerating electrically charged particles, the particle track of which comprises curved and straight sections, contains main magnetic field generating windings and at least one supplemental winding which is provided for focusing the particles on the particle track. It should be possible to accelerate relatively large particle streams to relatively high energy levels without the need for separate preaccelerators. In the region of at least one of the curved sections of the particle track the supplemental winding is designed as a conductor arrangement forming a quadrupole triplet for focusing the particles during their acceleration phase, the turns of the supplemental winding being arranged on both sides of the plane in which the particle track lies.

Abstract: Disclosed is an electron beam exposure apparatus which includes an objective lens focusing an electron beam, and a dynamic focus correction lens dynamically correcting the focusing by the objective lens. The apparatus comprises a control circuit which, in order to prevent a current variation induced in the objective lens by the dynamic focus correction lens, controls current supplied to the objective lens so as to cancel the current variation induced in the objective lens due to the coil current of the dynamic focus correction lens.

Abstract: An apparatus, such as an electron beam column (1), with high-speed deflection capability comprises a gun (2) which directs a beam (3) of charged particles by several magnetic lenses (9,11,13a,13b) towards a target (4). Magnetic deflection coils (12a,12b) produce a variable magnetic field for controllably deflecting the beam. The beam is surrounded by an electrically conductive surface (16a,16b,22) which in the vicinity of the deflection coils is constituted by a tube (22) comprising a single wire of, for example, nichrome preferably coated by an oxide film formed naturally and comprising a close-wound helix. This tube construction has a longitudinal resistance which significantly impedes the flow of undesirable eddy currents therein. On the other hand the tube can readily conduct along a helical path so that charge accumulation on the tube walls is avoided.

Abstract: A system for scanning a beam of charged-particles across a target is described which compensates for energy dispersion in the beam. A time-varying magnet with circular pole pieces is used to sweep the beam left to right. Two wedge-shaped magnet dipoles, one on each side of the center line are used to bend the beam parallel to the center line and compensate for beam energy dispersion.

Abstract: A scanning lens system for a particle beam scanning device having deflection elements and optical lenses for corpuscular radiation characterized by the deflecting elements being deflecting dipole elements which are disposed in the particle beam directly preceding at least one imaging lens so that the deflection chromatic aberration for arbitrary deflection angles should disappear and the lens field of a magnetic projection lens should be free of magnetic deflection elements. As a result of the arrangement, the isotropic deflection chromatic aberrations of the dynamically deflecting dipole elements compensates the isotropic off-axis chromatic aberrations of at least one imaging lens and at least one imaging lens forms an image rotation-free system.

Abstract: For carrying out electron beam welding, there is formed by means of a magnetic or electric field set up by an even number of at least two magnetic poles or electrodes, an electron beam, the pattern of which has elliptical or similar shaped cross sections. Electron beam welding is performed using such an electron beam. The direction of the major axis of the cross section of the electron beam at the surface of the material being welded and the direction of the major axis of the cross section of the electron beam at the bottom of the beam cavity, i.e., the root portion, are at right angles to each other.

Abstract: Normal or constant angle scanning of a target with high current ion beam is provided by positioning a space charge lens between a beam deflection system and the target. The space charge lens is positioned such that its focal point coincides with the virtual center of deflection. The space charge lens steers the scanned beam into parallel paths so as to maintain a constant angle of incidence upon the workpiece. In an electrostatic deflection system, two sets of y-axis deflection plates can be axially positioned on opposite sides of the x-axis deflection plates to provide a single virtual center of deflection. The apparatus is useful for performing ion implantation with higly uniform dose distribution.

Abstract: A deflecting magnet assembly for scanning an incident beam of charged particles in a scanning electron beam computed tomography scanner. The configuration (including angular orientation and coil end connection geometry) of the coils which comprise the magnet is selected to approximate the field of a pure dipole. In addition, a magnetic shield is used which equalizes the effective radii of the constituent coils and thereby simplifies construction of the assembly.

Abstract: An apparatus is described which makes possible the precise sputter etching and imaging of insulating and other targets, using a finely focused beam of ions produced from a liquid metal ion source. This apparatus produces and controls a submicron beam of ions to precisely sputter etch the target. A beam of electrons directed on the target neutralizes the charge created by the incident ion beam. Imaging of the target surface and ultra-precise control of the etching process is achieved by monitoring the particles that are sputtered from the target surface.

Abstract: In an Auger electron spectrometer for use in analyzing a surface of an object by the use of Auger electrons resulting from impingement of an electron beam, an electron gun comprises an electron beam source for generating the electron beam and an electron lens system for guiding the electron beam towards the surface. The electron lens system comprises a combination of a condenser lens and a permanent magnet member nearer to the surface than the condenser lens. The permanent magnet member may comprise either a plurality of permanent magnet pieces radially and azimuthally spaced apart from one another or a single permanent magnet piece. An ion gun may be disposed in the vicinity of the electron gun to generate an ion beam oblique to the electron beam and substantially concurrent with the electron beam on the surface. Magnetic pole pieces may be attached to each permanent magnet piece and bypassed to control a magnetic field generated by each permanent magnet piece.

Abstract: A machine for the internal butt welding of two tubes comprises a fixed axial electron gun, a rotary prism system for radially deflecting the electron beam and comprising a plurality of prisms and a deflection system for cyclically delivering the electron beam emitted by the electron gun successively to each of the prisms at a high frequency, the relative duration of utilization of each prism being modifiable in the course of the rotation of the rotary system.

Abstract: Ions emitted from an ion source are dispersed by a dispersion means, and those ions of a predetermined mass number are extracted from the thus-dispersed ions and are selectively implanted into a sample. A plane of incidence of ions in the mass-dispersing means is so formed that the angle of incidence of ions thereon has a negative value, and a plane of emission of ions therein is so formed that the angle of emission of ions of the predetermined mass number therefrom has a positive value.

Abstract: An electron microscope having plural stages of focusing lenses between an objective lens and an electron gun. The magnetic pole piece of the focusing lens of the final stage which is on the side of the objective lens takes a conic form which tapers toward the objective lens. The yoke of the objective lens on the side of the focusing lenses is centrally provided with a conic recess. This structure makes it possible to shorten the distance between the position of the magnetic field produced by an auxiliary lens and the front focal point defined by a front objective lens without introducing interference between the focusing lens of the final stage and the object lens proper. Thus, the observation of an electron micrograph covering a wide field of view facilitates accurate analysis of the physical properties of a designated microscopic region on a specimen.

Abstract: An apparatus for deflecting and focusing a beam of neutral particles for operating on extremely small workpieces, such as for doping or inscribing micro-electronic components, has a deflection system for generating at least one magnetic field through which the beam of neutral particles is directed for focusing and shaping of the beam on the workpiece. The lens system may include one or more six-pole or eight-pole lenses.

Abstract: The process of achromatic deflection by substantially uniform, crossed electric and magnetic fields with certain magnitudes. When the magnetic force is opposite to the electric force and twice as large, deflection is independent of particle velocity. An achromatic deflector produced by proper excitation of the poles of an achromatic quadrupole lens by means of suitable power supplies, without additional parts within the lens. An achromatic quadrupole lens having no yoke and only four electrodes, suitable for deflections varying at high frequency.

Abstract: An electron emission system includes a high-brightness field-emitter cathode. Advantageously, the tip of the cathode is shaped to minimize structural variations caused by surface tension forces. In addition, an electrode assembly associated with the cathode is designed to establish electric field forces that are opposite and at least approximately equal to the surface tension forces acting on the tip. The electric field forces can be adjusted to establish a highly stable operating condition without altering the value of electron beam energy for which the overall system was designed. Moreover, the current density of the beam at a writing surface can be selectively varied without changing prescribed operating parameters of the cathode. The resulting system is characterized by excellent emission stability, low noise and a useful operating life of at least several thousand hours.

Abstract: A transmission electron microscope having objective lens comprising upper, middle and lower magnetic pole pieces installed inside a yoke enveloping the first and second excitation coils. The polarity of the first magnetic field between the upper and middle magnetic pole piece is changed according to the selection of the two observation modes for an electron microscope image and for a converged beam diffraction pattern. However, the position of a specimen located at the midst of the second magnetic field between the middle and lower magnetic pole pieces is fixed during said two operation modes, so that analysis for a crystalline material of a very small area in the specimen is carried out by easy operation.

Abstract: An electron beam exposure apparatus having a control apparatus wherein the positioning error caused by transient response characteristics of an electron beam deflection device is detected by using a knife-edge type gauge and a reflecting electron detector. A compensation signal to compensate for the positioning error is produced in a compensation device, and the compensation signal is applied to the electron beam deflection device, whereby the deviation of the position of the electron beam, from the normal position, caused by the transient response characteristics, is reduced.

Abstract: Disclosed is an electromagnetic lens polepiece structure for use in an electron microscope comprising an upper polepiece and an opposite lower polepiece, each having a lens bore, wherein the lens bore portion of the upper polepiece protrudes toward the lens bore of the lower polepiece. The protrudant lens bore portion allows the provision of an X-ray conducting bore for the measurement without disturbing the magnetic field of the lens. This makes it possible a smaller lens bore and, consequently, the spherical aberration is reduced and the resolution of the microscope is improved. The protrudant lens bore structure allows the upper polepiece to be divided into two sections so that the bore section can be replaced easily for the measurement of X-rays.

Abstract: Process and apparatus for varying the deflection of the path of a charged particle beam over a path of length l in a space volume V, where there is a magnetic induction B (x,y,z), wherein a relative displacement of volume V with respect to the beam is produced in such a way as to vary the magnitude .intg.BDl calculated along the path and characterizing the deflection undergone by the beam on passing through volume V. In some of the embodiments rotating magnetic pole pieces are employed to vary the path. Fixed inclined faced pole pieces are also employed.

Abstract: A magnetic rotator lens for rotating the momenta of particles transverse to the direction of the beam is disclosed. The rotator lens includes a solenoid axially aligned with the beam and employs a magnetic permeable flux return structure surrounding the solenoid. The flux return structure includes beam entrance and exit plates with magnetic permeable and charged particle permeable portions forming the beam entrance and exit ports of the magnetic lens. In a typical example, the beam entrance magnetic permeable and beam permeable portion of the pole piece structure is formed by a grid of magnetic permeable members. The magnetic permeable and beam permeable portions of the pole structures terminate the axial magnetic field without introducing any substantial transverse components to the magnetic field which would otherwise produce undesired rotation of the beam shape.

Abstract: The apparatus, which may be an electron beam column, has a lens system for coarse- and fine-focus adjustments of the beam. The coarse-focus is effected by a magnetic lens (10) and the fine-focus by an electric lens (14) in parallel with the magnetic lens. The electric lens may comprise a plurality of electrically conductive cylinders (14a to 14d) inside the magnetic lens. The cylinders are so arranged that, by applying appropriate potentials to them, fine-focus adjustments can be carried out without causing the beam to rotate.

Abstract: A magnetic lens for imaging and rotation of a charged particle beam includes a pair of focus elements axially spaced along a lens bore through which the beam passes and a rotation element positioned axially between the focus elements. Each of the lens elements includes a polepiece having opposing portions surrounding the lens bore and axially spaced apart to define a circumferential gap, and a winding for energizing the polepiece and the gap. The focus elements are energized by equal and opposite magnetic fields which focus the electron beam without substantial beam rotation. The beam can be rotated, without substantial change in focus or magnification, by varying the magnetic field of the rotation element. The lens is particularly suited for use in a shaped electron beam lithography system.

Abstract: A variable axis immersion lens electron beam projection system shifts the electron beam while eliminating rapidly changing fields, eddy currents and stray magnetic fields in the target area. The electron beam projection system includes an electron beam source and a deflection means. A variable axes immersion lens for focusing the electron beam includes an upper pole piece, and a lower pole piece having a non-zero bore section, a zero bore section and an opening therebetween for inserting the target into the lens. The variable axis immersion lens provides an axial magnetic projection field which has zero first derivative in the vicinity of the target area. A magnetic compensation yoke, positioned within the bore of the upper pole piece produces a magnetic compensation field which is proportional to the first derivative of the axial magnetic projection field.

Abstract: An electron gun having a field emission cathode is provided with a focusing magnetic lens. In the vicinity of the cathode, a pole piece is defined by two parts. The orifice of the part closer to the cathode is smaller than that positioned downstream. The air gap between the two parts is small and the filament of the cathode extends up to a distance of from about 1.5 to 3 mm from the median plane of the air gap. The excitation of the coil of the focusing magnetic lens is provided such that the image of the electronic source is substantially positioned at the level of the orifice of an intermediate electrode surrounding at least the magnetic lens.

Abstract: An apparatus for the potential measuring technique has an objective lens for focusing primary electrons onto an object, and also has a spectrometer for the energy selection of secondary electrons to provide an improvement of the quantitative potential measurement on printed conductors of integrated microelectronic components with improved local resolution, higher probe current, and improved potential resolution. The objective lens is a magnetic lens in which the lens field lies largely outside the lens body, and the spectrometer is an electrostatic retarding field spectrometer arranged in the magnetic field of the lens.

Abstract: A segmented permanent ring magnet which is tunable to provide a preselected transverse magnetic field within the magnet interior is disclosed. The permanent magnet is constructed of material which has essentially linear behavior in the second quadrant of the B-H curve. The invention includes a method of tuning a segmented permanent ring magnet and a method of manufacture for the blocks of magnetic material which make up the magnet. The magnets of the present invention are particularly adapted for use as bias magnets in NMR imaging systems.

Abstract: A deflective focusing system includes a magnetic lens for focusing a charged particle beam, a plurality of rings made of magnetic material arranged substantially concentrically with the magnetic lens inside of the magnetic lens, the rings being arranged at spaced apart positions in the direction of the central axis of the magnetic lens so as to form a predetermined magnetic focusing field distribution, a one-stage electrostatic deflector having a plurality of deflection electrodes which are spaced apart in a circumferential direction of the magnetic lens, which are arranged substantially concentrically with the magnetic lens inside of the magnetic lens, and which extend in the direction of the central axis so as to form a predetermined electrostatic deflection field distribution, so that the charged particle beam passes through the concentrically arranged deflection electrodes to be deflected in accordance with a voltage applied to the deflection electrodes, and ring-like grounding electrodes disposed substan

Abstract: In a mass spectrometer, suitable for use in the determination of isotope ratios, having as a mass selector a sector magnet and detector means for detecting and measuring the intensity of ion beams at two or more positions in the focal plane of said sector magnet, the improvement comprises providing the exit (and optionally also the entrance) pole face of said sector magnet with a curvature such that the focal plane of said sector magnet lies substantially at right angles to the ion optical axis as it passes through said focal plane. With this arrangement, motion of detector means between positions in said focal plane by means of mechanical linkages controlled from outside the vacuum system of the mass spectrometer is facilitated.

Abstract: In an electron microscope, having intermediate lenses between the object and the projection lenses, the first and second intermediate lenses are used for rotating the final electron microscope image without changes in magnification. The desired angle signal indicating to azimuth angle .theta. is designated by an operator. The absolute magnetomotive force .vertline.J1.vertline. and .vertline.J2.vertline. of the first and second intermediate lenses are controlled by a lens control means in the relation that (.vertline.J1.vertline.-.vertline.J2.vertline.) is proportional to the azimuth angle .theta. and (.vertline.J1.vertline.+.vertline.J2.vertline.) is nearly proportional to the square of the azimuth angle .theta..

Abstract: A transmission electron microscope comprises an imaging electron lens system constituted in at least five stages by an objective lens, a group of intermediate lenses and a group of projector lenses to which excitation currents are applied with alternately different polarities. Lens current control means is provided for controlling the excitation currents in such a manner that the lenses belonging to the intermediate lens group and the projector lens group, respectively, serve as reduction lens when the microscope is operated in a predetermined range of low magnifications, while all the electron lenses serve as magnifying lenses in a predetermined range of high magnitudes. When the magnification is changed over, the lens current control means regulates the excitation currents in such a manner that the difference between the excitation currents supplied to the intermediate lens group and the projector lens group, respectively, is maintained substantially constant.

Abstract: An apparatus for electron beam irradiation of objects comprises an electron beam shaper 1 providing a ribbon-shaped beam 7 and a deflecting electromagnet 8 with a frame-type magnetic circuit 9 to direct the beam 7 onto an irradiated object 6 substantially at an angle of 90.degree.. The deflecting electromagnet 8 has two poles 10, 11 extending over the width of the irradiated object 6 and two windings 12, 13 embracing the poles 10, 11 and connected to a direct current source 14, the deflecting electromagnet 8 being arranged so that the trajectories of the electrons within the area from the shaper 1 to the deflecting electromagnet 8 are inclined to the frame of its magnetic circuit 9.

Abstract: The apparatus for focus-deflecting a charged particle beam includes two electron lenses for focusing the charged particle beam and a single deflector for deflecting the charged particle beam to a desired position, wherein one of the electron lenses on the object point side and the deflector are arranged so that their magnetic fields overlap each other, and the aberration generated by them offsets that generated by the other of the electron lenses.

Abstract: A composite objective lens for a particle beam lithographic system having deflection coils within the bore of the lens, a solenoidal excitation lens coil, a first set of two or more cylindrical pole pieces with a lens gap and a second set of cylindrical pole pieces arranged concentrically outside the first pole pieces but within the excitation lens coil. The second set of pole pieces is constructed of a high saturation material such as iron with a lens gap coextensive with the gap in the first set of pole pieces. The return flux from the deflection coils is carried by the inner cylindrical pole pieces only, while the flux generated by the lens coil is shared by both the inner and outer pole pieces. With this arrangement the linear relationship between the axial field strength and excitation current is maintained at all points along the axis of the lens, and saturation of the inner pole piece is avoided.

Abstract: An electron lens equipped with a ferromagnetic body disposed in at least one of the pole gaps of the electron lens which serves as a switchable bypass for magnetic flux. The said ferromagnetic body is magnetically saturated under strong excitation, thereby permitting switching between the double-gap lens and the single-gap lens to be effected by changing the excitation current.

Abstract: The specification describes a scanning electron microscope or similar equipment having a magnetic pole piece detachably provided in an opening of a lower pole of its electromagnetic objective lens so as to decrease the magnetic flux leaked through the opening when the specimen is placed below the lower pole. The above scanning electron microscope or similar equipment may also include a change-over system for varying the crossing point between a scanning charged particle beam and an optical axis depending whether the specimen is placed in the vicinity of the lens center of the objective lens or below the lower pole. Accordingly, the specimen can be placed either below the lower pole of the objective lens or in the vicinity of the lens center as the operator desires, thereby making the scanning electron microscope or similar equipment applicable for varied purposes.

Abstract: A method for producing various configurations of permanent magnet quadrupoles so that there is no coupling in the two transverse directions when focusing a charged particle beam is provided. Each configuration comprises a plurality of rotatable quadrupole disks, and means for rotating the quadrupole disks with respect to each other in a predetermined relationship.

Abstract: A liquid metal field ionization source is used in conjunction with a three element asymmetric lens system to provide an ion gun having greater magnitude beam currents focused on a smaller spot size than has been previously possible for intermediate energy beams. An ultra-clean emitter surface is achieved by means of an argon sputtering and/or plasma etching process applied to the emitter surface before liquid metal is applied to the emitter surface to ensure uniform and sufficient flow of liquid metal to the apex of the emitter. The three element asymmetric lens system has a very low chromatic aberration coefficient, enabling precise focusing of beams with large energy spreads. For beam accelerating ratios in the range from 0.2 to 6, the ion gun produces very high current densities in beams focused on very small spot areas, despite the relatively large energy spread of beams produced by liquid metal ionization sources.

Abstract: A first order achromatic magnetic deflection system for use in conjunction with a charged particle accelerator, is realized from a stepped gap manget wherein a charged particle propagated through the system is subject to at least two adjacent homogenous magnetic fields in traversing one-half of a symmetric trajectory through the system.

Abstract: In a scanning electron microscope, an objective lens has upper, lower and outer magnetic pole pieces. The bore diameter of the lower magnetic pole piece is smaller than those of said upper and outer magnetic pole pieces. The lens field between said upper and lower magnetic pole pieces is generated for observing a bulk specimen positioned at a long working distance. The lens field between said lower and outer magnetic pole pieces is generated for obtaining a high resolution image of a specimen.

Abstract: A control apparatus according to this invention is installed between an intensity specifying circuit and an excitation current source for a magnetic lens or deflecting coil. The intensity specifying circuit generates a target signal (B) corresponding to the desired magnetic flux density (B) in a magnetic lens or deflecting coil. The excitation current source outputs a current (I) which produces the desired flux density (B). Due to the effects of hysteresis the ferromagnetic yoke adjacent the lens or deflecting coil, the relationship between the target signal (B) and current (I) is nonlinear. The control apparatus has a memory in which data defining the hysteresis characteristics of the lens or coil being controlled has been stored.

Abstract: Means for compensating for third and higher order aberrations in an electron optic system comprises two sextupoles and a field lens, the latter being spaced intermediate the sextupoles. The resolving power of the optic system can be reduced to the range of about 0.5 Angstrom unit.

Abstract: An electron lens equipped with three magnetic pole pieces defining two gaps forms two magnetic fields. The magnetic fields are generated in said gaps by opposite and same strength excitation. S1/S2 ratio is in the range of 2.7 to 3.8, in which S1 represents the first gap length between the upper and middle magnetic pole pieces and S2 represents the second gap between the middle and lower magnetic pole pieces, so that radial (isotropic) distortion is eliminated and spiral (anisotropic) distortion is extremely reduced.

Abstract: A system for achromatically deflecting a beam of charged particles without producing net divergence of the beam comprises three successive magnetic deflection means (A,B,C) which deflect the beam alternately in opposite directions, the first (A) and second (B) by angles of less than 50.degree. and the third (C) by an angle of at least 90.degree.; particles with different respective energies are transversely spaced as they enter the third deflection means (C), but emerge completely superimposed in both position and direction, and may be brought to a focus (F) in each of two mutually perpendicular planes a short distance thereafter. Such a system may be particularly compact, especially in the direction in which the beam leaves the system, and is suitable for deflecting a beam of electrons from a linear accelerator (5,6,7) so as to produce a vertical beam of electrons (or with an X-ray target, of X-rays) which can be rotated about a horizontal patient for radiation therapy.

Abstract: An electron lens system wherein at least two coils are arranged in the vicinity of an electron-optical lens and are excited in directions opposite to each other, thereby making it possible to adjust the focal distance of the lens without including a rotation attributed to the electron-optical lens in an electron beam which passes through the lens.

Abstract: Deflection of an electron beam from axis 28 is controlled by electromagnetic deflection saddle coils 24, 26. Interference with deflection by surrounding mass of lens 22 is overcome by compensation coils 42, 44. The deflection coils and compensation coils are shaped and sized so that their own conductive properties do not distort the deflection magnetic fields. This is accomplished by equalizing between the deflection and compensation coils, the change in magnetic field during coil current penetration and by equalizing the penetration time constants for both sets of windings.

Abstract: The relation between the focusing distance (D) of the objective lens and the excitation current (I) of the objective lens is stored in a data memory. The initial focusing distance (D.sub.o) corresponding to the optimum excitation current is determined as a result of the focusing adjustment operation. Thereafter, the change (.DELTA.D) in the focusing distance due to the specimen shift along the optical axis of the objective lens is detected. The magnitude of the excitation current corresponding to the focusing distance (D.sub.o +.alpha.D) is read from said memory and a current of that magnitude is supplied to the objective lens.

Abstract: A microwave plasma ion source according to the present invention is designed such that a microwave electric field and a magnetic field are applied to a discharge gas introduced into a discharge region, to form plasma, from which ions are extracted. The above magnetic field is formed by means of an electromagnet provided on the low-voltage side of ion extraction electrodes and a high-permeability member provided in that section which is on the side of a waveguide and which permits the microwaves to be propagated freely.

Abstract: A high current electron source with a narrow energy band of the type employed in an electron beam printer has a beam generating system consisting of a cathode and focusing electrodes and an anode with at least one electrode functioning to astigmatically focus at least one electron crossover in the low velocity range. For varying the position and shape of the crossover in a simple manner for adaptation to particular use conditions of the high current electron source, the Wehnelt electrode of the electron source consists of one or more cylinder lenses or of an electrostatic multipole element, or an electromagnetic multipole element is disposed in the plane of a conventional non-ferromagnetic Wehnelt electrode with a circular aperture for the electron beam. By pole reversal of the Wehnelt voltage or of the coil current and by changing the size of the current, the shape and azimuth position of the first crossover in the cathode space can be varied to any configuration.

Abstract: The beam deflection system includes two dipole magnets for deflecting a charged particle beam in a deflection plane. The first magnet bends the beam in a path having a radius .rho..sub.1 and a deflection angle .theta..sub.1 greater than 180.degree.. The first magnet has an effective exit edge at an angle (90.degree.-.eta..sub.1) with respect to the beam path. The second magnet further bends the beam in a path having a radius .rho..sub.2 and a deflection angle .theta..sub.2 where 225.degree..ltoreq.(.theta..sub.1 +.theta..sub.2).ltoreq.280.degree.. The effective entry edge of the second magnet is at an angle of (90.degree.-.eta..sub.2) to the beam path. In the usual practical case the effective interior edges of the two magnets are roughly parallel, that is, .eta..sub.1 .perspectiveto.-.eta..sub.2 and the doubly achromatic properties are obtained to first order when the angles satisfy the following formula:.eta..sub.1 .apprxeq.-.eta..sub.22.eta..sub.2 .apprxeq..theta..sub.2 -(.theta..sub.1 -180.degree.