US2384087A - Current limiter - Google Patents

Description

R. R. GOODRICH CURRENT LIHITER Filed April 1. 1942 3nnenloi Robert H. Goodrtch (Itto rneg to a conventional thermionic cathode.

Patented Sept. 4, 1945 UNITED STATES PATENT OFFICE- M22323... Robert R. Goodrich, Merchantvilie, N. 1., assignor to Radi of Delaware Corporation of America, a corporation Application April 1, 1942, Serial No. 437,168

6 Claims.

This invention relates to the regulation of electrical current and particularly to improved current regulating means for electron discharge devices of the type normally employing operating potentials of several thousand volts.

While the invention will be disclosed as embodied in a current regulating system for a socalled point microscope," it will be apparent that there are many other applications for the invention and that the disclosure in this respect is merely illustrative for purposes of explaining the inventive concept. It is now appreciated by those skilled in the art of electron discharge tubes that, for certain applications, a "point cathode is to be preferred By way of example in so-called beam tubes wherein a beam of minimum cross-sectional area is desired, the use of a point cathode may reduce or obviate the necessity for employing auxiliary magnetic or electrostatic means for compressing the beam. (As to this see, by way of example, Ramberg et al. U. 8. Patent 2,271,990).

The principal objection to the use of a. point cathode is the tendency of the point to burn out the presence of residual gas in the tube. Attempts to obviate this difliculty, as by the use of a protective system of the type employing a resistor in series with the load and which depends for its operation upon an increase in current through said resistor, have not since such systems are based upon the feedbac principle and possess an inherent time lag which, however small, prevents the limiting action from taking place prior to the passage of the current which the said system is designed to control.

At first glance it might appear that the foregoing objections to conventional "feedback systems for regulating the current supplied to a point cathode might be overcome by the use in a "feed forward" system employing a conventional tube (say an RCA pentode type 6J7) possessing a substantially flat plate-current characteristic. objection to the use of conventional tubes in such a regulating system resides not only in the fact that they are usually incapable of standing up under the voltages which they would be obliged to sustain but in the fact that their interelectrode capacitance is so great as to provide a capaci ive path to the load for at least a surge of the current whose passage it is desired to preventbeen successfultrons from the The- 'vention is to Accordingly, the invention is to provide an improved current regulating system for high voltage discharge tubes, and one which obviates the foregoing and other less apparent disadvantages to known regulating systems.

Another and related object of the present inprovide an improved current regulating discharge tube having a low internal capacitance, a compact structure, and one nevertheless capable of use in connection with a load device which is normally subject to an operating potential of several thousand volts.

The foregoing and other-objects are achieved, in accordance with the invention, by the provision, in a "feed forward" regulating system, of a beam tube of special design and wherein the electron current which impinges a metal target or second anode is independent of variationsin voltage incident to changes in the resistance of the utilization circuit. 7

Certain preferred details of construction together with other objects and advantages will be apparent and the invention itself will be best understood by reference to the following specification and accompanying drawing wherein Figure 1 comprises a longitudinal sectional view of a current limiting beam tube constructed in accordance with the invention;

Figure 2 is a sectional view taken on the line 2-2 of Fig. 1, and

Figure 3 comprises a diagrammatic view partly in section showing the device of Fig. 1 connected for use, as a current limiter, with a so-called "point microscope."

In the drawing wherein like reference characters designate the same or similar parts in all figures, I designates a highly evacuated tubular envelope containing an electron gun 3 adjacent one end and, adjacent the other end, a second anode or target electrode I upon which the elecgun impinge. The target electrode I comprises a-fiat metal plate which extends in a plane normal to the long'axis of the gun and is shown supported upon a press 1 by means of a connector 0 which extends through the press to the exterior of the envelope. The plate or target 5 is provided with rounded edges la to prevent cold emission and, in order to more effectively dissipate heat, comprises a surface of an area considerably greater than required for the reception of the beam of electrons from the gun I. A plate ll of similar diameter and contour is mounted on the leading end of the gun in principal object of the present 2 I register with the target plate I to ensure a symtrons in their journey to the target. The plate i I is providedwith a central aperture I in through II in the. instant case comprises a helicallywound fiat pancake filament which is supported, as on conductive leads Ila, lib. within the cylindrical control electrode II, and has its emissive surface presented to a central aperture I511 in the said cylinder. A grid wire lib stretched ass-soar \i metrical held in the space traversed by the elec- I reduced. Since the limiter tube l is in series with the point cathode I} of the tube the voltage' across the limiter increases by the same amount that the voltage across the tube 25 drops, thus the overall voltage remains the same. In the absence of secondary emission eii'ects in the limiter tube the large increase in voltage thereacross'has no eil'ect upon the electron current which impinges the second anode I since the tubular'conduit l1 eflectively shields the electron source ll from the intensified electrostatic held between the end plate H and the second anode plate 5. Further, since the spacing between the across the aperture Na and conductively secured to the cylinder it permits the electron current to be controlled without altering the diameter 'of the beam. This provides a sharp control and is especially desirable in the instant, case since it prevents spreading of the beam and inhibits the generation of secondary or impact electrons which would result if the primary electrons struck the inner surface or the first anode II or the inner edge of the end plate H. The first anode [1- is provided with a pair of beam forming diaphragms l9 and 2|, having central apertures which are aligned with the aperture Ila in the 'end plate II.

variations, due to arcing, in the resistance 0! the said load: Diameter of grid and first anode cylinders 1%". Length of grid cylinder A"; length of anode cylinder 1''. Diameter of grid aperture 0.15"; diameter of grid wire 0.007". Grid-anode space 0.040". Spacing between end plate II and second anode 8 1". Spacing of focusing diaphragms l9, 2! 0.5". Diameter of beam forming apertures 0.125". Diameter of aperture in end plate I i 0.25". The parts were made of tantalum carefully outgassed and treated with about 100 kilovolts to burn oi! all cold emission points.

Referring to Fig. 3, the electron current which impinges the second anod 5 of the regulator tube i is applied as by means of a lead 5b to a needle like point-cathode 23 of an electron discharge device 25 which, in the instant case, comprises an electron-microscope similar to the one shown in Ramberg et a1. U. S. Patent 2,271,990. As disclosed in the said patent, one or more auxiliary electrodes 21 and 29, when supplied through leads 21a and 29a, respectively, with potentials several thousand (say, 5000) volts positive with respect to the cathode 23, serve to establish and maintain a "cold discharge" of electrons from the point 23a- Though it is usual. to outgas such a tube just prior to use, it may happen that during its operation'enough occluded gas willbe driven out of the metal or glass to render the spacev betw en egg +3 23a of the cathode 28 and i-tsnext adiacent eiectrcilc it at stalls-ins resi that the vintage area therebetween is m energizec],

plate H and 5 is so great that the capacitance therebetween is negligible there can be no capaci- 'tive discharge of current therebetween (as there would be in the case of a conventional plural grid tube) which can .be transmitted to the load device 25.

I claim as my invention:

1. A system consisting of -a current limiter comprising an evacuated envelope containing an electron emissive cathode and an anode, and a utilization circuit connected to said anode, said utilization circuit including a path of variable resistance, and electrode means of negligible capacitance between said cathode and anode for maintaining a constant flow of electrons therebetween substantially independent of variations in the resistance of said utilization circuit.

2. The invention as set forth in claim 1 and wherein said electrode means. comprises a conduit through which said electrons pass in their journey to said anode 3. The invention as set forth in claim 1 and wherein said utilization circuit comprises a point cathode and a discharge maintaining electrode mounted in spaced relation with respect to said point cathode.

4. The invention as set forth in claim 1 and wherein the spacing between said anode and the electrode next adjacent thereto is so great as to prevent a capacitive discharge of current therebetween when the resistance of said utilization circuit is varied. I

5. A current limiter comprising an electron emissive cathode, a tubular electrode and a platelike metal anode upon which electrons from said cathode impinge mounted in the order named in spaced relation along a common axis within an evacuated envelope, and leads individual to said tubular electrode and said anode for establishing a difference of potential therebetween, the

spacing between said anode and the adlacent end of said tubular electrode being so great as to prevent a capacitive discharge oi current therebetween when said device is subjected to extreme variations in the potential applied to said'electrodes.

6. The invention as set forth in claim 5 and wherein said tubular electrode terminates in a plate-like surface of a diameter corresponding substantially to that of said anode. whereby a b tia y u i e e rostatic seats snap shill ehetseen when said electrodes are seems a. seam es-

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