JPS62188148A - X-ray tube protected from welding - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

BACKGROUND OF THE INVENTION The present invention relates to x-ray tubes and x-ray systems, and more particularly to x-ray tubes and x-ray systems that use grid cutoffs to stop the generation of x-rays. .

In some x-ray applications, it is beneficial to have the ability to stop and start the x-ray beam at will. It is difficult to start and stop the x-ray beam quickly by simply controlling the anode voltage. This is because the anode voltage of X-ray tubes is often high, exceeding 100 kilovolts, and such voltage levels cannot be turned on rapidly.
It is difficult to turn off.

In view of the above, gridded x-ray tubes are often used in x-ray machines where it is desirable to stop and start the x-ray beam quickly. Gridded x-ray tubes operate like conventional x-ray tubes in that the electron beam emitted from the filament is directed toward the anode. In an x-ray tube, the anode is made of a refractory metal that emits x-rays when struck by an electron beam. Moreover, such gridded x-ray tubes also, like conventional triode vacuum tubes, stop the electron beam and thus stop emitting x-rays when a sufficiently negative voltage is applied to the grid. The advantage obtained by using a gridded x-ray tube is that the grid voltage required to stop the electron beam is much less than the anode voltage. for example,
Typically the grid voltage of an X-ray tube is between 2000 and 500
It is in the range of 0 volts. Such voltages can be controlled more efficiently than high anode voltages.

As a result, gridded x-ray tubes are commonly used today. However, the level of grid voltage required to stop x-ray emission depends in part on the spacing between the filament and the grid; the smaller the spacing, the lower the voltage required. The spacing between the grid and filament is usually minimized, as lower voltages are easier to control.

Unfortunately, causes such as anode spits, overvoltage on the anode, mechanical shock or vibration can cause the filament to start vibrating. The vibrating filament physically contacts the grid with very little vibration or deflection. During such contact, if the grid voltage is on, a significant current will flow between the grid and filament, which may cause the filament and grid to weld together, rendering the tube defective. I end up.

Therefore, an object of the present invention is to have the advantage that the X-ray beam of the gridded X-ray tube can be easily cut off, and
An object of the present invention is to provide an X-ray tube and an X-ray system that prevent welding between a grid and a filament.

SUMMARY OF THE INVENTION In one aspect, the invention includes an x-ray tube having a filament, a grid, and an anode. Additionally, a voltage limiter, such as a metal oxide varistor, is connected between the filament and the grid to attenuate grid overvoltages caused by power supply fluctuations or anode spits or the like.

By providing a voltage limiter, electrostatically induced vibrations of the filament are prevented and the occurrence of filament-to-grid welding is reduced.

One feature of the invention is that a series resistor is provided in series with the grid. In the event of a short circuit between the filament and the grid, the series resistor limits the current flowing to the grid, thereby preventing welds from forming. Since no grid current flows during normal operation of a negatively biased grid, relatively high value resistors can be used in series with the grid without interfering with normal grid operation.

The novel features of the invention are pointed out with particularity in the claims. The invention itself, however, as to its structure and method of operation, together with other objects and advantages of the invention, may be best understood from the following description, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, a gridded x-ray tube 10 is shown having a filament 11, a grid 12, and an anode 13 within an envelope 14. It is housed and maintained under high vacuum. The x-ray tube 10 of FIG. 1 operates in the normal manner. That is, the filament 11 is electrically heated and the anode
The interfilament voltage causes the electron beam to flow from the filament to the anode. The anode 13 is made of a refractory material with a high atomic number, at least in the region where the electron beam collides, so as to efficiently emit X-rays. When the grid 12 is biased to a sufficiently negative voltage with respect to the filament 11, the electron beam stops and so does the x-ray.

Typically, in medical x-ray tubes, the anode-to-filament voltage is greater than 100 kilovolts. The grid cutoff voltage is typically about 3.7 kV (kilovolts). Grid voltages above 4kV are somewhat higher than average, with approximately 2.7kV typically required to cut off 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 at each end of filament 11 are not shown. filament 11
It will be appreciated that the spacing between the filament and the cathode 12 is shown relatively close so that when the filament vibrates, physical contact occurs 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 this X-ray tube 20 are the same as components of X-ray tube 10, the same reference numerals are used for simplicity.

Further, referring to FIG. 4, filament 11 generates an electron beam. An anode 13 is provided in the path of the electron beam, which emits X-rays when the electron beam collides with it. A grid 12 is also provided in the path of the electron beam, and the electron beam is selectively stopped when a sufficient grid voltage is applied to the grid.

In addition to the components described above, a resistive element is provided in the x-ray tube 20. That is, voltage limiting means 21 are connected between the filament 11 and the grid 12, which attenuate voltages between them that exceed a preselected limit value.

As this voltage limiting means, for example a non-linear resistor such as a metal oxide varistor can be used.

One of the reasons why the filament vibrates is the overvoltage between the grid and the filament or the anode spit caused by power fluctuations, as described above.

The function of the voltage limiter is grid filament! - Make sure that the voltage between the two does not become high enough to cause the filament to start vibrating. It has been found that a 6 kV metal oxide varistor works well as a voltage limiter 21 since the grid voltage typically does not need to exceed 4 kV. It will be appreciated that the voltage limiter need not have a steep cutoff characteristic. It is only important that the grid-to-filament voltage is not high enough to induce vibrations.

Besides metal oxide varistors, other components such as Zener diodes can also be used as voltage limiters.

It is preferred that voltage limiting components be mounted as close as possible to the circuitry to be protected. Therefore, size and lead length are considerations when selecting components.

The grid voltage is applied to external connection means 22 including a series resistor 23 . The purpose of this series resistor is, as mentioned earlier,
The purpose is to prevent excessive current from flowing into the grid when a short circuit between the grid and the filament occurs due to vibration.

As is well known in the vacuum tube art, a negatively biased grid does not require any current under normal operating conditions. Therefore, resistor 23 can be made to a relatively high value without interfering with the cut-off function of the grid. Obviously, the higher the value of the resistance 23, the more efficient the protection against welding of the grid and filament. Values in the range of 500 kilohms to 1 megohms have typically been found to be effective in preventing welding of the grid to the filament. Since the contact between the filament and the grid is typically very brief, the average power rating of the resistor is not particularly important, so the resistor only needs to be capable of dissipating short pulses of power.

Referring now to FIG. 5, an x-ray system 30 using an improved x-ray tube 20 is shown.

An anode power supply 31 is connected between the filament 11 and the anode 13. As in a conventional
Supplied between the anodes.

A filament power supply 32 is provided to heat the filament to emit an electron beam.

Finally, an external grid power supply 33 is connected between grid 12 and filament 11 to provide a grid voltage for cutting off the electron beam.

As is clear from FIG. 5, the series resistor 23 is connected to the grid 1
2 and a grid power supply 33, and the voltage limiter 21 is connected between the grid 12 and the output of the filament power supply.

As previously mentioned, the best protection against filament-to-grid shorts is obtained by increasing the value of the series resistance. Moreover, increasing the value of the series resistance should theoretically not interfere with the operation of the grid. However, it has been found that the resistance should not be too high in value. For example, 2 megohms is considered too large. The reason why there is an upper limit to the value of the series resistor 23 will be explained below. A resistor 34 is present, shown in dotted lines in FIG. 5, which represents the leakage resistance of the x-ray tube, the low voltage resistance of the nonlinear resistor 21, and the insulation leakage resistance at the x-ray tube connector. As can be seen, the series resistor 23 and the leakage resistor 34 constitute a voltage divider, thereby allowing the grid power supply 3
The grid voltage supplied from 3 is determined. If the value of leakage resistor 34 is substantially greater than series resistor 23, the voltage on grid 12 will be close to the full output voltage of the grid power supply. However, if, for example, leakage resistance 34 and series resistance 23 are of the same value, the effective grid voltage will be half the output voltage of grid power supply 33. Therefore, care must be taken not to set the series resistor 23 to an excessively high value in order to allow the intended operation of the grid.

Although the invention has been described with respect to particular embodiments, other modifications and variations will occur to those skilled in the art. It is therefore to be understood that the present invention is capable of various modifications and variations within the scope of the appended claims.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of a conventional X-ray tube with a grid, FIG. 2 is a plan view showing the filament and grid of such a conventional X-ray tube, and FIG. 3 is a diagram showing the structure of a conventional X-ray tube with a grid. 4 is a block diagram of an improved x-ray tube according to the present invention; and FIG. 5 is a circuit diagram of an x-ray system using the improved x-ray tube. (Explanation of symbols) 11...Filament, 12...Grid, 13.
...Anode, 14...Envelope, 21...Voltage limiting means, 23...Series resistance.

Claims (1)

[Claims] 1. A filament that emits an electron beam, a grid that selectively stops the electron beam, and an anode that is provided in the path of the electron beam and emits X-rays when the electron beam collides with it. an envelope containing the filament, the grid and the anode under vacuum; and a voltage connected between the grid and the filament to attenuate the voltage between the grid and the filament above a preselected limit. X having a limiting means
wire tube. 2. The X-ray tube according to claim 1, wherein the voltage limiting means is comprised of a nonlinear resistor. 3. The X-ray tube according to claim 1, wherein the voltage limiting means is composed of a metal oxide varistor.
wire tube. 4. An x-ray tube according to claim 3, wherein said metal oxide varistor has a threshold voltage of about 6 kilovolts. 5. In the X-ray tube according to claim 1, the grid is further connected to an external voltage source via an external connection means, and the external connection means is provided in series with the grid to connect the grid to the grid. An x-ray tube that contains a series resistor that limits the flow of current. 6. The X-ray tube according to claim 5, wherein the voltage limiting means is constituted by a nonlinear resistor. 7. The X-ray tube according to claim 5, wherein the voltage limiting means is composed of a metal oxide varistor.
wire tube. 8. The x-ray tube of claim 7, wherein said metal oxide varistor has a threshold voltage of about 6 kilovolts. 9. An x-ray tube according to claim 7, wherein the series resistance has a value in the range of about 500 kilohms to 1 megohm. 10. An X-ray tube having a filament that emits an electron beam, a grid that selectively stops the electron beam, and an anode that is provided in the path of the electron beam and emits X-rays when the electron beam collides; filament power supply means for supplying a voltage to heat the filament; grid voltage power supply means for supplying a grid voltage to the grid; anode voltage power supply means for supplying an anode voltage to the anode; and an output of the filament power supply means and the voltage limiting means connected between the grid voltage power supply means and the grid, the voltage limiting means being connected between the grid voltage power supply means and the grid to attenuate the voltage across the output exceeding a predetermined limit value; An X-ray system having a series resistance limiting the flow of current between the power supply means and said grid.