EP0236573A2

EP0236573A2 - Weld-resistant X-ray tube

Info

Publication number: EP0236573A2
Application number: EP86117778A
Authority: EP
Inventors: Maurice David Dilick; Joshua Oluwadare Kolawole; John Warren Newman; Edward Taylor Rate, Jr.
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1986-01-03
Filing date: 1986-12-19
Publication date: 1987-09-16
Also published as: EP0236573A3; JPS62188148A

Abstract

Disclosed is an X-ray generating tube (20) having a filament (11), a grid (12) and an anode (13). Connected between the grid and filament is a nonlinear resistor (21) for preventing the relative voltage therebetween from exceeding a preselected limit. Connected in series with the grid is a resistor (23) for limiting current flow thereto. Together, the nonlinear resistor and the grid resistor help prevent grid filament shorts in the X-ray tube.

Description

Background of the Invention

This invention relates to X-ray tubes and systems and, more particularly, to X-ray tubes and systems which employ grid cutoff to terminate X-ray generation.
In certain applications of X-rays, it is useful to have the ability to stop and start the X-ray beam at will. Rapid initiation and termination of the X-ray beam is difficult to achieve by simply controlling the anode voltage. This is because the anode voltage in an X-ray tube is quite high, frequently in excess of 100 kilovolts, and voltages at that level are difficult to turn on and off rapidly.
In view of the foregoing, X-ray apparatus in which rapid termination and initiation of the X-ray beam is desired often uses gridded X-ray tubes. A gridded X-ray tube operates as does a conventional X-ray tube, in that a beam of electrons emitted by a filament is directed to an anode. In the X-ray tube, an anode is made of a refractory metal which when struck by the electron beam emits X-radiation. In addition, such a gridded X-ray tube also operates as does a conventional vacuum tube triode in that a sufficient negative voltage on the grid will terminate the electron beam and, thus, terminate emission of X-rays. The advantage obtained by use of a gridded X-ray tube is that the grid voltage required to terminate the electron beam is far less than the anode voltage. For example, typically, grid voltages in an X-ray tube will be in the range of 2,000-5,000 volts. Such voltages can be controlled much more easily and efficiently than can the high anode voltages.
As a result, gridded X-ray tubes are commonly used today. However, one problem results from the fact that the level of grid voltage required to terminate X-ray exposure is dependent, in part, on the filament-to-grid spacing; the smaller the spacing, the lower the required voltage will be. Inasmuch as it is easier to control lower voltages, the grid-to-filament spacing is usually minimized.
Unfortunately, certain factors, such as anode spits, overvoltages on the anode, and mechanical shock and/or vibration can cause the filament to begin vibrating. Very little vibration or deflection is required in order for a vibrating filament to physically contact the grid. Upon such contact, if the grid voltage is on, a substantial current flows between the grid and the filament, and this sometimes causes the filament and grid to weld together, resulting in failure of the X-ray tube.
It is an object of the this invention, therefore, to provide an X-ray tube and an X-ray system which provide the easy X-ray beam cutoff advantages of the gridded X-ray tube, but is resistant to formation of grid-to-filament welds.

Summary of the Invention

In one embodiment, the present invention includes an X-ray tube comprising a filament, grid and anode. In addition, a voltage limiter, such as a metal oxide varistor, is connected between the filament and the grid, such that grid overvoltages caused by power supply irregularities or anode spits or the like are dampened. The presence of the voltage limiter helps prevent electrostatically induced filament vibration and thus reduces the incidence of filament grid welds.
A feature of the invention is the inclusion of a series resistor in series with the grid. In the event that a filament-to-grid short does occur, the series resistor limits the current flow into the grid and thus helps eliminate the formation of welds. Inasmuch as normal operation of a negatively biased grid causes no grid current flow, a relatively high value resistor can be used in series with the grid without hampering normal grid operation.

Brief Description of the Drawings

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic diagram of a conventional gridded X-ray tube;
FIGURE 2 is a plan view of the filament and grid of such a prior art tube;
FIGURE 3 is a sectional view of the filament and grid shown in FIG. 2;
FIGURE 4 is a schematic diagram of an improved X-ray tube in accordance with the present invention; and
FIGURE 5 is a circuit diagram of an X-ray system employing the improved X-ray tube.

Description of the Preferred Embodiment

Referring first to FIG. 1 there is shown a gridded X-ray tube 10 including a filament 11, a grid 12 and an anode 13, all within an envelope 14 which maintains them in a high vacuum. The tube 10 of FIG. 1 operates in a conventional manner; the filament 11 is electrically heated and an anode filament voltage potential causes an electron beam to flow from the filament to the anode. The anode 13 is constructed of a high atomic number refractory material, at least in the area where the beam strikes it so that it efficiently emits X-rays. If the grid 12 is made sufficiently negative with respect to the filament 11, the beam of electrons will be terminated and thus the X-ray generator will be terminated.
Typically, filament-to-anode voltages in medical X-ray exceed 100 kilovolts. Typically, grid cutoff voltages are approximately 3.7 kV. Grid voltages in excess of 4 kV are somewhat higher than average and approximately 2.7 kV is normally required to cause cutoff in a typical medical X-ray tube.
Referring now to FIGS. 2 and 3, details of the filament and cathode are shown. For clarity, the filament supports on each end of the filament 11 are not shown. The relatively close spacing of the filament 11 and the cathode 12 is shown and, thus, it will be appreciated that filament vibration can cause physical contact between the filament and the cathode.
Referring now to FIG. 4, an X-ray tube 20 of the present invention is shown. Where components of the tube 20 are similar to components of the tube 10, similar reference numerals have been used for clarity.
Still referring to FIG. 4, filament 11 produces a beam of electrons. In the path of the beam of electron is anode 13 for emitting X-rays when struck by the beam of electrons. Grid 12 is also in the path of the beam of electrons and selectively terminates the beam when a sufficient grid voltage is applied thereto.
In addition to the foregoing components, certain resistive elements are present in the X-ray generating tube 20. A voltage limiting means 21 is connected between the filament 11 and the grid 12 for damping voltages therebetween which exceed a preselected limit.
The voltage limiting means can be a nonlinear resistance, such as a metal oxide varistor.
It will be recalled that one of the causes of filament vibration is overvoltage between the grid and filament resulting from poor power supply regulation or anode spits. The function of the voltage limiter is to prevent the grid-filament voltage from becoming so high that such vibration begins. Inasmuch as grid voltage typically does not need to exceed 4 kV, a 6kV metal oxide varistor has been found to work well as voltage limiter 21. It will be appreciated that the voltage limiter need not have a sharp cutoff. It is important only that the grid-filament voltage not be permitted to become so high that vibration is induced.
In addition to metal oxide varistors, other components such as Zener diodes could be used as a voltage limiter. It is desirable that the voltage limiting component be mounted as nearly as possible to the circuitry to be protected. Therefore, size and lead length are a consideration in alternative device selection. Grid voltage is applied to an external contact 22 which includes a series resistor 23. The purpose of the series resistor, as explained previously, is to prevent excessive current flow to the grid in the event that there is a grid filament short due to vibration.
As is well known in the vacuum tube field, negatively biased grids require no current to operate in their normal mode. Therefore, the resistor 23 can be of a relatively high value without impeding the function of the grid cutoff. Obviously, the higher the value of resistor 23, the more efficient will be the protection against grid filament welds. It has been found that a value in the range of 500 thousand ohms to 1 meg ohm is effective to prevent many grid filament welds. The average power rating of the resistor is not particularly critical beause the contacts between the filament and the grid are typically of very short duration and thus the resistor is called upon to dissipate only short pulses of power.
Referring now to FIG. 5, there is shown an X-ray system 30 employing the improved X-ray tube 20. Connected between the filament 11 and the anode 13 is an anode power supply 31. As in a conventional X-ray system, the anode power supply 31 provides filament-anode power for the electron beam for generating X-rays.
A filament power supply 32 is included to heat the filament so that the electron beam can be emitted.
Finally, an external grid voltage supply 33 is connected between the grid 12 and filament 11 to supply grid voltage to cut off the electron beam.
As will be apparent from FIG. 5, the series resistor 23 is connected in series between the grid 12 and the grid voltage supply 33, and the voltage limiter 21 is connected between the grid 12 and the output of the filament supply.
As was previously stated, best protection against filament grid shorts is obtained with a high value of series resistor. Furthermore, increasing the value of the series resistor should not, in theory, hamper the operation of the grid. However, it has been found that the resistor should not be of too high a value. For example, it is believed that 2 meg ohms is excessive. The reason that there is an upper limit on the value of the series resistor 23 is a follows. Shown in phantom in FIG. 5 is a resistor 34. This resistor represents the leakage in the X-ray tube, the low voltage resistance of the nonlinear resistor 21 and the leakage in the insulation in the connectors of the X-ray tube. As will be seen, the series resistor 23 and the leakage resistor 34 form a voltage divider which determines the grid voltage supplied by the grid voltage supply 33. If the value of the leakage resistance 34 is substantially larger than the series resistor 23, the voltage on the grid 12 approaches the full output of the grid supply. However, if, for example, the leakage resistance 34 and the series resistor 23 are of equal value, the effective grid voltage will be one half of the output of the grid voltage supply 33. Therefore, caution must be taken to ensure that the series resistor 23 is not too high to permit the grid to operate as intended.
While this invention has been described with reference to particular embodiments and examples, other modifications and variations will occur to those skilled in the art in view of the above teachings. Accordingly, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than is specifcally described.

Claims

1. An x-ray generating tube comprising:
a filament for emitting a beam of electrons;
a grid for selectively terminating the beam of electrons;
an anode in the path of the beam of electrons for emitting X-rays when struck by the beam;
an envelope containing said filament, grid and anode in a vacuum; and
voltage limiter means connected between said grid and said filament for damping voltages therebetween which exceed a preselected limit.

2. An X-ray generating tube in accordance with Claim 1 wherein said voltage limiter means comprises a nonlinear resistance.

3. An X-ray generating tube in accordance with Claim 1 wherein said voltage limiter means comprises a metal oxide varistor.

4. An X-ray generating tube in accordance with Claim 3 wherein said metal oxide varistor has a threshold voltage of about 6 kilovolts.

5. An X-ray generating tube in accordance with Claim 1 further comprising an external contact means for connecting said grid to an external source of voltage, wherein said external contact means comprises a series resistor in series with said grid for limiting current flow thereto.

6. An X-ray generating tube in accordance with Claim 5 wherein said voltage limiter means comprises a nonlinear resistance.

7. An X-ray generating tube in accordance with Claim 5 wherein said voltage limiter means comprises a metal oxide varistor.

8. An X-ray generating tube in accordance with Claim 7 wherein said metal oxide varistor has a threshold voltage of about 6 kV.

9. An X-ray generating tube in accordance with Claim 7 wherein said series resistor is of a value approximately in the range of 500 thousand ohms to 1 meg ohm.

10. An X-ray system comprising:
an X-ray generating tube having a filament for emitting a beam of electrons, a grid for selectively terminating the beam of electrons and an anode in the path of the beam of electrons for emitting X-rays when struck by the beam;
a filament power supply means for supplying voltage to heat said filament;
a grid voltage supply means for supplying grid voltage to said grid;
an anode power supply means for supplying anode voltage to said anode;
voltage limiter means connected between the outputs of said filament power supply means and said grid voltage supply means for dampening voltages therebetween which exceed a predetermined limit; and
a series resistor connected between said grid voltage supply means and said grid for limiting the current flow therebetween.