US1720172A - Electron-discharge tube and method of manufacturing the same - Google Patents

Description

July 12. DESSAUER 3,720,172

ELECTRON DISCHARGE TUBE AND METHOD OF MANUFACTURING THE SAME Filed Sept. 15, 1925 2 Sheets-Sheet l July 9, WW IF. DESSAUEW MIWJWE ELECTRON DISCHARGE TUBE AND METHOD OF MANUFACTURING THE SAME Filed Sept. 15, 1925 2 Sheets-Sheet 2 mjyfi I 5/ W 1 Q g 77 k 3; W7 w L. .1

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Patented July 9, 1929.

UNITED STATES PATENT OFFICE.

FRIEDRICH DESSAUER, F FBANKIORT-ON-THE-MAIN, GERMANY.

ELECTRON -DISCHARGE TUBE AND METHOD OF MANUFACTURING THE SAME.

Applicatioufiled September 15, 1925,'Seria1 No. 56,440, and in Germany August 15, 1923.

having a heated cathode. Although it has been alleged that a substantially pure electron discharge can beobtajned in X-ray tubes in spite of an excess of pressure over the limiting pressure of the Coolidge tube,

by filling them with helium, hydrogen or av mixture of the two gases, my experiments have not confirmed the correctness of this statement.

My present invention relates to a vacuum tube more particularly an' X-ray tube, in-

which I am able to obtain a pure electron discharge without relying upon the very high bulb vacuum thought necessary hitherto, and presenting material advantages in the manufacture and operation of the tube, as well as increasing the life thereof.

According to my invention, a constant, quiet, pure electron discharge is ensured by bringing the electrodes, and particularly the anode (anticathode or target) into a s e'cial condition in which they will (even 1f the vacuum within the 'bulb is not very high) emit only electrons, and no ions at all, or at least not enough ions to impair the electron discharge effect. The well-known expedient of bombarding the electrodes with cathode rays, or other methods for driving out gases from the electrodes will, if continued with sutlicient intensity and for a suflicient length of time, according to my invention, produce such a condition of the electrodes that, as long as the cathode is not heated, it will be impossible to effect a discharge through the tube, but when the cathode is heated. a pure electron discharge without any manifestations of positive ionization amount of gases), the discharge would be effected by gas ions, and particularly positive ions.

I have found, further, that it is advantageous to place the cathode within the narrow portion or neck of the tube and toarrange the anode and cathode close to each other.

The extreme evacuation of gases from the electrodes and particularly the anticathode, serves to minimize the occurrence of ionization and thus to prevent the formation of any discharge other than a pure electron dis charge. The high evacuation of gases from the electrodes is a factor sufficient of itself, that is, even without the other features, to produce the effect discovered by me and to make it available for practical purposes,

In order to facilitate the task of a tery thorougl1,removal of the gases from t-he'electrodes, the amount of metal in the tube should be as small as possible and all arts of the tube should be arranged "accordingly.

Several examples of vacuum tubes embodying my present invention are shown in the accompanying drawings. Fig. 1 is a longitudinal section through an X-ray tube of the improved type, with a diagrammatic representation of the circuits associated therewith; Figs. 2 and 3 are partial sections, upon a larger scale than Fig. 1, illustrating certain novel arrangements of the cathode; and Fig. 4 shows a tube of the type that I employed for experimental purposes and for demonstrating the correctness of my assertions, but

this tygei is also suitable for practical use.

The -ray tube shown in Fig. 1 does not differ, in its structural features or appearance, from the well-known Coolidge tube. However, instead of being evacuated to the trttremely high degree which is a charm-t ristic of the Coolidge tube, my tube contains a substantialamount of gas, that is the gas pressure is relatively high (low vacuum). and moreover the electrodes, and particularly the anti-cathode, have been subjected to a treatmentwhich insures a pure electron discharge, and in particular to a treatment which by evacuation orexhaustiou deprivw them in an extremely high degree, of tingases occluded in, or adhering to, the said electrodes.

In the evacuated glass bulb 1 is arranged a'target or anticathode 2 of tungsten or other suitable material, with its lead extending airtight through the glass, and a cathode 3 of any suitable-metal. This cathode has a central cylindrical recess in which is located a tungsten wire 4 wound in a spiral the plane of which is preferably perpendicular to the axis of the tube. One end of the tungsten wire 4 is connected to the supporting wire 5, which is insulated and passes through the bottom of the recess and through the glass in an air-tight manner, extendingto the outside thereof so as .to form a lead-in wire. The other end of the coil or spiral 4 is connected conductively with the metal of the cathode'3.

The cathode is carriedby the supporting wire 6 which extends airtight to the outside of the bulb and likewise serves as a lead-in wire. The tungsten wire 4 may be brought to incandescence by current applied from a transformer 7, the primary of which receives current from an alternating current generator 8 through a circuit including an adjustable resistance or rheostat 9. -At 10 I have indicated the high-potential transformer (having primary and secondary coils), operated by current from an alternating current generator 11. .The terminals of the secondary are connected with the anticathode 2 and with the cathode respectively, and in this circuit may be located an ammeter 19. An adjustable resistance 12 has been indicated in the primary circuit of the transformer 10.

In Fig. 2, the entire cathode 3 is located in the tubular end or neck of the X-ray tube. In Fig. 3, the cathode 3, while located largely or entirely within the body of the bulb, is surrounded by a sleeve or shield 20 of insulating material, for instance glass. the inner end of said sleeve being open to the interior of the The X-ray tube shown in Fig. 4 has been used by me to investigate the discharge phenomena and to demonstrate the effects I have asserted, but it is also suitable for any purposes for which X-ray tubes are customarily employed. The parts numbered from 1 to 12 inclusive and 19 are substantially the same as in Fig. 1. By a tubular neck 13 the interior of the bulb 1 is connected with a glass receptacle 14, closed or sealed air-tight to the surrounding air and containing two metal electrodes 15 provided with lead-inwires 16. This receptacle has a tubular extension 17 into the wall of which is sealed air-tight a palladium tube 18 open at its inner end but closed at its free or outer end.

The X-ray tube shown in Fig. 4 was exhausted or evacuated in the manner well known in this art, that is, by long continued heating the gases and vapors contained therein were driven out, and withdrawn by means of a high vacuum pump, such as a mercury vapor pump, and of a gas-trap cooled with liquid air. The filamentary cathode 4 was deprived of gas by the passage of an electric current, this operation at the same time heat- .manne'r.

ing the metal-body 3 of the cathode. The anti-cathode or target 2 was heated strongly and for a long time by bombardment with electrons issuing from the incandescent tungsten wire 4. This treatment was carried out with quite particular-care. The electrodes 15 contained in the auxiliary receptacle 14 were not subjected to any special gas-removing treatment.

After the exhaustion of the bulb had been completed, the latter was disconnected from the pump by'heating and sealing the previously open connection, in the well-known The palladium tube 18 was then heated with a gas burner so as to introduce hydrogen into the bulb in such amount that upon connecting the electrodes 15 with a .source of electricity producing a suitable potential drop, a discharge would pass through the gas, which could be ascertained by the appearance of a blue light and green fluorescence of the glass. My experiments showed that the potential required for this result was relatively low, corresponding about to a potential such as would produce a spark of one inch in free air. If thereupon the high potential transformer 10 was brought into action, no discharge could be obtained between the target 2 and the cathode, as long as the wire 4 was not heated, even if the entire resistance 12 was cut out. As long as the wire 4 remained cold, there was not the least manifestation of blue light or of green fluorescence of the glass. However, as soon as the tungsten wire was (by the action of the transforn'ier 7) brought to the temperature at which this material emits electrons, a pure electron discharge occurred between the incandescent cathode and the anti-cathode or target 2, and the ammeter (milliammeter) 19 indicated the passage of a current. The X-ray tube in this case operated as a rectifier or valve, in other words, it permitted the passage of only that portion or phase of the alternating current during which the tungsten wire 4 acted as a cathode.

During this discharge the milliammetcr was 'to the discharge and that in this manner an automatic hi h evacuation or exhaustion of the bulb is effected; for the tube exhibits the characteristic properties of a pure electron tube even immediately after the source of high potential has become operative to supply electricity to the main electrodes of the tube. Another experiment proving that the result observed must not be attributed to an automatic increase in the degree of vacuum, consists in operating the tube for some time so as to emit X-rays, and then applying the high potential to the auxiliary electrodes 15; this will result in the production, as before, of a blue discharge through the gas and fluorescence of the glass, at a discharge potential of the same order as referred to above, that is, corresponding to a spark of not morethan from to 1% inches in free air.

Should a portion of the gas contained in the X-ray tube be absorbed, a replenishing amount of gas may be introduced into said tube from time to time by heating the palladium tube 18, or in any other suitable manner.

In my experiments, I introduced hydrogen gas into the bulb through the palladium tube. Of course, the bulb may be filled with any other gas which is not absorbed during the operation of the X-ray tube, for instance with a rare inert gas. The rare gases, mentioned above, are especially well adapted for the filling in a tube according to this inven tion because there is little or no tendency for these gases to be absorbed during the operation of the device.

The number of positive ions emitted at a relatively high gas pressure (low vacuum) is very small and negligible as compared with the number of the electrons emitted, the latter forming the main conductors for the dis charge, so that the presence of the few positive ions will not impair the efliciency of the vacuumtube. It should be understood that if the electrodes are deprived of gas to a high degree, this high degasification alone is sufficient to insure the securing of the novel effect found by me and to render this effect applicable for practical uses.

The above described characteristic of highly degasified electrodes, consisting in the prevention of ionic discharges even under relatively high gas pressures (low vacua), is possibly due not only to thefact that such electrodes have been deprived to a particularly high degree, of the gases adhering thereto and occluded therein, but to some other changes produced in the electrodes or at least in the anode by the customary bombardment with electrons employed in the manufacture of the X-ray tube. It will therefore be understood that the success of my invention does not depend on the correctness of the theory T have advanced to explain the effects observed; in particular, it should be considered immaterial whether or not the thorough removal of gas from the electrodes is really the reason or the sole reason for the phenomenon observed, viz: that such electrodes will not permit any ion discharge even at pressures at which such ion discharges occur with the usual electrodes, not treated in the manner described herein. The amount of gas pressure within the bulb may preferably be within the limits of 6 X 10' and 10' mm. Hg.

Various modifications may be made without-departing from the nature of my invention as set forth in the appended claims.

I claim: 7

1. The method of preparing a vacuum discharge tube, which consists in electrically heating the cathode to incandescence and subjecting the anticathode to an electronic bombardment under high vacuum until the anticathode has been brought to a condition to minimize ionization and to insure a discharge of predominantly electron character notwithstanding the presence of a vacuum which would ordinarily give an ionic character to the discharge.

2. An X-ray device comprising an evacuated bulb containing a filling of gas of a pressure greater than 6X10' mm. Hg. which is the degree of vacuum normally necessary to eliminate gas ionization, a cathode and an extremely highly degasified anticathode located in the bulb, the extremely high degasification of the anode making it impossible for sustained current to flow through the tube when the incandescent cathode is unheated, whereby a substantially pure electron discharge occurs when the cathode is-heated.

3. As an article of manufacture an X-ray tube comprising an envelope containing an incandescable cathode, an extremely highly degasified anode and a filling of gas at a substantial pressure which would normally cause a discharge across cathode and anode to assumo an ionic character, the extremely high degasification of said anode insuring a dis charge of predominantly electron character notwithstanding said substantial gas pressure.

4. The method of preparing an electron discharge device comprising an envelope containing a first electrode constructed to be heated to incandescence and a second electrode, which includes subjecting the electrodes in said device to a degasifying treat ment under high vacuum, subsequently restoring the gas pressure within said envelope until the vacuum falls below the high vacuum required for the production of a pure electron discharge in high vacuum tubes and reg ulating the duration of the degasifying treatment prior to restoration of gas pressure so as to deprive the second electrode of gas to an extent rendering it incapable of supporting a continuous discharge through the tube at the restored pressure while said first electrode is unheated.

5. The method of preparing an X ray device comprising an envelope containing an incandescent cathode and an anode, which includes subjecting the electrodes to an intensive degasification under high vacuum, until the anode has reached a condition in which mally occurs, and restoring the gas pressure under a certain range of high voltages hcwithin said envelope until the vacuum falls tween the electrodes and a certain range of below the high vacuum normally required for 1 heating currents so few ions are emitted the production of a pure electron discharge.

5 therefrom that the said ions can produce no In testimony whereof I have hereunto set appreciable ionization by collision at gaseous my hand. pressures at which ionization by collision nor- FRIEDRICH DESSAUER.

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