US2715183A

US2715183A - Electron discharge devices

Info

Publication number: US2715183A
Application number: US787873A
Authority: US
Inventors: Klemperer Hans
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1947-11-25
Filing date: 1947-11-25
Publication date: 1955-08-09
Anticipated expiration: 1972-08-09

Description

Aug. 9, 1955 H. KLEMPERER 2,715,133

ELECTRON DISCHARGE DEVICES Filed Nov. 25, 1947 2 Sheets-Sheet l M U0 VM flaw Qw h Q m w H III/III [III II A Train 15') g- 9, 1955 H. KLEMPERER 2,715,183

ELECTRON DISCHARGE DEVICES Filed NOV. 25, 1947 2 Sheets-Sheet 2 SWEEP VOLTAGE '1- ///Pl/T b 576ml 2 /0UTPUT\ V/DEO AMPLIFIER 5 sca PE 25 xiii /0 m 9 nraernoN DISCHARGE DEVICES Hans Klemperer, Belmont, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., 21 corporation of Delaware Application November 25, B47, Serial No. 7 87,87 3

7 Claims. (Cl. 250-27) This invention relates to electron discharge devices, and more particularly to the type thereof known as storage tubes, in which an electron beam is used to place a charge on a non-conducting electrode disposed therein.

In a storage tube of the aforesaid type which is adapted to be used in a radar system, a signal such as a radar echo is stored in the form of an electrical charge distribution on the surface of the non-conducting electrode or storage plate. In moving target indicator (MTI) applications, an individual complete trace over the storage surface (one recording) is compared with the next succeeding trace, and an output is produced only when these two compared traces do not coincide with each other. As long as there are no moving targets in the area. being scanned, successive traces will be exactly the same and there will be no output signal. However, if moving targets are present, successive traces will not be exactly the same and an output signal will be produced.

An object of this invention is to provide an arrangernent for biasing the storage surface of a storage tube to a potential level which is substantially different from that placed on it by the recording beam, thereby enabling an increased output signal to be obtained. The foregoing and other objects of the invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawings, wherein:

Fig. l is a cross-sectional view of an illustrative embodiment of the present invention;

Fig. 2 is a schematic representation of an embodiment of the present invention; and

Fig. 3 is a diagram of an equivalent circuit of the storage tube of the present invention.

Referring now more particularly to Fig. 1, the numeral 1 indicates an evacuated vessel usually associated with electron beam projection type electron discharge devices. Within said vessel there is disposed an electron gun 2 (shown in broken outline), a pair of horizontallydisposed deflecting plates 3 and a pair of vertically-disposed defiecting plates 4. The electron gun 2 is located within the narrower portion 5 of the vessel 1, the method of projection of the electron beam therefrom being quite familiar to those versed in the art of cathode-ray oscillography.

Disposed within said vessel in the larger portion 6 thereof is a plurality of planar parallel electrodes 7., 8 and 9, and extending from the narrower portion 5 to a position covering substantially half of the larger portion 6 is an electrically-conductive coating 10. This coating is suitably connected to apart of the electron gun structure 2 in a manner well known to those skilled in this particular art and serves to electrically shield the electron beam represented by the broken line outline 11. The planes of electrodes 7-9 lie at substantially right angles to the axis of beam 11, as shown.

The electrode 7, which is preferably constructed of nickel wire of very small diameter in the form of a 100- mesh screen, is disposed perpendicularly to said electron atent beam 11, said electron beam passing through said screen electrode 7 and impinging on the electrode 3. This electrode is composed of a non-conducting or insulating material such as glass. The electrode 9 is made of electrically-conductive material and is in intimate contact with the rear surface of electrode 8, the electrode 9 preferably consisting of a conductive coating on the back of the electrode 8.

The electrode 7 is suitably supported by an electricallyconductive annular member 12 and said annular member is in turn supported within the envelope 1 by a current-carrying conductor 13 suitably fused to said envelope and welded or brazed to the annular member 12, said conductor 13 being terminated in an electricallyconductive cap 14 suitably fastened to said envelope and conductor 13.

The electrodes 8 and 9 are supported by a second annular member 15 which in turn is suitably attached and supported, as by welding or brazing, to an electrical conductor 16. Said last-named conductor passes through the envelope 1 and is fused thereto so that said envelope serves to support said annular member 15.

In order to maintain the electrodes 7 and 8 a predetermined distance apart, a pair of spacing members 17 and 18 is respectively attached, as by Welding or other suitable means, to the annular member 12 and the conductor 16, the other ends of said spacing members being embedded in an electrically-insulating bead 19 which may be composed of glass or other electricallyinsulating material. The conductor 16 is terminated in an electrically-conductive cap 20 suitably attached to said conductor 16 and the envelope 1.

Fig. 2 illustrates schematically the use of a storage tube, such as that previously described, in connection with the visual reproduction of a signal due to the presence of a moving target in a given area scanned by a pulse-echo objectdetecting or radar system. In other words, this figure represents the use of such a storage tube in a so-called MTI system.

Cathode 21 of electron gun 2 is connected to the negative .end of a suitable source 22 of direct voltage, for example, a battery, the positive end of which is connected to ground, so that the said cathode has a high negative potential with respect to ground. The potential of battery 22 is quite high, on the order of 1600 volts, for example. Cathode 2.1, it will be understood, produces the necessary electron population when suitably energized.

Accelerating anode 23 of the electron gun is connected to an intermediate point on battery 22 to provide the necessary bias thereon. In Fig. 2, for purposes of simplicity, the necessary focusing and control electrodes included in gun 2 are not shown, nor are their potential sources; however, it is to be understood that such electrodes are included in gun 2 in a manner familiar to those versed in the art pertaining to this type of electron discharge device. Although in Fig. 2 several sources of potential are shown in the form of batteries, the usual practice is to provide these potentials from a source of suitably-rectified alternating current.

Connected to one of the vertical deflecting plates 4 is a source of time sweep voltage 24. The circuit for generating said sweep voltage may be any of the many well-known circuits and is indicated on the drawings in block form to simplify the illustration. One of the output terminals of source 24 is grounded, as is one of the plates 4, so that the electron beam is deflected horizontally 'by the voltage supplied from source 24. Although it is not shown, it is to be understood that, in a radar system, the source 24 would be connected to the source of radar transmitter pulses to be triggered thereby at the time of each transmitted pulse.

Horizontal deflecting plates 3 are arranged to deflect the electron beam from gun 2 in a vertical direction. These plates are connected to receive the output from an input signal source 25, such as a radar receiver, which is shown in block form because such a receiver is conventional and is familiar to those skilled in the radar art. The output of receiver is a series of pulses corresponding to those reflected from reflecting objects in space within the field of search of the radar equipment.

The above-recited connections to the electron gun 2 and to the two pairs of deflecting plates are like those utilized in a so-called Type A radar indicator, in which the echo pulses cause upward deflections to occur along the sweep trace on storage surface 8 at distances from the transmitted pulse deflection proportional to the range of the target, the height of such deflections corresponding to the received signal intensity. Thus, the electron beam is swept across the storage surface 8 in a repeated trace pattern and the radar receiver signal is applied in such a manner as to deflect the beam in a direction perpendicular to the direction of sweep.

Coating 10 is connected to ground, as at 26.

Collector screen or grid 7 is connected through a resistor 27 to ground at 28, so that said screen is at ground potential. Opposite ends of load or output resistor 27 are connected to provide the input to an output amplifier 29, a direct current blocking condenser 30 being provided in series in one of the leads to amplifier 29. The amplified output of amplifier 29 may be applied to an oscilloscope 31, as shown.

The repeller electrode or metallic plate 9, which, as shown in Fig. l, is in intimate contact with the storage plate 8 made of insulating material, is connected through a high resistance 32, on the order of one megohm, for example, to the negative side of a direct voltage source 33, the positive side of which is grounded at 34. The potential of battery 33 is preferably on the order of 500 to 1000 volts. By this connection, the repeller 9 and thereby the whole storage plate 8 are raised to a high negative potential with respect to ground.

Although the repeller 9 is illustrated as being biased by an external source 33, it is possible to provide such bias in other ways. For example, the bias may be provided internally of the storage tube by subjecting the storage surface 8 to unconcentrated electron bombardment from a source of electrons inside the tube.

The secondary emission ratio of an electron target, whether an insulator or a conductor, depends on the voltage of the incident electrons. The secondary emission ratio of such a target may be defined as the ratio of the secondary emission current leaving the target to the primary electron current striking the target, and for each material there is a certain value V1 of primary electron voltage which gives for such material a secondary emission ratio of unity. When the voltage of the primary electron beam is less than the value V1, the secondary emission ratio is less than unity and the number of electrons striking the target exceeds the number leaving the target, while, when the voltage of such beam is greater than V1, the secondary emission ratio is greater than unity and the number of electrons leaving the target exceeds the number striking the target.

For explaining the action of the tube, we will disregard the negative potential of the repeller 9 for an instant. The electron beam is of high voltage with respect to ground or with respect to plate 8 if said plate is at ground potential, this high voltage being on the order of 1600 volts, for example. This voltage is substantially greater than the so-called critical voltage V1 defined above, which critical voltage gives a secondary emission ratio of unity for target 8. Therefore, the number of secondary electrons leaving the target 8, from the areas bombarded by the electron beam of gun 2, is in excess of the number of primary electrons striking said target in such areas. Since more secondary electrons are leaving the target than are striking it, a net positive charge or voltage tends to be produced on plate 8 in the areas bombarded by the electron beam from gun 2.

The electron beam current is made sufficiently high, with a given sweep or writing speed, to produce an equilibrium potential, or an equilbrium potential of zero current, throughout the areas bombarded by the main electron beam. This equilibrium condition results from the following action. As the surface of plate 8 builds up a positive potential, due to the secondary emission ratio being greater than unity, as described above, the positive charge on this plate tends to attract the secondary electrons produced back to the plate, since such electrons have a rather slow velocity. The positive potential of surface 8 also tends to increase the velocity of the primary electrons striking said surface, to thereby produce more secondary electrons, but this effect is greatly overshadowed by the attracting or retarding effect of the positively-charged plate on the secondary electrons, since the primary electrons are of very high velocity as compared to the velocity of the secondary electrons, and it therefore requires a much greater voltage change to produce an appreciable effect on the primary electrons than on the secondary electrons. This retarding or attracting effect on the secondary electrons increases as the voltage of plate 8 becomes more positive, until a point is reached at which the number of secondary electrons which succeed in escaping the retarding or attracting voltage of plate 8, and which therefore leave said plate, equals the number of primary electrons striking the plate, giving a zero net current at this equilibrium point.

It has been found that this equilibrium condition is reached when the potential of plate 8 is on the order of two volts positive with respect to screen 7 or ground. Therefore, since this equilibrium condition is reached in a single trace or sweep of the main electron beam across surface 8, a completed trace will leave the storage surface 8 covered with a line of discrete charges at equilibrium potential, or having a predetermined value of potential with respect to ground, on the order of two volts positive, for example.

By the above-described action, the beam, under these conditions, produces a trace which is slightly positive with respect to ground. Some of the secondary electrons liberated by the tracing or progressing beam are collected by the collecting screen 7 and some return to the storage surface. Those which return to the storage surface ordinarily return to those parts of the surface which were left positively charged, causing a slight negative charge bordering the positive trace and reducing the positive charge of that portion of the trace which the beam has just passed. This obliterating action tends to destroy the trace and greatly reduces the sensitivity of the storage tube, particularly since the potential difference between trace and no trace under these conditions is comparatively small.

By raising the repeller 9, and thereby the whole storage plate 8, to a high negative potential, the sensitivity of the tube is greatly improved.

The potential of the storage plate 8 and repeller 9 relative to the potential of cathode 21 will determine the speed of the electrons striking the storage surface 8. In the example given, the voltage of the repeller 9 may be 1,000 volts negative with respect to ground, while the voltage of cathode 21 may be 1,600 volts negative with respect to ground, so that a difference of potential of 600 volts exists between cathode 21 and plate 8, cathode 21 being 600 volts negative with respect to plate 8. This voltage diiference is substantially greater than the socalled critical voltage V1 defined above, which critical voltage gives a secondary emission ratio of unity for target 8; therefore, more secondaries are leaving than arriving at the point of incidence of the electron beam on the storage surface. Due to the negative voltage of repeller 9, secondaries will continue to leave the storage surface 8 in greater numbers than they arrive until the potential of the storage surface becomes positive with respect to ground, or screen 7, to a sufiicient extent to establish equilibrium, which, as has been pointed out above, is on the order of two volts positive with respect to ground or screen 7. Since this is so, the trace will still be slightly positive with respect to the potential of the collector 7, due to the above-explained action, so that the charge of the trace changes from the original unbombarded 1,000 volts negative with respect to ground to two volts positive with respect to ground. The change in charge of the trace is therefore increased (by applying a sufficient negative potential to repeller 9) from a few volts to the order of 1,000 volts. The beam trace will therefore produce a line on surface 8 along which the potential is uniform and predetermined, said potential being greatly dif erent from the potential of those areas not bombarded by the electron beam because the areas not so bombarded remain at the negative 1,000 volts potential (with respect to ground) applied to storage surface 8 by means of repeller 9 and battery 33.

Also, the secondary electrons have an increased tendency to stay away from the storage surface 8 when a volta e is applied to repeller 9, since they are repelled by the hi h negative potential surrounding the trace. Therefore, the secondaries do not return to the storage plate but are forced to the collector 7 or to the so-called second anode 19. As a result, the deleting action caused by secondaries upon earlier parts of the trace is greatly reduced, resulting in a sharper positive trace.

With the repeller 9 energized, the difierence of potential between trace and no trace is very large, and also the obliterating or deleting action of the secondary electrons is substantially eliminated. By applying a repeller potential the output signal is raised, roughly speaking, from microvolts to millivolts (maximum on the order of miliivolts), giving a greatly increased tube sensitivity. By the biasing of surface 8 to a high negative potential by repeller 9 and battery 33, I can obtain a larger output signal than if no biasing were used, since the potential change of record points on surface 8, produced by the electron beam in bringing such points from the biasing potential value to the small predetermined potential (with respect to ground) value or equilibrium value in the manner described, is much greater than that which would result from bringing such points merely from zero potential to the same small predetermined equilibrium potential value with respect to ground.

The voltage across resistor 27 is used as the input signal to amplifier 29, so that collecting screen 7 is capable of being utilized as a signal electrode. Fig. 3 shows an equivalent circuit of the storage tube.

Points

7, 8 and 9 designate the collector, storage surface, and repeller, respectively, these points being connected by capacitances and resistances as shown.

C1 represents the repeller-collector grid capacitance and is on the order of 50 micrornicrofarads, varying, of course, with the construction of the tube. C2 and C3 are the capacitances between the bombarded element on the storage surface 3 and the repeller and collector, respectively, and depend on the concentration of the cathoderay spot of the input electron beam 11 which strikes storage surface 8. C4 is the capacitance between collector grid 7 and the coating or second anode 1 3.

The resistance R2 shunting C2 corresponds to the conductivity of the storage plate 8 which is made of a material which is a very poor conductor, such as glass or mica, for example. The product R2 C2 is called the time constant of the storage tube and defines the memory time; time constants for various storage plate materials may range from a fraction of a second to hours or even days, depending upon the particular material.

The paths of the incident electron beam 11 and of the secondary electrons are represented by arrows; as indicated, some of the secondary electrons ejected from plate 8 return thereto, some proceed to collector grid 7, and some proceed to anode 10. The action of the secondary electrons upon plate 8 can be approximated by a series arrangement of a battery 36, a switch 35, and a variable resistance R3, the action insofar as anode 10 is concerned involving variable resistance RA in series rather than R3. The closing of the switch 35 corresponds to the incidence of the electron beam 11. Since the .electron beam leaves a positive charge on plate 8, battery 36 is poled as indicated.

The external repeller resistance 32 is on the order of one megohm; the external collector resistance 27 is considerably smaller, on the order of 50,000 ohms, to present an appropriate input impedance to the output amplifier 29.

The repeller resistance 32 is critical; it should not be much smaller than one megohm, since otherwise the output signal is reduced in size. The distance from the repeller 9 to the beam side of the storage surface 8 has a large influence upon the amplitude of the output signal; tubes with close spacing are more sensitive than tubes with a wider spacing.

The sign of the collector output signal voltage is positive. Referring to Fig. 3, the action of the electron beam 11 upon plate 8 with respect to the collector 7 (the closing of switch 35) is twofold. Firstly, liberated secondary electrons tend to induce negative signals when they strike the collector '7. However, a large fraction of the total number of secondaries liberated is lost through the holes in grid 7 to the second anode 10; in fact, the second anode current is ordinarily on the order of ten times the collector current. Secondly, dielectric displacement, caused by the change of potential of plate 8 from a high negative toward a positive potential by the beam 11 in the manner explained previously, is transferred through C2 and C1 in series, and also through C3, to the collector '7. This dielectric displacement current flows in the opposite direction to that produced by the movement of the secondary electrons which strike collector 7; this dielectric displacement current is larger than that produced by the movement of the secondary electrons, so that the net collector signal or output voltage is positive.

A negative output signal indicates that the electron beam 11 has stayed on the bombarded spot (on plate 3) long after the change in surface potential is efiected. Displacement currents have subsided, under these conditions, and the negative charge from arriving secondaries prevails at the collector '7. The weak negative signal is therefore an indication of improper adjustment of beam intensity; the output signal reverses to a weak negative signal when a very strong beam intensity is used.

On the other hand, a strong positive output signal requires a so-called free-swinging repeller 9, or one which has a very high repeller resistance 32 connected in series between the repeller and the potential source. The strong output signal is reduced with increased capacitance to ground C of the repeller 9. if the positive transient, produced by the action of the electron beam 11, is suppressed at the repeller by means of a considerably in creased C positive and negative impulses on the collector 7 cancel out almost completely; low ground capacitance of the repeller 9 and its associated leads is thereelectron beam, instead of being caused to swing across the surface of the storage plate, may be merely caused to impinge thereon at certain predetermined points. Also, this repeller type storage tube can be utilized for general laboratory work in cases where a discriminating memory action is required. For example, the tube could be used to obtain records of transient unforeseen electrical phenomena, such as lighting or arc-backs in rectifiers. In such applications, the storage tube, by its action as described previously, would cancel out any periodically repeating information, and Would supply a record only of anything unforeseen. A photograph could be taken of the oscilloscope pattern, in order to retain information regarding the unforeseen phenomena. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

1. An electron discharge device circuit comprising an electron gun for projecting a beam of electrons; a potential storing target having a potential storing surface in the path of projection of said beam; an electrode closely adja cent and capacity coupled to said surface; said potential storing surface being disposed between said electron gun and said electrode; a source of fixed voltage coupled to said electrode and said gun for applying a predetermined fixed voltage positive with respect to said gun to said electrode; an electron permeable collector means closely adjacent and spaced from said target surface on the side thereof facing said gun; and means directly electrically connected to said collector means including means for deriving an output signal therefrom.

2. An electron discharge device circuit comprising an electron gun for projecting a beam of electrons; a potential storing target in the path of projection of said beam having a potential storing surface facing said electron gun and an opposed surface; an electrode closely adjacent and capacity coupled to said potential storing surface; an electron permeable collector means closely adjacent and spaced from said storing target on the side thereof facing said gun; a source of fixed voltage coupled to said electrode and said gun for applying a predetermined fixed voltage positive with respect to said gun and negative with respect to said collector means to said electrode; and means directly electrically connected to said collector means including means for deriving an output signal therefrom.

3. An electron discharge device circuit according to claim 2 further including means for repetitively sweeping said beam across said storing surface of said target at a predetermined rate; and means responsive to an input signal for deflecting said beam in a direction perpendicular to the direction of said sweep.

4. An electron discharge device circuit according to claim 2 further including means including said source for initially biasing said storing surface negative with respect to said collector means; said surface charging toward a predetermined equilibrium potential value during impingement of said storing surface by said beam which is positive with respect to said collector means.

5. An electron discharge device circuit according to claim 2 wherein said source is of such magnitude as to give a secondary emission ratio greater than unity for said potential storing surface of said target so that the portion of said surface impinged by said beam tends to charge toward a predetermined equilibrium potential value which is positive with respect to said collector means.

6. An electron discharge device circuit comprising an electron gun for projecting a beam of electrons; a potential storing target in the path of projection of said beam having a potential storing surface facing said electron gun and an opposed surface; an electrode closely adjacent and capacity coupled to said potential storing surface; an electron permeable collector means closely adjacent and spaced from said storing target on the side thereof facing said gun; a source of fixed voltage coupled to said electrode and said gun for applying a predetermined fixed voltage positive with respect to said gun and negative with respect to said collector means to said electrode; means directly electrically connected to said collector means including means for deriving an output signal therefrom; means for repetitively sweeping said beam across said storing surface of said target at a predetermined rate; and means responsive to an input signal for deflecting said beam in a direction perpendicular to the direction of said sweep; means including said source for initially biasing said storing surface negative with respect to said collector means; said surface charging toward a predetermined equilibrium potential value during impingement of said storing surface by said beam which is positive with respect to said collector means.

7. An electron discharge device circuit comprising an electron gun for projecting a beam of electrons; a potential storing target in the path of projection of said beam having a potential storing surface facing said electron gun and an opposed surface; an electrode closely adjacent and capacity coupled to said potential storing surface; an electron permeable collector means closely adjacent and spaced from said storing target on the side thereof facing said gun; a source of fixed voltage coupled to said electrode and said gun for applying a predetermined fixed voltage positive with respect to said gun and negative with respect to said collector means to said electrode; means directly electrically connected to said collector means including means for deriving an output signal therefrom; means for repetitively sweeping said beam across said storing surface of said target at a predetermined rate; and means responsive to an input signal for deflecting said beam in a direction perpendicular to the direction of said sweep; said source being of such magnitude as to give a secondary emission ratio greater than unity for said potential storing surface of said target so that the portion of said surface impinged by said beam tends to charge toward a predetermined equilibrium potential value which is positive with respect to said collector means.

References Cited in the file of this patent UNITED STATES PATENTS 2,403,239 Rose .a July 2, 1946 2,437,173 Rutherford Mar. 2, 1948 2,454,410 Snyder Nov. 23, 1948 2,548,405 Snyder Apr. 10, 1951 2,618,762 Snyder Nov. 18, 1952