US3295013A

US3295013A - Electron tubes containing gas below critical pressure

Info

Publication number: US3295013A
Application number: US48454A
Authority: US
Inventors: Warren C Strnven
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1960-08-09
Filing date: 1960-08-09
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27

Description

Dec. 27, 1966 w. c. STRUVEN 3,295,013

ELEcTRoN TUBES CONTAINING GAS BELOW CRITICAL PRESSURE Filed Aug. 9, 1960 f WARREN C. STRUVEN BY g/W F www@ ATTORNFYS United Statesv Patent C) 3,295,013 ELECTRON TUBES CNTAHNING GAS BELOW CRITICAL PRESSURE Warren C. Struven, San Carios, Calif., assigner, by mesne assignments, to Varian Associates, a corporation of California Filed Ang. 9, 1960, Ser. No. 48,454

Claims. (Cl. 315-168) This invention relates to gas tubes and, particularly, to low-pressure gas tubes which are self-starting in ionizing the gas therein for the conduction of electricity.

Gas filled tubes conduct current only when the gas is ionized. The gas can be initially ionized by the aid of free electrons which are formed in the tube; for example, free electrons may be formed by a heated filament as in a thyratron or by an arc or spark as in ignitrons. In some applications which employ gas tubes, neither a filament nor a spark source in the tube is practical, especially when very small-size gas tubes are required. In the past, gas tubes have been made self-starting; that is, the tubes ionized the gas therein without the aid -of the filament or spark to form free electrons.

In general, a tube which is self-starting comprises two electrodes separated by a tubular insulating wall. The tube is filled with gas at a given pressure, and the sparking potential required across the two spaced electrodes is governed by the gas pressure. When the gas pressure is below critical pressure (hereinafter to be referred to as a low-pressure tube), the sparking potential is inversely proportional to the gas pressure. When the gas pressure is above critical pressure (hereinafter to be referred to as a high-pressure tube), the sparking potential is proportional to the gas pressure. Then the critical pressure of the gas is the pressure at which the minimum sparking potential will form a spark. Therefore, a low pressure tube operates by placing a high potential difference across the electrodes to take advantage of the free electrons, which are always available in a cold tube. Then any charged particle within the tube is attracted by one or the other electrode. If the charged particle is a free electron, it is attracted to one -of the electrodes (biased positive), and in moving to the positive electrode the electron gains energy and collides with one 'or more gas molecules, releasing an electron from its bond with the nucleus of the molecule. Then two or more electrons are available for further ionization of the gas. The number of free electrons multiply in the tube until the tubes current capacity is limited by its external circuit. As is well known in the art, the probability of an electron causing a collision with an atom increases if the distance between electrodes of a low-pressure tube is increased, and also if the pressure is increased up to critical pressure. A low-pressure tube, which must operate at a given ionization potential, would have a minimum critical length if it is to be self-starting.

An object -of this invention is to provide a gas-filled selfstarting tube which has a smaller lengththan was here-vl tofore obtainable with self-starting tubes.

Another object of this invention is to eliminate the need for supplying additional free electrons in a gas tube in order to make it conducting.

A still further object of this invention is to make an improved, ei'iicient gas tube.

Briey described, this invention provides a means for establishing within the tube an area in which the voltage gradient is lower than it is in prior art tubes of equal size. According to a preferred embodiment of the invention, the means is a third electrode spaced intermediate the rst and second electrodes of a conventional tube.

The third electrode is biased at a potential intermediate l the first and second electrodes. The specific position and Mice bias of the third electrode with respect to the two conventional electrodes is such that the voltage gradient established between the third electrode and one of the conventional electrodes is lower than any voltage gradient existing in a conventional two-electrode tube of equal length. By way of definition, a low voltage gradient refers to an area in which the equipotential lines of a given value are spaced far apart, whereas a high voltage gradient refers to an area where the equipotential lines of the same given value are spaced close together. A

The invention contains other objects and features of advantage, some of which, wifh the fore-going, will be set forth in the following description of the invention. The invention is not limited to the disclosed embodiment, as variant embodiments thereof are contemplated and may be adopted within the scope of the claims.

Referring to the drawing:

FIGURE l shows an outline of the novel tube with a suitable power supply attached thereto.`

FIGURE 2 is an enlarged cross-sectional view of the tube of FIG. 1 and showing equipotential lines; and

FIGURE 3 is a cross-sectional View on the same scale as FIG. 2 showing a longer tube made in accordance with the prior art and showing equipotential lines.

Referring to the drawing in greater detail and to FIG- URE 3 in particular, the prior art will first be described in order to provide a basis for understanding the invention. FIGURE 3 shows a conventional gas tube 101 which is of a length whereby it is self-starting. This length is determined by the distance which is required between

electrodes

102 and 103, as determined by the gas pressure in the tube and the ionization potential placed across the electrodes. The electro-

des

102 and 103 are cup-shaped and are sealed gas-tight each to one end of a tubular ceramic insulator 104 by

anges

106 and 107 attached to

electrodes

102 and 103, respectively.

Ceramic backing rings

103 and 109 are sealed to the

anges

106 and 107, respectively, opposite the insulator 104. The interior of the tube forms a gas tight Chamber 121, and the chamber 121 contains a. gas at a pressure which is below critical pressure. Therefore, the tube is a lowpressure tube.

In some applications the tube shown must withstand very high D.C. potential difference between

electrodes

102 and 103 before it conducts electricity. Then,the pressure therein must be well below atmospheric and below critical pressure. Since the outside of the tube is at atmospheric pressure, the tube is designed so that the external spacing between

electrodes

102 and 103 is .greater than the internal spacing. To make the tube 101 conducting, the potential between the two electrodes is increased rapidly to higher than the potential which the electrodes can normally hold off at the low pressure. The anode electrode 102 is biased positive with respect to the cathode electrode 103. The cup well of electrode 102 is located within the tube `to provide for a lower voltage gradient within the well of the electrode 102 than between the

electrodes

102 and 103. This is illustrated by the

numbered lines

10, 20, 30', 35, 40, 50, 60, 70, 80, and 90, which lines represent in percent the relative potential of the line with respect to the

electrodes

102 and 103. IOf course, the surface of electrode 102 is at 0 percent and the surface of electr-0de 103 is at 100 percent, The voltage gradient in the region between electrode 102 and the 40 percent equipotential line is extremely critical if self-starting is to be achieved, because, as is known in the art, the probability of ionization of an electron does not increase continuously with increasing electron velocity but reaches a maximum at about electron volts. A smaller voltage gradient within the well of the cup of electrode 102 allows the electrons to remain close `to the 1GO-volt energy level through a longer distance, thereby 3 increasing the probability .that they will collide with gas 'molecules and ionize them into charged nuclei and more free electrons which in time will ionize more molecules, etc., until a glow discharge is formed in thev compartment 121 and the tube is thus onducting electricity.

As stated above, some applications require smaller size tubes than the tube 101, but a tube having a length shorterthan the critical length will not self-starting. The critical length of a tube is determined by the mini- 'mum distance required between electrodes of a tube containing a gas at a given low pressure, and at a given ionization potential. FIGURE 2 shows a tube 101 built according to the invention, which is similar to tube 101 eircept shorter. Like numbered members of tube '1, but which numbers are primed, perform the same functions as and are identical lto the non-primed numbered members of tube 101, except that insulator 104 is shorter than insulator 104 since tube 101' is shorter than tube 101. Tube 101l forms a chamber 121' which is lled with a .gas having a pressure below the critical value, and preferably having the same low pressure as in tube 101.

Of course, when .the same potential diiierence is placed across the

electrodes

102 and 103 of the tube 101 as in tube 101, the equipotential lines (which are not illustrated) will be closer together than similar lines in tube 101, since the spacing is shorter. Therefore, the voltage gradient in the tube and, especially, in and adjacent to the well of electrode 102, will be too high to allow a free electron to start gas ionization under the same operating conditions at which tube 10-1 will operate because the free electron will rapidly accelerate through the 100- volt region and will not collide with a sufficient number of molecules to start ionization.

n order to .make tube 101' self-starting without decreasing the potential, tube 101' has a novel structure in the form of a grid 122, which is permeable to charged particles, The grid is mounted on a terminal ring 123 which protrudes through and is sealed ,to insulator 104. As a practical matter, insulator 104' is made in two sections with ring 123 sandwiched between them. Now a potential can be placed on the grid 122 which will redistribute the location of .the equipotential lines in tube 101'. In the embodiment illustrated, the grid 122 made of wire mesh is disposed where the 35 percent equipotential line of tube 101 would occur in tube 101' if tube' 101' were the same length as .tube 100. Further, grid 122 has a shape which is the same shape as the 35 percent equipotential line in tube 101. Now, if the potential of grid 122 is made equal to 35 percent of the potential difference between electrodes 102' and 103', .the

equipotential lines

10, and 30 between grid 122 and electrode 102' will be the same in shape and spacing as the corresponding

equipotential lines

10, 20 and 30 in tube 101. The equipotential lines 40', S0', 60', 70', 80', and 90 between grid 122 and electrode 103 must then be closer together than the

equipotential lines

40, 50, 60', 70, 80, and 90 in tube 101, because the spacing between grid 122 and electrode 103' is much shorter than the distance between the equipotential line 35 and electrode 103. Since the

equipotential lines

10, 20, and 30 within the tube 101' are not altered from the

equipotential lines

10, 20, and 30 within the tube 101, tube 101' has the same starting ionizing characteristics as tube 101. Both tubes are self-starting, start under the same conditions, and require no auxiliary electron source, but tube 101' is made much smaller than tube 101.

A tube built in accordance with .the teachings of tube 101 and having an overall length of less than two inches was made self-starting, While a tube built in accordance with the teachings of the prior art tube 101 could not be made shorter .than three inches and still be self-starting.

The grid 122 in tube 101 provides an additional advantage in that a standard low-voltage power supply 130 (FIGURE 1) can be placed between it and electrode 102 and a standard high-voltage power supply 131 can 4 be placed between electrodes 102 and 103'. Standard power supplies have a slower time-rising potential at a lower voltage and a faster time-rising potential at a higher voltage. The novel tube 101' has another advantage in that the rate of the rise of potential between

electrodes

102 and 103 does not need to be limited in order to start ionization. Therefore the current in tube 101' can be controlled more accurately than in tube 101. The time-rising potential in tube 101 must be limited in value because the gases in tube 101 must have time to ionize.

Although in the preferred embodiment a wire-mesh grid 122 has been shown at a particular location .between electrodes 102' and 103', and 'has been described as being connected to a potential which makes it the 35 percent equipotential between the electrodes, the grid 122 can be an annular electrode or a radial vane type grid and it can be positioned closer to or further away from electrode 102'. However, if positioned closer to electrode 102', the .grid may have to be connected to a potential which is equal to less than 35 percent of the potential dilerence between electrode 1012' and 103. The essential requirement is that the shape, position, and potential of grid 1-22 are such that it causes a lower voltage .gradient within the well of electrode 102' than would be present without the grid.

I claim:

1. A gas tube comprising an envelope, two spaced electrodes disposed within said envelope, one of said electrodes being a cold cathode and the other being an anode, a gas at a pressure .below critical pressure in said envelope, said gas pressure being below critical pressure for all distances between surfaces of said two electrodes inside said envelope, and third electrode means within said envelope for forming a voltage gradient which is lower adjacent said anode than adjacent said cathode.

2. A .gas tube comprising an envelope, two spaced nonemissive electrodes disposed within said envelope, means for biasing one of said electrodes positive with respect to the other, a gas at a pressure below critical pressure in said envelope, said gas pressure being below critical pressure for all distances between surfaces of said two electrodes inside said envelope, a third electrode permeable to charged particles positioned between said .two spaced electrodes, and means for biasing said third electrode at a p0- teutial intermediate said two spaced electrodes, said one positive electrode being lcup-shaped with the well of the cup facing the other of said two spaced electrodes.

3. A gas tube comprising an envelope, two spaced electrodes disposed within said envelope, one of said electrodes being a cold cathode, a gas at a pressure below critical pressure in said envelope, said .gas pressure being below critical pressure for all distances between surfaces of said two electrodes inside said envelope, a grid cornprising a plurality of spaced wires positioned between said two spaced electrodes, means for providing a substantially large potential difference between said two spaced electrodes, and means for biasing said grid at a potential intermediate said two spaced electrodes.

4. A gas tube comprising an envelope, a gas at a pressure below critical pressure in said envelope, two spaced non-emissive electrodes disposed within said envelope, and non-ernissive electrode means within said envelope for forming a voltage gradient area in said tube which is less than any voltage gradient area which exists in the absence of said'electrode means, said gas pressure being vbelow critical pressure for al1 distances between surfaces of said two spaced electrodes inside said envelope.

5. A gas tube comprising a tubular dielectric insulator, a irst electrode sealed to one end of said insulator, a. second electrode sealed to the other end of said insulator, said tubular insulator and said first and second electrodes forming an envelope, a gas at a pressure below critical pressure in said envelope, and third electrode means within said envelope for forming a voltage :gradient which is lower adjacent one of said yfirst and second electrodes than adjacent the other of said first and second electrodes.

6. A gas tube comprising a tubular dielectric insulator, a iirst electrode sealed to one end of said insulator, a second electrode sealed to the other end of said insulator, said tubular insulator and said first and second electrodes forming an envelope, a gas at a pressure below critical pressure in said envelope, an electrical lead-through disposed between the ends of said insulator and protruding through said insulator to form a terminal, and a grid mounted on said lead-through.

7. The gas tube of claim 6 wherein said iirst electrode is a positive electrode and is cup-shaped with the well of the cup facing said sec-ond electrode.

8. The gas tube of claim 7 wherein said grid is convex on the side facing the well of said cup-shaped electrode.

9. A gas tube comprising a iirst tubular dielectric insulator; a cup-shaped electrode sealed to one end of said rst insulator and disposed with the well of the cup facing the other end of said irst insulator; a terminal ring sealed to the other end of said lirst insulator; a second tubular dielectric insulator sealed at one end to said terminal ring and axially aligned with said irst insulator; a second electrode sealed to the other end of said second insulator; said cup-shaped electrode, said first insulator, said terminal ring, said second insulator, and said second electrode forming a ygas-tight envelope; a grid disposed Within said envelope and mounted on said terminal ring; and a gas at a pressure below critical pressure in said envelope,

10. Gas tube apparatus comprising a tube having an envelope, two spaced electrodes disposed within said envelope, one of said electrodes being a cold cathode, a gas at a pressure below critical pressure in said envelope, said gas pressure being below critical pressure for all distances between surfaces of said two electrodes inside said envelope, power supply means for providing a substantially large potential difference between said two electrodes which will not start a discharge between said two electrodes in the absence of means for changing the voltage `gradient formed between said two electrodes, and additional electrode and power supply means for forming an area between said two electrodes having a voltage gradient which is substantially lower than the voltage gradient formed by said two electrodes alone, and the voltage gradient formed between said two electrodes by the action of all three of said electrodes causing a discharge between said two electrodes.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Conduction of Electricity Through Gases, J. I. and G. P. Thompson, University Press, Cambridge, Great Britain, 1933, pp. 481, 482, Vol. II.

IAME'S W. LAWRENCE, Primary Examiner.

RALPH G. NILSON, GEORGE N. WEST'BY, JOHN W.

HUCKERT, Examiners.

C. R. CAMPBELL, Assistant Examiner.

Claims

3. A GAS TUBE COMPRISING AN ENVELOPE, TWO SPACED ELECTRODES DISPOSED WITHIN SAID ENVELOPE, ONE OF SAID ELECTRODES BEING A COLD CATHODE, A GAS AT A PRESSURE BELOW CRITICAL PRESSURE IN SAID ENVELOPE, SAID GAS PRESSURE BEING BELOW CRITICAL PRESSURE FOR ALL DISTANCES BETWEEN SURFACES OF SAID TWO ELECTRODES INSIDE SAID ENVELOPE, A GRID COMPRISING A PLURALITY OF SPACED WIRES POSITIONED BETWEEN SAID TWO SPACED ELECTRODES, MEANS FOR PROVIDING A SUBSTANTIALLY LARGE POTENTIAL DIFFERENCE BETWEEN SAID TWO SPACED ELECTRODES, AND MEANS FOR BIASING SAID GRID AT A POTENTIAL INTERMEDIATE SAID TWO SPACED ELECTRODES.