Method Of And Apparatus For Producing Energetic Charged-particle Extended-dimension Beam Curtains And Pulse-producing Structures Therefor

Abstract

This disclosure deals with extending the high voltage operation of energetic charged-particle extended-dimension beam curtain generators, preferably electron beam curtain generators, without permitting breakdown between evacuated electrode structures, by employing specially shaped high voltage pulses of substantially comparable very steep rise and fall times, preferably by resonant transformer action, while limiting the much longer pulse time duration to a value insufficient to permit such breakdown.

Description

The present invention relates to methods of and apparatus for producing energetic charged-particle extended-dimension beam curtains and pulse-producing structures therefor, being more particularly, though not exclusively, directed to pulsed energetic electron beam curtains.

Extended-dimension charged-particle beam curtain generators have heretofore been proposed for enabling the treatment of large areas by such curtains without the necessity for scanning or the like, as in the case of pencil or smaller focused beam systems, as described, for example, in U. S. Letters Pat. No. 2,887,599 and in copending application Ser. No. 153,769 of B. S. Quintal, entitled, "Apparatus For and Method of Producing an Energetic Electron Curtain," and assigned to Energy Sciences Inc., the assignee of the present invention. While these energetic charged particles may be electrons or ions, as is well known, they shall be described herein in connection with the preferred electron beam curtains.

In the treatment of large areas by energetic electron or ion beams, either by the application of scanning, in the case of beams of small cross-section (pencil beams) or of extended-dimension electron-emitting curtain surfaces, a dc voltage is generally used for accelerating the charged particles. The extended-dimension electron beam approach is highly attractive because of its simplicity, compactness, and its constant and linear beam trajectory. The extended electron or ion source approach to beam processing systems is also adaptable for the treatment of non-planar products; e.g. of a coaxial nature, as described, for example, in copending application of S. Nablo, Ser. No. 151,640, entitled "Apparatus for the Bilateral Isotropic and Cylindrically Symmetric Irradiation of Objects Using Energetic Electrons." Energetic electron beams, and sometimes ion beams, are being increasingly used for the processing of material such as the curing of metal coatings, the cross-linking of plastics, and the sterilization of materials, as three major processes of economic interest.

While such curtain systems are ideally suited to lower-energy applications, e.g., below 300 kV, higher voltage operation is limited by vacuum breakdown problems in the evacuated electrode structure system. The present invention has as one of its primary objects, accordingly, the extending of high voltage operation and the removing of this voltage limit on operation by adopting a mode of operation wherein the accelerating voltage is applied in the form of a (repetitive) pulse, critically shaped and of duration sufficiently short that vacuum breakdown processes do not have time to develop fully. The pulse duration range of primary interest herein is from one to several hundred microseconds.

A further object of the invention is to provide a new and improved method of and apparatus for producing energetic charged-particle extended-dimension beam curtains, including those of energetic electrons, not subject to the higher voltage breakdown problem, and for providing novel pulse-producing structures particularly adapted therefor.

An additional object is to provide a novel charge-particle pulse-producing resonant transformer of more general application, as well.

In summary, from one of its aspects, the invention contemplates a technique for extending the high voltage operation of energetic charged-particle (preferably electron) extended-dimension beam curtain generators embodying electrode structures disposed in an evacuated housing and susceptible to breakdown discharges with high voltage applied thereto, said method comprising simultaneously producing charged particles along an extended dimension, applying a high voltage pulse to the electrode structures to form the particles into a pulse of a high voltage energetic charged-particle extended-dimension beam curtain, and adjusting the said high voltage pulse to substantially comparable very steep rise and fall times, while limiting the much longer pulse time duration to a value insufficient to permit such breakdown. Preferred operational and constructional details, including preferred pulse-generating structures are hereinafter set forth.

Other and further objects are explained hereinafter and are more particularly delineated in the appended claims.

The invention will now be described with reference to the accompanying drawings, FIG. 1 of which is a graph plotting interelectrode and housing vacuum breakdown characteristics in such energetic electron generators along the ordinate as a function of the gap spacing between the evacuated electrodes and/ or housing structures (in centimeters), curve A being for dc or continuous voltage operation and showing the rapidly reached breakdown for short gaps in the 200-300 Kilovolt (kV) range, and graph B showing the greatly extended high-voltage operation using the discovery of the present invention in connection with specially shaped and tailored fast pulse voltages;

FIG. 2 is a longitudinal section of an exemplary energetic electron curtain apparatus, together with schematic circuit, operated in accordance with and embodying the invention in preferred form;

FIG. 3 is a voltage waveform diagram illustrating the resonant transformer pulsing of the structure of FIG. 1;

FIG. 4 is a section similar to FIG. 2 of a modification; and

FIG. 5 is a view similar to FIG. 4 of a coaxial version thereof for simultaneously irradiating objects from widely different directions.

The process which limits vacuum insulation performance at large electrode gaps is called "current loading" created by the aggregate of small pulses of charges passing between the electrode structures (microdischarges) which leads to deterioration of the vacuum, with ensuing gas discharges.

In Smith, W. A. and Mason, T. R. "Preliminary Measurements of Time Lags to Breakdown of Large Gaps" Proceedings 2 nd Int. Symp. on Insulation of High Voltages in Vacuum, p. 97 (1966 ) and Smith, W. A. et al, "Impulse Breakdown and the Pressure Effect," Proceedings 3 rd Int. Symp. on Discharges and Electrical Insulation in Vacuum, p. 203 (1968 ), for example, there are described experiments with the pulse performance of vacuum gaps at voltages up to 340 kV and compared with continuous voltage performance. These pulses were relatively steeply rising, but vastly more slowly falling as decaying pulses. The pulse voltage breakdown strength proved to be about 60 % higher than the continuous voltage strength, and breakdown developed on an average of 24 microseconds after the start of the pulse. A discovery underlying the present invention, however, resides in the vastly novel improvement attainable with pulses, typically of duration less than 10 microseconds, with comparably steep rising and falling edges or times and longer duration pulse width limited, however, to inhibit the formation of vacuum discharges at these lower voltages, so typical of continuous voltage operation. This has now made near megavolt (FIG. 1) and megavolt, single-gap acceleration systems possible, greatly extending the range of the electron curtain processes. A further advantage of such tailored-pulse form of operation is that the feed-through bushing into the vacuum can be capacitively graded through suitable geometric shaping and can then be made smaller than the corresponding dc bushings in the continuously operated curtain.

One particularly attractive method of accomplishing this particular pulsed operation is through the use of a "double resonant" pulse transformer network such as has been described by E. A. Abramyan et al, for use with "pencil beam" accelerators, in "High-Current Transformer Acclerators," published by the Institute of Nuclear Physics, Novosibirsk U.S.S.R., 1970, and in "High-Current Accelerators for Scientific Industrial Use," published by Techsnabexport, Moscow, 1971, and in U. S. Letters Pat. Nos. 3,390,303 and 3,450,996. This form of transformer operates particularly effectively when primary and secondary circuits are both brought into resonance at the same frequency with a coupling factor of about 0.6, at which the output waveform has the shape shown in FIG. 3, the operative half cycle being the second (which has the greater amplitude) for electron beam acceleration, such second half cycle being negative, as shown. The transformer can be made to operate up to about 80 KHz and above, which would give a half-cycle duration of about 6 microseconds. As described by Abramyan et al, the filament power and grid controls may be fed to the high voltage terminal by having the pulse transformer secondary consist of multiple windings in parallel, such as bifilar windings. The secondary winding may also be the outer shield of a multiconductor cable, the internal conductors carrying low voltage power and signals to the high voltage terminal. A further advantage of such pulse form of operation is that the feed-through bushing into vacuum can be capacitively graded through suitable geometric shaping and can then, as before stated, be constructed smaller than the corresponding dc bushing in the continuously operated curtain.

Referring to FIG. 2, such a pulse transformer network is shown at PT, having a primary winding P, preferably frustoconically shaped and comprising substantially self-supporting relatively large-size copper or other conductive strips, mounted within a conductive evacuated housing structure 15, at one end thereof, substantially in line with an electron beam curtain gun EG, with the housing 15 serving as the anode of the gun and supporting an electron-pervious egress or exit window 17, as described in said copending Quintal application. A conventional pulse driving circuit P. D. (say, for 50 kV pulses) is connected to the primary P, the primary coaxially surrounding the multiple parallel-winding cylindrical secondary S, shown sealed from communication with the vacuum V (say, of the order of at least 10.sup..sup.-3 Torr), within the anode housing 15, by a ceramic or other insulating cylinder C containing an insulating gas I, such as SF.sub.6 , or an insulating fluid, as of oil, or the like. The left-hand terminal of the secondary winding may be connected to the anode housing 15 at the ground G, and the low voltage for the before-mentioned filament and grid control may be developed between the multiple windings to be converted at 21, as is well-known. The cathode electron or charged particle source is illustrated as a longitudinally extending filament 1 disposed within a channel 3 provided with a control grid 7 extending longitudinally parallel to an coextensively with the cathode 1 and transversely thereof to the walls of the channel 3, as described in said Quintal application. Coaxially surrounding the cathode 1 and control grid 7 is an electrostatic shield or Faraday cage 11 having a further longitudinal grid structure 13 aligned with the cathode 1 and control grid 7 to form the beam into an extended-dimension energetic electron curtain that may exit as it expands through the window 17. The high pulse voltage from the secondary is applied at the electrode 13 from the high voltage or right hand terminal of the secondary winding S, this being done as the cathode source 1 is caused to emit electrons in response to an appropriately timed control grid pulse applied to the control grid 7, as described by said Abramyan et al.

This operation is illustrated in the waveforms of FIG. 3, the resonant transformation in PT producing oscillatory ringing 11' (say, up to 80 KHz), with the first negative half-cycle 11" being used for the high voltage pulsing. The control grid is pulsed, as described, for example, by Abramyan et al, and is shown at 13', to produce the substantially monoenergetic electron pulse within the voltage pulse duration which has a somewhat longer time of some microseconds, say 6 or more, consistent with the holding off of electrode-gap vacuum breakdown, discussed in connection with FIG. 1.

Though the resonant pulse transformer PT (or other substantially similar-performance pulse-forming network) is mounted in-line with the gun EG in the embodiment of FIG. 2, it is shown mounted within an intermediate transverse extension, in the embodiment of FIG. 4; and other mounting positions are also possible. In this modification, the end of the sealed chamber formed by the secondary S and its external insulating cylinder C is provided with a sealed voltage bushing B for enabling the necessary voltage connections to be effected within the gun structure EG. This construction, moreover, is particularly advantageous, as shown in FIG. 5, for irradiating objects 0, such as longitudinal plastic-covered wire-to-be-treated as it is drawn longitudinally past the window 17. Where simultaneous irradiation from widely different directions or coaxially thereabout is desired, as described in said Nablo application, additional curtain guns or gun portions differently directed radially about the object, such as EG' with window 17' etc. may be provided, also pulsed from the pulse transformer P-S. It will be noted that there is no requirement for a high vacuum feed-through bushing in these embodiments since the pulse transformer itself protrudes into the vacuum V, maintained as at 2. The transformer secondary itself has to be well graded dielectrically and thus also grades the dielectric surface which supports the high voltage terminal in vacuum. Typically, the pulse transformer secondary S, housed inside a ceramic tube C with vacuum on the outside and high pressure gas or insulating fluid on the inside, has its high voltage terminal shaped so that several radial electron beams can be accelerated to treat such a coaxial product as the wire or cable O of FIG. 5. The electron beams and their support structures can also be configured to treat special shapes other than coaxial.

In actual tests a structure of type above-described was designed to operate at 150 Kv dc. With such dc or continuous operation, the system started to condition (show signs of electron loading in the vacuum) at about 80 Kv, and took 2 hours of conditioning to reach 150 Kv. When operated in accordance with the present invention with 8 microsecond pulses at 5 pulses per second, 200 Kv was attained immediately and over 300 Kv was attained within a few minutes of conditioning. The vacuum was about 2 .times.10.sup.-.sup.6 Torr, with about 10 centimeters of spacing between the high voltage accelerating grid and the window in the grounded outer coaxial housing of about 30" diameter.

Further modifications will also occur to those skilled in this art, and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. Apparatus for producing energetic charged-particle extended-dimension beam curtains having, in combination, a dimensionally extending source of charged particles, first electrode means positioned on one side of the source and extending therealong and thereacross, means for applying potential between the source and the first electrode means to generate a dimensionally extending charged-particle beam, further electrode means aligned with the source and first electrode means for shaping the beam into a curtain, an evacuated housing structure surrounding the source and electrode means and having a charged-particle-pervious window substantially aligned with the first and further electrode means for transmitting the curtain outside said housing structure, means comprising pulse network means disposed contiguous to said housing structure and connected with said further electrode means for developing a high voltage pulse of substantially comparably steep rise and fall times but of duration insufficient to permit breakdown within the housing structure between any of the source, both the electrode means, and the housing structure, and means connected with the first electrode means for enabling the drawing of charged particles from said source during the developing of said high voltage pulse, whereby a substantially greater charged-particle curtain voltage above several hundred kilovolts is produced without any such breakdown than the apparatus can produce without breakdown during either of direct-current and capacitor-discharge-controlled-wave-form pulsing operation thereof.

2. Apparatus as claimed in claim 1 and in which said pulse network means comprises primary and secondary transformer windings the former of which is connected to pulser means and the latter of which is connected to said further electrode means to pulse said source, with the windings being adjusted to resonate at a common frequency and coupled to develop an oscillatory ring comprising a high voltage pulse as a half-cycle thereof.

3. Apparatus as claimed in claim 2 and in which said primary winding is disposed within said evacuated housing structure exposed to the vacuum thereof and the secondary winding is substantially coaxially disposed therewithin but insulatingly sealed from said vacuum.

4. Apparatus as claimed in claim 3 and in which said transformer windings are mounted substantially in-line with the dimensional extension of said source and electrode means.

5. Apparatus as claimed in claim 3 and in which said transformer windings are mounted at a position intermediate the ends of the said source and electrode means and extend therefrom.

6. Apparatus as claimed in claim 3 and in which said primary winding comprises relatively large-size winding turns formed substantially frusto-conically, said secondary winding comprises multiple substantially cylindrical smaller windings, the insulating seal comprises a dielectric cylinder enveloping the secondary windings, and an insulating medium comprising one of gas and insulating fluid is sealed within the secondary windings.

7. Apparatus as claimed in claim 2 and in which the transformer resonant frequency is adjustable to values up to the order of substantially 80 KHz to produce half-cycle duration high voltage pulses of duration down to the order of substantially a few microseconds.

8. Apparatus as claimed in claim 7 and in which said source comprises a longitudinally extending electron cathode, said first electrode means comprises a substantially longitudinally extending control grid means, said further electrode means comprises further grid means, and said housing structure comprises a conductive anode means with electron-pervious window means for exiting the produced electron curtain pulse.

9. Apparatus as claimed in claim 8 and in which said pulse network means comprises primary and secondary transformer windings the former of which is connected to pulser means and the latter of which to said further electrode grid means, with the windings being resonated at a common frequency, and coupled to develop an oscillatory ring comprising the said high voltage pulse as a negative half-cycle thereof.

10. Apparatus for producing energetic charged-particle beams having, in combination with an evacuated housing containing a source of charged particles, pulse network means comprising primary and secondary transformer windings the former of which is connected to pulser means and the latter of which develops a high voltage pulse, means connected to said secondary winding for pulsing said source and for producing a charged-particle beam, the windings being adjusted to resonate at a common frequency and coupled to develop an oscillatory ring comprising said high voltage pulse as a half-cycle thereof, means for supporting said primary winding within said evacuated housing exposed to the vacuum thereof and means for supporting the secondary winding substantially coaxially disposed therewithin but insulatingly sealed from said vacuum.

11. Apparatus as claimed in claim 10 and in which said transformer windings are mounted substantially in-line with said source.

12. Apparatus as claimed in claim 10 and in which said primary winding comprises relatively large-size winding turns formed substantially frusto-conically, said secondary winding comprises multiple substantially cylindrical smaller windings, the insulating seal comprises a dielectric cylinder enveloping the secondary windings, and an insulating medium comprising one of gas or insulating fluid is sealed within the secondary windings.

13. Apparatus as claimed in claim 10 and in which the transformer resonant frequency is adjustable to values up to the order of substantially 80 KHz to produce half-cycle duration high voltage pulses of duration down to the order of substantially a few microseconds.

14. Apparatus as claimed in claim 13 and in which said source comprises a longitudinally extending electron cathode, having a corresponding longitudinally extending control grid means, said means for pulsing said source comprises further grid means, and said housing comprises a conductive anode means with electron-pervious window means for exiting the produced beam.

15. A method of extending the high voltage operation of energetic charged-particle extended-dimension beam curtain generators embodying beam-curtain-forming electrode structures disposed in an evacuated housing and susceptible to breakdown discharges with high voltage applied thereto, said method comprising simultaneously producing charged particles along an extended dimension, applying a high voltage pulse to the electrode structures and forming the particles into a pulse of a high voltage energetic charged-particle extended-dimension beam curtain, and adjusting the said high voltage pulse to substantially comparable very steep rise and fall times while limiting the much longer pulse time duration to a value insufficient to permit such breakdown.

16. A method as claimed in claim 15 and in which said adjusting and limiting is effected by resonantly transforming an impulse to generate said substantially comparable rise and fall time high voltage pulse as a half-cycle of such resonance.

17. A method as claimed in claim 16 and in which the resonance frequency is adjustable to values up to the order of substantially 80 KHz to produce half-cycle duration pulses down to the order of substantially a few microseconds.

18. A method as claimed in claim 16 and in which said charged-particle producing step comprises generating electrons, and said half-cycle of resonance is selected as a negative half-cycle thereof.

19. A method as claimed in claim 15 and in which an object-to-be-irradiated is drawn past said curtain, and a further similar energetic charged particle extended-dimension beam curtain is similarly pulsed and substantially simultaneously directed upon said object from a different direction than the first-named curtain.