RU2487433C1 - Cathode pack of vacuum tube for high-voltage operation - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: cathode pack of vacuum tube purposed for high voltage operation includes photoelectronic converter (PEC) with photocathode as an electron source with dynode system for secondary electrons multiplying and leads to feed voltage to electrodes. In order to free anode space from electrode leads to prevent glowing, discharging and other phenomena occurring in a strong electrical field in presence of other conductors and to preserve dimensions and shape of the device it is suggested turning PEC electrode leads to 180°, to manufacture additionally a photocathode chamber with entrance gate where photocathode is placed and to place this chamber inside circumference of outside leads.

EFFECT: reliable, durable and compact cathode pack of vacuum tube operating at high voltage values.

1 dwg

Description

The present invention relates to electrovacuum devices operating at high voltages, in particular to x-ray tubes, and can be used for technical and medical applications.

As an electron source in electrovacuum devices operating at high voltages, for example, glow and cold cathodes are used in x-ray tubes.

So, X-ray tubes (RTs) with filament cathodes have been widely produced for various applications since 1926 (V.I. Rakov, 1952 Electronic X-ray tubes). As a rule, X-ray tubes have a vacuum shell, a heated cathode, capable of emitting electrons directed by a strong field to the anode. After being bombarded by electrons exiting the cathode, the anode emits X-rays.

However, the incandescent cathode is the unit that most often fails; it cannot provide not only durability, but also long-term emission stability.

The so-called “cold” cathodes for x-ray tubes are known (application US 2002063500, op. 05.30.2002, WO 0042631 op. 07.20.2002, GB 1109744 (A) op. 04.10.1968, WO 0042631 op. 20.07. 2002, GB 1109744 (A) op. 04.10.1968). Their action is based on the phenomenon of field emission (field) emission, which occurs at very high electric field intensities, while the anode voltages can reach 100-300 kV. These tubes operate in a pulsed mode and do not apply to tubes of low and medium power.

As a prototype, the cathode assembly of the x-ray tube described in patent document LV 14047 (B), op. 01/20/2010, with a cathode based on the built-in photoelectronic converter and an external photon source for exciting photo and secondary electron currents in the vacuum volume of the RT, which also refers to “cold” cathodes.

Such a cathode assembly is a photoelectronic converter (PEC) with a photocathode as an electron source and a dynode multiplication system, with leads for supplying voltage to the electrodes. It is proposed to use a photomultiplier of the multiplier type as a PEC.

The operation of such a photoelectronic converter is as follows: a voltage is applied to the electrodes of the photomultiplier, which is distributed so that the electrons from the photocathode are directed to the dynodes of the multiplier, and then to the anode. Light entering the photocathode causes a stream of electrons. Electrons are multiplied by dynodes and under the influence of a strong electric field they bombard the anode, causing x-ray radiation.

We took this cathode assembly for an x-ray tube as a prototype. The prototype has the following advantages compared to the traditional incandescent cathode:

- reliability.

An incandescent cathode is the unit that most often fails. The proposed MTBF time between failures with a cathode assembly - a prototype is an order of magnitude higher, which makes it possible to use RTs in long-term operation, for example, in field conditions, for space and medical applications.

- stability.

An incandescent cathode cannot provide long-term emission stability. The PEC-based cathode has the ability to regulate the emission due to feedback from the LED or voltage regulation on the secondary multiplier.

However, this prototype:

- does not allow the use of photomultiplier tubes of a traditional design as an electron source for devices requiring tens or hundreds of kilovolts for their operation, such as are required to obtain, for example, x-ray radiation due to discharge phenomena at the terminals located in the high voltage field ;

- the cathode assembly described in the prototype does not allow efficiently focusing the flow of photoelectrons from the photocathode to the dynode multiplication system, and then generating a stream of secondary electrons to the anode, and therefore does not provide a high gain and an output current of sufficient magnitude to the anode of the x-ray tube.

This invention proposes the construction of a cathode assembly containing a photoelectronic converter (PEC), which, under the influence of an external light source, can serve as a source of electron flow and can be used in devices with high (tens and hundreds of kilovolts) anode voltage, for example, x-ray tubes.

The invention solves the problem of creating a reliable and durable cathode assembly of an electrovacuum device for operation at high voltages.

This problem is solved in that in the known cathode assembly of an electrovacuum device designed to operate at high voltages, including a photoelectric converter, as an electron source, with a photocathode and a dynode system for multiplying secondary electrons, as well as outputs for supplying voltage to the electrodes, a photoelectronic converter additionally has a photocathode camera with an input optical window, on the inner surface of which there is a photocathode, and the terminals for supplying voltage from an external at regular enrollment photoelectronic transducer electrodes arranged around the chamber photocathode, wherein the photocathode chamber diameter smaller than the diameter of the secondary electron multiplying dynode system.

In this case, a conventional photomultiplier cannot be used as a photomultiplier photomultiplier, because instead of the traditional photomultiplier collector, which picks up the output signal, an amplified electron beam must be formed and directed to the anode, while the electrode leads will be located in the high voltage field zone and, as it turned out , such a device cannot operate in a strong electric field due to glow, discharges, and other phenomena that occur in a strong electric field in the presence of other conductors.

In order to free the anode space from the terminals of the electrodes, it is proposed to expand the terminals of the PEC electrodes by 180 °, additionally make a photocathode chamber with an input window on which the photocathode is located, placing it inside the circumference of the outer terminals to preserve the dimensions and shape of the device. At the same time, such a design of the photomultiplier made it possible to correctly place antimony and alkali metal sources inside the chamber, so that the process of firing the balloon and other high-temperature operations would not damage the quality of those elements on which the manufacturing process of the main photosensitive electrode, the photocathode, and not lose the gain multiplication system. In addition, the design of the photocathode chamber provides for the protection of the electrode leads from the deposition of antimony and alkali metals, which could create conductive films between the electrodes. Thus, this design of the cathode assembly allows, while maintaining the compactness of the device, to accomplish the task.

Another difference is that the photoelectronic converter contains a system for focusing the flow of photos and secondary electrons, which allows you to generate and effectively control the electronic flow to obtain the desired parameters of the device.

The drawing schematically shows a drawing of an x-ray tube with the claimed cathode assembly.

As an example of a specific implementation, consider the cathode assembly of an x-ray tube.

The x-ray tube shown in the drawing consists of a cathode assembly, which is a photomultiplier (1), a sealed housing (2), and an anode (3).

FEP consists of:

- Photocathode chamber (4), which is a glass cylinder with a length of 20 mm and a diameter of 22 mm, at the end of which through an insidious ring (5) an entrance window of optical glass is hermetically welded, on the inner surface of which a photocathode (6) is applied. Inside the cylinder there are focusing and accelerating electrodes, two antimony evaporators located in a certain way, sources of potassium and cesium (not shown in the figure). The cylinder of the photocathode chamber (4) is welded into a glass disk with a diameter of 42-44 mm. Around the cylinder at a diameter of 34 mm, 20 treacherous leads (7) are welded into the glass disk, which serve to connect the electrodes located inside the sealed volume with power supplies from the outside (the leads are shown only from the outside).

- The dynode system (8), which serves to enhance the flow of photoelectrons. We have chosen a louvre system of 12 dynodes based on Cu-Al-Mg bronze. The type of blinds is selected from a multiplier designed for high currents.

- A system of electrodes (9) that form the output electron stream, which makes it possible to obtain the necessary adjustable focus on the tube anode.

The operation of the cathode assembly, including the photomultiplier, is as follows.

The supply voltage is applied to the electrodes: a photocathode (6), focusing and accelerating electrodes (not shown), dynodes (7), output forming electrodes (9). The total supply voltage of the photomultiplier cathode assembly is about 1000-2000 V.

Another power source supplies a high (10-50 kV) voltage between the output electrodes and the anode (3).

Under the influence of light, the photocathode (6) emits an electron stream, which is formed by the electric field of the focusing and accelerating electrodes and falls on the first dynode of the louvre multiplier system (7). Then a cascade multiplication of electrons occurs, after the 12th dynode we obtain an amplified electron flux, which falls under the action of the field of the focusing electrode system (9) and is accelerated by the anode field (3).

Below are the results of measurements of the parameters of the photomultiplier part of the fluoroscopic tubes, showing the possibility of obtaining normal photoelectric parameters in this design of the cathode assembly:

Photocathode light sensitivity, μA / lm 100 ÷ 219 Gain at a voltage of 1600 V, times (0.2 ÷ 7) · 10 ⁶ Dark current, A (1 ÷ 20) · 10 ^-9

Several models of PSF were tested in the PMT mode at currents of 1000 μA. The test results showed the ability of the cathode assembly to operate at high currents.

The developed design of the cathode assembly with deployed leads and focusing systems, including a photomultiplier, made it possible to make an X-ray tube capable of operating at voltages up to 50 kV, to obtain an electron beam with a focus of the required size and an X-ray spectrum.

An X-ray tube with the proposed cathode assembly can provide pulsed and continuous operation, stability of the output signal in the continuous mode due to feedback on the control of the light source, durability, high speed, low power consumption.

The inventive cathode assembly can be used in other types of electrovacuum devices operating at high voltages.

Claims (2)

1. The cathode assembly of an electrovacuum device for operating at high voltages, including a photoelectronic converter as an electron source, with a photocathode and a dynode secondary electron multiplication system, terminals for supplying voltage to the electrodes, characterized in that the photoelectronic converter further has a photocathode chamber with an input optical window , on the inner surface of which the photocathode is located, and the terminals for supplying voltage from an external source to the electrodes are located around the photo atodnoy chamber, the diameter of the photocathode chamber smaller than the diameter of the secondary electron multiplying dynode system. 2. The cathode assembly of an electrovacuum device for operating at high voltages according to claim 1, characterized in that the photoelectronic converter comprises a system for focusing the flow of photos and secondary electrons to the electrodes.