US2975319A - Tacttron - Google Patents

Description

March 14, 1961 A. HlX

TACITRON Filed Feb. 27, 1958 2 Sheets-Sheet 2 BY @y TACITRON Antonin Hix, Prague, Czechoslovakia, assigner to Tesla, narodni podnik, Prague, Czechoslovakia Filed Feb. 27, 1958, Ser. No. 717,954 Claims priority, application Czechoslovakia Mar. 6, 1951 3 Claims. (Cl. 313-186) This invention relates to tactitrons, that is, gaseous discharge tubes of the kind having at least one anode and a control grid arranged in the posit-ive column of discharge or in the area of anode potential drop, for discontinuous control, and particularly for interruption of the current by means of a comparatively small negative grid bias even in the case of maximum anode current. The interruption of the current is caused by the so called Langmuir layer,that is, a layer of positive ions which surround the negatively charged electrode in the plasma or ionized area where there is an equalnumber of positive and negative particles so that the electrode appears to be neutral. The effective transparency of the grid is diminished -by these layers of positive ions and the thickness of such layers depends, in turn, on the negative charge of the electrode 'with reference to the potential of the plasma, on the pressure of the gas filling the discharge tube, on the degree of ionization of this gas, and on the nature of the discharge. If the transparency of the grid is diminished below a certain level, an interruption or cutoi of the discharge is eiected.

According to the theory known so Ifar, see, for example, Johnson, Olmstead, Webster The Tacitron Proc. of the IRE, September 1954, pp. 1350-1362, the

effective transparency of the grid is given by the expression 'nd 2 (HW/) where wisc the notation being:

n-number of grid openings per cm.,

d-grid wire diameter,

Vg-negative grid bias,

-electron current density on the anode surface, M-ion mass,

m-electron mass,

Te-electron temperature,

Ti-ion temperature.

(a) Mechanical reasons, viz., the mechanical strength of the grid;

"United States Patenti C) (b) Thermal conduction considerations, that is, if` the grid wires are too thin, thermal conduction becomes Worse and this may cause grid emission; and

(c) The tube drop, see,- for example page 1357, Fig. 12,

which may increase too much the grid openings becometoo small.

lt can be readily seen from the above formula that the only to Way to reduce the effective grid transparency isto increase the value of the term U in the formula.`

increases if M the ion mass is increased, because the ratio Te/Tp, i.e. the ratio of the electron and ion tem' peratures increases simultaneously, see Knoll, Ollendorf, Rompe Gasentladungstabellen published by Julius Springer, 1935, Berlin, p. 99. theory gases with the highest atomic weight should be used in tacitrons. In fact, all known tacitrons now generally use an xenon lling, or a filling of mercury vapour.

In comparison with conventional thryratrons, tacitrons` For example: The discharge may be extinguished or interrupted by a negative grid' bias; their noise is negligible; and their application is notf have many advantages.

limited by the time required for deionization of the gas tilling. xenon or another heavy noble gas illing resides in the fact that the current or power which may be controlled |by a taci-tron, the size of which is comparable with the size of a conventional thyratron, is considerably smaller, because, in the case of greater anode currents, the proper mode of the discharge is not preserved and the tacitron cannot be extinguished by application of a grid bias. Furthermore, tacitrons filled with noble gases cannot be used for higher voltages (in the order or kv.), as will be discussed later.

The object of this invention is to provide a tacitron which is tree of the above mentioned disadvantages of tacitrons lilled with xenon or other noble gases, while experiments that, by using hydrogen which is the lightest,

known gas, it is possible to achieve better results, to increase considerably lthe controlled power output and to decrease the necessary control power. Experiments have been carried out to compare the properties of the same system using first a xenon lling, and secondly a hydrogen lling. With the xenon filling a power of up to 60 W. could be extinguished or interrupted by means of the grid bias; while, with the hydrogen lilling it was possible to switch oli a power of 1 kw. by means of the grid bias; and, in pulse operation, as much as 15 kw. could be controlled. Thus the power controlled by the hydrogen filled tacitron was l5 to 200 times larger than the power controlled `with the tacitron having the usual xenon filling.

For a more detailed comparison of tacitrons with xenon and hydrogen llings, some characteristics are shown in the accompanying drawing, wherein: Fig. 1 shows `the so called cutoii characteristics, Fig. 2 shows the maximum controlled voltage and tube-drop as a function of the lling pressure, and Fig. 3 is au axial section- In accordance with this' The disadvantage o f existing tacitrons withal'view of atypical tacitron which is filled withuhydrogen in Vaccordance with theA present invention.

Referring to Fig. 3 in detail, it will be seen that a tacitron, to which the present'invention relates, includesV a sealed glasseenvelopez `rnounted on a base12'andhavin'g alsealed tube 11at fits lower end through which"hydro gen is pumped into the envelope during manufacture. The envelope 2 contains jan 'upper' ceramic insulator 3 and a lower ceramic insulator, vand a central oxide'cathode 4 extends between, and is supported by, the upper insulator 3 and a mica cathodeV holder '7. A grid 5, formed, for example, of `a tine wire mesh or network, extends around cathode 4 between insulator-3 and a grid carrier 10, and an anode 6 surrounds grid 5 and is con-V example, xenon or mercury vapour, but, in accordance:

with the present invention, the envelope 2 'is filled with hydrogen which replaces the usual noble gas, as previous- 1y mentioned. Thecutoi characteristics shown in Fig. 1, present the dependence of the maximum controlled source voltage V, on the extinguishing grid bias Vg with diierent anode currents Ia. From these characteristicsiit is possi-ble to deduce the negative grid bias required for interrupting the anode current at a certain source voltage. It can be seen for example that in the case of a xenon filling a 100 ma. current with a source voltage ofv 400 v. may be interrupted by a negative grid bias of 15 v., while with a hydrogen filling a negative grid bias of only 3.5 v. is required under the same conditions. With a xenon filling it is not possible to interrupt currents larger than 200 ma., while with a hydrogen filling it is even possible to interrupt a current of 1 a., with a source voltage higher than l kv.

In Fig. 2 the unbroken curves present the dependence orf the tube drop on the lling gas pressure. It can be seen from these curves that tacitrons lled with xenon (or other noble gases) cannot be used for controlling ahigher voltage. This would require a decrease of the iilling pressure, but the tube drop Vo would be increased simultaneously. The tube drop may not exceed theV so called destructive voltage, which means aV tube drop, where'a destruction of the oxide cathode sets in by ion bombardment. noble gaes and mercury vapour is about 20 v., vwhile with hydrogen it has a value of about'600 v. Fig. 2 shows that the applicable minimum pressure forxenon is about x10-2 mm. Hg or torr, which corresponds to a tube drop of 18 v. and a maximum controlled voltage V,- of 2 kv. With hydrogen the tube drop has a The destructive voltage in the case of 4. value far below thevalue of the destructive voltage, even at a pressure of 3 l01 mm. Hg or torr, which corresponds to a maximum controlled voltage of 10 kv. It is evident that the results achieved with a hydrogen lling diter very favourably from the results achieved with a xenon iilling. The results are still very favourable even if the hydrogen llingicontains a certain amount of other gases. n

Hydrogen has also a good "eiect'on the emission of the oxide cathode. The use of a hydrogen filling enables the construction of tacitrons 'as large as va"co'n'ventional thyratron fora similar output power; with a hydrogen lling it is also possible todesign "tacitror'is' for very high output `powers 'in the order bfftens of kv.' Such power outputs could not be achieved with existing tacitrons iilled with xenonor' other 'noble gases.

What I claim is:

1. In combination in a gaseous discharge device of the tacitron type, an envelope having therein a cathode electrode, an anode electrode spaced from said cathode electrode, an apertured grid electrode positioned intermediate said cathode and plate electrodes, and a filling of hydrogen, said grid electrode being adapted to interrupt current ow between said cathode and plate electrodes byv closing the aperatures of said grid electrode by a space-charge sheath which is opaque to electrons attempting to `move from said cathode electrode to said anode electrode, the vmaximum dimensions of the apertures of said grid electrode being substantially equal to the thickness ofsaid spacefcharge sheath.-

2. In `combination in a tacitron, an envelope having therein a cathode electrode, an anode electrode, an aperture'dl grid electrode positioned between said cathode and' anode electrodes for interrupting current flow between said cathode and anode electrodes by causing the vformation in the envelope of Langmuir layers, and a hydrogen gas lling, the dimensions of the apertures of said grid electrode being comparable to the thickness of said layers.

' 3. A combination as in claim 2, wherein the vpressure of said hydrogen gas filling is at least 3X1()-1 millimeters otf mercury.

References Cited inthe tile of this patent UNITED STATES PATENTS 2,003,012 Sashoff May 28, 1935 2,061,255 Rockwood Nov. 17, 1936 2,184,756 Rockwood Dec. 26, 1939 2,220,089 Druyvesteyn et al. Nov. 5, 1940 2,572,881 Rothstein Oct. 30, 1951 2,678,403 Germeshausen May 11, 1954 OTHER REFERENCES The Tacitron, by E. 0.' Johnson et al., Proceedings of the I.R.E., September 1954, pages 1350 to 1362.

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