US2897400A - Adjustable bias for electron beam apparatus - Google Patents

Description

July 28, 1959 r w. F. WESTENDQRP 2,897,400}

"ADJUSTABLE BIAS-FOR ELECTRON BEAM APPARATUS Filed Oct. 24, 1956 (b) T If 9 f r Inventor: i I i 4- i I i i W/Wem-E Wesfendorp,

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His A horrnsy 2,897,400 ADJUSTABLE Brit's jFo ELECTRON 'BEAM APPARATUS Willem F. Westenil'orp, Schenectady, -N.Y., assignor to General Electric Company, a corporationof New York Application ctober24, '1956, Serial No. 618,139

6' Claims. (C1. 3 15 p This invention relates 'to'a network for producing a bias voltage for electron 'beam apparatus and more particularly 'to an adjustable network "for producing a biasvo ltage waveform which is readily'contro'llable'as to relative duration and which is independent 'of the operating voltage and'current of'the electron'beam appar'atiis'to'whichit'is app1ied.

This invention may be adapted in various forms for applicationgenerally to many devices employinga cyclic .lly varying, input voltage. It is, however, especially suited for use'with high-energy electrombe'amapparatus of theresonance transformer type-such'as disclosed in mylatent 2,144,518,issuedJanuary'U, 1939. In such apparatus, an electron gunprovides highenergyelcctrons for a directed bea'rn inresponsetoa sinusoidalapplied voltage. Electrons from the *g'unpass through a focusing electrode andflare then accelerated by s'ucc'essivelyhigher 'pote'ntials applied to a plurality of accelerating electrodes ir-W n h beain path. 1

I t'isap'parent" that electrons accelerated from this form of equipment would have a Wide range of energies including those associated with therelatively low pote'n'tial p'or'tions of ithe'appliedf voltage and t'ho'se' ass'oc'iated' with peak portions. Generally, the accelerating 'voltage has-a substantially sine wave formyafid the electron-'ener'gy-distribution follows this same function. Such'a Wide velocity ftlistribution renders focusing of the beam eirtreinelydif- 'ficult and detracts from theefiiciency of-the apparatus i because the' lower energy elections do no usefuhworleand give rise to deleterious heating effects.

My copending applicationsj Serial No.- 5 l8 '-,199, filed June 27,"19 55 now Patent No; 121st1.65s;afid seriarno. 595,594, filed ]uly 2,; 1956,? new Patent"No.-'2,844757, 'f and assigned" to the same assignee 1 as' tiiis" application, pertain tocircuitsfor biasingl electron beam apparatus to obtain' a beam of .electronsvvhich is subStantialIYmOnO- energetic. Apealgvolt'age' having proper-'phase, amp-1itude, and periodicity with respect to' the f accelerating l volta ge is applied 'as' a bias to the controlelec'trode so that electrons flow inthe equipment only during a peak portion of the 'cycleof theapplied' acceleratingyoltage.

Such circuits are eminently" s uited' 'for application to 'X-ray equipment and to certain formsof highenergy electronlbeam irradiation equipment. However, these circuits employ a reference for the bias voltage which is determined from a fractional portion of theoperating potentials, so that the: phasing of" the bias circuit is dependent upon the load current of the=apparatus. Further, it would be desirable to provide a bias circuitinwhich the relative duration of'the bias potential with respect to the "useful half cycle of the operatingpotential" is readily controllable without necessitating complicated ad- "justment'of established circuit parameters.

Therefore, it is an object of this inventionto' provide a circuit to produce for electron "bearnapparatus 'a'bias voltage having a Wave-'formwhich i's 'inh'erentlyphased correctly with'respect to the-cyclically"varyingioperating Patented July 28, 1959 prises a full Wave rectifier bridge inseries with the oper- *atingvolt'agesupply of the apparatus and'responsive to the dielectric or charging current that flowsbetween the high voltage components of the apparatus andLground. The rectifier bridge is connected to energize in parallel a wave-form generating circuit and a referencewoltage circuit'which cooperate to produce a bias-voltagewaveform adjustable as to duration and inherently phased correctly with respect to the operating voltage.

Fe-atu'res of'the invention which are 'believe'dzto the "novel-are po'inte'd out with particularity in the appended claims.

However, for a better understanding a of the 'invention, together with further o'bjects and advantages there'of,-'reference should be had tothe following fdescmp 25 ing, wherein: I

ti'on taken in conjunction with the accompanying draw- Fig. 1 is -a--scher naticrepresentation-of a resonance transformer electron" 13 em apparatus embodying this invention, and

Fig. 2 illustrates wvave forms useful in expla ining' the characteristics of-the apparatus of Fig. 1.

'Referring-nowto Fig. If-thereis illustrated-at l a v resonanttransformer 'X-rayapparatus. l It is -un'cler"s tood, "however, thatmy invention may be -advantageously ap- 35 biasingvoltage in conjunction wit-h -a-' cyclically varying operating voltage. The biasing network of this invention is designated generally at2. The -Xray apparatus Linplied to other apparatus-wherein it is de'sirable to :apply a cludes a metallic enclosure or shield 3surroundingtheelements'of the apparatus and grounded as -at{4. The electron gun '5 comprises a filament 6 continuouslyheated to 'provide 'an emission limitedelectronsupply. -Suit'ably mounted relative to thefilament is a control or focusing "electrode 7 for shaping the electron beam andarraye d along the path of the beam is a series ofaccelerating electrodes -8. Operating voltage for the'X-ray apparatus-is supplied from a low-alternating'voltage source 9by means of a resonance transfonner comprising a primarywind ing' 10 "and stacked and series-connected secondary coils 1 1. Between shield 1 and the apparatus there isinterposed a hemispherical high voltage shield l2- vvhiehg in "conjunction'with the shield 3, provides "an-equivalent capacitance path to ground as indicated'by the'equiv'alent capacitance 13.

In operation, the transformersecondary winding 11 is-resonant with the capacity to. ground and produces potentials in the order of 1 million volts. Thisioperation is described in detail in my'aforementionedPatent 2,144,518. These potentials are applied to the accelerating electrodes 8 by means of conductors 14 from respective taps 15 on the coil stack 10 to' apply .in'synchronism discretely increased accelerating voltages' in sequence-alongthe beam direction. The electron beam from the gun emerges from. the apparatus through'the tube'16 passing through Window 17. ThefiIam'entG is energized from a filament supply windingll' on top of the transformer secondary 11.

It will be readily apparentthat the beam-energies --wi11 tend to vary from energies associated with the low potential portions of the sinusoidal accelerating voltage "applied to electrodes 8 to those potentialsassociated with 2: peak; potentials.

Since ut-hez low energyai portions of the beam may be absorbed in the window 17 causing deleterious heating effects and producing no useful result, it is desirable that the beam current flow only during the peak voltage portions of the operating cycle. Biasing network 2 is provided according to this invention to apply a negative potential to control electrode 7 to cut off the electron beam in synchronism with the sinusoidal operating voltage. I

The biasing network 2 comprises a rectifierbridge 18 connected in series with the high voltage terminal of the transformer secondary 11 and the shield 12 to provide a full wave rectified current at output terminals 19 and 20 having the indicated polarity and dependent upon the charging current flowing through the distributed capacity '13. A reference voltage circuit 21 is connected between terminals 19 and 20 and includes a rectifier 22 or other asymmetrically conducting device in series with a capacitor 23 so that the capacitor is charged by the full wave rectified current from the rectifier bridge 18. A shunt resistor 24 in parallel with capacitor 23 provides a discharge leakage path for the capacitor to establish the steady-state volta e developed on the capacitor. Connested in parallel with the capacitor circuit and in parallel with terminals 19 and 20 is a reactor circuit 26 comprising an inductance 27 and a series resistor 28. The function of the reactor circuit is to establish a nearly constant direct current which circulates through the rectifier bridge 18 and the reactor circuit 26. The output volt age from the biasing network may be applied to the primary winding 29 of a step-up transformer 30, the secondary Winding 31 of which is connected between the filament 6 and the control electrode 7. Since the output voltage of the biasing network supplied to the transformer primary winding 29 has a substantial direct current component, it is necessary to provide a blocking capacitor 34. It is also desirable to provide in the circuit of the secondary winding 31 a direct current voltage restoring circuit. This circuit may take the form of a capacitor 32 connected in series with the winding 31 and the unilaterally conductive device 33 connected across the winding 31 and capacitor 32 and poled in the proper direction so that it does not suppress the negative voltage peaks produced by the biasing circuit.

The biasing network of the present invention, as an important feature, provides a bias whose duration and phase are independent of the operating voltage and current of the tube involved and which is timed to provide for conduction during a limited portion of the time that the device may be conducting and located near the crest of the voltage applied to the beam-forming electrodes so that the beam is made up of electrons having relatively high and relatively uniform acceleration. The present circuit takes advantage of the high, relatively constant alternating current voltage that exists between the high voltage shield, usually employed as an enclosure for the high voltage apparatus for distributing and determining the voltage gradient between this apparatus, and the enclosing tank which is usually operated at ground. In an apparatus operated at a million volts, for example, a dielectric current, that is, the charging current for the distributed capacitance illustrated schematically ,at 13 between the shield 12 and the tank 3, may be in the order of 30 milliamperes. In the biasing network illustrated, this current is utilized to produce the desired hold-off bias and, as will become apparent, is ideally phased for this purpose with respect to the accelerating potential applied to the tube 5. The current is passed through the full wave rectifier bridge 18. The output of this bridge circuit causes a current to flow through the reactor circuit including reactor 27 and resistor 28. The alternating current impedance of reactor 27 is sufficiently large to suppress largely the alternating com ponent in the reactor circuit and to establish a substa-n= tially constant current curve, indicated by I in Fig. 2(0). The total rectified current from the network 18 is illustrated by the curve i, Fig. 2(c). During that portion of the current cycle where this current i exceeds the current I the excess current flows through the capacitor circuit including the rectifier 22 and capacitor 23. After capacitor 23 has been initially charged in a previous current cycle, this occurs at a predetermined voltage of capacitor 23 so that as the value of current i in Fig. 1(a) exceeds the value of current I the voltage of capacitor 23 is impressed on the primary winding 29 of transformer 30 through the blocking capacitor 34 and thus establishes the value of the negative bias voltage E illustrated in Fig. 2(b). The output transformer 30 of the biasing circuit is a step-up transformer which also serves to isolate the bias voltage supply circuit from the focusing electrode 7. The transformer draws a small magnetizing current and the voltage wave shape in the primary winding circuit is essentially the same as that applied to the focusing electrode. Since the circuit including the distributed capacity 13 between the shield 12 and the enclosing tank 3 is essentially capacitive, the current i of Fig. 2(c) leads the voltage E impressed on the discharge device 5 by almost exactly This places the intervals between the negative peaks of voltage E symmetrically about the zero point of the capacitive current i and as readily observed from a comparison with Fig. 2(a) places these intervals about the maximum point of the accelerating voltage applied to the discharge device 5. Accordingly, the periods during which the negative bias is removed from the electrode 7 occur about the portions of the supply voltage near maximum amplitude. This is illustrated in Fig. 1(a) Where the beam current I is illustrated as a current pulse located centrally with respect to the voltage maximum applied to the discharge device and having a duration equal to the interval between the negative bias pulses E It will be apparent to those skilled in the art that beam current flows only during those half cycles of the impressed voltage during which the cathode is negative with respect to the electrode 16.

It is apparent that the value of the resistor 28 directly affects the magnitude of the current I that is reached before the rectifier 22 becomes conductive and the capacitor is again effectively placed in circuit to receive charging current through rectifier 22. It is also apparent that the resistor 24 has an elfect since it determines the rate of discharge of the capacitor during the period that the rectifier 22 is nonconductive and so determines the actual charge on the capacitor just prior to the instant that conduction of device 22 starts. The periods during which the negative bias voltage is produced and the periods between these negative voltage pulses are therefore determined essentially entirely by the relative magnitudes of resistors 28 and 24. The magnitude of the voltage, during the negative pulses is dependent on the absolute magnitude of these resistors.

In the foregoing description and particularly in the curves of Fig. 2, the voltage and current waves have been illustrated as if there were zero change in current through the inductance and zero change in voltage on the capacitor 23. These are idealized conditions, but theoretical analysis and tests have indicated that these assumptions are justified. There is, of course, a small decrease in the current 1;, during the intervals between the negative voltage pulses of curve E during which time the energy of the inductance tends to circulate current through the rectifier and a gradual increase in current through the inductance as the charge on the capacitor 23 is fully restored during the negative voltage pulses. The change in the voltage of the capacitor during the beginning of the voltage pulse depends to a considerable extent upon the magnitude of the resistor 24 and the amount that the voltage on the capacitor has decreased during the preceding period of nonconduction of the rectifier 22, that is, the period between the negative biasing voltage peaks. However, the sides or edges of these negative voltage peaks are relatively steep and the etfect on the phase of the negative voltage pulse is negligible.

From the foregoing description, it is apparent that the present invention makes unique use of an electrical quantity existing in the equipment, namely, the dielectric or charging current that flows from the high voltage apparatus through the shield to the grounded enclosure, as a means for providing a timed voltage pulse with respect to the accelerating voltage applied to the beam. By a relatively simple selection of resistance values in the inductive and capacitive branches of the biasing circuit, it is possible to adjust the duration of the period of conduction and this conduction period is essentially symmetrical with respect to the instant of the maximum amplitude of the applied voltage wave and is independent of the operating voltage and current of the discharge device. With the resistance 28 having a value of 8000 ohms and the resistance 24 having a value of 12,000 ohms the period of conduction of the tube 5 is of approximately 42 degrees duration and increases to approximately 66 when these resistances have values of 4000 ohms and 20,500 ohms, respectively. These values of resistances are chosen to give approximately the same values of the negative bias peaks E It is these two resistance values which may be adjusted to give the desired duration of conduction while maintaining substantially constant magnitude of the bias voltage peaks. Accordingly, the present invention overcomes many of the disadvantages inherent in prior art biasing circuits and may be carried out with a small number of readily obtainable components.

While I have described a particular embodiment of my invention, it will'be apparent to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects and I aim, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A system for producing a biasing voltage for high voltage electron beam apparatus in timed relation to an alternating supply voltage for said electron beam apparatus, comprising a network connected to respond to the dielectric current flowing between the high voltage parts of said apparatus ground exteriorly of said electron beam apparatus, said network including means providing a reference voltage and means responsive to the magnitude of said dielectric current for energizing said beam apparatus from said reference voltage to prevent the flow of electron beam current for that portion of the dielectric current wave that exceeds a predetermined value.

2. A network for producing a biasing voltage for high voltage electron beam apparatus in timed relation to an alternating supply voltage for said electron beam apparatus comprising a biasing network connected to respond to the charging current flowing between said high voltage apparatus and a shield, said network comprising rectifier means for converting said charging current to unidirectional pulsating current, means energized from said rectifier means for establishing a substantially constant current of smaller magnitude than the amplitude of said pulsating current and means for maintaining said electron beam apparatus nonconducting during the interval that said pulsating current exceeds said substantially constant current.

3. A network for producing a biasing voltage for high voltage electron beam apparatus in timed relation to an alternating supply voltage for said electron beam apparatus comprising a biasing network connected to respend to the charging current flowing between said high voltage apparatus and a shield, said network comprising rectifier means for converting said charging current to unidirectional pulsating current, circuit means including a series-connected resistance and inductance energized from said rectifier means for establishing a substantially constant current of smaller magnitude than the amplitude of said pulsating current and means for maintaining said electron beam apparatus nonconducting during the interval that said pulsating current exceeds said substantially constant current.

4. A network for producing a biasing voltage for high voltage electron beam apparatus in timed relation to an alternating supply voltage for said electron beam apparatus comprising a biasing network connected to respond to the charging current flowing between said high voltage apparatus and a shield, said network comprising rectifier means for converting said charging current to unidirectional pulsating current, first circuit means energized from said rectifier means for establishing a substantially constant current of smaller magnitude than the amplitude of said pulsating current and second circuit means including a unilaterally conducting device and a capacitor in series and in parallel with said first circuit means for maintaining said electron beam apparatus nonconducting during the interval that said pulsating current exceeds said substantially constant current, said second circuit means including a resistor providing a discharge path for said capacitor.

5. A network for producing a biasing voltage for high voltage electron beam apparatus in timed relation to an alternating supply voltage for said electron beam apparatus comprising a biasing network connected to respond to the charging current flowing between said high voltage apparatus and a surrounding shield, said network comprising rectifier means for converting said charging current to unidirectional pulsating current, means energized from said rectifier means for establishing a substantially constant current of smaller magnitude than the amplitude of said pulsating current including resistance means for determining the magnitude of said constant current, and means for maintaining said electron beam apparatus nonconducting during the interval that said pulsating current exceeds said substantially constant current.

6. A network for biasing resonance transformer electron beam apparatus, said network comprising a rectifier bridge circuit for connection in series with the resonance transformer of the apparatus to produce a full-wave rectified current through said network, a reactor circuit connected across said bridge and including an inductor and a first resistor in series to establish a direct current of substantially constant predetermined magnitude through said bridge and a reference voltage circuit connected in parallel with said reactor circuit and including a capacitor and a rectifier in series and a second resistor shunting said capacitor to maintain a charge on said capacitor and thereby produce a reference bias voltage for said apparatus, said capacitor being efiectively connected to the output of said network through said rectifier when the current through said rectifier bridge exceeds said predetermined magnitude.

References Cited in the file of this patent UNITED STATES PATENTS 1,690,906 Morrison Nov. 6, 1928 2,286,091 Hang et al. June 9, 1942 2,405,477 Westendorp Aug. 6, 1946 2,448,771 Chn'staldi Sept. 7, 1948 2,537,862 Samuel Jan. 9, 1951 2,591,918 Cole Apr. 8, 1952 2,798,963 Saget July 9, 1957

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