US2409955A - Gas-filled electric rectifier - Google Patents

Description

0a, 22,1946. 9 T, R SA 2,409,955

GAS FILLED ELECTRIC RECTIFIER Filed March 13, 1943 Jams: 9 [NYE/V7191? acwsmv'rafis 14 JAM 4 96 4 M ATTORNEY Patented Get. 22, 1946 GAS-FILLED ELECTRIC RECTIFIER Henry Thomas Ramsay, Hatch End, England, assignor to The General Electric Company Limited, London, England Application March 13, 1943, Serial No. 479,121 In Great Britain March 13, 1942 6 Claims.

This invention relates to electrical apparatus of the type comprising a rectifier of the kind having a thermionic cathode, an anode, a shield forming part at least of a labyrinth separating the cathode from the anode, an auxiliary electrode, and a gas or vapour (e. g. mercury) filling the space between the said members, the arrangement being such that, when the auxiliary electrode is at cathode potential a discharge between cathode and anode cannot start under any voltage normally applied between them, but when the auxiliary electrode is made positive to the cathode, so as to' produce an ion-forming discharge between the auxiliary electrode and the cathode, the main discharge starts between cathode and anode. Rectifiers of the said kind are described in British Patent Specification No. 541,919 and in U. S. patent application Serial No. 466,651 filed November 23, 1942. It is to be understood that the auxiliary electrode may be a substantial part of the shield (as in the specification of the said application), or it may be so small that it has practically no effect in shielding the cathode from the anode (as in the said British Specification No. 541,919)

The term labyrinth implies that the starting of an ion-forming discharge from the cathode is substantially independent of the potential difference between anode and cathode. A necessary condition for such independence is that no straight line can be drawn from anode to cathode that does not intersect a conductor that is negative to the anode during the starting period. A straight line passing through an aperture a grid is not to be held to intersect the grid. But this condition is not suiiicient. A suflicient condition is that, if the anode and cathode potential is held constant and the control electrode is made progressively more positive to the cathode, the potential of the control electrode at which an ion-forming discharge first occurs (evidenced by a very sharp rise in the current between control electrode and cathode) is substantially independent of the anode-cathode potential. This proves that the anode and control grid do not co-operate in the starting of the first ion-forming discharge. The anode potential is effective only in converting this first discharge into a discharge between anode and cathode in virtue of theintroduction of ions into the labyrinth.

In apparatus of the type hereinbefore specified there is a finite interval between the application of a positive voltage to the auxiliary electrode a d the development of the full discharge between anode and. cathode. The interval depends on the 2 mercury pressure, the arrangement of the electrodes, and the voltages applied to them, at least over a certain range. It is sometimes desirable that the interval should be very short, for example 1 micro-second; then the discharge during the interval is apt to damage the cathode, especially when it is activated and of the form described with reference to the drawing accompanyin'g the said British Specification No. 541,919. An object of this invention is to provide means for mitigating this damage in these circumstances. This object can be attained by producing, by means of a discharge, ionization in the neighbourhood of the cathode before the application between the cathode and the control electrode of the voltage that starts the main discharge between the cathode and the anode. This ionization must not, of course, itself enable the main discharge to start; this condition can be fulfilled by causing the preliminary discharge that ,produces the said ionization .to pass between the cathode and another auxiliary electrode, different from the aforesaid auxiliary electrode and wholly on the cathode side of the said shield, or between the cathode and part of the shielditself, which part is then to be regarded as another auxiliary electrode. The same result can sometimes be produced by causing to pass between the cathode and the first-mentioned auxiliary electrode a preliminary discharge carrying a current too small to enable the main discharge tostart; but no advantage of this alternative is known.

The preliminary discharge may pass continu ously during the Whole operation of the apparatus, or during the whole period, between successive mam discharges. But it is often more convenient to apply between the cathode and the other auxiliary electrode a pulse of voltage immediately preceding that applied between the cathode and firstauxiliary electrode to start the main discharge; such a pulse will generally last longer than the pulse that starts the main dis charge, but will occupy only a small fraction of the period between main discharges.

Embodiments of the invention will be described with reference to the accompanying drawing, in which Figs. 1 and 3 show respectively relevant parts of two'diiierent forins'of the improved rectifier in axial section, together with certain circuit elements, represented diagrammatically, and N Fig. 2 is a sectional plan on the line 2-2 in Fig. 1.

Referring to Figs. 1 and 2, I is the cathode, 2 the anode, and 3 is a hollow metal cylinder surrounding the cathode. Leads IA and [B for heating current connect to the ends of the cathode and a lead IC to the centre of the cathode. These three leads pass through insulating bushings in the base of the cylinder 3. The auxiliary electrode is made up of a hollow metal cylinder 4 round the inner end of the anode and a plate 5 covering but spaced from the end of a tube 6 projecting from the apertured base of the cylinder 4 into an aperture in the top of the cylinder 3. The parts 4, 5 and 6 are electrically connected together and are maintained normally negative to the cathode by a source of E. M. F. I acting through a choke 8 and a lead It]. The plate 5 is the main operative element of the labyrinth: it shields the cathode completely from a field due to the potential difference between anode and cathode.

The cylinder 3 is insulated from the parts 4, 5 and 6 and is connected to the central cathode lead IC through a pulse-generator 9'. A pulsegenerator 9 is connected between the cathode and the lead In.

In order to produce the preliminary discharge, the pulse-generator 9 applies a Voltage pulse to the cylinder 3 positive with respect to the cathode, lasting say 80 micro-seconds and immediately preceding the application of a positive starting pulse applied to the auxiliary electrode 4, 5, 6 by the pulse-generator 9. The choke 8 prevents short circuiting of this pulse. An ion-forming discharge is thus started between cathode and auxiliary electrode. The ions produced by this discharge enable the main discharge to flow round the edge of the plate 5 and through the tube 6. When the discharge has been extinguished by reduction of the anode potential, the auxiliar electrode is negative and collects the residual positive ions. The example shown may be so arranged that the preliminary discharge current to the cylinder 3 is about 2 amps, and, when the auxiliary electrode 4, 5, 6 is pulsed about 100 volts positive with respect to the oathode, a main discharge carrying about 100 amperes (limited by the anode-cathode circuit) will then start within about 1 micro-second of the application of the starting pulse.

Fig. 3 shows a modification in which the preliminary discharge passes between the cathode and part of the shield itself. The numerals I, 2, 3, 5, 9 and 9 represent parts corresponding to those similarly denoted in Fig. 1. The shield is constituted by the cylinder 3, an inner tube 3A surrounding the cathode, and the plate 5. The centre point of the cathode is connected to the cylinder 3, and through this to the plate 5. A lead IilA,connected to the cathode by the pulse generator 9, enters the cylinder by a bush and leaves it by a bush 12; where it is within the cylinder it is surrounded by a quartz tube 13' which shields it from the cathode. The upper endof the lead IBA projects from the top of the case and forms the control electrode M. The

inner tube 3A is insulated'from the cylinder 3 and connected thereto and to the centre point of the cathode through the pulse-generator 9. The preliminary ionization in the neighbourhood of the cathode is established by application of a positive pulse to the tube 3A by the pulse-generator 9', immediately preceding the application of the starting pulse to the auxiliary electrode l t by the pulse-generator 9.

Iclaim:

1. An electrical circuit arrangement comprising a thermionic gas-filled rectifier having a thermionic cathode, an anode, ashield consisting of one or more parts interposed between said cathode and said anode so as to provide a labyrinth capable of rendering the starting of a discharge between said cathode and said anode substantially independent of the potential difference between said cathode and said anode and an auxiliary starting electrode separate from said cathode and said anode, means for periodically rendering said auxiliary electrode positive with respect to said cathode so as to produce between said cathode and said auxiliary electrode an ionforming discharge capable of enabling the main discharge between said cathode and said anode to start, and means for producing by a preliminary discharge ionization in the neighbourhood of said cathode during the period immediately preceding such application to said auxiliary electrode of the voltage that starts the main discharge.

2. An electrical circuit arrangement comprising a thermionic gas-filled rectifier including a thermionic cathode, an anode, and separate from said cathode and said anode an electrode system constituting a labyrinth capable of rendering the starting of an ion-forming discharge fromsaid cathode substantially independent of the potential difference between said cathode and said anode, means for periodically rendering one part of said electrode system positive with respect to said cathode so as to produce between said cathode and said part an ion-forming discharge capable of enabling the main discharge between said cathode and said anode to start, and means for rendering another part of said electrode system positive with respect to said cathode so as to produce by a preliminary discharge ionization in the neighbourhood of said cathode immediately preceding such application of starting voltage to said one part of the electrode system.

3. An electrical circuit arrangement comprising a thermionic gas-filled rectifier including a thermionic cathode, an anode, and separate from said cathode and said anode an electrode system constituting av labyrinth capable of rendering the starting of an ion-forming discharge from said cathode substantially independent of the potential difierence between said cathode and said anode, said electrode system comprising a first auxiliary electrode, and a second auxiliary electrode disposed nearer to said cathode than is said first. auxiliary electrode, means for periodically rendering said first auxiliary electrode positive with respect to said cathode so as to produce between said cathode and said first auxiliary electrode an ion-forming discharge capable of enabling the main discharge between said cathode and. said anode to start, and means for renderingsaid second auxiliary electrode positivewith respect to said cathode so as to produce by a preliminary discharge ionization in the neighbourhood of said cathode immediately precedingsuch application of starting voltage to said first auxiliary electrode. v i

4. An electrical circuit arrangement comprise ing a thermionic gas-filled rectifier including a thermionic cathode, an anode, a shield forming at least part of a labyrinth separating said cathode from said anode, a first auxiliary electrode constituting at least a substantial part of said shield anda second auxiliary electrode on the cathode side of said shield, means for periodically rendering said first auxiliary electrode positive with respect to said cathode so as to produce between said cathode and said first auxiliary electrode an ion-forming discharge capable of enabling the main discharge between said cathode and said anode to start, and means for rendering said second auxiliary electrode positive with respect to said cathode so as to produce by a preliminary discharge ionization in the neighbourhood of said cathode immediately preceding such application of starting voltage to said first auxiliary electrode.

5. An electrical circuit arrangement comprising a thermionic gas-filled rectifier including a thermionic cathode, an anode, a first auxiliary electrod having an apertured diaphragm disposed between said cathode and said anode and also having a bafile covering but spaced from the aperture in said diaphragm, a second auxiliary electrode in the form of a hollow metal cylinder disposed around said cathode and having an apertured cap at its end nearer said anode, means for periodically rendering said first auxiliary electrode positive with respect to said cathode so as to produce between said cathode and said first auxiliary electrode an ion-forming discharge capable of enabling the main discharge between said cathode and said anode to start, and means for rendering said second auxiliary electrode positive with respect to said cathode so as to produce by a preliminary discharg ionization in the neighbourhood of said cathode immediately preceding such application of starting voltage to said first auxiliary electrode.

6. An electrical circuit arrangement comprising a thermionic gas-filled rectifier including a thermionic cathode, an anode, a shield disposed between said cathode and said anode and providing a labyrinth separating said cathode from said anode, an auxiliary electrode 50 positioned that a starting discharge occurring between said auxiliary electrode and said cathode will produce ions in said labyrinth, means for periodically rendering said auxiliary electrode positive with respect to said cathode so as to produce said starting dis-.

charge, and means for rendering part of said shield positive with respect to said cathode so as to produce by a preliminary discharge ionization in the neighbourhood of said cathode immediately preceding such application of starting voltage to said auxiliary electrode.

HENRY THOMAS RAMSAY.

Images