US2520218A - Gas-filled rectifying tube - Google Patents

Description

` ug. 29, 1950 Y M. KUlPERs 255207218 GAS FILLED RECTIFYING TUBE Filed July 2, 194e AGENT Patented ug. 2.9-, 1950 GAS-FILLED RECTIFYING TUBE Minne Kuipers, Eindhoven, Netherlands, assigner i to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application July 2, 1946, SerialNo. 680,891 In the Netherlands March 6, 1943 Section 1, Public Law 690. August 8, 1946 Patent expires March 6, 1963 5 Claims. (Cl. Z50-27.5)

This invention relates to gas filled rectifying tubes having two or more anodes and wherein the incandescent cathode is disposed within a cathode screen, more particularly a rectifying tube for high voltages i. e. several thousand volts.

Rectifying tubes having a plurality of anodes are often used since with the same power they occupy less space than a corresponding number of tubes having one anode. Since, moreover, the feed transformer has a single heating winding the construction of a rectifier equipped with a rectifying tube having a plurality of anodes is particularly simple and cheap. One advantage of a tube having a plurality of anodes with respect to the tube having one anode and the same power is that the alternating voltage superposed on the rectified voltage is smaller and in addition that the load conditions for the cathode are more favourable, since the ratio of the maximum current to the average current is smaller.

One drawback of tubes having a plurality of anodes consists in that there often occurs arcing back in one discharge path or a direct breakdow-n between the two anodes and in addition th-at the anodes are liable to strong disintegration, due to which the tubes soon blacken and become unserviceable. With tubes having one anode substantially no ions are formed in the negative phase, so that practically only the ions available by the end of the positive phase find their way to the anode. In tubes having a plurality of anodes, however, ions are being steadily formed in the discharge paths in the positive phase. These ions gain at least partly access to the discharge paths of the negative phase which involves a stronger bombardment by ions and this brings about the aforesaid drawbacks of arcing back, breakdown and blackening of the tubes.

To avoid the aforesaid drawbacks in tubes having two or three anodes it has already come to be known to place one or more screens between the anodes, which screens extend to the cathode screen. These tubes, however. are only suitable for eiective anode voltages of several hundreds of volts, since the screens do not prevent ions from wandering directly from one anode to the other. At low voltages this is generally not very troublesome, since almost all ions reach the cathode where they are neutralized, but at higher voltages the ions often pass directly from one anode space to the other.

In regard to voltages exceeding several hundreds of volts, rectifying tubes having a plurality of anodes were hitherto often designed as a cylindrical or spherical glass bulb wherein the cathode was provided, if required within a cathode screen. whereas the -anodes were fitted in a corresponding number of anode arms of the bulb having a great length and being once or several times curved. As a result of the long curved arms any ions wandering to one of the anodes are collected by the wall of the anode arm so that the ion bombardment on the anode remains small. However, the arms also collect many ions in the positive phase so that the arc voltage and consequently the losses become high. Sometimes itl is necessary to provide auxiliary electrodes in the arms or to equip the arms on the outside with a conductive layer to prevent the ignition voltage from becoming too high. In addition the arms require much space and render the tube breakable and expensive.

Furthermore it is known to house the incandescent cathode of a tube having two anodes within a screen and to divide this screen, by means of a screen placed at the middle, into two parts each of which cooperates with one anode.

25 By placing the screen at the middle of the cathode it is to be ensured that the ions cannot find their way. through the cathode space, from one discharge path to the other. In this case. there are consequently two different cathodes as it were that are not in the same favourable load l lets must be provided.

The present invention has for its object to prevent these disadvantages and drawbacks in a rectifying tube having two or more anodes. in which the incandescent cathode is housed within a cathode screen, by choosing the distance d from any anode to the cathode screen in such a manner that a correct product of this distance and the pressure in the tube is smaller than the value at the minimum of the Paschencurve for the filling in question of the tube, the cathode screen and. it required, other screens being fitted in the tube in such a manner that asaaais their way from the space for one anode to the space for another anode. Consequently, the charge carriers in the space for one anode can reach the cathode space only where they are collected and neutralized by the cathode. With the same power'. tubes designed according to the invention have a much smaller circumference than tubes having arms. I

In a particularly suitable construction the spaces in front of the anodes communicate only through long. narrow and preferably curved slits,

' the cathode screen' being usually closely surrounded by the wall of the tube. As a result ofthe narrow slits charge carriers are not able to move freely from one anode space to mother, since they are 'soon collected by the walls thereof.

In tubes for very high voltages it is advantageous to cover the apertures for the passage of the discharge `by means of little screens preferably housed in the cathode screen. The screens covering the apertures for the discharge prevent ions from entering the cathode screen at a high velocity, 'due to which they would fly pest the cathode and reach any of the other anodes through one of the other apertures. Moreover. the screens have the advantage of preventing activating material disintegrating from the cathode from finding its way to the anode.

In a suitable form of construction of a tube according to the invention the anodes are mounted symmetrically. The temperature and consequently the arc voltage of each discharge path is the same during operation of the tube, so that uneven arc voltages do not cause additional higher harmonics in the rectified current.

In another suitable form of construction the anodes are closely surrounded by the glass wall and the front surface of the anodes is adapted to the form of the cathode screen, the diameter of the front surface of the anodes at least amounting to four times both the distance d from the anodes to this screen and the diameter of the discharge apertures in the cathode screen, and the product pd of the pressure in millimeters of the gas in the tube times the distance in centimeters from the anode to the screen being between 0.00015 and 0.015. In this construction the distance from the anodes to the cathode screen is practically constant, and as a result of the anodes being closely surrounded by the glass wall and of the small size of the discharge apertures and the distance from the cathode screen to the anodes in comparison with the diameter of the anodes it is achieved that practically no charge carriers are able tol gain access to the back of:

the anodes, so that the tube is free from arcing back even at very high voltages.

In order that the temperature of the anodes and the surrounding glass wall may be kept low, in spite of the compact construction of the tubes according to the invention, the following form of construction offers especial advantages. In

y this form the anodes, which are designed as cylindrical boxes closed on one side. are sealed with the open edge to the glass wall, so that the outer side of the anodes is cooled by conduction and convection, for instance exclusively by natural circulation of the air or by forced cooling, thereby cooling the inner anode surfaces. Though the screens for covering the discharge apertures may be directly connected to the cathode screen or. as an alternative. may be so provided as to be insulated therefrom without being connected to a definite voltage, it is especially advantageous to apply s positive voltage to these screens, for which either a constant direct voltage or s pulsating direct voltage may be made use of. In fact. since the screens have a positive voltage they prevent, even under the worst conditions, posi- 'tive ions from being drawn to an anode having a negative potential, and this because the screens repel the ions. In the positive phase they have the advantage of operating as a grid facilitating the first ignition of the tube. Similarly to all grids in gas-nlled tuba the screens ako act to s greater or less degree as an auxiliary snode by htlh the ignition of the main current is facili- In order that the invention may be more clearly understood and lreadily carried into eect it will now be described with reference to the accompanying drawing. wherein Figures 1 and 2 are a vertical and a horizontal section respectively of a tube according to the invention, in which the symmetrically mounted anodes are sealed to the wall of the tube. Figure 3 representing a tube according to the invention. in which the discharge apertures in the cathode screen are covered by distinct screens, and Fig. 4 is a sketch of the so-called Paschen curve.

In Figures 1 and 2 the reference member I denotes the glass wall of the tube. The cathode 2 is provided within the cathode screen I3 which is closely surrounded by the wall I. Three anodes are placed at a short distance from the cathode screen and consist of hollow chrome iron boxes 4 each having a layer of carbonized iron Il as an active surface. Each anode is equipped with a connecting terminal 1. The cathode is secured to the terminal wires s which 1nA their tum ire .secured to the chrome iron hoods I welded t`o the tube. The cathode screen is secured to the terminal wires l by means of the clips l and the insulating rings i. The cathode screen 3 is furnished with a supply wire il leading to an outlet. The stock of mercury I2 is contained in the chrome iron cap ii. The anodes are sealed with the open back Il to the glas of the tube. As a result thereof the metal of the anodes directiy contacts the open air so that the temperature is very low. In front of each anode the cathode screen is furnished with an aperture I3 for the passage of the discharge. Any ions penetrating inside the cathode screen are collected and neutralized by the cathode. There is substantially no penetration by the anode field. into' the cathode screen through the apertures i3 so that along this path substantially no ions can find their way from the space for one anode to the space for another anode. To prevent charge carriers from moving directly from one anode to the other the cathode screen is equipped with three circular metal screens i1 embracing an edge of the similarly shaped glass rings II. It is practically impossible for charge carriers to pass through the narrow slits 8a between rings II and I1, so that a breakdown between two anodes is not possible also along this path.

In Figure 3 parts corresponding to those of Figures l and 2 bear the same reference numerals.-

The apertures i3 are covered by screens Il which are insulated from the cathode screen and each of which has a separate outlet. For the discharge m each anode a comparatively narrow annular slit is left. Ions coming from one of the anodes at a high velocity are collected by the cathode screen 3 or one of the screens i8. Any ions penetrating into the cathode screen generally have a low velocity only, so that they are readily neutralized by the cathode. `As clearly appears from the drawing not only the cathode screen is closely surrounded by the glass wall, but also the upright side of the anodes. Consequently discharges cannot occur at the back of the anode. The front surface of the anodes provided at the sides of the tube has a curvature in a plane normal to the plane of the drawing, in such a manner that the distance between the front surface and the cathode screen has a substantially constant value. The stock of mercury I2 is provided in a bulb I9 into which reach the gauge-covered ends 20 of the sealed exhaustion tubulation. The tube, whose size is about 1%; times as large as that of the tube shown in the drawing, accommodates an effective alternating anode voltage of 10 kv. and a current of 2.5 ampper anode i. e. the tube is capable of rectifying a power of 75 kw. In this tube the screens I8 receive a positive voltage of to 20.

In Fig. 4 is illustrated a typical Paschen curve substantially as it appears when plotted on a double logarithm scale. Various Paschen curves are published in Nederlandsch Tijdschrift voor Natuurkunde (Netherlands Journal for Physics) in the article by F. M. Penning entitled Het Verband Tusschen de Doorslagkromme van Paschen en de Elementaire Processen, vol. 5, pages 146-151 (see Figure 3 of the article). Such a curve specifically prepared for mercury, for example, may be found in the article by F. Llewellyn Jones and W. R. Galloway in Proc. Phys. Soc. London, 1938, vol. 50, page 27, especially in Fig. 2 on page 210. It is preferable that the product p X d, where p is the pressure in the tube and d is the distance of separation between any anode surface and the opposed cathode screen surface be less than the minimum point on the Paschen curve.

What I claim is:

1. A gas filled rectifying tube comprising an envelope, a cathode within said envelope, a catlrr ode screen within the envelope and surrounding said cathode, a plurality of anodes having active surfaces substantially parallel to and facing surfaces of said screen on the side of the screen remote from said cathode, said screen having screen apertures interposed between said cathode and each said anode, said envelope and said screen dening anode chambers communicating with each other through elongated narrow passageways.

2. A gas filled rectifying tube comprising an envelope, a cathode within said envelope, a cathode screen within the envelope and surrounding said cathode, a plurality of anodes having active surfaces substantially parallel to and facing surfacesof said screen on the side of the screen remote from said cathode, the product of the gas pressure p and the distance d of separation between said anode and screen surfaces being less than the minimum point on the Paschen curve for the gas filling of the tube, said screen having screen apertures interposed between said cathode and each said anode, said envelope and said screen defining anode chambers communicating with each other through elongated narrow passageways.

3. A gas filled rectifying tube comprising an envelope, a cathode within said envelope, a cathode screen within the envelope and surrounding said cathode, a plurality of anodes having active surfaces substantially parallel to and facing surfaces of said screen on the side of the screen remote from said cathode, said screen having screen apertures interposed between said cathode and each said anode, said envelope and said screen defining anode chambers communicating with each other through elongated narrow passageways, and individual screen members interposed between said screen apertures and said cathode.

4. A gas filled rectifying tube comprising an envelope. a cathode within said envelope, a cathode screen surrounding said cathode, said envelope and said screen defining a plurality of anode chambers, a plurality of anodes symmetrically arranged about said cathode in said chambers and separated from said cathode by said screen, said screen having screen apertures interposed between said cathode and each said anode, said anode chambers communicating with each other through elongated narrow passageways, and individual screen members interposed between each of said screen apertures and said cathode.

5. A gas lled rectifying tube comprising an envelope, a cathode within said envelope, a cathode screen within said envelope and surrounding said cathode, said screen and said envelope defining a plurality of anode chambers, an anode in each of said chambers each having active surfaces substantially parallel to and facing surfaces of said screen on the side of said screen remote from said cathode, said screen having screen apertures interposed between said cathode and each said anode, said anode chambers communicating with each other through elongated narrow curved passageways, and individual screen members interposed between said screen apertures and said cathode.

MINNE KUIPERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

Images