US2217483A - X-ray apparatus - Google Patents

Description

Oct; 8, 1940.

W. K. KEARSLEY X-RAY APPARATUS Filed Dec. 20, 1938 v Inventor:

William K.Kear-s|ey,

b EMA/ 61M y His Attorney.

Patented Oct. 8, 1940 X-RAY APPARATUS William K. Kearsley, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 20, 1938, SerialNo. 246,868;

6 Claims.

The present invention relates to X-ray apparatus and more particularly to improved means for controlling the operation of a high voltage X-ray tube.

In the use'of X-ray apparatus for radiographic purposes it is frequently desirable to obtain exposures as short as one-tenth second or less. This requires that the control switch .employed must be of low inertia so as to be capable of opening and closing in such a short interval. This result is difiicult to obtain if the circuit interrupter is of high current capacity, as it must be, for example, if interposed directly in the low voltage portion of the load circuit.

It is an object of my present invention to provide an improved switching system in which the discharge controlling switch may be of very low current capacity and hence of low mechanical inertia. To this end the invention employs a circuit arrangement in which current interrup tion is actually accomplished in the high voltage, low-current discharge circuit, although controlled by a switching means located in the low voltage primary circuit.

The particular features which I desire to protect herein are pointed out in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to' the drawing in which Fig. 1 represents schematically an X-ray apparatus embodying the invention; Fig. 2 is an enlarged detail, partly in section, of certain of the elements of Fig. 1, and Fig. 3 shows an alternative application of the invention.

Referring particularly to Fig. 1 there is shown a self-rectifying vacuum X-ray tube it of the general character of that described in my prior Patent 2,039,908 granted May 5, 1936. This includes an anode ll, a cathode and a control electrode, the last two elements being shown in detail in Fig. 2. As appears in that figure, the electron emissive element of the cathode comprises a filament 2 with which is associated cup-like enclosure I3. One end of the filament is connected to a conductor It, the opposite end being connected to the part I3, which in turn is Surrounding the cathode there is provided a cup-shaped focusing and control electrode 23 which is supported from a mounting member 24. mounting member 24 permits a separate potential to be applied to the electrode. As is explained in my prior Patent 2,039,908, the electrode 23 is effective when negatively biased to prevent the passage of an X-ray producing discharge through the tube !0 in spite of the existence of a favorable potential across the tube terminals. In this respect the electrode acts'as a sort of control grid which may be used to hold off the occurrence of a discharge as long as desired.

Referring again to Fig. 1, it will be seen that the energizing circuit for the X-ray tube includes a low voltage source, represented as a pair of power lines 21, and a high voltage dis--' charge circuit coupled to, the source 21? through a voltage transformer 28. Heating current for the filament I2 is supplied by means of a second transformer 29 which also has its primary 39 connected to the voltage source 2?. The secondary winding 3| of the filament transformer is terminally connected to the conductor 26 so as to provide a reversible biasing potential for the electrode 23.

Interposed in circuitbetween the primary 3i) and lines 21 there is provided a reversing switch 33. In one of its positions, specifically, the position shown, the switch is is efiective to cause the bias applied to the electrode 23 by the transformer 29 to be negative during periods when the polarity of the transformer 28 is favorable to the production of a discharge through the X-ray tube It. That is to say, when the switch is in the position illustrated, the polarity of the electrode 23 becomes negative with respect to' the cathode i2 whenever the anode Ii is positive. Consequently, under these conditions no appreciable discharge can take place even when the high voltage circuit is energized. However, if the switch 33 is thrown to its other position, that is to the left, the polarity relationship of the voltage applied to the electrode 23 will obviously be reversed. With this arrangement, the bias applied to the electrode is positive during periods when a discharge tends to occur and such a discharge can therefore take place.

To avoid an undesirable enlargement of the size of the focal spot produced on the anode it due to the positive potential of the electrode 23, a high resistance 35 is provided in the control circuit. On half-cycles when the electrode is A lead-in conductor 26 connected to the negative this resistance has no effect, since no I current is flowing through it. On positive halfcycles, however, electrons from the filament l2 go to the electrode and bring its potential approximately to that of the filament so that distortion of the electron beam is avoided.

The switch 33 may obviously be actuated either manually or automatically, and in Fig. 1 there is shown an automatic control including a timing device 31. In operation of the apparatus this timing device is preliminarily adjusted for the length of exposure which is desired by the operator. Thereafter, a master switch 38, which may be, for example, a foot-switch, is closed by the operator so as to supply potential both to the discharge circuit and to the control circuit. It is assumed that the timing device 37 includes a switch, which being initially closed, permits immediate energization of a solenoid 45 connected to the switch 33. By the action of this solenoid the switch 33 is thrown to its left-hand position so as to permit a discharge through the tube [0. However, after expiration of a desired time interval, determined by setting of the device 31, the solenoid 40 is deenergized so that the switch 33 may be restored to its initial position by means of a spring 42 provided for that purpose. The action of the switch 33 in reversing the potential applied to the electrode 23, serves immediately to terminate the discharge.

Since the switch 33 has to accommodate only currents of low value, it may be of low inertia so as to permit extremely rapid action. Furthermore, since the only element required to be added to those which are ordinarily present in the normal operation of an X-ray tube is the secondary winding 3| of the transformer 39, it

will be seen that little or no increase in the weight or complexity of the apparatus is involved in the use of my improved control means.

In Fig. 3 I have shown a further application of the invention in connection with an X-ray tube 49 which is used with separate rectifying elements. In this alternative embodiment there is illustrated diagrammatically a conventional twoelement X-ray tube having an anode 50 and a filamentary cathode This is combined in a full-wave rectifying circuit with a series of electronic rectifiers respectively numbered 53, 54, 55 and 56. The main energizing circuit includes a transformer 58 connected to a low voltage supply source 59.

In the arrangement shown at least two of the rectifiers, specifically the rectifiers 55 and 55, are provided with control electrodes or grids 6i and 62. Their cathodes (63 and 64) are supplied with heating current from a transformer 51 which also serves as a source of control potential to be applied to the grids GI and 62. As shown the transformer 61 is provided with special secondary windings 68 and 69 effective to supply biasing potentials of opposite polarities to the grids BI and 62. Resistors l3 and M connected in the grid circuits, prevent the grid current from becoming excessive at any time.

The relation which the polarity applied to each grid bears to the voltage impressed between the main electrodes of its associated rectifier is controlled by a reversing switch It connected in circuit with the primary of the transformer 61. With this switch in its right hand position as shown, the switch is effective to cause the biasing potential applied to each control grid to be of negative polarity during periods when the polarity of the transformer 58 is favorable to the production of a discharge through the X-ray tube. Under these conditions, the rectifiers are rendered non-conductive in the normally conductive direction.

When the switch is thrown to its reversed or left-hand position, the polarity of the biasing potential applied to the grids 6| and 62 is reversed so that the tubes 55 and 56 are free to conduct when a favorable potential is applied to them. Consequently, with the switch in the said left hand position a full wave rectified potential may be applied to the X-ray tube 49 so as to cause a discharge therethrough.

As in the arrangement described in Fig. 1, a foot-switch 75 or other type of safety device may be used to assure that the high voltage discharge circuit is deenergized except when X-ray production is actually desired. This switch is closed, of course, before the reversing switch '10 is thrown to its left-hand or discharge-producing position and is opened only after the discharge is terminated. Consequently it never has to interrupt a current of large magnitude. If desired, a timing device of the character of the device 3'! may be employed to regulate the exposure period.

While I have described my invention in connection with particular embodiments thereof, it will be understood that various modifications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

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

1. In combination, a high voltage discharge tube including an anode and a cathode, a low voltage source of alternating current, a circuit supplied from said source for impressing a high potential between the said anode and cathode, a discharge-controlling means effective when biased to an appropriate polarity to prevent a discharge between the anode and. cathode, said means being conductively connected with the high potential circuit, a transformer having its primary winding connected to the low voltage source and its secondary winding connected to the discharge-controlling means for impressing a cyclically reversible biasing potential thereon and switching means in circuit with the transformer primary and normally operative to cause the biasing potential applied to the said discharge-controlling means to assume a dischargepreventing polarity during periods when the potential impressed between the anode and cathode of the said tube is favorable to the production of a discharge therethrough but actuable at the will of an operator to cause the potential so applied to assume a discharge-favoring polarity during such periods.

2. In combination, an X-ray tube including an anode and cathode, a low Voltage source of alternating current, a high voltage circuit supplied from said source for impressing a potential of X-ray-producing magnitude between the anode and cathode, a discharge-controlling means effective when biased to a predetermined polarity to prevent a discharge between the anode and cathode and effective when biased to the opposite polarity to favor such a discharge, said means being continuously connected with the high voltage circuit, a transformer having its primary winding connected to the low voltage source and its secondary winding connected to the discharge-controlling means for impressing a cyclically reversible biasing potential thereon,

and a reversing switch interposed in circuit between the said transformer primary and the low voltage source, the said switch being effective in one position to cause a discharge-preventing bias to be applied to the said discharge-controlling means during periods when the polarity of the voltage applied between the anode and cathode of the X-ray tube is' favorable to production of a discharge therethrough, and being effective in its other position to cause a dischargefavoring bias to be applied to the said means during such periods.

3. In combination, an X-ray tube having an anode, a cathode and a focusing element associated with the cathode, the said focusing element being effective when negatively biased with respect to the cathode to prevent a discharge through the X-ray tube, a low voltage source of alternating current, a high voltage circuit supplied from said source for energizing the X-ray tube at X-ray-producing potentials, a transformer having its primary winding connected to the low voltage source and its secondary connected to the focusing element in the high voltage circuit for impressing a cyclically reversible biasing potential on the element, and a switch interposed in circuit between the said voltage source and the transformer primary, the said switch being efiective in one position thereof to cause the biasing potential applied to the focusing element to assume negative polarity with respect to the cathode during periods when the voltage applied to the X-ray tube is favorable to the production of a discharge therethrough, and efiective in its other position to cause the potential so applied to assume a polarity other than negative during such periods.

4. In combination, an X-ray tube including an anode and a cathode, a low voltage source of alternating current, a high voltage circuit supplied from said source for impressing a potential of X-ray-producing magnitude between the said anode and cathode, a discharge-controlling means effective when appropriately biased to prevent a discharge between the said anode and cathode, the said means being conductively connected with the high voltage circuit, a transformer having its primary winding connected to the low voltage source and its secondary winding connected to the discharge-controlling means for impressing a cyclically reversible biasing potential thereon, a reversing switch interposed in circuit between the said transformer primary and the low voltage source, the said switch being ray tube to be of unidirectional character, control means associated with said rectifier and effective when biased to an appropriate polarity to cause the rectifier to be non-conductive in its normally conductive direction, a transformer having its primary winding connected to the low voltage source and its secondary winding connected to the said control means in the high voltage circuit for impressing a cyclically reversible biasing potential on said means, and means in circuit with the said transformer primary and operative in a first condition to cause the biasing potential applied to the control means of the rectifier to assume a conduction-preventing polarity during periods when the polarity of the voltage applied to the X-ray tube is favorable to the production of a discharge therethrough, the last-introduced means being actuable at the will of an operator to a second condition in which it is effective to cause the polarity of the said biasing potential to assume a condition-favoring polarity during such periods.

6. In combination, an X-ray tube, a low voltage source of alternating current, a high voltage full-Wave rectifying circuit supplied from said source for energizing the X-ray tube at an X- tials of opposite polarity thereto, and areversing switch connected between the primary of the said transformer and the low voltage source, the said switch being effective in one position to cause the biasing potential applied to each of the said control electrodes to be of negative polarity during periods when the polarity of the source is favorable to a discharge through its associated rectifier, and being effective in an-. other position to cause the said biasing potential to be of positive polarity during such periods.

WILLIAM K. KEARSLEY.

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