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Publication numberUS3158733 A
Publication typeGrant
Publication dateNov 24, 1964
Filing dateSep 12, 1962
Priority dateSep 12, 1962
Publication numberUS 3158733 A, US 3158733A, US-A-3158733, US3158733 A, US3158733A
InventorsSibley Clifton B
Original AssigneeNat Res Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Focus control for electron beam heating
US 3158733 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

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FOCUS CONTROL FOR ELECTRON BEAM HEATING Filed Sept. 12 1962 l2 FEEDBACK SIGNAL Z LLI O: O: D 0,

Z 9 Q LIJ l O U FOCUS CURRENT Fig. 2

INVENTOR.

BY CLIFTON B. SIBLEY United States Patent 3,158,733 FOCUS CONTROL FOR ELECTRON BEAM HEATING Clifton B. Sibley, Needham, Mass., assignor, by mesne assignments, to National Research Corporation, a corporation of Massachusetts Filed Sept. 12, 1962, Ser. No. 223,125

4 Claims. (Cl. 219117) This invention relates to an improvement in high vacuum electron beam heating. It is known to heat material by concentrating a charged particle beam such as an electron beam or an ion beam of high intensity on a point or small area of a workpiece in an evacuated chamber. This type of heating, hereinafter referred to as electron beam heating, is of particular utility in such applications as welding reactive metals to produce gas-free welds of extreme purity, etching and drilling.

A principal limitation of electron beam heating is the difiiculty of monitoring the focusing of fixed and scanning type beams. This is accomplished by intermittent operation with sample checks between operations or, during operation (through a sight port) by visual inspection of the glow at the work. Both of these techniques are awkward and inefiicient.

It is therefore the object of this invention to provide means for monitoring the focus of a high intensity electron beam by sensitive instruments which respond to a parameter of operation which varies as a measurable function of focusing and apparatus for implementing this method, manually and automatically.

In accordance with the invention, reliance is placed on thermionic emission from the Work piece. Thermionic emission is very sensitive to temperature and is conveniently detected under the vaccum conditions of electron beam heating processes. An emission pickup electrode is arranged to attract low energy electrons emitted from the workpiece by thermionic emission. This produces a current through the pickup electrode which can be measured to provide a convenient and reliable indication of focus.

It is a further object of this invention to provide an improved high vacuum electron beam furnace with apparatus for monitoring beam focus.

These and further objects will in part be obvious and in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings which schematically show a preferred embodiment of the invention and wherein:

FIG. 1 is a diagrammatic, partly sectional side view of an electron beam furnace incorporating the features of the instant invention, shown in a preferred embodiment;

FIG. 2 is a curve of collection current vs. focus coil current, indicating the qualitative relation of these parameter-s.

Referring now to FIG. 1, there is shown an electron beam metal machining furnace incorporating the instant invention. The furnace comprises a vacuum tight chamber having a work support 12 supporting a workpiece to be machined. Reference may be had to US. Patent 2,981,823 to Candidus, for discussion of the work support 12. A large capacity vacuum pumping system 14 is employed to evacuate the chamber 10 to pressures on the order of 0.1 micron depending upon the requirements of the particular workpiece.

Patented Nov. 24, 1964 The electron gun 20 comprises a filament 22 for heating a cathode 24. The cathode is preferably a concave tantalum or tungsten element. Reference may be had to: Hablanian, pp. 42-53 of Proceedings Third Symposium on Electron Beam Technology (Alloyd Electronics Corporation 1961), for further discussion. The indirectly heated cathode is maintained at a high negative bias, on the order of 20 k.v., relative to the anode. An apertured accelerating anode 26 is placed between the cathode and the workpiece. It will be appreciated that the cathode can alternatively be a simple directly heated filament, such as a helix of tungsten wire or a wire hairpin. A focusing electrode 28 surrounds the cathode.

The gun also includes a tube 30 with an apertured partition 32 and a magnetic focusing coil 34 for directing the electron beam to the workpiece while separating the cathode from the work-piece. A plurality of partitions and coils may be provided, as shown in said Candidus patent.

An electrode plate, or grid, 40 is located in the chamber 10 and includes an aperture 42 which is placed in the intended path of the electron beam. The size of the aperture 42 is large enough to allow for minor faults in focus ing and/ or direct-ion of the beam and allows the beam to pass through uninterrupted. The electrode may, alternatively, be a single loop of wire or the walls of the chamber 10. However, the plate configuration is preferred. The member 40 is biased positive relative to the grounded workpiece to pick up an electron current due to thermionic emission. The collection current, at volts bias and microamperes beam current, ranges from 10 to 50 microamperes, peaking at the point of maximum watt density at the working spot. This current is measured by a meter 44. A decrease in the current reading of meter 44 indicates that the beam is out of focus since thermionic emission will then drop off. When improper focus is noted it can be corrected, by adjusting the focus coil power sup ply 36. The voltage applied to the collector is not critical beyond a minimal value of 25 volts. This minimum value varies with the position of the collector 40.

FIG. 2 shows a curve of collection current plotted against focus current. Generally, the point of best focus corresponds to the peak of the collection current curve and a decrease in collection current indicate improper focus. Although the signal peak level changes as the work is moved, the peak level still corresponds to maximum watt density at the working spot.

There are several background signals which are unavoidable-secondary emission from the work, X-rays, infra-red rays and negative ions. However, the high temperature sensitivity of thermionic emission gives the electron collection current a distinguishable signal which can be calibrated to indicate proper focus.

It will now be appreciated that the meter 44 may be connected to a feedback circuit 38. Typically, the feedback could send a signal to focus coil power supply 36. Where optimum collection current is off the peak of the curve, an increase or decrease of collector current will indicate the appropriate corrective action. Where optimum collection is at the peak of the curve (which is usually the case) the automatic control must also include phase sensitive means as well as means for detecting a decrease in the magnitude of the collection current. The feedback 38 may be a conventional null seeking circuit with balancing motor (as shown, for example, in Batcher and Moulic, Electronic Control Handbook, Caldwell-Clements, New York, 1946, p. 298) with the feedback signal taken olf the shaft of the balancing motor. The feedbacks used in peak seeking radar circuits would also be suitable.

Since certain changes can be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an electron beam machine for performing metal machining operations such as welding, melting, etching and the like, wherein a self-accelerated gun emits a beam of electrons to impinge on a small area of the workpiece with a high watt density whereby low energy electrons are thermionically emitted, the improvement comprising: a pickup electrode located in line-of-sight relation to the area of the workpiece subject to impingement by the beam, means for biasing the electrode positive to the workpiece so that the electrode attracts said thermionically emitted electrons and electric circuit means including indicator means connected to said electrode for generating an output signal varying as a function of degree of focus of said beam of electrons.

2. The improved machine of claim 1 wherein the electrode comprises a plate mounted above the workpiece, said plate having an aperture therein to permit passage of said electron beam.

3. In an electron beam machine for performing metal machining operations such as welding, melting, etching and the like, wherein a self-accelerated gun emits a beam of electrons to impinge on a small area of the workpiece with a high watt density whereby low energy electrons are thermionically emitted, the improvement comprising: a pickup electrode located in line-of-sight relation to the area of the workpiece subject to impingement by the beam, means for biasing the electrode positive to the workpiece so that the electrode attracts said thermionically emitted electrons and electric circuit means for transmitting an output signal from said electrode varying as a function of the electron current flowing through said electrode, and wherein an electromagnetic focus coil is provided for adjusting the focus of the electron beam downstream of the gun and the improvement further comprises means responsive to the current through said electrode for automatically adjusting the focus of said beam of electrons.

4. The improved machine of claim 3 wherein the electrode comprises a plate mounted between the workpiece and gun, said plate having an aperture therein to permit passage of said electron beam.

References, Cited in the file of this patent UNITED STATES PATENTS 3,054,896 Jones et al. Sept. 18, 1962 FOREIGN PATENTS 1,094,891 Germany Dec. 15, 1960

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3054896 *Jan 12, 1961Sep 18, 1962Jones William HApparatus for electron beam heating control
DE1094891B *Oct 23, 1957Dec 15, 1960Karl Mueller & Co O H GVorrichtung zur Messung der Strahlstromstaerke von Teilchenstrahlen
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3267250 *Apr 19, 1963Aug 16, 1966United Aircraft CorpAdaptive positioning device
US3283120 *Mar 20, 1964Nov 1, 1966United Aircraft CorpApparatus for working materials with a beam of charged particles
US3291959 *Nov 9, 1962Dec 13, 1966United Aircraft CorpProcedure and equipment for the automatic focussing of the charge carrier beam in devices for the working of materials by means of a charge carrier beam
US3308264 *Feb 2, 1966Mar 7, 1967United Aircraft CorpAdaptive positioning device
US3351733 *Aug 21, 1963Nov 7, 1967Hitachi LtdWelding method
US3435187 *Jul 14, 1966Mar 25, 1969Welding Research IncElectron beam welding method with feed back control
US3437780 *May 24, 1965Apr 8, 1969Us NavyDevice for measuring and equalizing the gain characteristics of channel image intensifier arrays
US3472997 *Aug 26, 1966Oct 14, 1969Us NavySecondary electron collection system
US3542995 *Jun 3, 1969Nov 24, 1970Atomic Energy CommissionMeasurement and control of focus in electron beam welding
US3576438 *Apr 28, 1969Apr 27, 1971Bell Telephone Labor IncFocus monitor for electron microscope including an auxiliary electron gun and focusing lens
US3601575 *Jan 24, 1969Aug 24, 1971Steigerwald Gmbh K HMethod and apparatus for viewing the impact spot of a charge carrier beam
US3627971 *Oct 24, 1969Dec 14, 1971Commissariat Energie AtomiqueMethod continuous inspection of the quality of a weld obtained by the electron beam process
US3634645 *Apr 21, 1970Jan 11, 1972Westinghouse Electric CorpWork treating with electron beam
US3689798 *Jul 16, 1971Sep 5, 1972Mo Energeticheskij InstitutDevice for automatically controlling electrical conditions of an electron beam unit
US3736422 *Feb 8, 1971May 29, 1973Siemens AgApparatus for improving the signal information in the electron beam examination of sample topography
US4309589 *Mar 21, 1979Jan 5, 1982National Research Institute For MetalsMethod and apparatus for electron beam welding
US4948971 *Nov 14, 1988Aug 14, 1990Amray Inc.Vibration cancellation system for scanning electron microscopes
US5773784 *Sep 26, 1995Jun 30, 1998Mitsubishi Denki Kabushiki KaishaElectron beam processing apparatus
Classifications
U.S. Classification219/121.26, 250/396.00R, 219/121.23
International ClassificationH01J37/02, H01J37/21
Cooperative ClassificationH01J37/21
European ClassificationH01J37/21