US3689796A - Electromagnetic lens for high speed electron beams - Google Patents

Electromagnetic lens for high speed electron beams Download PDF

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Publication number
US3689796A
US3689796A US836040A US3689796DA US3689796A US 3689796 A US3689796 A US 3689796A US 836040 A US836040 A US 836040A US 3689796D A US3689796D A US 3689796DA US 3689796 A US3689796 A US 3689796A
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Prior art keywords
coil
lens
magnetic
electron
magnetic lens
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Expired - Lifetime
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US836040A
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English (en)
Inventor
Rolf Wideroe
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BBC Brown Boveri AG Germany
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Bbc Brown Boveri & Cie
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/46—Arrangements of electrodes and associated parts for generating or controlling the ray or beam, e.g. electron-optical arrangement
    • H01J29/58—Arrangements for focusing or reflecting ray or beam
    • H01J29/64—Magnetic lenses
    • H01J29/66—Magnetic lenses using electromagnetic means only
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—HANDLING OF 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

  • a lens of the magnetic field type for high speed electron beams such as those which emerge from an electron accelerator comprises a multi-tum toroidal coil having a trapezoidal configuration, which is energized by pulse currents.
  • the coil is wound from ribbon-form conductor material, such as aluminum and the conductor parts on the conical surface zones of thercoil which are traversed by the electron rays have a smaller thickness than the conductor parts at the inner and outer peripheral surface portions of the coil.
  • Cooling means in the'form of fins in contact with a fluid coolant can be provided in heat transfer relation with those conductor parts of the coil which are not traversed by the electron rays.
  • the traversed invention relates to a magnetic lens for high speed electron beams in which an axially symmetrical and azimuthally orientated magnetic field is produced in a toroidal chamber, disposed coaxially relative to the lens axis and being transversed by part of the rays of the pencil of electron rays.
  • said lens is used in a preferred embodiment for increasing the depth dose when irradiating the human body by means of high speed electrons.
  • said lens is disposed directly on the surface of the body to be irradiated.
  • a current is induced in the wall of the hollow member to generate therewithin an axially symmetrical, azimuthally orientated magnetic field.
  • Parts of the wall of said hollow member, constructed from relatively thin aluminum plate, serve as windows for the passage of part of the rays of the pencil of electron rays, said part being. focusingly deflected by the magnetic field which is established in the toroidal chamber.
  • the known magnetic lens however, has certain disadvantages. Firstly, the effective inductance of the magnet coil is relatively high so that short excitation current pulses with a duration of, for example 100 p. sec can be produced only with a relatively substantial effort. Furthermore, the losses in the known magnetic lens due to remagnetization of the iron core are relatively high.
  • the object of the invention is to provide a magnetic lens which does not have the aforementioned disadvantages, which permits better focusing of the electron rays and which moreover has a lower weight or is less expensive to produce than the known magnetic lens.
  • the magnetic lens according to the invention is characterized in that the toroidal chamber is surrounded by a toroidal coil with a plurality of turns, serving as excitation winding for the azimuthally orientated magnetic field.
  • FIG. 1 is a view of the improved magnetic lens in longitudinal section
  • FIG. 2 is a fragmentary view showing a sector of a toroidal coil shown in FIG. 1;
  • FIG. 3 is a circuit diagram illustrating one suitable circuit for supplying the toroidal coil in FIG. 1 with pulsed excitation current
  • FIG. 4 illustrates a modified construction wherein cooling means are provided for the coil producing the magnetic field
  • FIG. 5 illustrates a modified construction wherein a pair of magnetic lenses are located along the axis of the beam to effect first a divergence of the beam and then a convergence.
  • FIG. 1 shows a section of the magnetic lens. It comprises a toroidal coil 1 of aluminum and being of approximately trapezoidal cross-section, mounted in electrically insulated manner in a housing 2.
  • the toroidal coil 1 itself comprises a ribbon form conductor of aluminum.
  • the conductor parts 3 on the conical surface zones of the toroidal coil 1 are of smaller thickness (for example 1 mm) than the remaining conductor parts 4,5.
  • the joints 6 between two adjacent coil turns are filled with insulating compound.
  • the coil is fed via a cable which is connected to the ends 7 of the toroid coil 1.
  • An interchangeable, non-conductive irradiation tube 8 is mounted on the ray exit side of the magnetic lens.
  • the magnetic lens is accordingly advantageously excited by pulse currents which are synchronized with the electron ray pulses delivered by the accelerator.
  • the aforementioned pulsed excitation currents are produced by a supply apparatus whose circuit diagram is illustrated in FIG. 3.
  • the toroid coil 1 is connected via a low-induction cable 9 and a controllable discharge rectifier 10 to a low-inductance capacitor 1 1.
  • a serial connection comprising an inductance l2 and a controllable charging rectifier 13, also the output of an adjustable rectifier stage 14, whose input is supplied from the A.C. mains, is connected in parallel to the aforementioned capacitor 11.
  • the adjustable voltage output characteristic of the rectifier is indicated schematically by the arrow 14'.
  • the method of operation of the apparatus described hereinabove is as follows: after switching on the supply unit the capacitor 1 1 is charged by the variable rectifier stage 14 to a specified and pre-determined D.C. potential. Simultaneously with the control pulse which initiates the delivery of a pencil of electron rays from the accelerator, the discharge rectifier 10 is supplied via its control input 15 with an opening or gating pulse thus causing the capacitor 11 to be connected via the cable 9 to the toroid coil 1 and for said capacitor to discharge via the aforementioned coil. Owing to the effect of inductance which tends to maintain the coil current, the charge polarity of the capacitor 11 is reversed. During said reversal, a pulsed excitation current will flow in the toroid coil 1.
  • the charging rectifier 13 is supplied via its control input 16 with an opening or gating pulse so that the capacitor 11 is re-charged to its original polarity via the charging inductance 12.
  • the reduction of capacitor voltage due to circuit losses is compensated by additional charging from the rectifier stage 14.
  • the period of the second charge reversal is extended so that it is completed immediately prior to the initiation of the next excitation current pulse. In this way, the maximum power drawn from the mains is minimized.
  • the control pulses which serve to control the discharge rectifier are timed relative to associated control pulses which cause the pulse delivery of electrons from the accelerator so that the pencils of highly magnetic electrons traverse the magnetic lens only if at least approximately the maximum excitation current flows in the toroid coil 1.
  • the central rays 17 (FIG. 1) of the pencil of electron rays traverse the central opening of the magnetic lens in substantially un-changed form.
  • a part 18 of the rays of the pencil of rays however traverse the cavity surrounded by the toroid coil 1 in which cavity there exists an axially symmetrical, azimuthally orientated magnetic field whose field strength is proportional to the excitation current.
  • the thin-walled conductor parts 3 of the toroid coil 1 are traversed by high-energy electrons which are substantially un-attenuated.
  • the electrons are deflected proportionally to the magnetic field strength and proportionally to the length of path traversed by them within said cavity.
  • the angle of deflection it would be necessary for the angle of deflection to be approximately proportional to the distance of the electron beam to be deflected from the lens axis. Since, however, the magnetic field strength in the cavity is inversely proportional to the distance from the lens axis, the distances traversed by the electrons in the cavity would have to diminish approximately in accordance with a square law with a decreasing distance from the lens axis.
  • the square law relationship is approximated in the magnetic lens by the conical construction of the flanks of the toroid coil 1.
  • the focal length of the magnetic lens can be varied within relatively wide limits by altering the maximum excitation current, that is to say by varying the output voltage of the rectifier 14-.
  • the toroid coil may, for example, as illustrated in FIG. 4 be provided at its cylindrical circumference with cooling fins 19 which extend into an annular cooling duct 20 between the toroid coil 1 and the housing wall 21 through which cooling air is blown by means of a suitable blower, not illustrated
  • the remaining parts of the conductor of the toroid coil are additionally supplied with blown cooling air.
  • a focal length of 30 cm is obtained for electrons having an energy of 35 MeV and a divergence angle of 3.82, for example, with a coil current of 4,300 Amperes.
  • an excitation current pulse duration of 100 p. sec and a frequency of S pulses/s the 1 mm thick conductor parts of the coil will have a mean power loss of 410 watts which can be easily dissipated by the air cooling system.
  • the inductance of the toroid coil amounts to approximately 8.5 l0 henries.
  • a capacitor having a capacitance of 119 p. farad is required to produce excitation current pulses of 100 11. sec duration.
  • the maximum capacitor voltage amounts to approximately 1,200 V.
  • a charging inductance 12 of 0.237 henries provides a charging half wave whose duration corresponds to a frequency of 30 cycles and a charging current having a maximum peak value of 27 amperes.
  • two serially disposed lenses are required for producing a pencil of suffrciently large diameter, the first of said lenses being disposed as dispersion lens at the output of the accelerator to increase the divergence of the emergent pencil of rays, while the second lens, disposed on the object to be irradiated, focuses said pencil of rays.
  • the two lenses are advantageously operated from the same supply unit. To this end, the excitation current pulses supplied to the lenses and therefore the focal length thereof must be individually adjustable but must be proportional relative to each other.
  • FIG. 5 illustrates an arrangement of two magnetic lenses as described above, in conjunction with a linear accelerator 21 which delivers an'electron ray pencil having a very small divergence angle- For producing a pencil of sufficiently large diameter, lenses 22, 23' are.
  • the lens 22 being disposed, as a dispersion lens, at the output of the accelerator 21 to increase the divergence of the emergent pencil of rays, while lens 23 disposed at the object 24 to be irradiated, focuses the pencil of rays.
  • the two lenses 22, 23 are advantageously connected to the same electrical supply source 25, the excitation current pulses of which are individually adjustable but proportional relative to each other and the coil currents being in opposite directions.
  • a magnetic lens structure for producing an axially symmetrical, azimuthally oriented magnetic field serving to control a beam of high energy electrons
  • said lens structure includes a toroidal space disposed co-axially with the lens axis and through which space the outer rays of the electron beam are passed
  • said magnetic field is produced within said toroidal space by a coil having a plurality of turns surrounding said space, said coil having a trapezoidal cross-sectional configuration and in which the minimum dimension thereof in the axial direction faces towards said lens axis.
  • a magnetic lens arranged as defined in claim 1 and which further includes an excitation circuit for said trapezoidally coil, said excitation circuit delivering adjustable pulse currents and which are produced by the charge reversal of at least one capacitor.
  • said excitation circuit includes at least one controllable dischargerectifier for effecting the charge reversal of said capacitor.
  • said excitation circuit includes a charging inductance and controllable charging rectifier for effecting reverse charge reversal of said capacitor after charge reversal.
  • a magnetic lens arrangement wherein an electron accelerator is utilized for producing the electron beam which passes through first and second magnetic lenses each as defined in claim 1 and arranged in series, said first lens serving to increase the divergence of the electron beam emerging from the accelerator and said second lens serving to focus the divergent beam.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Electromagnetism (AREA)
  • Electron Beam Exposure (AREA)
  • Particle Accelerators (AREA)
US836040A 1968-06-27 1969-06-24 Electromagnetic lens for high speed electron beams Expired - Lifetime US3689796A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CH963668A CH485317A (de) 1968-06-27 1968-06-27 Elektromagnetische Linse für Strahlen schneller Elektronen

Publications (1)

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US3689796A true US3689796A (en) 1972-09-05

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US836040A Expired - Lifetime US3689796A (en) 1968-06-27 1969-06-24 Electromagnetic lens for high speed electron beams

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US (1) US3689796A (de)
CH (1) CH485317A (de)
DE (1) DE1918822A1 (de)
FR (1) FR2014285A1 (de)
GB (1) GB1268714A (de)
NL (1) NL6810820A (de)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111261314B (zh) * 2020-01-17 2020-10-09 桂林狮达技术股份有限公司 带电粒子束加工设备聚焦系统校准标定方法

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2005330A (en) * 1930-08-19 1935-06-18 Thomas W Sukumlyn Electron emission device
US2295403A (en) * 1940-11-15 1942-09-08 Rca Corp Apertured electron lens and method of alignment
US2305761A (en) * 1938-03-08 1942-12-22 Bodo V Borries Electron-optical lens
DE734995C (de) * 1940-08-08 1943-05-04 Aeg Laufzeitroehre mit Hohlraumresonatoren

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2005330A (en) * 1930-08-19 1935-06-18 Thomas W Sukumlyn Electron emission device
US2305761A (en) * 1938-03-08 1942-12-22 Bodo V Borries Electron-optical lens
DE734995C (de) * 1940-08-08 1943-05-04 Aeg Laufzeitroehre mit Hohlraumresonatoren
US2295403A (en) * 1940-11-15 1942-09-08 Rca Corp Apertured electron lens and method of alignment

Also Published As

Publication number Publication date
DE1918822A1 (de) 1970-03-05
FR2014285A1 (de) 1970-04-17
GB1268714A (en) 1972-03-29
NL6810820A (de) 1969-12-30
CH485317A (de) 1970-01-31

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