CA1128724A - Apparatus for cold-cathode electron beam generation for sterilization of surfaces and similar applications - Google Patents

Abstract

ABSTRACT
The disclosed invention provides apparatus for electron-beam-irradiation of surfaces for purposes of sterilization and the like. An electrically triggered repetitive pulse generator having a stacked array of discharge gaps is connected with a corres-ponding staggered co-extensive array of capacitors and is disposed in a pressure vessel. An evacuated electron gun having an electron-pervious window is electrically connected to the pulse generator so as to draw electron beams exiting the window in response to generated pulses. The pressurized vessel is separated from the electron gun by a high-voltage insulator.
With the apparatus of the invention voltage variations have less effect and reliability is improved.

Description

This is a division of Canadian Application Serial No.
338,618 filed October 29, 1979.
The present invention relates to apparatus for electron-beam genera-tion for sterilization and o-ther irradiation of sur-faces, materials and workpieces oE various -types, beiny more particularly concerned with the cold-cathode pulsed electron-beam generation of relatively low energy electrons (say, of the order of 50-450 keV) wi-th a high degree of reliability.
_ckground of Invention Relatively low energy electron beams have been used successfully for such applications as surface sterilization and the surface treatment of containers and other articles, materials or workpieces, as described, for example, in Uni-ted States Letters Patent No. 3,780,308, of Energy Sciences Inc., the assignee of the present application. Bulk electron-sterilization techniques are disclosed in U.S. Letters Pa-tent No. 3,779,796, of said Energy Sciences Inc. In such applications as packaging material sterilization, direct-current beam generators of the type marketed under the trademark "Electro-curtain", by said Energy Sciences Inc., nave been employed; such jr/
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low energy electron beam generation being described, for exarnple, in U.S. Letters Patent Nos. 3,7~2,412; 3,745,396; and 3,769,600.
There are advantages, in some applications, as mentioned in said Letters Patent, in the use of repetitlve-pulse-produc-tlon of such relatively low energy electron beams wlth ~he aid o cold-cathode electron sources, and wi-th capacitor~discharge pulsing techniques of the type previously used in other types of pulse generators, including the Marx-type capacitor storage-spark-discharge generators long-applied to high-energy physics systems, among the more recent of which is the pulsing of lasers, as des-cribed in Physics Today, April, 1975. (Also, E. Aul-t et al, IEEE
J. Quant. Elec., Vol. 10, p. 624, on [1974]).
Amon~ the considerations in applying such techniques to the problems of the present invention, however, are the very serious consequences of even temporary erratic pulsing or the missing of pulses, which, when occurring in a pro~uction-line sterilization application, for example, can result in the poten-tially dangerous effect of failing to sterilize at all, or im-properly or inadequately sterilizing the workpiece as a result of poor beam uniformity, directivity and the like. A new level of reliability over prior uses of these pulse techniques in other applications is thus required for the purposes oE the present invention. Further, prior systems using such techniques were often directed to laboratory and experimental applications which did not require the longevity of operation and industrial reli-ability underlying the commercial requirements of production-line sterilization and the like.

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It has been determined that one of the keys to the unreliability (for present purposes) of previous pulse techniques of this character has been the absence of a sufficiently wide triggering range of the spark-discharge gaps. Previously, ~ixe~
gap trigger generators have operated at rela-tively nar~ow triy-gering ranges of approximately 15 percent below the sel-breakdown voltaye; or, where dynamic range variation has been re~uired, with manual adjustments of gap spacing or by multiple triggered gaps, clearly unsuitable for production-line operation. Near the upper end of the triggering range, occasional prefires will cause low output voltages; while near the lower end of the triggering range, occasional misses occur. In accordance with the present invention, on the other hand, triggering range capacity has been extended upwards of about 30 percent--operation foùnd necessary for long service industrial life time.
Among the novel pulsing circuit features of the invention, are significantly improved and tailored conductive shield struc-tures for increasing the stray capacitance to ground along the capacitor stack, and large-area spark gaps of the "rail" type with novel trigger location and operation. Underlying the invention, moreover, is the discovery of a technique -for obtaining a novel substantially linear depth-dose profile characteristic, and an intermediate region of operation thereupon, that startlingly renders the effects of the electron beam impulses significantly less sensitive to possible voltage variations during the pulse generations, thus promoting substantially uniform irradiation of surfaces and workpieces (sometimes herein generically -termed "products") passing the apparatus.

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-~ ,' Accordingly, an object of the present invention isto provide a new and improved apparatus for the generation of relatively low voltage, energetic electron beam pulses which are not subject to the above-described ]imitations and disadvantages, but that possess -the increased reliability needed -for many in-dustrial applica-tions, such as steriliza-tion, and that, in large measure is at-tained by a significantly increased triggering range and by operation in a most-favored region of a relatively low-slope depth-dose profile characteristic of the generated beam.
Another object of the invention is to provide a new and improved pulse-generating capacitor bank construction that allows for a greatly increased range of voltage varia-tion within which operation of the system is permissible, and with a concomitant increase in reliability.
Other and further objects will be described hereinafter and more particularly delineated in the appended claims.
The invention of the parent applica.ion contemplates a process of and apparatus for the irradiation of objects by ener-getic electron particles wherein the reliability of pulse genera-tion has been so greatly enhanced as to make such techniques avail-able to a broader range of commercial applica-tions. In one of its important aspects, that invention embodies a method of insuring the reliability of the production of repetitive impulses of elec-tron-beam energy for production-line sterilization and similar purposes, that comprises, repetitively generating electric-dis-charge pulses; applying the pulses repetitively to draw elec-tron beam impulses from a cold cathode to and through an electron-pervious window means; disposing the window anode adjacent a por-tion of a region along which jr/

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7,~l products-to-be-electron-beam~irradiated are passed; adjusting the impedance presented by the cold cathode-window anode to the impedence presented by the pul.se generating step to produce a substantially linear elec-tron-beam dose versus penetration dep-th characteristic curve of rela-tively low slope in the region near the one-half dose region of the characteristic curve, thereby to reduce the sensitivity of the electron beam impulses to pos-sible voltage variations during the pulse generating step in order to insure substantially uniform irradiation of the products passing along said region.
The above method may be carried out by way of an apparatus for electron-beam-irrad:iating surfaces passed along a predetermined region, for purposes of steriliæation and -the like, having, in combination, electric-discharge repetitive pulse-generating means; electron gun means comprising cold cathode means and electron-pervious window anode means connected to the pulse-generating means repetitively to draw electron beam impulses from the cold cathode means to and through the window anode means;
means for disposing the window anode means ad~acent a portion of the region along which products-to-be-electron-beam irradia-ted are passed; and means for adjusting the relative impedances of the pulse-generating means and the electron gun means to produce a substantially linear electron-beam dose versus penetration depth characteristic curve.
On the other hand the apparatus of the present divisional application broadly provides apparatus for electron-beam-irradia-ting surfaces passed along a predetermined region, jr/

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for purposes of sterilization and the like, having, in combination, electrically -triggered repetitive pulse-generating means having a stacked array of discharge gaps connected with a corresponding staggered co-extensive array of capacitors and disposed in ~
pressurized vessel; evacuated electron gun means provided ~i-th electron-pervious window means; means for electrically connecting the pulse-generating means to -the electron gun means -to draw there-from electron-beams exiting the window means in response to the pulses generated by the pulse-generating means; and high-voltage insulating means separating the pressurized`vessel from the evacuated electron gun means and supporting the electrically connecting means.
The invention will now be described with reference to the accompanying drawings, Fig. 1 of which is a graph contrast-ing a dose-depth profile characteristic attained in accordance with the invention with prior characteristics;
Fig. 2 is a side elevation of an apparatus construc-tion in accordance with a preferred embodiment, using the process underlying the invention;
Fig. 3 is a view of the lower right-hand por-tion of Fig. 2, upon an enlarged scale, and partly sectionalized longitudinally, to illustrate details of the cold-cathode electron beam generators;
Fig. 4 is a schematic circuit diagram of a preferred Marx-type pulse generator for driving the beam generators of Fig. 3;
Fig. 5 is a longitudinal section, upon a larger scale, of the upper capacitor-spark gag Marx pulse generator of Fig. 2 and of the circuit type shown in Fig. 4; and Fig. 6 is a transverse section taken along the line 6-6 of Fig. 5, looking in the direction of the arrows.

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Before discussing the preferred apparatus and techniques of operation lt is in order to explain the before-mentioned dis-covery of the rather critical mode of operation of the cold-cathode-generated beam and its tailoring to a pre~erred substan~
tially linear dep-th-dose profile charac-teris-tic with a hiyhly advantageous intermediate region of opera-tion -therealony that reduces sensitivity to possible voltage variation during pulse generation and insures substantially uniform irradiation.
Referring to the graph of Fig. l, (delivered dose, as a percentage of front product surface, piotted along the ordinate, and range of penetration or depth into the product or other surface wall plotted along the abscissa in mg/cm ), the remar~ably linear, relatively low-or moderate-slope curve labelled "OPTIMUM`' ~having significant curvature only at its lower or right-hand end) represents the type of dose-depth pro-file characteristic attainable with the no-~el cold-cathode opera--tion of the invention, as distinguished from those attainable wi-th prior art techniques discussed, for example, in said Letters Pa-tent No. 3,780,308 (see more particularly Fig. l thereof). With ad-justments below and above such optimum conditions, as represented by the steep, non-linear dash-line curve "10W" and the steep, non-linear dash-dot curve "HIGH" this characteristic is not at-tained. It is also not attained by machines such as the before-mentioned "Electrocurtain" type D.C. generators, operating with the rather steep, non-linear curve "D.C." of Fig. l. By operating with as low a slope as possible at the intermediate (near or approximately one-half) dose point P, ~say of the order of 45 slope, more or less, as distinguished from the steep angle slopes, including almost 90 slopes, of prior type characteristics of ~r/

Fig. 1) it has been discovered that such an optimum linear depth-dose profile will enable the generation of substan-tially constant electron beam impulses with substantially reduced sensitivity to a wide range of possible voltage variations durlng the pulse genera-tion, thus remarkably insuring substantia~ly unio~l~ ir-radiation of the products passing by the apparatus.
Such reduced sensitivity, as before stated, does not exist for the steep slope, non-linear profiles of the prior art as indicated at "LOW", "HIGH" and "D.C." in Fig. 1, the slope of the curve being, indeed, a measure of the sensitivity to volt-age changes. Through the obviating of such steep (and non-linear) profiles, the present inven'ion enables reduced sensitivity to volt-age variation as before stated. Opera-tion up to and near -the intermediate one-half dose point P enables the required depth of sterilization penetration (say, of the order of 20-25 mg/cm , Fig. 1, or 8-10 mils of pene-tration in paper wall and the like).
That is, the surfaces of the irradiation-penetrated product most remote from the electron beam window are treated near the one-half dose.
The use of pulsed cold-cathode opera-tion, where ap-propriate, as distinguished from thermionic cathode operat`ion, moreover, results in simplified electronics, lower insula-tion requirements, decrease in size due to pulse stress considerations, decrease in vacuum requirements for reliable operation, and a substantial decrease in cost of the apparatus. Through the ad-ditional use of multiple pulse overlap to avoid the deleterious effects of even sta-tistical spark-gap prefire or miss, such ap-paratus can provide a new order of reliability and uniform per-formance that enables the production-line results of the invention.

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Referring to the generalized system of Fig. 2, a pair of linear cold-cathode electron beam generators 2 and 2' i5 shown mounted in general opposition, w:ith their respective electron-permeable windows 1 and 1' irradiating a web W and/or articles carried -thereby, as schematically indicated b~ the arra~s of arrows emanating from 1 and 1', with the web W passing con-tinuously into the plane of the drawiny (-through conventional nitrogen or other gas-contained chamber or zone, as discussed, for éxample, in said Le-tters Patent). An array of stacked capa-citor-spark-gap Marx-type generator elements, later described, is disposed within an upper pressurized vessel M for driving the cold-cathode generators. A vacuum pump V is provided for -the - evacuated generator chambers 2 and 2', with the pulse feed con-ductor section F applying the periodic pulses to the cold-cathode diode structures, later more fully described, and with a cooling system comprising a heat exchanger H, pump P' and liquid reservoir R.
Turning, now, to the details of the irradiating genera-tors 2 and 2', this sec-tion of the apparatus i~ shol~n, in Fig. 3, on a larger scale than in Fig. 2, and in longitudinal section. The driving pulses from the Marx genrator in the upper pressurized vessel M are provided between an inner conductor 3 and the outer grounded vessel wall, and are fed via a vertical conductor ex-; tension 3' within the evacuated chambers F to a pair of horizon-tal conductor supports 4 and 4' supporting the respective cold-cathode mounting structures, of which the mount 5 is shown within the chamber 2 (it being understood -that a similar structure is provided within chamber 2'). The mount 5 supports the longi-tudinally extending field-initiated cold cathode gun 6 (as~ for jr~ - 8 -~ ~8~

example, of longitudinal parallel foil strips, such as of copper, graphite, or copper-graphite composite), facing the longi-tudinal electron-pervious window l; it being understood that the ca-thode of the gun within the chamber 2' will be upwarcll~ poin-ting toward the window 1'. I'he grounded outer conductor-wall windo~ of ~.he chambers 2 and 2' constitut:c the anodes of the cold ca-thode diode guns thus provided. UseEul field-initiated cold cathode gun configurations are descrlbed, for example, by Loda and DeHart (HQ Defense Nuslear Agency), "Tnvestigatiorl of pulsed cold cathode electron guns for use as a laser discharge sustainer", Physics International Company, DNA 2777F, May, 1972, PIFR-326.
The conical insulating bushing 7 supporting the~
conductors 3-3' on opposite sides of the apex seals -the gas-pressurized chamber M of the spark-gap driving circuits from the vacuum section F-2-2' of the electron beam generators, providing a most convenient high-voltage bushing, as well.
In accordance with the present invention, while the windows 1 and 1' of the electron genera-tors 2 and 2', generally oppose one another, they are rotated slightly relative to one another so that the exiting beams are offset or staggered, though overlapping partially (say, of the order of one beam width) to avoid direct bombardment into one another or other beam inter-ference, and, in sterilizing application, to eliminate the pos-sibility of transfer of organisms from one side of the web passed therebetween to the other.

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It has been found! moreover, that there ls a most important and determinatiye reiatlonship or connection between the irnpedance match effected between the cold-cathode gun and the driver circuits, and the nature both of the depth-dose profile characteris-tic attained from the xesul-tlng elec-l;ron beams and the pulse spectrum thereof. If the cold-cathode diode gun impedance is too low, the electron spectrurn has been found to be dominated by low-energy electrons and ~he depth-dose profile deviates from the described "OPTIMUM" profile, lO - as shown at "10W" in Fig. l; whereas, if the gun impedance i~s too high, an excess of both low-and high-energy electrons results, with the depth-dose profile curve showin~ a low half-dose point (as shown at "HIGH" in Fig. l)~ but great penetrating power and energy waste thereaf-ter. Through appropriate spacing of the plasma cathodè 6 and anode walls, as well as the number and dimensions of the cathode foil strips, the match can be adjusted to attain the desired "OPTIMUM" profile characteristic, and ad~ustment of pulse repetition rate can achieve operation which produces the novel results previously described.
It now remains to describe the preferred details of the capacitor-spark gap driver circuit, a simplified schematic diagram of which is illustrated in Fig. 4. Capacitor banks Cl-C2-etc. with associatea spark gaps Sl-S2-S3, etc. forming a Marx-type generator, are charged from a high frequency inverter 30 working directly from line current rectified by a rectifier network 32, as opposed to conventional D.C~ charging schemes where more~than half of the input power is absorbed in the charging resistors. The high-frequency inverter 30, with a ms/

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high transformer ~,C, voltage output (say~ 15 Kv rms at 10-20 KHz), driven a pair of converl~ional doubling circuits 36 of opposite polarity, with both polarities charyed simultaneously through comparatively small series capacitors 38 (say of the order of lOQpicofarads) ,that pump up the much laryer cap~citor~
Cl, C2 etc. through isolating resistors Rl. Stray capacitance and leakage reactance of the inverter ou-tput transformer are - used to effect self-resonating in the inverter. The capacitor bank is arranged to charge both positively and negatively simultaneously balanced to ground. Such balanced charging reduces D.C. insulation requirements by one half. The output (transformer) of the inverter 30 is thus exposed to only the load of the small pump capacitors 38 and pumps charge through the same in both directions. However, each pump capacitor 38 connects to the bank capacitors Cl, etc. through a diode, so that the bank is charged in only one direction. The inverter can therefore operate into the bank at zero voltage~ because the current out of the inverter is limited by the reactance -' of the small capacitance.
~ sensing resistor 42 measures the,voltage on the capacitor bank and feeds back a signal to the trigger generator 44 for comparison with a present reference. When the charge-sensing signal reaches the preset level, the trigger generator 44 produces an output pulse, commonly in the range OL 50 kv, applied to a trigger pin 46 which, in accordance with the invention, is situated in the first full gap or second stage S2 of the pulse generator system. By triygering other than in the first stage Sl, which is conventional in such generators as descrihed in the pr~vious~y cited references, it has been ;~ -11-ms/

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found that a substantial increase in the effective triygering range of the system is obtained~ the advantages of which have been previously mentioned, The first gap Sl, because it is greatly over-volted, breaks down after gap S2 and -then -the Marx system fires down the line, over-vol~ing S3 throuyh C2~ etc.
until the final driving pulse is delivered to the load RLoAD~
schematically representing the cold-cathode electron gun diodes, A preferred construction ls shown in Fig. 5 and in the transverse section thereo~ in Fig. 6, where the capacitors Cl, C2, etc. of the bank are shown supported by vertical columns 20 on alternately opposite or staggered sides thereof (to reduce interstage coupling), wi-th the spark gaps Sl, S2, S3, etc. in a ver-tical column therebetween, flanked by columns of the charging resistors R2, etc. The triggering pin 46 is shown associated with the second gun S2, as before explained. Further, the assembly is surrounded by a downwardly and outwardly tapered conical conductor or shield S (actually in octagonal sections~
Fig. 6) which has been found to be as close a shielding arrange-ment as can be provided without breakdown problems and which materially reduces the volume occupied by the magnetic field set up during pulse generation, thus reducing the inductance significantly and desirably increasing capacitance to ground.
This configuration has been found to aid in increasing the triggering range, as before discussed.
In practical apparatus of this type, highly successf~l production-line sterilization has been obtained with 75 nano-seconds pulses (full width at half maximum amplitude), produced at a repetition rate of 20 pulses~second at 225 Kv peak voltage and 2 kiloamps peak current, The electron beam width ms/

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2~'7~fl at windows 1 and 1I was about 4,0 cm, Synchronization of the line speed of the web W with the pulse repetition and dose-depth adjustments was effected such -that for a 5 megarad surface dose, the line speed of -the web W was adjus-ted to about 10 feet/
minute, and the windows 1 and 1' of the gun cylin~er ~enerators 2 and 2' were tilted at about a 15 offset from facing one another. Under these conditions, a linear dose-depth p~ofile close to that illustrated at "OPTIMUM" in Fig. 1 was ob-tained, - and with at least about a 10-pulse overlap provided which, though the reliability of the sytem was very high! avoided even the remote statistical possibility of a spark-gap prebreakdown or pulse miss, resulting in non-sterility As an example, B-pumilis, a radiation-resistant spore, was effectively destroyed (D10-value of 250 kilorads i.e. 20 log treatment). Voltage-pulse ranges of 200 = 50 KV! with pulse widths (measured as before indicated) of the order of 80 = 20 nanoseconds, and with pulse repetiton frequencies of the order of 20 = 10 pulses per second have been found most useful for certain sterilization purposes of the invention. Units involving products fed at higher line speeds (web speeds of about 25 meters per minute) are operable at repetition frequencies of the order of 100 pulses per second. As before stated, low energy electrons of the order of 50 to 450 KeV are useful for the purposes of the invention~ being generated by electric-discharge pulses of the order of 100 to 500 kV; and with pulse widths at one-half maximum of the order of 50 to 150 nanoseconds, and repetition frequencies of the order of 20 to 100 pulses per second.
While the invention has been described in connec-tion with its important application to cold-cathode beam s-terilization, ms/

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features of the same may be used in othe~ applications where similar advantages are desired~ and t~e novel aspects of circuit and constructional details may also be used elsewhere as desired; further modifications occuring to -those skilled in the art being deemed to fall within the spirit and scope o~
the invention as defined in the appended claims.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for electron-beam-irradiating surfaces passed along a predetermined region, for purposes of steriliza-tion and the like, having, in combination, electrically triggered repetitive pulse-generating means having a stacked array of discharge gaps connected with a corresponding staggered co-extensive array of capacitors and disposed in a pressurized vessel; evacuated electron gun means provided with electron-pervious window means; means for electrically connecting the pulse-generating means to the electron gun means to draw there-from electron-beams exiting the window means in response to the pulses generated by the pulse-generating means; and high-voltage insulating means separating said pressurized vessel from said evacuated electron gun means and supporting said electrically connecting means.