US1912866A - Electron tube for high operating voltages - Google Patents

Description

June 6, 1933. R, STRIGEL ET AL ELECTRON TUBE FOR HIGH OPERATING VOLTAGES Filed July 1, 1929 QQS l atented June 6, i933 UNITED STATES PATENTQOFFI'CE wns'rmvn; GERMANY, ASSIGNOR rro srnrunns-scrrncxnnrwnnxn AK'IIENGESELL- j scHAruor BERLILT-SIEMENSSTADT, GER-MANY, A CORPORATION or GERMANY ELECTRON Turn non uteri OPERATING VOLTAGES Application filed July 1,1929, Serial No. 375,190, and in Germany June 30, 1928.

Our invention relatesto improvements in electron tubes and more particularly to. an electron tube for high operating voltages, the main application of such tubes being for instance to X-ray devices, to apparatus for detecting faults .in power cables, and in general to devices operating at voltages higher than 300,000 volts. i

As is well known in theart, the voltage, impressed between the anode and thermionic cathode of highly exhausted vacuum electric deviceson the reversal of the alternating halt wave, during which no current should flow between the electrodes, is automatically limited on account of discharges caused by the ionization due to prevailing intenseelectrostaticvfields, especiallyin the neighborhood of the electrodes. According to the degree of evacuation these discharges start at a field intensity of 10*-10 volts per cm. .lVith the raising of the voltage at which current flowis stillprevented, and with the customary. arrangement in which the cathode is located within an anode cylinder, this calls for continually increasing dimensionsof the cathode; instead of the customary finehot wire filaments itis necessary to use rods or hollow cylinders for the cathode. In this way comparatively large heating currents at very low heating voltages are necessary, and difficulties thus arise in sealing the required heavy metal conductors inthe glass wall of the bulb. The loss of heat at the ends of the heated cathodemay also amount to a multiple of the heating energy required for heating the cathode itself, so that quartz and glass parts located in thevicinity of the seals or employed as insulating material are liable to fuse. i l

The object of our invention is to eliminate these difficulties. For simplicity sake the cycle portion atwhich current flows between theanode and the cathode will be termed the flow. phase, and the cycle portion at which no current flows the stop phase, and

the prevailing voltages, respectively will be termed ffiow voltage?" and stop voltage.

H In reducing our invention to practice,a simple form consists in placing between the anode and the thermionic cathode a grid of wide mesh which is so designed that no field intensities greater than 10. volts per cm are. able to develop. This condition is complied with, for instance, by a grid of thick rods or tubes consisting of a metal having tion of the field intensity with increasing cathode radius, in case of a coaxial arrangement between ahot filament and a cylindrical anode. V y p Fig. 2, a cross-section through the electrode structure of an electron tube,

Fig. 3,a longitudinalsection through the structure shown in Fig. 2,

Fig. 4, a cross-section through a modified electrode structure,

Fig. 5, a longitudinal section through the electron structure according to Fig. 4, Figs. 6, 7 and 8 show other modifications in longitudinal section according to the principle of two oppositely disposed plate electrodes, and in which the anode consists of a circular dished element, and

Figs. 9 and 10represent wiring diagrams.

Like parts are indicated by like letters of reference throughout all the figures of the drawing. p

It is well known in the art that with a coaxial arrangement between the heated filament and the cylindrical anode, the behavior of the field intensity at the cathode surface withanincreasing cathode radius (the anode radius is assumed to remain constant) is such as represented by the graph in Fig. 1 of the drawing. This graph shows that with an infinitely small cathode radius as one extreme, or with an infinitely small spacing between anode and cathode (cathode radius equal anode cylinder radius) as the other extreme, the field strengthbecomes infinite for both extremes. Between these extremes prevails a rather broad minimum of the electrostatic field strength (1' .1 in Fig. 1). It is of advantage to dimension the grid rods so that the field intensity at their surface lies within this minimum value in order that for the values of a high electrostatic field strength lying within the minimum of 10 and 10 volts/cm, a discharge will not be caused by the ionization through the inherent electrostatic field. V

An arrangement of this kind is illustrated in Figs. 2 .and 3 of the drawing. .The anode a is here, as customary a hollow cylinder or shell. The grid consists according to our invention of thick rods 9, which in this eX- ample are of circular cross-section. They terminate in end disks 2, only the upper disk being shown in Fig. 3. The cathode is is a hairpin-shaped thermionic filament which is kept in shape by a central support m and a spring Figs. 4 and 5 of the drawing illustrate a preferred modification of the rods 9 of the grid. The anode a is here, as customary, a hollow cylinder or-shell. The grid consists, according to our invention, of thick rods 9 of drop-shaped cross section, their blunt convex sides facing the anode, and their pointed portions facing the oathode. They terminate in end discs 2, only the upper disc being shown in Fig. 5. The cathode is is a hairpin-shaped thermionic filament which is kept in shape by a central support m and a spring This construction has the advantage over the circular cross section, insofar as the voltage factor f of the tube (Z and (L denoting respectively infinitely small changes of the anode potential and the grid potential,

. wherein the electron stream is assumed to be constant) is in this instance somewhat increased, and the rid current or the charge of the grid is slig itly reduced.

During the flow phase the grid is brought to a potential which suffices to neutralize the space charge which exists between filament and grid. The voltage between grid and anode, on the other hand, must amount to a multiple between the voltage difference of the grid and the heated filament, so that the voltage factor of the tube or the influence of the anode voltage attains such a value, that the electrons accelerated in the grid filament field are removed from the grid space.

During the stop phase the voltages may distribute themselves in the same proportion over anode, grid and filament. The main portion of the voltage drop thus prevails between anode and grid, while between grid and filament only a fraction of it prevails. The filament may in this case be comparatively thin without fear of the occurrence of ionization through a strong field between the grid and the filament.

It is, however, still more favorable if dur ing the stop phase the filament. and the grid are brought to the same potential. In this case the filament is located in a practically field-less space. All the field lines emanate from the rods of the grid instead of from the filament.

However, even in the arrangements so far described the voltage may be applied during the stop phase without detrimental ionization is still limited. In an, arrangement in which the cylindrical anode has a diameter of 6 cm, for instance, the minimum of the field strength at the grid rods g would at an applied voltage of 300 kV lie already at a value of the order of 10 volts per cm. A further increase of the anode diameter would not change materially the order of the value, since the anode diameter enters logarithmically into the field strength calculation. Our invention may, nevertheless, be used for extremely high voltages if the following constructions are employed.

As fundamental arrangement of the electrodes should be chosen: two plate shaped elements facing each other. 7

According to Fig. 6 the anode a consists, for instance, of a circular piece of sheet metal which is bent up at the edge to form a dish or tray, the edge being turned up with a large radius of curvature. The electrode, which functions as grid, consists of an inverted dish-shaped rim g and of comparatively thick parallel rods 9 which take the place of the dish bottom.

Instead of these rods, metal channel bars or tubes of the shape shown in Figs. 7 and 8 respectively may be employed. In the latter figure,the grid bars 9 are of drop-shaped cross section, as shown in Figs. 4 and 5, but they are made hollow of sheet material. In both of these sheet metal modifications of the grid bars, illustrated in Figs. 7 and 8, the bars face the anode with their blunt portions and the cathode with their pointed portions. Belowthese rods of the grid is stretched the thermionic cathode is in filament form in the manner well known inthe art. In an arrangement of this kind the grid electrode and the filament are during the stop phase preferably brought to the same potential.

The correct distribution of the potential may be attained in various ways:

(1) Without special auxiliary apparatus by capacitive coupling between the individual electrodes, the capacity grid-cathode being preferably bridged by a resistance R, as shown in Fig. 6, in order to permit leakage of the grid charge which in this case is capable of attaining considerable values;

(2.) by a transformer 25 connected between grid 9 and hot filament h. The external lead may be connected, either to the grid or to the filament, as. shown in Fig. 9 of the drawing;

(3) by connecting two electric valves '0 '0 in the circuit "transformer-grid-filament,

as shown in Fig. 10 of the drawing; in this Way the filament and the grid may during stop phase be brought to the same poten- Various modifications and changes may be made without departing from the spirit and the scope of the invention.

We claim as our invention:

1. A thermionic valve for high alternating current voltages, comprising an evacuated vessel containing a thermionic cathode, an anode and a grid consisting of individual elements of relatively larger circumferences than the constituent elements of the cathode, the cross section of each of said grid elements having a blunt convex side facing the anode and a pointed portion facing the cathode.

2. A thermionic valve for high alternating current voltages, comprising an evacuated vessel containing a thermionic cathode, an

anode and a grid consisting of individual a solid grid bar elements of relatively larger circumference than the constituent elements of the cathode, the cross section of each of said grid elements being drop-shaped, the blunt side facing the anode and the pointed portion facing the cathode.

3. A thermionic valve for high alternating current voltages, comprising an evacuated vessel containing a thermionic cathode, an anode and a grid consisting of individual hollow sheet metal grid bar elements of relatively larger circumference than the constituent elements of the cathode, the cross section of each of said grid elements being dropshaped, the blunt side facing the anode and the pointed portion facing the cathode.

4-. A thermionic valve for high alternating current voltages, comprising an evacuated vessel containing a thermionic cathode, an anode and a grid consisting of individual channeled grid bar elements of relatively larger circumference than the constituent elements of the cathode, the blunt portion of each of said channeled bars facing the anode and the open portion of each of said bars facing the cathode.

In testimony whereof we afiix our signatures.

ROBERT STRIGEL. MAX STEENBEGK.

Images