US2250301A - Space charge compensation - Google Patents

Description

July 22, 1941. w. F. FATHAUER SPACE CHARGE COMPENSATION Filed June 6, 1940 INVENTQR. v WaZZe/F Fal /2621166 7 11.. adapted for excitation for the emission of electrons which, impinging upon the anode, result in Patented July 22, 1941 SPACE CHARGE COMPENSATION Walter F. Fathauer, Highland Park, 111., assignor to- General Electric X-Ray Corporation, Chicago, 111., a corporation of New York Application June 6, 1940, Serial No. 339,109.

2 Glaims.

My invention relates general to electronics and has more particular reference to the control of X-ray'tubes;

An important object of the present invention is to maintain the relationship of voltage to current as a constant function during operation of devices of the character mentioned, operation of the tube in accordance with the present invention being accomplished at all times in accordance with a pred'etermned voltage current relationship which may be graphical-1y illustrated as a single curve;

Another important object is to provide means to compensate for space charge efiects in the operation of an electronic device, comprising ananode and a cathode adapted to be excited for electron emission, so that the relationship of voltage across the device to anode current comprises a constant function regardless of the extent cathode excitation.

Among the advantages obtained by causing operation of the device, in accordance with an invariable voltage current relationship, is that the performance of the device in terms of anode current may be accurately predetermined or predicted for all operating voltages, thereby minimizing. uncertainty and making for precise results, which are of importance especially where X-ray technique is involved;

The foregoing and nun'ierous other important objects, advantages and inherent functions of the invention will become apparent as it is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention;

eration of an electronic device embodying an anode and a cathode adapted for excitation for the emission of electrons; and

Figure 21 isa diagram of connections for the control of an electron flow device, in accordance with the teachings of my present invention.

To illustrate the invention, I have shown in the drawing an X-ray tube ll comprising a sealed envelope i3 containing a cathode I5 and an anode The cathode I5 is shown as a filament the production of X-rays. The electrons emitted 'by the cathode are attracted toward and impinge. upon the anode only so long as it remains electrically positive with respect to the cathode.

If and when the anode becomes electrically negative with respect to the cathode, it becomes electron repellent.

Upon excitation of the cathode L5, electrons will be emitted: thereby and will flow toward and impinge upon the plate I? and form a. current conducting path between anode and cathode, along which flows the tube current under the influence of electricalpotential applied between anode and cathode. The electrons emitted by the cathode, unless driven toward the anode under the influence of potential of appreciable magnitude, congregate in-the space near the cathode and collectively constitute a cloud of electrons, which is-known as-the space charge. These electrons, being negatively charged, tend to repel other electrons leaving the cathode, and a state of equilibrium will be established between the pressure of oncoming electrons and the pressure exerted by the electrons which have already escaped from the cathode. A- number of electrons will, of course, reach the anode and give up their charge, thereby creating X-rays. When the voltage between anode and cathode is exceedingly low, many of the-electrons donot reach the anode even though emitted by the cathode, because the velocity with which they are emitted by the cathode is insufficient to carry them to the anode through the space charge of mutually repellent electrons surrounding the plate. Under excessively low voltage conditions, increasing the number of electrons emitted by the cathode will have very little eiiect' on the anode current, and to increase anode current, it is necessary to increase the voltage between anode and cathode. The effect of the space chargeis tohamper the movement of electrons toward the anode from the cathode and thus limit anode current, the space charge effect being particularly noticeable under low voltage conditions, and being shown at the lower end of the graph in Figure 1 of the drawing. As voltage across anode and cathode is increased, current increases relatively slowly until the space charge effect isovercome, current thereafter increasing more rapidly in accordance with a straight line function.

The current flow through the tube also is limited by the available supply of electrons emitted by the cathode and by the behavior of the electrons after they leave the cathode. The available supply of electrons is determined by the intensity of cathode excitation, proportionally more electrons being emitted as cathode excitation is increased. For every degree of excitatiom'there is a limit to the number of electrons that may escape from the cathode, so that maximum possible anode current is determined by the degree of excitation, ordinarily expressed in terms of filament current. To attract all of the emitted electrons to and cause the same to impinge on the anode requires a definite optimum potential difference between anode and cathode, there being a difierent optimum for every degree of cathode excitation throughout the operating range of the tube. When all of the electrons emitted by the cathode impinge upon the anode, the tube is in what may be designated as a condition of saturation. When a tube is in saturated condition, current flow between anode and cathode is limited to a certain maximum value, even though voltage across the tube may be increased, current limitation being caused by the fact that after saturation, only as much current as can be carried by the emitted electrons may flow between cathode and anode.

As tube voltage is increased toward a condition of saturation, the voltage-current relationship is affected by an effect similar to the space charge effect previously mentioned, and the current increase, in response to voltage increase, gradually is reduced as the saturated condition is reached, as illustrated at the upper end of the graph, Figure 1 of the drawing, in Which the curves H and L represent the current-voltage curves for the same tube, operating respectively with high and low cathode excitation. It will be apparent that the tube may be adjusted for operation at any point between the limits defined by the curves H and L, depending upon the adjustment of applied voltage and cathode excitation, anode current thus being determined by the adjustment of a plurality of variables. In practical X-ray technique, the necessity of carefully adjusting both applied voltage and filament excitation consumes valuable time, and the fact that anode current of any value between points M and N may be attained at the same applied voltage, represented by the line V, makes for confusion. I propose to simplify X-ray technique by providing voltage current control through a single, manually operable handle so that for each position of the control handle, there will be a corresponding flow of anode current, which will always be the same, thus avoiding confusion and the chance of obtaining excessive current flow through improper control of filament excitation. I propose to accomplish the control by decreasing filament excitation as the tube voltage is increased, and vice versa, so that, at low voltages, the space charge effect is to some extent overcome by maximum filament excitation, while current is limited at high voltages by saturation, as indicated by the curve C in Figure 1, in which only one value of anode current, represented by the point P, is obtained in response to tube voltage, represented by the line V.

As shown in Figure 2 of the drawing, power for actuating the device H is supplied from a suitable source shown as a power line l9, preferably supplying alternating current power. The power is delivered from the line l9, preferably through a control switch 2| and through an adjustable auto-transformer 23, t the primary winding of a power transformer 21, the secondary winding 23 of which is interconnected between the anode and cathode of the tube for the purpose of supplying electrical power for the operation of the tube. The power source is also connected through the auto-transformer and an adjustable current control device 3| with the primary winding 33 of a current limiting transformer 35, the secondary winding 31 of which is connected in series with the primary winding 39 of a filament transformer 4|, the secondary winding 43 of which is connected in series with the filament I5 of the X-ray tube. The primary winding of the filament transformer is powered from the source [9 through the auto-transformer 23 and an adjustable resistor 45, so that the equipment may be adjusted by means of the resistor 45 for operation in accordance with predetermined conditions, the adjustable device 3| serving to alter the value of cathode excitation during the operation of the device.

One side of the line H3 is connected through the switch 2| with one side of the primary winding 25 of the transformer 21 by means of a conductor 41, to which conductor one end of the auto-transformer coil 23 is electrically connected. The other side of the line is connected through the switch 2| with an adjustable contact member 49 co-operatively associated with the autotransformer coil 23. It is, of course, not essential to utilize an adjustable connection at 49, but the line connection may be made permanently on the winding 23, as at the end thereof. It is desirable, however, to provide an adjustable connection in order to set the apparatus to deliver a predetermined voltage across the coil 23.

The coil 23 has associated with it an adjustable contact member 5|, which is electrically connected by means of the conductor 53 with the side of the primary winding 25 of the transformer. '2'| remote from the conductor 4'! so that. by adjusting the position of the adjustable contact 5| on the auto-transformer coil 23, any desired potential within the operating limits of the apparatus, as determined by the setting of the member 49, may be applied on the primary winding of the transformer 21 to thereby apply a desired operating voltage between the anode and cathode of the tube. The system also includes a cathode exciting circuit extending from one side of the line I9 through the switch 2| and thence through a conductor 51, the adjustable rheostat 55, the primary winding 39 of the filament transformer, and the secondary winding 3? of the current limiting transformer 35, and thence to the conductor 41 which serves as a common return conductor in the system. The system, furthermore, includes a circuit for regulating current flow in the cathode exciting circuit, said regulating circuit extending from one side of the line I9 through the switch 2|, and a conductor 59 connected with a slide contact 6| having adjustable relationship with the resistor 3|, which is connected through the primary winding 33 of the limiting transformer 35 to the common return conductor ll. By varying the relationship of the contact member 6| with respect to its associated resistor 3|, the limiting transformer 35 may be caused to alter the current flowing in the cathode exciting circuit. The cathode exciting circuit includes the primary winding of the filament transformer, current flow through which may also be adjusted by means of the rheostat 45. The slide members 5| and 6|, which respectively control the voltage applied between the anode and cathode of the tube and the flow of cathode exciting current, are interconnected, as by mounting the same on a common manually operable member 53, which, in the illustrated embodiment, is shown as a lever member on which both of the contact devices 5| and 6| are mounted. By moving the member 63, both contact devices and 6| may be caused simultaneously to be adjusted respectively on the autotransformer coil 23 and on the coil 3|, thereby simultaneously altering the voltage applied between anode and cathode and the cathode exciting current delivered through the transformer 4|; and the parts are arranged so that the cathode exciting current is increased as the tube voltage is decreased, and vice versa, the relationship of the cathode exciting current to tube voltage being an inverse function of any desired character, depending upon the selected constants of the circuits involved.

It will be seen from the foregoing description that I have provided means whereby to control the operation of an electronic device through a single manual control element which serves to determine cathode excitation as a function of tube voltage, to the end that the device may be operated on a constant tube voltage-anode current curve. The control assures the production of a definite anode current and corresponding X-ray intensity, in response to any value of tube voltage applied, thereby eliminating confusion and simplifying the operation of the apparatus. At the same time, by providing for the increase of cathode excitation at low voltages, the space charge eifect is reduced, and by reducing cathode excitation at high voltages, excessive anode current is prevented.

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 combination, with an X-ray tube having an anode and an excitable cathode, of a transformer having a secondary winding connected between said anode and cathode and a primary winding connectible with a power source through adjustable power supply means for varying the voltage applied through said transformer between the anode and cathode, a current transformer having a secondary winding connected with said cathode and a primary winding connectible in circuit with said power source, said circuit including the secondary winding of a control transformer, the primary winding of which is connectible, in association with an adjustable current flow control device, with said power source, and means drivingly interconnecting the adjustable portions of said current flow control device and said power supply means.

2. The combination, with an X-ray tube having an anode and an excitable cathode, of a transformer having a secondary winding connected between said anode and cathode and a primary winding connectible with a power source through adjustable power supply means for varying the voltage applied through said transformer between the anode and cathode, a current transformer having a secondary winding connected with said cathode and a primary winding connectible in circuit with said power source, an adjustable ci1' cuit control device in said circuit, said circuit including the secondary winding of a control transformer, the primary winding of which is connectible, in association with an adjustable current flow control device, with said power source, and means drivingly interconnecting the adjustable portions of said current flow control device and said power supply means.

WALTER F. FATHAUER.

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