EP0368489A1 - Noyau pour une lentille magnétique multipolaire - Google Patents

Noyau pour une lentille magnétique multipolaire Download PDF

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Publication number
EP0368489A1
EP0368489A1 EP89310664A EP89310664A EP0368489A1 EP 0368489 A1 EP0368489 A1 EP 0368489A1 EP 89310664 A EP89310664 A EP 89310664A EP 89310664 A EP89310664 A EP 89310664A EP 0368489 A1 EP0368489 A1 EP 0368489A1
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EP
European Patent Office
Prior art keywords
core
poles
magnetic
lens
multipole lens
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP89310664A
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German (de)
English (en)
Inventor
Frank Watt
Geoffrey William Grime
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Individual
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Individual
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Publication date
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Publication of EP0368489A1 publication Critical patent/EP0368489A1/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
    • H05H7/04—Magnet systems, e.g. undulators, wigglers; Energisation thereof
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/08—Deviation, concentration or focusing of the beam by electric or magnetic means
    • G21K1/093—Deviation, concentration or focusing of the beam by electric or magnetic means by magnetic means

Definitions

  • This invention relates to a core for a magnetic multipole lens and more particularly to a quadrupole lens used to focus charged particles, for instance in a proton microprobe.
  • microprobe which is a highly versatile tool used in analytical science to provide elemental analysis of samples and information on the spatial distribution and concentration of elements within the sample.
  • the first working microprobe system was developed in the early 1970s and since then microprobes have been used as an analytical tool in a wide range of applications including the biological sciences, medicine, the earth sciences, metallurgy and industry, solid state physics and electronics, archaeology and many more.
  • a wide range of analysis techniques are used with a nuclear microprobe.
  • the principle techniques are: particle-induced X-ray emission (PIXE), nuclear reaction analysis (NRA), Rutherford back-scattering (RBS) and elastic recoil detection analysis (ERDA).
  • PIXE particle-induced X-ray emission
  • NAA nuclear reaction analysis
  • RBS Rutherford back-scattering
  • ERDA elastic recoil detection analysis
  • a magnetic lens The basis of a magnetic lens is that charged particles (eg. electrons, ions etc) can be focused using a magnetic field with the form:
  • Such a field distribution can be generated by four magnetic poles arranged symetrically about the z-axis and excited alternately North, South, North, South. This forms a quadrupole lens.
  • this lens is to converge the beam in one plane and diverge it in the orthogonal plane.
  • two or more lenses of alternating polarity are required to give convergent focusing in both planes simultaneously.
  • the magnetic lens For use in a microprobe, the magnetic lens should focus the beam of particles into as small an area as possible whilst maintaining sufficient current (eg. several tens of picoamperes) through that area for the various analytical techniques which are used.
  • the spot diameter produced by such lenses had been reduced to 1um diameter with a current of 80pA of 4MeV protons.
  • the applicants set an improved standard with a conventional quadropole lens with 90% of a 25pA beam estimated to be focussed within a 0.5um sq.
  • Electrostatic lenses are considered to be impracticable as fields in the MV/m region would be required.
  • the type of magnetic solenoid lens used in an electron probe has also been considered but it has been calculated that a 1MeV proton probe would need 230 times the field used in a 30KeV electron probe. Fields of this order cannot yet be produced although it is possible using superconducting coils to generate fields of several Tesla. Lenses using superconducting coils have been made and although initial results have not been totally successful, the technique is still being developed.
  • a plasma lens consisting of ionised atoms and free electrons confined into a cylindrical shape using a suitable set of magnetic fields has also been constructed and development of this is continuing.
  • the most successful focusing system to date has been the magnetic quadrupole lens and a cross-sectional view of a conventional quadrupole lens is shown in Figure 1.
  • the form of the magnetic field produced is determined by the shape and position of the pole pieces 1 and the yoke 3 forms the return path for the magnetic circuit as well as the mechanical support for the pole pieces 1.
  • the magnetic circuit is energised by the exciting coils 2.
  • Figure 1 shows the magnetic field produced in the aperture of the lens.
  • the effect of a single quadrupole lens is to converge a beam in one place and diverge it in the orthogonal place so forming a line focus.
  • This is illustrated in Figure 2.
  • Two or more lenses are thus required to focus a beam to a spot and this is typically performed using a doublet or triplet of quadrupole lenses.
  • a number of aberrations limit the spot size to which a beam can be focussed by a quadrupole lens.
  • the dominant intrinsic and parasitic aberrations are: astigmatism due to the differing focusing strengths in orthogonal planes, chromatic aberration due to the energy spread of particles within the beam to be focused, spherical aberration which arises as a result of the slightly different forces experienced by particles travelling at an angle to the axis of the lens, aberrations due to misalignment of the quadrupoles relative to the optical axis of the system and aberrations due to the quadrupole construction.
  • the magnetic (or electrostatic) field lines would be perfect hyperbolae generated by hyperbolic pole faces arranged with true four-fold symmetry. In practice, however, it is not possible to construct these ideal profiles.
  • the pole faces must be truncated at some point to allow space for the coils, while for ease of machining the pole faces may be made up from cylindrical or flat surfaces.
  • constructional tolerances can lead to errors in the pole profile and deviation from symmetry due to imprecise pole positioning.
  • the distortion of the field may be treated mathematically as a superposition of higher-order field harmonics on the basic quadrupole field.
  • an octupole component may be envisaged as having been produced by eight alternating poles arranged symmetrically about the axis.
  • the quality of imaging depends on the quadrupole field purity, expecially for nuclear microprobe applications where it is crucial to minimise the higher order multipole contamination.
  • the quadrupole lenses must be constructed so that the stringent requirements of a pure quadrupole field which can be precisely aligned are achieved.
  • the task of designing a quadrupole can conveniently be approached in three stages: (i) determining the basic dimensions of the lens to achieve the required focusing performance, (ii) determining the correct pole shape to produce a field with the required quality and mounting the poles and coils inside the yoke with the necessary degree of symmetry, and (iii) mounting the complete lens on a rigid base which permits precise alignment. These three stages will be considered in turn.
  • the basic dimensions must first be determined; in particular, the length and the radius of the aperture are important, since these will affect all subsequent operations.
  • the effective length and the focusing strength of the lenses are determined by the space available for the instrument, the desired imaging properties and the target distance.
  • the bore radius of the lens does not affect the imaging directly, and so may be chosen from a range of values. The following factors affect the choice of radius:
  • the next stage of the design is to select the shape of the pole face.
  • the ideal pole profile would be hyperbolic, with the pole extending to infinity. Obviously this is not possible, and the poles must be truncated to allow space for the coils to be mounted. In addition, the precise machining of hyperbolic surfaces is difficult even with numerically controlled equipment. The penalty of departing from the ideal profile is the introduction of higher-order multipole harmonic contamination, as discussed above.
  • the pole profile only affects the 12- and 20-pole components, of the field. Departures from symmetry, however, introduce lower-order components which can have a large effect on the imaging, hence rather than absolute accuracy in the in the production of the pole profiles, more emphasis should be placed upon ensuring that all four poles are indentical and are mounted with precise four-fold symmetry.
  • the shape of the yoke is relatively unimportant, since it simply provides a return path for the magnetic flux. It also provides mechanical support for the poles and coils, so it should be designed with this requirement in mind.
  • the use of a cylindrical yoke simplifies the rotational adjustment of the lens, and the yoke should have sufficient rigidity to avoid distortion when it is resting on it mounting.
  • the material of the yoke and poles should be high-quality magnet iron selected for high saturation fields and high permeability.
  • Typical lenses use SKF 'Remko' steel for the poles and 'Maximag' for the yoke, and some quadrupoles are made from 'Vacoflux' magnet steel. It is important that the material should be homogeneous, since variations in permeability of saturation field could also cause localised fluctuations in the quadrupole field.
  • the number of ampere-turns per coil can be calculated from a knowledge of the radius and the required strengh; this may be achieved either with few turns and high current or many turns and low current.
  • the use of a few turns of thick wire is preferred, since the turns can be laid individually by hand to ensure a consistent winding profile.
  • Typical coils each have 19 layers of 27 turns of 1 x 2 mm2 cross section wire, while coils of 3250 turns have also been used.
  • the assembly of the various components of the lens takes place from the centre outwards, so that the pole position is firmly established before the yoke is fitted.
  • the position of the pole tips are extablished be means of precision, jig-bored, phosphor-bronze rings before the heels of the poles are ground to match the inside surface of the yoke.
  • the poles may be located by means of precision spacers rather than by rigid fastenings. This method avoids some of the stresses which may result from a rigid assembly.
  • the final stage of construction is to make a support stage for the lens which will allow the orientation to be set to the required degree of accuracy.
  • the most critical adjustment is the rotation about the axis, so one approach is to mount the cylindrical quadrupole on roller bearings on a sub-table.
  • the quadrupole can then be rotated by means of a micrometer driving a spring-loaded lug on the yoke, while the sub-table can be mounted on three adjustable ball-bearing feet to allow vertical and horizontal adjustment of translation and tilt.
  • This system has been used with some success, although it is obvious that there is considerable scope for mechanical variation in the design of quadrupole mounting systems.
  • the present invention represents a radical departure from such developments and avoids or reduces many of the problems discussed above.
  • the present invention provides a core for a magnetic multipole lens, the core comprising at least four poles between which a magnetic field is to be produced and a yoke connecting the poles together and providing a return path for the magnetic circuit within the core, the core being formed from a single piece of magnetic material so that the poles and yolk are integral with each other.
  • the core 5 shown in Figure 3 is made from a single piece of magnetic material such as SKF 'Remko' high quality magnet iron made by Uddeholm Strip Steel AG of Sweden. As the core is made from a single piece of iron, errors due to the assembly of individual components are eliminated so deviations from exact symmetry are reduced.
  • the magnetic circuit is continous (variable magnetic flux leakage caused by air gaps or machining at joints can cause asymmetry in the magnetic field).
  • the core is formed by cutting it from a billet of magnetic material by a computer numerically controlled (CNC) wire spark erosion technique using an Agiecut wire cutting machine manufactured by Agie of Switzerland.
  • CNC computer numerically controlled
  • a wire is held adjacent the workpiece, immersed in a liquid such as water, and a voltage is applied to the wire causing sparking along its length between itself and the workpiece. This sparking erodes the workpiece and the wire is continually fed along its length.
  • the position of the wire and hence the shape of the profile cut from the workpiece is determined by movement of guides positioning the wire under the control of a computer.
  • the cutting can be performed within tolerances of less than 5 microns and complex shapes can be cut by appropriate movement of the wire.
  • the core is formed by the following steps:
  • Figure 4A shows U-shaped laminations 6 and bridging pieces 7 made from copper strips.
  • the bridging pieces 7 are soldered between the end of one lamination and the opposite end of the adjacent lamination as indicated by the arrows in the Figure.
  • Glass fibre spacers 8 are positioned between the laminations 6 to insulate them from each other. It will be appreciated that a stack of such laminations 6 and spacers 8, together with the connecting pieces 7, form a continious coil as shown in Figure 4B.
  • a coil consisting of 12 such turns of copper strip is able to carry a current of 50-80A.
  • the wire erosion cutting technique allows the core to be much more accurately formed than has been possible with conventional machining techniques.
  • the accuracy of cutting is also easier to reproduce so several cores can be cut to a similar degree of accuracy.
  • the wire spark erosion cutting technique also allows complex shapes to be accurately cut within confined spaces so it is possible to form the poles with hyperbolic faces which are very difficult to form by conventional machining techniques.
  • Figure 5 shows a perspective view of a triplet of quadrupole lenses each having one-piece cores 5 and coils formed from copper laminations 6 as described above.
  • Each lens is mounted in a stage 9 which allows precise adjustment of its position by horizontal and vertical translation and tilt (with a 5 micron accuracy) and axial rotation (within a 50 micro-radian accuracy) to permit the lenses to be aligned with each other.
  • the stages are of conventional design and so will not be described further.
  • the lenses are operated by conventional, matched power supplies.
  • Multipole lenses are also used as correctors for various aberrations in ion or electron optical systems.
  • This field is generated by 2n alternating poles arranged symmetrically about the axis.
  • Magnetic quadrupole and multiple lenses are also used in beam transport systems for high energy ion beams for focusing ion beams to very small diameters ( ⁇ 1 micron) for use in nuclear probes.
  • the one-piece core and magnetic multipole lenses using such a core may also be used in these applications.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Optics & Photonics (AREA)
  • Plasma & Fusion (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Particle Accelerators (AREA)
EP89310664A 1988-11-08 1989-10-17 Noyau pour une lentille magnétique multipolaire Withdrawn EP0368489A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB888826162A GB8826162D0 (en) 1988-11-08 1988-11-08 Core for magnetic multipole lens
GB8826162 1988-11-08

Publications (1)

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EP0368489A1 true EP0368489A1 (fr) 1990-05-16

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GB (1) GB8826162D0 (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2691602A1 (fr) * 1992-05-22 1993-11-26 Cgr Mev Accélérateur linéaire de protons à focalisation améliorée et impédance shunt élevée.
CN104681230A (zh) * 2014-12-16 2015-06-03 中国原子能科学研究院 一种加速器用束流均匀化六极磁铁
CN104703378A (zh) * 2015-03-17 2015-06-10 中国原子能科学研究院 一种永磁束流均匀化六极磁铁
CN108735328A (zh) * 2018-07-28 2018-11-02 中国原子能科学研究院 质子束流线上四极透镜的安装准直装置及安装准直方法
CN114388219A (zh) * 2022-01-21 2022-04-22 北京高能锐新科技有限责任公司 用于正负电子对撞机增强器的无铁芯二极磁铁

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3482136A (en) * 1966-04-13 1969-12-02 High Voltage Engineering Corp Charged particle beam spreader system including three in-line quadrapole magnetic lenses
DE1907342A1 (de) * 1969-02-14 1970-09-03 Nagel Wilhelm Friedrich Permanent magnetischer Quadrupol-Kombinationsmagnet
LU46306A1 (fr) * 1963-07-15 1972-01-01 Acec Lentille magnétique multipolaire et son procédé de fabrication

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
LU46306A1 (fr) * 1963-07-15 1972-01-01 Acec Lentille magnétique multipolaire et son procédé de fabrication
US3482136A (en) * 1966-04-13 1969-12-02 High Voltage Engineering Corp Charged particle beam spreader system including three in-line quadrapole magnetic lenses
DE1907342A1 (de) * 1969-02-14 1970-09-03 Nagel Wilhelm Friedrich Permanent magnetischer Quadrupol-Kombinationsmagnet

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
"Principles & applications of high-energy ion microbeams", 1987, pages 122-124, Ed. E. WATT and G.W. GRIME; IOP Publishing, Bristol, GB *
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH, vol. B40/41, part I, 2nd April 1989, pages 669-674, Elsevier Science Publishes B.V., Amsterdam, NL; D.N. JAMIESON et al.: "The new Oxford scanning proton microprobe analytical facility" *

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2691602A1 (fr) * 1992-05-22 1993-11-26 Cgr Mev Accélérateur linéaire de protons à focalisation améliorée et impédance shunt élevée.
CN104681230A (zh) * 2014-12-16 2015-06-03 中国原子能科学研究院 一种加速器用束流均匀化六极磁铁
CN104681230B (zh) * 2014-12-16 2017-03-29 中国原子能科学研究院 一种加速器用束流均匀化六极磁铁
CN104703378A (zh) * 2015-03-17 2015-06-10 中国原子能科学研究院 一种永磁束流均匀化六极磁铁
CN108735328A (zh) * 2018-07-28 2018-11-02 中国原子能科学研究院 质子束流线上四极透镜的安装准直装置及安装准直方法
CN108735328B (zh) * 2018-07-28 2023-10-24 中国原子能科学研究院 质子束流线上四极透镜的安装准直装置及安装准直方法
CN114388219A (zh) * 2022-01-21 2022-04-22 北京高能锐新科技有限责任公司 用于正负电子对撞机增强器的无铁芯二极磁铁
CN114388219B (zh) * 2022-01-21 2022-09-09 北京高能锐新科技有限责任公司 用于正负电子对撞机增强器的无铁芯二极磁铁

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