US2111757A - Vacuum discharge vessel - Google Patents

Description

March 22, 193.

W. DALLENBACH VACUUM DISCHARGE VESSEL Filed Aug. 13, 1934 Patented Mar. 22, 1938 UNITED STATES PATENT oFFicE many, assignor to N.

V. Machinerieen-en Apparaten Fabrieken Meat, Utrecht, Netherlands Application August 13 1934., Serial No. 739,678

In Germany June 22, 1934 4 Claims.

-5 vide between the anode and cathode, directly before the anode, a partition dividing the inner space of the discharge vessel into a smaller space adjoining the anode surface and a larger one. The partition possesses one or several small apertures equal in size to the mean free length of path of the particles of the gas or vapor content, and means are further provided for operating the tube at a current intensity which, owing to the constriction of the discharge by the diaphragm in the space adjoining the active anode surface, involves a decrease in the number of positive ions, so that a drop in voltage increasing with the current intensity will take place in that space.

A further object is to limit the distance from the partition to the anode to a maximum equal to a few lengths of path of electrons in the gas or vapor.

Another object is to construct the apertures in the form of elongated channels.

Another object is to produce the partition from metallic material and to connect it to the anode by means. of aresistance.

A further object is to employ control electrodes for influencing the current passing through the opening of the partition.

Still anotherv object is to connect in series with the discharge tube a resistance or choker dimensioned so that the tube will carry out slow automatic oscillations similar to those of a flash lamp.

Further details will become apparent from the specification and the accompanying drawing, in which Figure 1 is a diagram of a characteristic of a discharge tube according to the invention; Fig. 2 shows the construction of such a tube by means of a mercury cathode and an auxiliary ignition electrode; Fig.3 shows a cathode ray tube with deflecting electrodes; Fig. 4 shows a tube provided with two anodes which alternately may receive the cathode ray by means of control electrodes; Fig. 5 shows a tube, in which the partition made of metallic material is connected to the anode by a resistance; Fig. 6 shows an anode with a partition, in which a number of apertures having the form of small channels are provided;

and Fig. 7 shows a diagram of connections for ob-- taining slow fiashlike oscillations.

In vacuum discharge tubes having a rarefied gas or vapor filling it can be observed that the drop in voltage will decrease at first when the current intensity increases from low values and increase again at very high intensities. This increase is particularly marked and takes place at still permissible current intensities if the anode is enclosed in a chamber communicating with the remainder of the inner space of the discharge vessel only by means of a small diaphragmlike opening. .The smaller the contents of this chamber and the narrower the communicating opening between the chamber and the rest of the tube space, the steeper will be the current-voltage characteristic beyond a certain current load and the smaller the current intensity at which the characteristic will begin to rise again.

Fig. 1 shows the current-voltage diagram of such a gas discharge tube. A more accurate examination shows. that in following the voltage characteristic up to the point where it begins to ascend a branch will be reached which rises in vertical direction. This means that the current J reaches saturation, the saturating current intensity dependingupon the gas pressure within the tube.

In Fig. 2, a tube of this kind is shown. A is the anode; B, the diaphragm surrounding the anode; and K a cathode. of the mercury or hot type. In case of a mercury cathode, an exciter anode E is, as a rule, required to maintain the exciter arc towards the cathode K, though the exciter anode becomes superfluous if enough current is takenfrom the cathode.

The saturating current intensity for the cur, rent passing from the cathode to the anode is brought about as follows:

In the diaphragm opening a so-called striction cathode is formed which all electrons coming from the cathode have to pass. At the point of location of this cathode a fixed relation exists between the electron current passing from the cathode to the anode and the positive stream of ions passing in reversed direction from the anode to the cathode. The ratio of the electron current to the ionic current must be Jr m m being the mass of positive ions and me the mass of electrons. The value for mercury vapor will therefore be 605, i. e., the electron current passing from the cathode to the anode is 605 times greater than the oppositely directed ionic current. As the chamber formed by B tightly shuts oii the anode from the remaining portion of the vacuum vessel, the positive ions projected by the positive ionic current Jp through the diaphragm opening from the chamber into the discharge space C must necessarily be supplied later on in some way, which is effected by the diffusion of neutral mercury atoms from the discharge tube through the diaphragm opening into the inside of the chamber. This diifusion is due to the temperature motion of the mercury atoms. The number of neutral gas particles which diffuse at low gas pressure through a diaphragm opening is known to be dependent solely on the number of particles per unit of Volume and the temperature, i. e., the intensity of motion of these particles. As long as the density of the gas, particularly the density of the mercury vapor, and the temperature of the gas in front of the diaphragm opening remain unchanged, the inflow of particles through the diaphragm opening into the inside of the anode chamber will therefore be constant and the discharging current intensity fixed. When the saturating current intensity has therefore been reached, 1. e., as soon as the depletion of ions inside the anode chamber takes place and the point P has been reached in the characteristic according to Fig. 1, only the inflowing gas particles will be available for covering the positive ionic current. As this positive ionic current intensity, according to the equation stated, has a fixed relation to the electron current, the saturation character that is actually observed will follow therefrom.

The electron current passing through the stric tion cathode formed in the diaphragm opening runs through the space towards the anode as a regular ray. The striction cathode acts there fore in the hollow space E as a novel type of cathode (plasma cathode), as it is fed by the irregularly moving electrons, the so-called plasma, in the space C. In view of the extraordinarily high current densities which are possible in a gas discharge, current densities can be attained by means of this plasma cathode, which considerably exceed the densities of the usual hot cathodes. It thus becomes possible to produce with this plasma cathode in the chamber B a cathode ray of much higher density than could be attained with, say, a hot cathode.

The possibilities of application of this novel cathode are quite varied. For example, it may be used as cathode ray oscillograph by employing a fluorescent screen or a window of metal foil with a fluorescent screen arranged behind it instead of the anode A. Such an arrangement is shown in Fig. 3. O is the window; D represents the deflecting electrodes and L the fluorescent screen. It is further possible to produce extraordinarily intense X-rays with this arrangement, and the canal rays formed by the ions passing from the anode through the diaphragm may be used also in the ordinary way.

For controlling the electronic ray electrodes D may be provided inside the chamber B to produce a transverse field, as indicated in Fig. 4.

To obtain a modulation by means of the controlled electronic current the anode is preferably divided into the two parts A1 and A2 (Fig. i) which, according to the voltage applied, take up at the auxiliary electrodes D the electron current in whole or in part, or not at all.

Fig. 4 also shows how such a discharge vessel may be used as counter-contact amplifier in the manner usual in electron tubes. If the auxiliary electrodes D are supplied with control voltages by the line S, they can be taken oii amplified at the outlet A.

If the input and output conductors are connected to corresponding poles and constructed as a resonance circuit, the discharge vessel will function as counter-contact generator whose oscillation frequency may be varied within wide limits.

The discharge vessel described may further be used for limiting current. As indicated in the characteristic of Fig. 1, the current intensity cannot increase beyond the value at point P as long as the gas pressure maintains a certain value, and the tube can thus be employed as steadying resistance in front of apparatus which, for example, are always to be operated at a certain current intensity regardless of the feeding voltage. For this purpose, hot cathode tubes are known, but in contrast with the latter considerable current intensities can be kept constant in this manner, according to the invention, by simple means and at low power input. As cathode K for the discharge and the formation of the plasma cathode proper in the opening F of the diaphragm B a hot or a mercury cathode with or without the exciter anode E may be employed in the usual way. If a mercury cathode is chosen, it is desirable to keep the mercury vapor pressure constant near the diaphragm opening, which can be done for instance by disposing the tube in a thermostat.

Another possibility consists in filling the tube with a rare gas of such high pressure that, in comparison, the mercury vapor pressure is of no importance. The higher the gas pressure chosen, the smaller must be the diaphragm opening in view of the saturation current intensity involved.

Further use can be made of the arrangement if gas discharge lamps are to be operated connected to a supply network without a series re sistance or choke. The omission of the series resistances and chokes saves the otherwise required reactive power and thus increases efiioiency. These tubes permitting the construction of small luminous tubes connected directly to the network, can be employed with special advantage in lighting.

If in this way any impedance in series with the electric discharge vessel is to be avoided and the latter to be connected directly to the network, it is necessary that, compared with the partial voltage drops of the gas discharge decreasing with increasing current intensity, the drop in voltage increases to such an extent that the entire drop in voltage between the anode and cathode increases with the current intensity.

The invention is thus of special importance for all electric discharge vessels serving as receivers, particularly independent receivers, of electric energy.

To insure a drop in voltage increasing with the current intensity in consequence of the diaphragm, etc., constricting the cross section of the discharge it is advisable to keep its distance from the active anode surface within certain limits. If this distance is too great, a striction cathode will form in the diaphragm opening and cause slow cathode radiation from the diaphragm opening to the inside of the space adjoining the active anode surface, but as long as the kinetic energy of this electronic radiation at collisions with the gas particles found between the diaphragm and the anode can be fully utilized for forming new pairs of ions, the drop in voltage in the portion of space between the diaphragm and anode will not increase with the current intensity owing to the diaphragm. Only when the slow cathode radiation coming from the diaphragm impinges on the anode surface beforeit could fully utilize its kinetic energy for the formation of new pairs of ions, i. e., when the reduced number of collisions between the soft cathode rays coming from the diaphragm and gas particles begins to limit the utilization of the kinetic energy ofthis radiation, will the drop in voltage between the diaphragm and the anode increase with the'current intensity. It is therefore preferable to make the path of electronic radiation from the diaphragm or the like to the active anode surface not too long, and experiments have shown that, at the highest, it should amount to a few mean free lengths of path of electrons in the gas or vapor.

The back diffusion of neutral gas or vapor particles may further be impeded by causing the discharge-from the diaphragm or the like to the anode surface to take place in a channel whose diameter is comparable with that of the diaphragm and which terminates near the anode surface. The back difiusing particles are compelled by this arrangement to move for several lengths of path inside a channel whose diameter is also comparable with the length of path. From the laws of the kinetic gas theory it is known that such a channel will increasinglyinterfere with the free motion of the particles in proportion to its length. The diaphragm opening lengthened by the channel extending to the anode surface has, moreover, a particularly good effect according to the invention, as the soft electronic radiation from. the diaphragm opening in the directionof the anode finds only a relatively narrow space for. collisions with neutral gas particles for forming new pairs of ions.

1 Instead of employing a single opening as F in Fig. 2 one may provide a'plurality of diaphragm openings resembling a honeycomb or the squares in a chessboard in the bottom of the body B, or, as indicated in Fig. 5, the bottom of the body B maybe formed by a porous member G in which channels are provided. In this case, several parallel discharges towards one and the same anode A will takeplace simultaneously, which is possible because each of these partial discharges has, according to the invention, a positive voltage characteristic and thus can exist simultaneously in parallel connection with a discharge of the same kind.

As the high current density renders consider able heating at the diaphragm possible, it may be necessary to produce the diaphragm or the like which constricts the cross section from a high melting material.

It will bedifiicult. as a rule, to cause ignition of the discharge from the anode A to the cathode K without a special exciter anode, since the diaphragm opening F brings about a'considerable increase in the spark potential. It is therefore advisable to arrange the diaphragm B or the like which constricts the cross section of discharge in the form of a conductor which, as shown in Fig. 5, by a special current lead H and a resistance R. can be connected to the anode lead. When the anode takes on positive voltage values, a discharge between the diaphragm body B and the cathode will firstignite. Owing to this discharge, a sufficient amount of electrons will be brought near the diaphragm openings, so that the voltage ofthe anode will sufiice to initiate the main discharge towards the anode, which, once established, will continue to exist, even if the current supply to the diaphragm B would be interrupted. The resistance It may therefore be relatively high, so that after the anode A has taken over the current conduction, only a relatively small current or induction current flows through the diaphragm carrier B, the conductor H and the resistance R. The resistance B may of course be entirely disposed inside the discharge vessel to dispense with the conductor H. This is shown in Fig. 6 where the diaphragm body B provided with a plurality of openings F is positioned directly on the anode A by means of an interposed member L of poorly conducting material, and the openings F are elongated by the channels M and extend up toward the active anode surface to prevent back difiusion of neutral gas or vapor particles. The intermediate member L serves also for supporting the metallic diaphragm body B. It is possible to construct the body B entirely from poorly conducting material and to position it on the anode or its current-carrying conductor.

If a vessel is operated with alternating current, a drop in voltage will take place, at least near the peak value of the current in the space adjoining the active anode surface. After the drop in voltage required for initiating greater current intensities has been exceeded, the current intensity will instantly increase to an amount correspondingto the positive branch of the current-voltage characteristic and, if this branch rises relatively steeply, only an immaterial increase of the operating current intensity will take place, so that the line diagram of the current intensity will have approximately rectangular shape.

The main feature of the invention is to successfully influence the drop in voltage of the tube by relatively simplein-built means near the. anode in such a way that it will increase in wholeor at least in part with the current intensity so as to avoid series impedances which complicate and increase the cost of the plant and involve additional losses or a poor power factor. If the electric discharge vessel serves for producing light, the diaphragm arranged near the anode surface may simultaneously be used for producing the brightness of the discharge or an approximately point- 7713 and m represent the mass of the electron and positive ion. e=l.59 10 coulomb is the elementary charge. For mercury is For a discharge in mercury vapor the following formula will be obtained wherein Ne refers to the number of positive charges which pass through the screen opening of the cross section F, and J refers to the strength of the discharging current.

Owing to the continual outflow of positive ions, a much higher vacuum will be produced inside the chamber B than outside thereof. Assumed that the gas pressure is very low, e. g., 0.03 mm. mercury column, and the free length of path of the gas particles comparable with the diameter of the opening F, this difierence in pressure can be calculated in a very simple manner; it must be so great that the difference between the number of particles entering the chamber from outside and the number leaving the chamber owing to the temperature motion is equal to the number of positive ions passed out by the current intensity. According to the kinetic gas theory, a cross sectional area F will be passed per time unit by a number of particles g 1/ HT 12 being the gas pressure in mm. mercury column and T the absolute temperature, and [.L the molecular weight of the gas. The excess particles, i. e., those entering the chamber in excess of the outflowing ones, are covered by this formula 3.535.10 (P2-Pl) 02 al- 1/; F m

wherein m and T2 represent the pressure and the temperature outside the chamber and p1 and T1 the corresponding values inside the chamber. For mercury vapor =200 is assumed. If it is assumed that T1=T2=625 abs., the following formula will result i. e., if in the cross sectional opening a current density of about 100 amps. per cm is assumed, it will result in a difierence in pressure amounting to 1/100 mm. mercury column between the inside and outside of the anode chamber. This decrease in pressure is already so considerable that the electrons entering through the opening F to the inside of the anode chamber are no longer in a position to form a sufiicient number of positive ions with the result that the number of the latter will decrease, as mentioned above. There will be a negative space charge and increased drop in voltage.

According to the simple physical laws involved, each current intensity and difference in pressure desired will correspond to a certain size of the cross-sectional opening, the exact ascertainment of which is leftto experiments. In an actually constructed tube the saturation current intensity amounted to about 1 amp. at a temperature of the mercury cathode of about 17 C., a mercury vapor pressure of about 10- mm. mercury column and a diaphragm diameter of 5 mm.

If a gas discharge vessel according to the invention is operated with current intensities near the point P (Fig. 1) where the characteristic changes from an approximately horizontal to an approximately vertical course and if a series resistance to be found by testing or a correspondingly dimensioned series choke DR (Fig. 7) are inserted in the circuit, the tube will carry out slow oscillations, which is probably due to the following causes:

A certain current intensity prevails at first under the influence of which gradual evacuation of the chamber B is effected. When this evacuation has reached a certain degree, this discharge will break, as inside the chamber the current and the voltage will rise, owing to the decrease in the number of ions. When the current intensity decreases, the chamber will fill up with gas from the space E and the current intensity can rise again. This process is repeated periodically. The frequency of the oscillations ranges from many seconds to fractions of a second and can be regulated within these limits by the size of the chamber, the opening thereof, the gas pressure or the value of the saturation current intensity depending on the gas pressure. In this way it becomes possible to operate lamps working without outer elements, such as condensers, devices for an auxiliary discharge current, etc., at a predetermined interrupting frequency. Lamps of this kind may suitably be used for advertising purposes, signal plants, etc.

I claim:--

1. An electric discharge vessel containing an ionizable medium under sufficiently high pressure to support a current discharge comprising at least one anode, a cathode, a partition between the anode and the cathode dividing the discharge vessel into a smaller space directly adjoining the active anode surface and a larger space, said partition having an opening therein for passage of said discharge, the diameter of said opening being of the magnitude of the mean free path of the particles of the ionizable medium, whereby the discharge is constricted at this point to a fraction of its cross section in the remainder of the space and the current density is increased, the latter causing a paucity of ions and a potential drop corresponding to a current increase, and an auxiliary electrode adjacent the path of discharge provided with a suitable potential for controlling said discharge.

2. An electric discharge vessel containing an ionizable medium under sufliciently high pressure to support a current discharge comprising at least one anode, a cathode, a partition between the anode and the cathode dividing the discharge vessel into a smaller space directly adjoining the active anode surface and a larger space, said partition having an opening therein for passage of said discharge, the diameter of said opening being of the magnitude of the mean free path of the particles of the ionizable medium, whereby the discharge is constricted at this point to a fraction of its cross section, in the remainder of the space and the current density is increased, the latter causing a paucity of ions and a potential drop corresponding to a current increase, and two electrodes between said anode and said partition on opposite sides of the path of discharge for controlling its direction.

3. An electric discharge vessel containing an ionizable medium under suiiiciently high pressure to support a current discharge comprising at least one anode, a cathode, a partition between the anode and the cathode dividing the discharge vessel into a smaller space directly adjoining the active anode surface and a larger space, said partition having a plurality of openings therein for passage of the discharge, the diameter of said openings being of the magnitude of the mean free path of the particles of the ionizable medium, whereby the discharge is constricted at this point to a fraction of its cross section in the remainder of the space and the current density is increased, the latter causing a paucity of ions and a potential drop corresponding to a current increase, and an auxiliary electrode-system provided with a suitable potential adjacent the path of discharge for controlling said discharge. 7

4. An electric discharge vessel containing an ionizable medium, comprising an anode, a cathode, a partition of conducting material between the anode and a cathode dividing the vessel into a smaller space directly adjoining the active surface and a larger space, said partition being connected with the anode by a resistance outside the vessel, said partition having an opening therein, the diameter of the opening being of the magnitude of the mean free path of the particles of the ionizable content, said structure causing a paucity of ions and a potential drop corresponding to a current increase, and an auxiliary electrode-system provided with a suitable potential adjacent the path of discharge for controlling said discharge.

WAL'IER \DALLENBACH.

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