RU2063108C1 - Multiple-pole magnetic lens - Google Patents

Abstract

FIELD: electric engineering. SUBSTANCE: device has poles, windings and chokes of magnetic circuit, which provide two dipole flat parallel magnets. Middle planes of inter-pole spaces coincide with lens focal plane. Projection of poles of magnets to focal plane provide shape of sectors which vertex is directed towards symmetry axis of lens and are displaced with respect to it. Chokes of magnetic circuit may be shared by both magnets and be bonded by non-magnetic pins. EFFECT: non-linear focusing of beams of charged particles. 2 cl, 4 dwg

Description

 The invention relates to electrical engineering, to the device of magnetic lenses used for nonlinear focusing of charged particle beams. Such focusing, in particular, is used along with the magnetic beam sweep method to ensure uniform irradiation of the target with charged particles. High uniformity of irradiation is required both in some scientific experiments and in a number of practical applications of charged particle beams (radiation therapy, sterilization, material processing, etc.) For those applications in which the irradiated object (target) moves perpendicular to the axis of the beam, for example, with using a conveyor, sufficiently uniform exposure in only one plane.

 Multipolar magnetic lenses with six, eight and a large number of poles are known for the above purpose. Such lenses form a magnetic field, the induction of which increases nonlinearly with distance from the axis of the lens.

 Most often, for nonlinear focusing, which aims to equalize the distribution of the beam density on the target, eight-pole lenses (octupoles) are used (see E. Kashy and B. Sherrill. Nuclear Instruments and Methods in Physics Research B26 (1987) 610-613; B. Sherrill , J. Bailey, E. Kashy and C. Leakeas. Nuclear Instruments and Methods in Physics Research B 40/41 (1989) 1004-1007) In an eight-pole lens, the magnetic field induction grows cubically according to the distance from the axis of the lens and when passing through the axis changes direction. After the beam of charged particles passes through the octupole, the dependence of the particle density in the beam on the distance to the beam axis, combined with the axis of the lens, changes significantly, since a stronger magnetic field acts on the particles remote from the axis. This allows the target to obtain a more uniform distribution of particle density in that direction, which coincides with the plane of nonlinear focusing of the lens.

 The disadvantage of an eight-pole lens is the complexity of its design, large size and weight, as well as a large power supply. The eight-pole lens contains eight magnetic poles, eight field windings covering the poles, and eight connecting poles with the core of the magnetic circuit. At the same time, only a small part of the volume of the magnetic field excited by the lens, which includes only one of the four planes of symmetry of the lens, is used for focusing.

 The aim of the invention is to remedy these disadvantages, simplifying the design of the lens, reducing its size and weight and reducing power supply.

в котором α угол сектора (радиан),
Е-полная энергия частиц (электронвольт),

приведенная скорость частиц (с скорость света),
В индукция поля в зазоре (гаусс),
L расстояние от линзы до мишени (см).This goal is achieved by the fact that the poles, field windings and the yoke of the lens are arranged in such a way that they form at least two dipole plane-parallel magnets, mounted in such a way that the middle planes of the pole clearances of the magnets are aligned with the focusing plane, the windings of one dipole magnet are turned on in opposite directions the windings of the other, the poles in the projection onto the focus plane have the shape of sectors, the vertices of which are directed towards the axis of the lens and offset relative to this axis, and the angle at the apex of the sector selected from the relation

in which α is the angle of the sector (radian),
E-total particle energy (electron volt),

reduced speed of particles (with speed of light),
In the field induction in the gap (gauss),
L is the distance from the lens to the target (cm).

For example, for an electron beam with E 10 ⁷ eV at B 1000 G and L 100 cm, we obtain α≈0.59 rad ≈ 34 ^° .

 This goal is also achieved by the fact that dipole magnets have common magnets for both magnets pulled together by non-magnetic yoke pins connecting the poles with the opposite windings.

 In FIG. 1 and 2 are schematic drawings of the lens in two proposed design options. Fig. 3 is a diagram explaining the effect of the lens on the trajectories of charged particles of the beam, and Fig. 4 shows the distribution of particle density of the beam in front of the lens and on the target irradiated by the beam.

в межполюсном зазоре.The lens (figure 1) is composed of two identical dipole magnets, each consisting of a pair of plane-parallel poles 1, a pair of field windings 2 and one yoke of the magnetic circuit 3. Pole 1 in the projection onto the middle plane A-A 'of the interpolar gap h, combined with the focus plane lenses have the shape of a sector with an angle at the apex α. The top of the sector is offset relative to the axis of the lens OO 'by a distance d. The excitation windings 2 of one magnet are turned on in the opposite direction to the windings of the other, which is indicated in FIG. 1 by arrows indicating the direction of the magnetic field induction vector

in the pole gap.

 Another embodiment of the lens design is shown schematically in FIG. 2. The difference from the embodiment of FIG. 1 is that the yoke 3 of the magnetic circuit connects the poles with the on-off windings and is pulled together by non-magnetic studs 4. The remaining lens components in FIG. 2 and the designations on it are the same as in FIG. 1.

 The operation of the lens is illustrated in FIGS. 3 and 4. FIG. 3 schematically shows in a projection onto the focus plane the areas of action of the magnetic field bounded by the sector poles 1 of the lens and several trajectories of charged particles of the beam that is focused by the lens and sent to target 5. For clarity 3 and 4 show a beam of particles whose trajectories at the entrance to the lens are parallel to its axis. Particles passing near the axis of the lens (trajectories 6 and 6 ') are not affected by the magnetic field of the lens, the trajectories remain parallel to the axis. The trajectories 7 and 7 ', 8 and 8', the removal of which from the axis of the lens exceeds the amount of displacement d of the vertex of the sector poles of the lens, are deflected by its magnetic field. This deviation is the stronger, the farther the trajectory is from the axis, since with increasing distance from the axis the width of the magnetic field increases due to an increase in the width of the sector pole. The magnitude of the angle α of the sector α and the induction of the field B are selected so that the extreme paths of the beam 8 and 8 'after passing through the lens hit the target 5 at a point lying on the axis O-O'. Intermediate trajectories 7 and 7 'after deflection by the lens will come to the target with a smaller deviation towards the axis.

.Such a change in the shape of the trajectories of charged particles by the lens leads to a significant change in the distribution of particle density on the target compared with the distribution of their density in front of the lens. Figure 4 schematically shows the density change for the case when the parallel particle beam has at the entrance to the lens an initial distribution 9, close in shape to a Gaussian, of the form
ρ (x) = ρ _o exp (-x ² / x $\genfrac{}{}{0ex}{}{\text{2}}{\text{o}}$ ), (2)
and limited in x coordinate to x 2x _o . At the point x x _{o, the} density is equal to half the density on the axis of the beam

.

In order to obtain a nearly uniform distribution of particle density on target 5, the displacement of the top of the sector pole is chosen to be d x _o , and other lens parameters are determined from the condition that the extreme trajectory starting at x 2 x _o hits the intersection of the surface of target 5 with the axis of the lens . Calculations show that the fulfillment of this condition corresponds to the above relation (1), which relates the parameters of the lens and the beam. In this case, the density distribution 10 on the target is obtained by adding the undistorted central part (region 0≅х≅х _o ) of the distribution 11 at the entrance to the lens and the inverted “tail” part of this distribution 12 (region х _o ≅х≅2х _o see FIG. 4). As a result, the resulting irregularity of the target irradiation does not exceed ± 5%, as calculated, and the width of the irradiation field in the focusing plane is 2d.

 Numerical computer calculations performed for a beam with particle paths diverging from the axis showed that in this case the proposed lens provides the same irregularity of the target irradiation. However, with a diverging beam, the width of the irradiation field increases significantly; this width can be several times greater than the distance 2d between the vertices of the sector poles of the lens. Using the invention will allow at least halving the size, weight and power supply of the lens in comparison with the octupole. As a result, it will become expedient to replace widely used magnetic beam sweep systems, requiring powerful sawtooth current generators for their power supply, with a more economical and reliable direct current magnetic system based on the proposed four-pole lens with sector poles. YYY2

Claims (2)

 1. A multi-pole magnetic lens including poles covered by field windings and a yoke of a magnetic circuit connecting the poles, characterized in that the poles, windings and yokes form two dipole plane-parallel magnets mounted so that the middle planes of the inter-pole gap of the magnets are aligned with the focusing plane of the lens, the windings of one dipole magnet are turned on in the opposite direction to the windings of another magnet, and the poles in the projection onto the focus plane are in the form of sectors whose vertices face the axis of symmetry PS and offset relative thereto.  2. The lens according to claim 1, characterized in that the yokes of the magnetic circuit are made common by the day of both magnets and are pulled together by additionally inserted non-magnetic studs.

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