US3066238A - Asynchronous beam scanning device - Google Patents

Description

NW, 27, 1%.? H. H. ARNEDT 9 3 ASYNCHRONOUS BEAM SCANNING DEVICE Filed March 25, 1959 2 Sheets-Sheet 1 AMPLITUDE CONTROL INVENTOR. FEQZ RICHARD H. RNDT OEFLECTION FLUX ACCELERATNG WLWE ELECTRON ACOELERATING Nov. 27, 1962 R. H. ARNDT 3,066,238

ASYNCHRONOUS BEAM SCANNING DEVICE AT TORN E Y United States Patent 3,066,238 Patented Nov. 27, 1962 like 3,066,238 ASYNCHRONOUS BEAM SCANNING DEVICE Richard H. Arndt, Milwaukee, Wis., assignor to General Electric Company, a corporation of New York Filed Mar. 2.9, 1959, Ser. No. 801,320 5 Claims. (Cl. 315-22) 1 described primarily in connection with an electron beam generator.

Electron beam generators usually employ a long insulating evacuated tube for accelerating the electrons through a large potential difference existing between an electron gun including a hot cathode emitter at one end of the tube and an anode at the other. The anode includes an electron permeable window through which the beam passes from the tube onto a substance being irradiated. The beam is focused in the tube so as to create a beam diameter on the window in the range of 1 cm. in diameter which generally increases as it emerges to the atmosphere as the result of electron scattering in the window and from encountering atmospheric gas particles. lnevitably, the electron density is greatest near the beam center and appreciably attenuated near its margin.

Not only does the electron current density differ over adjacent increments of a beam cross section, but the various electrons are not, ordinarily, monoenergetic. The energy differences result from slight variations in the accelerating voltage amplitude. Consequently, a substance subjected to electron irradiation may undergo varying amounts of ionization or dosage at diiferent depths and over different surface zones.

In order to distribute the available ionizing energy from an electron beam it has been proposed to scan the beam lengthwise and crosswise of the window by conventional electrostatic or electromagnetic deflecting means. in some generators, where the beam is pulsed and where the accelerating potential varies sinusoidally over the pulse duration, as is the case where a resonant transformer is used, it has been customary to create a deflecting field which causes the beam to start moving lengthwise from the same point on the window and to terminate at the same point for each successive current pulse. Stated in another way, it is the practice to initiate successive pulses when the time varying deflecting voltage flux has the same magnitude and to terminate the pulses when the deflecting flux has the same magnitude. When the deflecting flux is sinusoidal, an attempt is usually madeto time the pulses for coincidence with the linear part of the deflection sine wave such as is approximated by going from a positive to negative value on the flux wave. The purpose of this is to move the beam with nearly linear velocity or with a slight retardation near the ends of the scan. This is accomplished by chosing the scan frequency properly and by phasing the pulse such as to endure while the scanning flux wave goes slightly onto the sharply curved parts of the sine wave existing near its peaks. Then a build-up of electrons is obtained at the window ends and the usual decline in beam current intensity near the beginning and end of each pulse is compensated.

While moving a beam diameter over the window repeatedly in the same pattern and at linear velocity is satisfactory in some applications, it is intolerably disadvantageous in other cases, because the maximum beam current intensity and the peak accelerating voltage always occur when the beam is in the same position on the window. The result of this is overheating and often melting the window, ovcrdosing the substance being irradiated in the zones where current maximums are repeated, underdosing other portions of the substance, and varying the degree of beam penetration. The peak current zones determine the maximum current rating of the window, and indeed, the generator itself. They also have a bearing on the amount of window cooling that is necessary and on the window energy losses.

Accordingly, a primary object of the present invention is to provide a charged particle beam generator that yields more useful radiation with a given power input.

Another object is to provide a beam generator scanning system that distributes the high and low intensity beam zones over the entire usable lengths of the beam transmitting window with the added effect of obtaining more uniform ionization in the substance being irradiated.

Still another object is to provide a beam generator with scanning means that enable simplifying the controls and eliminating the necessity for establishing careful phasing between the current pulses and the deflection flux.

Achievement of the foregoing and other more specific objects will be evident throughout the course of the ensuing specification.

In general terms, the present invention involves apparatus and a system for obtaining uniform energy distribution over an electron permeable window and a product being irradiated, characterized by emission of electrons in symmetrical or unsymmetrical pulses to form a beam which is scanned both transversely of and along the window length over a predictable, but not successively repetitious pattern. This is achieved by a time varying scanning field rising and falling at a frequency that is predetermined and different than the pulse rate of the beam so that maximum beam energy will occur at different beam positions for each scan.

An illustration of the invention will now be set forth in conjunction with the drawings in which:

FIG. 1 is a schematic representation of a resonant transformer type of electron beam generator embodying the invention;

FIG. 2 is a fragmentary section, taken on a line corresponding with 2-2 in H6. 1, showing part of the amide and electron exit window of an electron beam generator tube and products being irradiated;

FIG. 3 is a plan view of the electron exit window showing by a broken line how the beam changes position along the long dimension thereof;

FIG. 4 is similar to P16. 3 except that it shows by a broken line the beam path resulting from scanning across the narrow dimension of the window;

FIG. 5 is a diagram showing the relationhip of the accelerating voltage, the current pulses and the time vary-- ing flux that deflects the beam lengthwise of the window; FIG. 6 is a plot of the beam travel along the lengthof the window during successive scans in accordance with the instant invention; and,

FIG. 7 shows the variations of the accelerating voltage and electron density with respect to the position of the electron beam on the window where ordinary linear sca'ndesigned to withstand voltages between its opposite ends in the range of 1 million volts and upward. It includes a plurality of glass rings 11 which are sealed in end-to-end relationship by intervening metal spacers 12 and it is enclosed at its upper end by a cathode mounting assembly including a focusing electron gun 13 having a hot cathode 14 and a control grid 15. Wires 16 pass through vacuumtight insulators 17 in order to carry heating current to the cathode 14.

Tube is terminated at its bottom end by a metal ring 18 to which is joined a round metal tube 19 and a flared tube section 20. The flared tube 20 terminates in an adapter 21 that is closed by an electron permeable exit window 22 usually made of very thin titanium, aluminum or other metal of low atomic number. The elements recited in this paragraph constitute the accelerating tube anode and as can be seen at 23 they are ordinarily grounded so as to be at zero potential level with respect to the cathode 14. By this means the electron beam, represented by the dashed line 24, acquires suflicient energy to pass through the window 22 and irradiate a product 25 carried on a conveyor belt 26, for example, below the window.

The electron accelerating voltage is derived from the high voltage secondary winding of a resonant transformer 31. The secondary 30 usually surrounds the accelerating tube 10. At its high potential upper end, winding 30 is connected to one cathode lead 16 and at its lower end 32 it is grounded at the potential level of the tube anode which is grounded at 23. Suitable taps 33 on the secondary winding are connected with corresponding intermediate electrodes 34 within the tube 10 so as to effect a gradual potential gradient between the ends thereof. More detail on the construction of the resonant transformer and an accelerating tube having the character of that here used as an example are obtainable from US. Patent No. 2,144,518.

The accelerating tube 10 and concentrically surrounding resonant transformer windings 30 may be enclosed in a metal tank 27, only a fragment of which is shown in FIG. 1, and which is filled with a dielectric medium such as oil or pressurized gas.

Means are provided for impressing a biasing potential between cathode 14 and a control grid 15 so that tube 10 will only conduct when the accelerating voltage wave is near its positive peaks. Biasing energy is derived from the sinusoidal voltage which appears across the capacitance formed between tank 27 and a cap 37 located above tube 10. The charging current associated with this capacitance voltage is fed into a bias control 40 through a wire 28 from cap 37 and a wire 29 from the high potential end of secondary winding 30. In bias control 40 is a rectifier and rectangular pulse forming circuit, not shown, whose output pulses are applied in phase with the accelerating voltage between cathode 14 and grid 15 through wires 38, 39 and the bias is adjusted so the tube will conduct in pulses that occur and persist only when the accelerating voltage is near the positive peaks of its sine wave curve.

The relation of the tube beam current pulses to the accelerating voltage, which appears between cathode 14 and window 22, may be seen in the upper portion of FIG. 5. The accelerating voltage curve 44 may have an amplitude in the range of over 1 million volts and a frequency of 180 cycles per sec., in this instance. Since the tube 10 conducts only when the anode or window 22 is positive with respect to the cathode 14, current pulses 45 appear only during positive half cycles of the accelerating voltage. By proper adjustment of the bias circuitry, the current pulses may be biased to cut-off until voltage curve 44 reaches a value near its peak, at which time a substantially rectangular wave current pulse 45 is formed out of what would usually be a sinusoidal current wave form were it not biased. In this example, the current pulse width may be taken as 72 on the 180 cycle per see. time scale for convenience, although a different conducting angle may be desirable in other cases. Thus, only the electrons possessing nearly the highest possible energy are conducted and window heating is reduced since the low energy electrons are not present to be absorbed in the window 22 and manifested as heat.

The resonant transformer 31 includes a primary winding 46 of relatively few turns compared with the second ary, which primary is supplied with current through an amplitude control, symbolized by the device 47, which is in this case, supplied from a cycle per sec. generator 43. Generator 48 includes the usual excitation controls (not shown) and is mechanically driven by a schematically represented synchronous motor 49.

Another alternating current generator 51 produces 1005 cycle per sec. current, in this instance, for supplying scanning coils 50 that deflect the beam 24 lengthwise of window 22. Generator 51 is driven by motor 49 through a speed changer 52 so that the generated frequency shown in the lower curve 53 bears a predetermined relationship to the accelerating voltage and current pulses in FIG. 5. The wave shape of generator 51 may be sinusoidal and used directly in that form or it may be modified by a wave shaping circuit, which is conventional and therefore only symbolized by the device 54 in FIG. 1. In the same circuit there may also be included a variable scanning voltage amplitude control inductor 55. If wave shaping is adopted, the supply to coils 50 may be saw-toothed as suggested by curve 53 in the lower portion of FIG. 5.

In the present invention, the voltage from generator 51 is applied to means for scanning the electron beam 24 over the long dimension of the window 22 as suggested in FIG. 3 where the beam spot 56 is seen .0 execute that movement on the window. The scanning means may be electrostatic but in this case they preferably take the form of electromagnet coils 50 located on opposite sides of anode tube 19.

Another set of scanning coils 59, at right angles to coils 50, are energized by a high frequency voltage which develops a flux that deflects the beam spot 56 extremely rapidly across the narrow dimension of the window 22 during each current pulse and whose component of motion is illustrated in FIG. 4. Any suitable conventional oscillator (not shown) may be used to supply coils 59. In one commercial model where the window is scanned at 180 cycles per sec. along its long dimension, the cross scan frequency is in the range of 200 kilocycles per sec. Generally, however, the cross scan frequency may be selected in view of the beam spot diameter, the window size and the desired amount of overlap of the beam spot. This will, in turn, usually be governed by the character of the product being irradiated, the rate at which it is conveyed, and its dosage requirements.

Scanning the beam in two directions as suggested in FIGS. 3 and 4 is established procedure, the result desired being to distribute the electron beam as uniformly as possible, by superposition of the scanning movements, over articles such as 25 which are being irradiated in FIGS. 1 and 2. The articles may be stationary or transported on a conveyor belt 26 in order to remove them from the radiation field after they have received a predetermined dose.

According to prior practice it is customary to scan the beam on the long window dimension by a voltage derived from a resonant transformer at the same frequency as the current pulse rate and the accelerating voltage. In some instances the scanning frequency representing some close multiple of that frequency is employed, but always in an effort to have the pulse coincide with a substantially linear part of the scanning wave so that the beam would move at uniform velocity. However, these are not optimum conditions for obtaining uniform energy and electron density on the irradiated product, for as can be seen from FIG. 7 with respect to the beam spot window position, the accelerating voltage 44 and the current pulse 45 magnitude both drop off in their regions of initiation and termination. Hence, on repeated scans, the beam current and accelerating potential peaks always occur when the beam is in the same window position. This causes repeated overloading of the window in some regions and a deficiency of radiation in other regions during successive scans. The disadvantages of such synchronized scanning procedures are overcome by the present invention.

In accordance with the invention, the electron beam 24 is deflected by using the entire flux wave developed by coils 50 rather than only the linear part as was done heretofore. This results in a beam current trace that is started and ended at a different position on the window for each successive tube current pulse. Moreover, the coinciding, most energetic, current and voltage peaks appear at different window positions so that window heat load is more uniformly distributed. To achieve this result it is necessary that the flrx which deflects the beam along the length of the window be such that it does not advance or retard the successive traces so much as to cause a repeating or coinciding trace in only a few cycles.

In FIG. 5 it will be observed that the lengthwise scanning flux voltage curve 53 is of such frequency that it passes through more than a whole cycle during a tube current pulse 45. Thus, during the first pulse under consideration, the beam moves over a period of time indicated by the dashed portion of the deflecting voltage curve 53. During this time lapse the beam is moved from a point near the center of the window, to the far right end, back to center, to the far left end, and then back to beyond the center. The peak current and accelerating voltage coincide near the window center as indicated by the proximity of dot 60 to the horizontal axis of no deflection. On the next pulse, also indicated by a dashed line, deflcction begins at a diiferent point and proceeds in an op posite direction with the peak current and voltage occurring at the opposite side of the window center as indicated by the dot 60'. There is also present the high frequency crosswise scan, as suggested above, which gives the effect of converting the beam spot to a band whose width is that of the spot diameter and whose length is the width dimension of the window. Thus, if the trace of FIG. 4 is superimposed on FIG. 3 the eflfect is that of a band like beam scanning the length of the Window 22.

To facilitate understanding the beam energy distribution pattern and the relation of the scanning flux to the accelerating voltage and pulse timing, the following specific numerical example is set forth. If in FIG. 5 the tube current pulses 45 occur at a rate of 180 per sec. and if they endure for 72 on the time scale of the 180 cycle accelerating voltage, each pulse will endure for U900 seconds.

72/pulse 1 sec./cycle 360lcycle 180 =1/900 sec./pu1se The minimum lengthwise scan frequency which will complete at least one full cycle during this time is:

1 fmin- T.T 17566- 900 cycles/sec.

=5 105/180 scan cycles/tub cycle 6 and the phase retarding between successive current pulses 18,

(6-5 /180) cycles/cycle 1 tube current. cycle where 6 is the next highest integer or whole numbe' of scan cycles. In order to determine when the time :elationship between the current pulse and the scan frequency repeats itself, the amount of retardation is multiplied by the integer n,

=5/l2 cycle of tube current 5/ l2 n=integral number of cycles, or

n: 12 cycles In FIG. 6, the resulting beam trace pattern along'"the window length is graphed with respect to time on the scanning frequency scale. In this graph, the points on each trace at which the pulse is started on the window are identified by dots, the peak voltage points by X, and the direction of beam movement by arrows. The space between traces indicates the lapsed time between the end of one pulse and beginning of the next. It will be noted that the first and thirteenth traces are identical and that each successive starting point and peak occurs at a different window location. Generally, it is desirable to select a scanning frequency that will pass through more than one but less than two complete cycles during a single current pulse. In irradiating some products, however, preferred beam energy distribution may be obtained where the beam is scanned lengthwise of the window several times or during more than two complete scan cycles for each pulse. This is an important advantage of the novel unsynchronized scanning method disclosed herein, because it allows positioning the beam energy peaks so as to meet the dosage requirements of products having various irregular shapes and densities and whether or not they move at various speeds on a conveyor belt.

If the traces in FIG. 6 are looked upon as physical representations of the beam path, the time axis may be disregarded and all traces may be shifted to lie in a common plane vertical with the paper and extending along the long axis of Window 22 As pointed out earlier, however, the beam spot is simultaneously and continually deflected crosswise of the window at a very high frequency so the-beam spot may be considered to take the form of a band that travels back and forth on window 22 in a direction and with a velocity indicated in FIG. 6.

As shown in FIG. 5, deflecting voltage curve 44 may be saw-toothed, in which case the beam spot is deflected at a substantially linear velocity along the window 22. If absolute linearity is not required, which is often the case, a sinusoidal wave form of the same frequency from generator 51 may be applied directly to scanning coils 50 and the wave shaper 54 may be eliminated. Although, in this illustration, the relation of the scanning frequency to the pulse rate is maintained by driving both generators 51 and 48 from a common mechanically connected motor, it will be understood that a constant frequency scanning voltage may also be obtained from a crystal controlled oscillator, for example, which drives an electronic power amplifier, neither of which are shown. If this alternative is elected, only one generator such as 48 may be necessary since scanning power can be derived from it.

The invention is not limited to using any particular current pulse rate, accelerating voltage frequency or scanning frequency and the values used herein are to be considered illustrative only. The exact parameters chosen in any case will depend upon the nature of the product being irradiated, the required depth dose, the conveyor speed and other variables that are taken into account by those skilled in the art of irradiation. An important feature of the present invention, however, resides in asynchronously relating the accelerating voltage and current pulses with the deflecting flux to the end that the maximum beam intensities will appear consecutively at different zones on the window and product. Although an illustrative example has been set forth, it will be understood that the invention may be variously embodied and it is to be interpreted in accord with the scope of the claims which follow:

It is claimed:

1. Electron irradiation apparatus comprising an evacuated electron accelerating tube including an anode and an oblong electron permeable exit window at one end and an electron emitting cathode spaced therefrom, means for impressing a cyclically varying electron accelerating potential between said cathode and anode for projecting an electron current beam through the window, means for biasing said cathode so that beam current flows in pulses periodically with respect to the accelerating potential, means for deflecting said electron beam at a high rate across the narrow dimension of the window, coil means for creating a magnetic field that deflects the beam at a lower rate than said high rate along the greatest dimension of the window, means for energizing said coil means with current that varies at a frequency in cycles per second that is greater than the reciprocal of the pulse duration in seconds and which current has a different magnitude at the initiation of each beam current pulse in a predetermined number of such pulses, whereby the beam appears at different deflected positions on the Window when each successive pulse in a predetermined series of pulses is initiated.

2. Electron irradiation apparatus comprising an evacuated tube including an electron emitting cathode, an anode and an oblong electron permeable exit window, a high voltage alternating current source connected between the cathode and anode for accelerating a beam of electrons to high energy and projecting it through the window, means for biasing said cathode to emit electrons in pulses periodically in a substantially constant phase relationship with respect to the accelerating voltage which varies in magnitude with time, a first means for deflecting said electron beam at a high rate across the narrow dimension, second means for creating an electron deflecting field for scanning the pulsed beam lengthwise of the window, means for energizing said deflecting means with an alternating voltage that is unsynchronized with the pulses and that passes through more than one cycle for each pulse duration, whereby the beam is deflected to a different window position at the beginning of each in a predetermined number of successive pulses and whereby the accelerating potential peaks occur at successively different window positions of the beam.

3. A method of imparting a substantially uniform integrated dose of high energy penetrating electrons across a substance and at any selected depth in the substance, comprising the steps of developing in an evacuated chamber pulses forming an electron beam that emerges therefrom through an oblong electron permeable window, said electron pulses being generated at a constant frequency and constant phase angle with respect to an alternating field that accelerates them, projecting said electron pulses through an alternating flux that deflects them in a narrow band over the narrow dimension of the window, projecting said pulses through another alternating flux field that passes through fewer cycles in the same time than the first named flux, said other flux having a different magnitude for each of a number of consecutive pulses of electrons passing therethrough and the same magnitude after a predetermined number, whereby there is a coincidence between the same electron density of the pulses and the same accelerating field at consecutively different positions along the long dimension of the window and at the same position after said predetermined number.

4. The invention set forth in claim 3 wherein the frequency of said other alternating fiux field is such as to deflect each electron pulse back and forth along the length of the window over a total distance that is greater than the window length.

5. The invention set forth in claim 3 wherein the frequency of said other alternating field flux is such that consecutive pulses of electrons impinge upon said Window at different positions along its length, and certain of said pulses are initially deflected in opposite directions along the window length as compared with preceding and following pulses.

Refcrences Cited in the file of this patent UNITED STATES PATENTS 2,602,751 Robinson July 8, 1952 2,730,566 Bartow Jan. 10, 1956 2,961,561 Westendorp Nov. 22, 1960 2,977,500 Boeker Mar. 28, 1961 FOREIGN PATENTS 334,193 Germany Mar. 11, 1921 145,084 Great Britain Sept. 19, 1921

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