US2340363A - Control for focal spot in X-ray generators - Google Patents

Description

Z. J. ATLEE ETAL Filed March 3, 1942 CONTROL FOR FOCAL SPOTS IN X-RAY GENERATORS Feb. 1. 1944.

0 0 0 .w v w w w mm R o m% w a 9 H 015 J 4 m m a F m P m .0 l Mm J w W Z w W m a r i HN a me m wmmw WW amwam w E H 0 H0 wpwwvm E w a wfi xwm G N MW 5 a Ma m m5 LQSWkKB-WQAK I w m o w w m w w w m w o Patented Feb. 1, 1944 CONTROL FOR FOCAL SPOT IN X-RAY GENERATORS Zed J. Atlee, Elmhurst, and Frank R. Abbott,

Berkeley, Ill., assignors to General Electric X- Ray Corporation, Chicago, 111,, a corporation of New York Application March 3, 1942, Serial No. 433,208

7 Claims.

Our present invention relates in general to electronics and has more particular reference to the operation of X-ray generators, the invention applying specifically to the control of an X-ray generator in order to maintain the focal spot configuration as nearly as possible in optimum condition throughout the operating range of the generator.

An important object of the invention is to overcome the space charge efiect in an X-ray generator when in operation at low voltages, so as to allow substantial current to flow between the anode and cathode of the order of the current flowing when the generator is operated at high voltages; a further object being to overcome the space charge effect, when the generator is in operation at low voltage, by applying a positive biasing potential on the cathodecup with respect to'the electron emitting filament of the cathode.

A further important object of the invention is to provide for the operation of an X-ray generator so as to maintain, throughout the operating range of the instrument of the generator, a focal spot pattern as'nearly as possible, and preferably uniformly, in accordance with the optimum or ideal focal spot pattern, the ideal pattern'comprising a large, sharply defined area,

on the drawing an X-ray generator ll com'prising a cathode l3 and a co-operating anode l5 enclosed in suitable hermetically sealed envelope and the ideal generatorloperating conditions be.-.. 3

ing such that the spot maintains a constant shape and definition through the operating range of the generator.

Another important object is to control the focal spot pattern in an ,X-ray generator by applying positive anode bias; ,a further object being to obtain control of focal spot distribution by positively biasing the cathode focusing cup with respect to the electron emitting filament of the cathode; a still further object being to regulate or adjust the positive biasing potential as an inverse function of generator operating voltage so that maximum positive bias is ,applied when the generator is in operation at low voltages applied between anode and cathode, the biasing potential being reduced proportionally when the operating voltage is increased.

These and numerous other important objects, advantages and inherent functions of theinvention will become apparent as the invention is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

' e r n 1 1 h? draw n e-i'is a diagrammatic representation of means It. The cathode includes a focusing cup I! and electron emitting filaments l9 mounted in the cup in position to discharge electrons toward and upon the anode l5. The anode 15 comprises a tubular member 2| having a cavity 23 facing toward the'cathode and has a target 25 at the bottom of the cavity in position to re.- ceive electrons emitted by the cathode when the tube is in operation.

It should be understood that X-ray generators are conditionedfor operation by exhausting from the envelope means It substantially all gaseous and other impurities by evacuating the envelope, as by means ofa molecular exhaust pump, and finally sealing the envelope in evacuated condition. X-rays are generated at the target 25 as a result of impingement thereon of electrons emitted as a stream 26 by the cathode filaments is when electrically excited. Such electrons are impelled toward the target under the influence of the driving force provided through the application of operating voltage applied between the filament and anode from an external source, the focusing cup I! of the'cathode, through electrostatic action, serv ing to confine and direct the electron stream in a desired path toward the anode target. The shape and configuration of the cathode cup, to some extent, determines the surface area or focal spot'of the target 25 within which the electron stream impinges upon the target. Impingement of the electron stream upon the target 25 results in the generation of Xerays which pass from the target laterally through the walls 2liof the anode structure, which are;

preferably formed with a window 21 to facilitate the passage of X-rays therethrough. The X- rays thus transmitted from the target form ,a beam or cone 29, the sectional shape of which is determined by the pattern of the anode focal spot.

It is desirable that the sectional configuration of the X-ray beam 29 be uniform throughout the operating voltage range of the generator, and it is further desirable that the boundaries of the beam 29 be sharply defined and that X-ray intensity be uniform throughout the beam. These factors, in turn, are determined by the uniformity and definition of the target focal spot. We have noted that the size, configuration. and marginal definition of the focal spot of any given' X-ray generator changes as the operating voltage is varied between anode and cathode. For example, as shown in the upper row of diagrams in- Figure 5, the character of the focal spot in a given X-ray tube may improve as the tube operating voltage increases, the focal spot having desirable characteristics when the tube is operating at relatively high voltages and having impaired characteristies at lower voltages. This phenomenais-due apparently to changes in electron focus on the target resulting from changes in the operating voltage, it being understood that electrons have optical characteristics analogous to light rays and that the focused pattern of the electrons on the target depends upon the shape of the electron source, which, in the illustrated embodiment, comprises a pair of parallel filaments, the shape of the focusing cup l1, and the voltage applied between filaments and target. In this connection, the top diagram in the left-hand column of Figure 5 represents a sharply focused electronic pattern or image of theelectron emission elements on the target, while the other diagrams of Figure 5 represent the filament patterns in varying derees of soft focus.

In order to improve the character of the focal spot pattern when the tube is in operation at lower voltages, we provide for the application of a positive bias potential between the electron emitting filaments I9 and the focusing cup l1, and we have discovered that electron flow may be controlledin this fashion to produce substantially the same focal spot pattern when the generator is operating at low voltage as is produced without biasing the cathode when the generator is operating in a high voltage range.

The three lower rows of diagrams in Figure 5,

respectively, illustrate the focal spot patterns I developed through the operating range of the generator when biasing the cathode cup, respec tively, at 300, 600 and 900volts, and it will be noted that the character of the focal spot pattern is progressively improved by increasing the biasing potential while the tube is in operation at low voltage; but that the character of the focal spot pattern deteriorates as the biasing potential is increased when the generator is in operation in the higher voltage ranges. The particular generator which produced the focal spot patterns illustrated in Figure 5 was a generator built for normal operation at voltages between filament and anode of the order of 100 kv. p., and it will be noted that when operating at normal voltage, the application of a positive biasing potential has little, if any effect upon the character of the focal spot pattern.

When in normal operation at a voltage of the order of 100 kv. p. and without cathode bias, the anode current which may flow is limited bythe space charge between anode and cathode, and the focal spot has the usable pattern shown in the second diagram in the first row of Figure 5.' .As the operating voltage'is increased, the.'eifectof the space charge is minimized, with the result that the focal spot pattern is improved, as shown in the third and fourth diagrams in the first row of Figure 5. If, however, the tube is placed in operation at a voltage substantially below its rated voltage, the effect of the space charge increases to such an extent that the focal spot pattern deteriorates, becoming sharp and narrow.

The graph shown in Figure 3 illustrates the space charge limitations of the generator operating at low voltage of the order of 60 kv. p. with the filament carrying 4.5 amperes. The curve shows the variation of anode current as cathode bias is increased. From this curve, it will be noted that saturation is attained at 39 milliamperes tube current, with about 800 volts positive cathode bias; and. there is relatively little increase in tube current above biasing potential of 350-400 volts, at which point in the curve the anode current is of the order of 33 milliamperes. Three hundred fifty to four hundred volts, with anode current at 33 amperes, may therefore be selected as the upper limit for standardizing cathode biasing procedure, although, of course, the range may be increased,- if desired. The curves shown in Figures 3 and 4 were made from data obtained in operating the particular tube which produced the patterns shown in Figure 5. Corresponding curves, however, may be obtained for any generator and, of course, may vary from the curves shown, depending upon the optical and electrical characteristics of the selected generator.

The curves in Figure 4 show the relationship between cathode biasing current in milliamperes to cathode filament current in amperes for various bias Voltages, with the tube in operation at 60 kv. p. The-upper curve represents conditions with cathode bias at 1000 volts. The middle curve represents conditions with cathode bias at 660 volts. The lower curve represents conditions with cathode bias at 330 volts. These curves are of interest, since the upper curve approaches 200 milliamperes required in the cathode biasing circuit at 4.5 amperes in the cathode filament, that is to say, the operating condition for the curvein Figure 3. This means. simply that with 600 volts bias at a filament, current of 4.5 amperes, 200 milliamperes pass to the cathode, releasing 120 watts of energy, inaddition to the normal cathode heat generated due to filament heating alone, which, at 4.5 amperes of filament current, amounts to about 40 watts. The biasing trans former, therefore, should be several times larger than the filament heating transformer. The focal spot patterns shown in Figure 5, with no cathode bias, are satisfactory when the generator is in operation at voltages in excess of kv. p. With cathode biased at 300 volts, the spot pattern is only slightly improved when the generatoris in operation at 60 kv. p., changed when the generator is operating .at.100 kv. p., and shows deterioration'whenthe generator is operating at voltages in excess of 100 kv. p. With'the cathode biased at 600 and 900 volts,- the spot pattern improves withthe generator operating at voltages below 100 when the generator is in operation at the higher voltages between cathode and anode.

In order to control the X-ray generator for operation with satisfactory focal spot pattern throughout its ,full operating range, a positive bias of at least 600 volts is needed on the cathode when the generator is in operation within avolt age range up to voltages of the order of 100kv. p.

is substantially unkv. p. but deteriorates A bias of this magnitude, however, when the tube .'is inoperation at higher voltages. across anode and vcathode, results in the production of focal spot patterns of diminished size so that'for "most effective operation,'the bias should, when the generator is in operation at the high voltage range, be either removed entirely or diminished in order to reduce tube current to thereby avoid overloading the anode heat dissipating capacity of the tube, the following schedule being a'satisfactory guide illustrating the desirable variation in bias (expressed in terms of tube current) at various voltages within the operating range of the generator:

Bias rating in terms of anode current Operating voltage (kv. p.)

ment l9 and the anode 2|, the primary winding 39 of the transformer 3| being connected, by conductors 4| and 4 3, with a suitable source of generator operating power.

The control system also includes filament exciting means which may comprise a filament exciting and biasing transformer 65, a portion d1 of the secondary winding 49 of which is connected to energize the cathode filament l9 through suitable conductors 31 and 5|. Biasing potential may be applied between the filament l9 and the cathode cup l1 by means of the transformer 45, a portion 53 of the secondary winding 49 of which may be connected to the filament, through the conductor 5!, and to the cathode cup |1 through a conductor 55, which conductor preferably includes switch means 51 for con-trolling the biasing circuit. This switch 51 preferably comprises a shiftable blade electrically connected with the cathode cup H, the blade being movable in order to make contact with the winding 53 in order to apply cathode bias, or to make contact directly with the conductor 5| in order to remove the cathode bias by electrically connecting the oathode cup with the filament circuit. The switch 51 may be controlled by a solenoid 59 operable, when energized, to throw the switch blade 51 into bias applying position. When the solenoid 59 is de-energized, suitable spring means may be utilized to throw the switch 51 into position connecting the cathode cup directly with the filament circuit. Preferably, the switch operating means includes a stem insulated, as at 69, to assure electrical isolation of the switch from the operating circuit of the solenoid 59. The primary winding 6| of the transformer 45 is electrically connected, as by means of the conductor 23 and a conductor 63, with a suitable external electrical power source.

The generator H, the transformers 3| and 15, the switch 51 and its operating solenoid 59, and the several circuits for connecting the aforesaid control elements with the generator, are preferably all enclosed in and insulated from a container or grounded casing 65; and the mid point of the Winding 33 of the transformer 3| may be electrically connected with the grounded casing 65, as by means of a conductor 61.

. The transformers 3| 'and'45 and the switch control solenoid 59 may be energized in any suitable or preferred fashion, but we prefeerably utilize a supply system comprising an autotransformer and control switches having rheo'stats, which are preferably enclosed in and insulated from'a. grounded container 69, which may be located remotely from the generator casing 65. The. autotransformer may comprise a suitable winding 1|, one end of which may be connected by means of the conductor 13 with one side of a suitable external power source, the conductor means i3'leading to the primary windings oithe transformers 3| and being also electrically connected to the power source through the conductor 13. The other side of the power source may be connected, .through a. conductor 15 and a control switch 11, to the winding 1| intermediate the ends thereof. The winding 1| may be connected, by means of an adjustable connection 19 through a control switch 8| and an adjustable rheostat 83, to the conductor 4| forenergizing the primary winding of the transformer 3|. The winding 1| likewise may be connected, through an adjustable rheostat 85, to the conductor 53 for the purpose of energizing the filament heating and cathode biasing transformer The switch operating solenoid 59 may be connected to the winding 1| through a switch 81 and may also be connected to a remote portion of the winding 1| through means for adjusting the voltage applied to the solenoid, and said means may conveniently comprise the adjustable connection 19. By closing the switch 81, the solenoid 59 will be energized to throw the biasing switch 51 into position applying a bias upon the cathode. This bias, because of the adjustable rheostat 83, may be varied as a function of filament current. The bias will be maintained so long as the solenoid 59 remains energized. The solenoid, however, may comprise a device adapted to operate at a predetermined voltage in order to throw the switch to a position discontinuing the bias on the cathode. The adjustable contact 19 is a control for determining the operating voltage, applied between anode and cathode, of the tube through the transformer 3|, and by utilizing this control means 19 in the operating circuit of the solenoid 59, the system may be adjusted to operate the switch 51 for the removal of the cathode bias whenever the element 19 is manipulated to increase the operating voltage of the generator above a predetermined value, thereby automatically accomplishing the control of the generator in accordance with the teachings of our present invention. It will be apparent, however, that the control of the bias need not necessarily be interlocked with the control of generator operating potential but may be had by separate means if desired.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. The method of operating an X-ray generator in which the cathode comprises a focusing cup and an electron emitting element which consists in applying positive bias on said cup with respect to said element when the generator is'in operation at low voltage and diminishing the consists in positively biasing said cup with respect to said element as an inverse function of the operating voltage of the generator.

3. The method of operating an X-ray generator in which the cathode comprises a focusing cup and an electron emitting element which consists in positively biasing said cup with respect to said element of the generator when in operation at relatively low voltage and reducing the biasing eifect as the operating voltage is increased.

4. The method .of operating an X-ray generator in which the cathode comprises a focusing cup and an electron emitting element which consists in maintaining uniform focal spot pattern throughout the operating voltage range of the generator by positively biasing said cup with respect to said element inversely with respect to generator operating voltage throughout the operating voltage range of the generator.

5. The combination, with an X-ray generator comprising an anode, a cathode including an electron emission element and a mounting head, means to apply generator operating voltage between the emission element and the anode, and

means for adjusting said operating voltage, of biasing means for applying a positive bias potential to said head with respect to said emission element, and means operable to control said biasing means whereby to reduce the intensity of bias progressively as the operating voltage is increased.

6. The combination, with an X-ray generator comprising an anode, a cathode including an electron emission element and a mounting head, means to apply generator operating voltage between the emission element and the anode, and means for adjusting said operating voltage, of biasing means for applying a positive bias potential to said head with respect to said emission element comprising a biasing circuit, and means to control the biasing circuit to vary the bias inversely with respect to variation in the generator operating voltage.

7. The combination, with an X-ray generator comprising an anode, a cathode including an electron emission element and a mounting head, and means to apply generator operating voltage between the emission element and the anode, of energizing means for delivering element energizing electrical current to said emission element, biasing means for applying positive bias on said head with respect to said emission element, means for adjusting the energizing current delivered to said emission element, and means for varying the bias as a function of the variation in filament current.

ZED J. ATLEE. FRANK R. ABBOTT.

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