US3536951A - Electron and heavy particle beam scanning systems - Google Patents

Description

06b 27, 1970 I J. DE soLA MOSELY 3,536,951

ELECTRON AND HEAVY PARTICLE BEAM SCANNING SYSTEMS Filed Nov. 5. 1967 WAVE FORM 50W CHAMBER GENERATOR I 79 l I 20 I 5 I T/ME BASE I ADJUSTMENT I a CONTROLLER VI/47.69007. #TVWFWH hr WATER /N HRJLLLILWLLIH I ,4 hugauyuu wu 13 12 II Qw AIR 75 A/R I I Ii Hal F/G.4.

United States Patent 3,536,951 ELECTRON AND HEAVY PARTICLE BEAM SCANNING SYSTEMS John de Sola Mosely, 17 Nottingham St., London, W. 1, England Filed Nov. 3, 1967, Ser. No. 680,475 Claims priority, application Great Britain, Nov. 4, 1966, 49,571/ 66 Int. Cl. H01 29/72 US. Cl. 315-24 Claims ABSTRACT OF THE DISCLOSURE Charged particle beam scanning apparatus, especially for an electron beam, in which a well defined beam is scanned across a window, that part of the scan which passes through the window irradiating a work material situated beyond the window. Scanning is effected by beam deflection means which ensures that the flyback trace does not fall on the window. Moreover failure of the beam deflection means will result in the beam falling on a wall surrounding the window.

This invention relates to scanning systems as used for scanning electrons or other particles in the form of a beam over a surface to be treated and has particular reference to such systems applied to industrial processes in which materials on a stationary or a moving support are subjected to scanning by a broad beam of electrons.

It is known to effect curing of chemical compounds by subjecting them to impingement by an electron beam passing through a window and to effect scanning of the beam it is usual to use a magnetic deflecting system energised by a triangular waveform. The speed of the scanning trace is theoretically linear and should result in a predictable dose rate to material passing beneath the window. For various reasons, the dose rate is in fact likely to be variable and primarily because, due to the use of a triangular waveform, saturation is common on one halfcycle, if not on both half-cycles, with the result that the dwell time at the ends of the window produces burning of the window and uneven dosage of the material being treated. Again, due to the particular sort of waveform used, the material being treated will be either under-dosed or over-dosed.

The object of the present invention is to provide a system which will produce a scan having a predictable slope, rise time and amplitude which can be adjusted smoothly and at will and which can be so associated with the process as to establish a control between the scanning system and the process being carried out.

According to the present invention, in a system for subjecting material to irradiation by a beam of particles such as electrons incident upon a window which is permeable to such particles, the scanning trace produced by a beam deflecting voltage of sawtooth waveform is caused to commence its linear trace before registering with the window and to extend beyond the window before commencing the return or flyback trace, the flyback trace being so deflected or suppressed that the beam is not incident on the window.

According to a feature of the invention, it is provided that in order to avoid deleterious effects due to incidence of a stationary beam upon the window, the beam deflecting means are so biassed that the effective scanning trace is spaced apart from the undeflected axis of the beam.

According to a further feature of the invention, the axis of the electron beam generator is placed at a convenient distance normal to the permeable window, such that in the event of the failure of the scanning system,

the beam will fall upon awater-cooled portion of the window assembly and not on the window, thereby rendering unnecessary the provision of a shutter in front of the permeable window while affording protection of the equipment from damage and the processing area from unwanted radiation.

According to a further feature of the invention, the beam deflecting means may be so biassed that the beam is rendered incident upon a material the nature of which,

is such that X-rays are produced.

One form of apparatus embodying the invention will now be described in greater detail by way of example with reference to the accompanying drawings in which:

FIG. 1 is a purely diagrammatic representation of the apparatus,

FIG. 2 is a diagrammatic plan view of a detail of the apparatus; and

FIGS. 3 and 4 are waveforms which will be referred to in detail.

Referring to the drawings, the block 1 represents a waveform generator supplying a scan chamber 2 having a scan horn 3 attached to it. The scan chamber is 00111- posed of material which will resist a corrosive atmosphere and have a negligible hysteresis elfect on the scan magnets and the scan horn is composed of stainless steel. The mouth of the horn is closed by a hollow wall 4 the exterior of which is provided with fins 12 to provide heat radiating surfaces which assist cooling. The wall is provided with an elongated cut-out for a window 5 composed of electron permeable material such as thin rolled titanium or aluminium. The thickness of the window is determined by the desirability that it should pass as great a power as possible and with small heat loss, while being strong enough to withstand the high degree of vacuum which must obtain in the scan horn.

The scan chamber 2 houses a high velocity electron beam generator and beam deflecting means supplied from the waveform generator 1. The normal beam axis 6 of the beam generator is olfset from the permeable window 5 thus providing a fail/safe device which is effective from the standpoint of the equipment and radiation damage. Because of the high temperatures engendered during operation of the generator, the hollow wall 4 is maintained cooled by a continuous flow of preferably turbulent cold water by means of a water inlet 13 and a water outlet 14 and the permeable window is cooled by forced air. For the latter purpose, a high velocity air blowing manifold, indicated at 15, enshrouds the underside of the scan horn and is constructed so that air is directed on to the window. A second manifold, indicated at 16, is provided to suck the air from the window and to direct it away from the material being irradiated.

The beam deflecting waveform generator is designed to produce an adjustable sawtooth waveform as represented in FIG. 3, the flyback period of which is very short compared with the rise time and in operation, the beam is caused to commence its linear traverse from a point 7 shown in FIG. 2, across the water cooled wall 4 before registering with the electron permeable window and to complete its linear traverse at a point '8 beyond the window before commencing its flyback or return stroke 9. The time base of the adjustable sawtooth waveform is varied by the time base adjustment controller 20. A square pulse as shown in FIG. 4 is also produced by the deflecting waveform generator 1 and is used as a beam deflecting pulse applied to the beam deflecting means which may be electrostatic or electromagnetic to ensure that the flyback trace does not fall across the window of the generator and is only incident on the cooled wall 4.

The beam generator is provided with the usual beam focussing means to produce a scanning spot of selected dimensions and with an additional electron lens system which causes the beam emergent from the window to diverge and produce a broad band of electrons incident upon the material to be irradiated. The incident band is preferably of the order of from one to two inches in width and the material to be irradiated is placed on a moving conveyor 11 or on a fixed support. In the case of a moving conveyor the speed of travel governed by driven rollers 17 and 18 is related to the beam intensity and the nature of the material to be treated and in the case of a fixed support for the material, the dwell time of the beam is determined by switching means which are controlled so as to switch the beam on and off as desired.

Adoption of the method of operation as described results in greater elficieny in an irradiating system applied, for example, to the curing of chemical compounds and in particular to paints. A scanning system embodying the invention reduces or eliminates to a large extent the compromise in efficiency associated with the use of a triangular waveform scanning system. The use of an adjustable time base facilitates correlation of the precise dose rate with variation in speed on a conveyor carrying material to be irradiated, whether caused accidentally or deliberately. The interdependence between conveyor speed and the time base is indicated in FIG. 1 by broken line 19.

The system is also applicable to the generation of X- rays and for this purpose, the beam deflecting means would be operated so as to cause the beam to impinge upon a material within the generator which results in the production of X-rays.

I claim:

1. Charged particle beam scanning apparatus for irradiating work material including in combination:

(a) a charged particle beam generator having a normal beam axis and producing a well defined charged particle beam;

(b) a window permeable to said charged particle beam, said window being disposed at right angles to but displaced from said normal beam axis;

(c) a wall surrounding said window;

(d) beam deflecting means which provides a voltage of sawtooth waveform which scans said beam in a scanning trace and a flyback trace in a first direction parallel to said window across said wall and which deflects said scanning trace of said beam in a second direction at right angles to said first direction by a distance corresponding to said displacement of said window from said normal beam axis, said scanning trace scanning said window between points on said wall; and

(e) a work material carrier exposed to said scanning trace which passes through said window.

2. Apparatus according to claim 1, in which said wall is provided with cooling means.

3. Apparatus according to claim 2, in which said wall is hollow, said cooling means including a water inlet and outlet in said wall.

4. Apparatus according to claim 3, in which said wall is provided with heat radiating fins.

5. Apparatus according to claim 2, in which forced air cooling means are provided for said window.

6. Apparatus according to claim 1, in which said work material carrier includes conveyor rollers for a sheet of said work material, said rollers being provided with driving means.

7. Apparatus according to claim 6, including a time base adjustment controller associated with said beam deflecting means for varying the time base of the sawtooth waveform, said controller and said driving means being operatively associated to maintain constant the dose rate of said material by said scanning beam for variations in the speed of said rollers.

8. Beam scanning apparatus for irradiating work material including in combination:

(a) An electron beam generator having a normal beam axis and producing a Well defined electron beam;

(b) a window permeable to said electron beam, said window being disposed at right angles to but displaced from said normal beam axis;

(c) a wall surrounding said window;

((1) beam deflecting means which provides a voltage of sawtooth waveform which scans said beam in a scanning trace and a flyback trace in a first direction parallel to said window across said wall and which deflects said scanning trace of said beam in a second direction at right angles to said first direction by a distance corresponding to said displacement of said window from said normal beam axis, said scanning trace scanning said window between points on said wall;

(e) a time base adjustment controller associated with said beam deflecting means;

(f) conveyor rollers for conveying a sheet of said Work material past said window; and

(g) driving means for said conveyor rollers, said driving means being operably associated with said time base adjustment controller to adjust said time base in dependence upon the speed of said rollers.

9. Apparatus according to claim 8, in which said wall is hollow and is provided with cooling means comprising:

(a) heat radiating fins on said hollow wall; and

(b) a water inlet and a water outlet from said hollow.

10. Apparatus according to claim 9, in which said window is provided with forced cooling means.

References Cited UNITED STATES PATENTS 3,066,238 11/1962 Arndt 313--74 X 3,096,437 7/1963 Muray 25049.5 X

RODNEY D. BENNETT, JR., Primary Examiner T. H. TUBBESING, Assistant Examiner

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