US2217186A - High current space discharge device - Google Patents

Description

Oct. 8, 1940. c. (5. SMITH 2,217,186

HIGH CURRENT SPACE DISCHARGE-DEVICE Original Filed March 5, 1925 Zyz.

Ch les 6 51112 221 Patented Oct. 8, 1940 2,217,186 HIGH CURREN SPACE DISCHARGE DEVICE Charles Grover Smith, MedfonLMass assignor,

by mesne assignments, to Raytheon Manufacturing Company, Newt of Delaware on, Mass, a corporation Original application March 5, 1925, Serial No.

Divided and this application October 26, 1931, Serial No. 571,186. Renewed June 24,

8 Claims.

This application is a division of my copending application Serial No. 13,145 filed March 5, 1925. The invention relates to highcurrent gaseous conduction space discharge devices using a thermionic cathode and is an improvement on the space discharge device disclosed in my copending application Serial No. 696,337 filed March 1, 1924, now Patent 1,929,122, in which a space discharge of high current capacity and low voltage drop is maintained by the use of a solid thermionically active cathode in a vessel filled with gas at substantial pressure so that during the discharge the cathode space is screened against loss of heat and ionization, thereby reducing the cathode losses and enabling discharge of heavy currents at low voltages.

In one aspect the present invention comprises a low pressure or high vacuum tube containing electrode elements including an anode and a cathode having a much higher pressure maintained in the region of its discharge surface by electric or magnetic forces, or both, for the purpose of obtaining ample electronic emission. The gas which fills the tube is preferably inert with respect to the electrodes. This difference of pressure is maintained by an obstruction between the electron source and the electron receiving area, the obstruction having an opening therethrough for the passage of electrons and the opening being so restricted that a difference of pressure (or state of ionization) may be maintained on opposite sides of the obstruction. As will appear hereinafter the obstruction is preferably formed as a part of the cathode. Under constant conditions of operation the pressure within the cathode is preferably static in contradistinction to pressure resulting from continuous fiow into the cathode and restricted flow out of the cathode. This is conveniently effected by trapping the gas within the hollow cathode with an electric field, the field being so related to the electron discharge opening in the cathode that gas molecules or ions, or both, may wander into the cathode but are restricted from escaping, at least until the pressure within the cathode builds up to a relatively high value.

This electric pumping action is preferably produced, at least in part, by the thermal ionization of the molecules in the region of the opening in the cathode with heat derived from the cathode or other suitable source. The opening through which the gas enters the cathode is preferably the same as that presented to the anode for the electronic discharge. The said field may be produced in many ways, as by one or more auxiliary electrodes, one or more anodes positioned close to the mouth of the cathode, etc., but it is preferably efiected by making the cathode mouth elongated and producing a potential gradienttherealong, the outer end of the mouth being positive relative to the inner end. This pumping and trapping action is most effective when the diameter of the cathode opening is substantially within the limits of the mean free path of the molecules of gas outside the cathode; and owing to the low pressure outside the oathode this mean free path is long, thereby permitting a fairly large cathode opening.

In another aspect the invention consists in heating a hollow cathode independently of the fiow of current between cathode and anode, thereby to ionize the gas within the cathode independently of the cathode-anode current (that is, thermally instead of or in addition to collision), and to augment the emission and conduction of electrons from the interior of the cathode. The cathode is preferably heated sufficiently to ionize the vapor therein (about 20 C. when using caesium vapor), by an auxiliary source of current connected thereto. By suitably connecting the auxiliary source it may also serve to produce the aforesaid potential gradient along the mouth of the cathode. While the entire cathode may be heated as herein described the heat is particularly effective in the region of the cathode opening and for many purposes other regions of the cathode may be at a lower temperature.

In a further aspect the invention involves the use of an electric field for directing the electronic discharge from the interior of the cathode through the mouth thereof to the anode which is separated from the cathode by a path of substantial distance, the field preferably being combined with a magnetic field eiitending longitudinally of the opening to restrain the electrons from impinging upon the walls of the cathode.

In a still further aspect the invention consists in using an alkali vapor, preferably caesium vapor, as the aforesaid gas in a hollow cathode. This gas is readily ionized by thermal means and the volume trapped within the cathode becomes intensely ionized, thereby greatly increasing its current-carrying ability. The attendant radiation of this volume of highly ionized gas is also believed to be advantageous since it liberates electrons from neutral molecules and does so more or less independently of superposed electric or magnetic fields. During operation the caesium vapor, which is a readily ionizable gas, is

presented as a gaseous atmosphere at a substan- A coil 54 may be employed to direct the electial pressure, sufiiciently high to produce copious ionization upon the passage of a discharge therein, and to supply during operation the requisite number of ions necessary to neutralize the space charge of the electron current between the cathode and anode to the desired extent.

The use of caesium vapor or the like is sharply distinguished from the use of a caesium coating on a cathode since the cathode preferably operates at a temperature higher than that at which such a coating can adhere. The liberat'ionof electrons from the cathode results from the intensely ionized volume of vapor in the cathode together with thermionic emission in addition. Owing to the fact that the action of the vapor is substantially confined to the comparatively small volume in the cathode, where most of the vapor is trapped under relatively high pressure, the required quantity of alkali is very small.

' For the purpose of illustration. embodiments of the invention are shown in the accompanying diagrammatic drawing, in which:

Fig. l is a longitudinal central section of an electronic device with associated circuits shown;

Fig. 2 is a detailed section of a modification of a part of Fig. l; and

Fig. 3 is a top plan of Fig. 2.

The embodiment shown in Fig. 1 comprises an an evacuated tube or envelope containing a thermionic cathode 4| closed at both ends except for a tubular neck42 at its upper end, two electrodes 43 in the form of plates adapted to operate as anodes for the cathode 4|, an auxiliary electrically-conductive electrode or ring 44 between the mouth of the cathode and anodes, the ring having a larger diameter than the neck 42, another auxiliary electrode or cylindrical shield 45 surrounding and enclosing the aforesaid parts and a heating filament 45 connected at its upper end to the shield 45 and at its lower end, through a disk 41, to the cathode lead 48. The passage extending through the ring 44 permits electrons in the discharge path between the oathode and anodes to pass from the cathode 4| through said passage to the anodes 43. The shield 45 likewise has an opening through which the anodes 43 are presented to the cathode 4|, and thus through which saidelectrons may pass. The shield 45 serves to prevent extension of the discharge from: the space within it to the space bordering the walls of the vessel. The positive and negative terminals of a source 48 are connected to the shield 45 and lead 48 respectively,

.this source establishing a positive potential on the ring 44 and heating the filament 45 to incandescence whereby the interior electron-emitting surface of the cathode is heated to temperature of thermionic emission during operation.

.Since the potential of electrodes 44 and 45 is controlled by the source .49, these electrodes are electrically independent of the cathode 4| and the anodes 43. The tube is preferably completely evacuated except for a small amount of caesium or other alkali. The alkali may be in solid or vaporous form but ordinarily it is at least partly solid.

For the purpose of causing a discharge to pass between the cathode and anodes and for producing oscillations a source of direct current 50 is connected at its negative end to the lead 48 and at its positive end to the anodes 43 respectively tronic discharge from the interior of the cathode through the neck 42; and alternating current coils having their axes extending perpendicularly to the plane of the anodes may be employed to control the frequency of the oscillations. The coils 55 create a magnetic field transverse to the discharge path between the cathode 4| and the anodes 43. Some of this field is located between the cathode 4| and the auxiliary electrodes 44 and 45. The walls of the neck 42 and the sides of the ring 44 comprise surfaces adjacent the path of electron flow to the anodes 42 in the discharge, and the transverse magnetic field tends to force the path of the discharge over a ainst these. surfaces. The coil ,54 creates a magnetic field longitudinal to the discharge path coaxial with the path of electron at said surfaces and substantially in line with the passage through the ring 44 and the shield 45.

When the interior of the cathode 4| is heated (for example above 1200 C.) the vapor inside the cathode, and particularly in the region of the neck 42, is highly ionized and the vapor is pumped into the cathode until a relatively high pressure is established therein. The theory of this pumping action is believed to be substantially as follows: a

As a result of the continuous molecular movement of the vapor in the tube, neutral molecules (that is unionized molecules) wander into the nmk of the cathode where they are ionized by the intense heat in this region. These ions are thus trapped in the neck and impelled to the interior of the cathode by the potential of the cathode. This trapping action is augmented by making the diameter of the neck 42 less than the mean free path of the vapor molecules outside the cathode because substantially all molecules tending to enter the cathode collide with the wall of the neck and become ionized as a result of the high temperature of the wall, whereby substantially all neutral molecules which are capable of escaping are converted into ions which may be trapped. The difference in pressure between the interior and the exterior of the cathode may be regulated by varying the voltage of source 45. A higher internal pressure may also be produced by making the diameter of the neck 42 smaller.

With thecathode 4| full of highly ionized alkali vapor under substantial pressure an electronic discharge from the interior of the cathode through the neck 42 to the anodes is readily produced by a relatively low potential applied between the anodes and cathode, reverse current being prevented upon reversal of potential because .the space near the anodes does not contain highly ionized vapor under pressure.

The longitudinal magnetic field produced by coil 54 reacts upon the electronic stream, tending to direct it axially through the neck 42 and tendi ing to restrain the stream from contacting with the wall of the neck.

In this embodiment the electric field employed to pump vapor into the cathode and maintain a pressure therein is produced by the ring 44 and the region of the cathode is heated by filament 46, the shield 45 serving to localize the heat from filament 46.

Instead of having a single discharge opening in the cathode, a single ring 44 and a single pair of anodes 43. these parts may be multiplied as illustrated in Figs. 2 and 3 where cathode 4| has four outlets 42', the ring 44' has four openings 5| therethrough and two pairs of anodes 42' are connected in parallel to each side of the outgoing line.

Figs. 2 and 3 also illustrate a coiled spiral conductor 60 surrounding the cathode, in approxi- .mately the position of the shield 45 in Fig. 1.

By sending direct current through this conductor the aforesaid electron directing magnetic field may be produced and by employing sufiiclent current the conductor may also serve to heat the cathode and adjacent region.

It is to be understood that features of construction shown in each illustrative embodiment may be employed in each of the other embodiments and that many other modifications and adaptations may be made within the scope of the appended claims.

I claim:

1. A gaseous conduction device comprising an envelope containing a cathode, an electrode adapted to operate as an anode therefor and spaced therefrom, a gas filling at a pressure sumciently high to produce copious ionization upon the passage of a discharge therein, an auxiliary electrode interposed between said cathode and anode, said auxiliary electrode having a passage therethrough to permit electrons to pass therethrough to said anode, means for creating a. magnetic field transverse to the discharge path between said cathode and anode, said field being located between said cathode and said auxiliary electrode, and means for creating a magnetic field longitudinal to said discharge path and substantially-in line with said passage.

2. A gaseous conduction device comprising an.

envelope containing a cathode, two electrodes adapted to operate as anodes therefor and spaced therefrom, a gas filling at a pressure sufilciently high to produce copious ionization upon the passage of a discharge therein, an auxiliary electrode surrounding said cathode and interposed between said cathode and anodes, said auxiliary. electrode having a passage therethrough to permit elec-= trons to pass therethrough to said anodes, means for creating a magnetic field transverse to the discharge path between said cathode and said anodes, 'said field being located between said cathode and said auxiliary electrode, and means for creating a magnetic field longitudinal to said discharge path and substantially in line with said passage.

3. An electrical space discharge device comprising a vessel having therein an anode and a thermionic cathode having an electron-emitting surface designed to be heated to the temperature of thermionic emission during operation, an auxiliary electrode, an ionizable gas inert with respect to said electrodes in said vessel at a pressure sufilcient to supply during operation the requisite number of ions necessary to neutralize the space charge of the electron current between said cathode and anode, and a shield constituting an enclosure around said anode said cathode and said auxiliary electrode for preventing extension of the discharge from the space within said enclosure to the space bordering the walls of said vessel.

4. An electrical space discharge device including a plurality of elements comprising two anodes, a. thermionic cathode, said anodes and cathode being adapted to support a space discharge between them, an ionizable gas in said device, and auxiliary electrode means interposed between said cathode and said anodes, one of said elements having a surface adjacent the path of the electron flow to said ano'des in said discharge, and means for producing a magnetic field coaxial with said path of electron flow at said surface.

5. An electrical space discharge device including a plurality of elements comprising two anodes, a thermionic cathode, said anodes and cathode being adapted to support a space discharge between them, an ionizable gas in said device, and auxiliary electrode means surrounding said cathode, one of said elements having a surface adjacentthe path of the electron fiow to said anodes in said discharge, and means for producing a magnetic field coaxial with said path of electron flow at said surface.

6. An electrical space discharge device including an envelope containing a plurality of elements comprising an anode and a thermionic cathode, said anode and cathode being adapted to support a space discharge between them, an ionizable gas in said device, one of said elements having a surface adjacent the path of the electron flow to said anode in said discharge, and coiled conductor means within said envelope for producing a magnetic field in the discharge space coaxial with said path. of electron flow at said surface.

'I. An electrical space discharge device including an envelope containing a plurality of elements comprising an anode and a thermionic cathode, said anode and cathode being adapted to support a space discharge between them, an ionizable gas in said device, one of said elements having a surface adjacent the path of the electron flow to said anode in said discharge, and

field, one of said means including a coiled conductor in said envelope.

8. A space discharge device comprising a gastight vessel containing two electrode elements separated by a path of substantial distance, a gas in said vessel, means for causing a discharge to pass between said electrode elements, the pressure of said gas being sufiiciently high to produce intense ionization upon the passage of said discharge, electrically-conductive means comprising an additional electrode, electrically independent of said first named electrode elements and positioned adjacent the discharge path between said electrode elements within said envelope, and means for impressing a magnetic field on the discharge space immediately adjacent said electrically-conductive means and transversely to said discharge path to force the

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