US2996643A

US2996643A - Art of heating electron tube cathodes

Info

Publication number: US2996643A
Application number: US827598A
Authority: US
Inventors: Frederick C Johnstone; Jr Jackson W Kendall
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1959-07-16
Filing date: 1959-07-16
Publication date: 1961-08-15
Anticipated expiration: 1978-08-15
Also published as: FR1262893A; GB923183A; CH366602A; NL250741A

Description

Aug. 15, 1961 F. c. JOHNSTONE ETAL 2,996,643

ART OF HEATING ELECTRON TUBE CATHODES Filed July 16, 1959 FIG! 2.

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A 7' TORNEYS fornia Filed July 16, 1959, Ser. No. 827,598 Claims. (Cl. 315-405) This invention relates generally to the art of heating an electron tube cathode and more particularly to a method of and means for rapidly heating an electron tube cathode.

In all electronic equipment using vacuum electron tubes the largest quantity of power is consumed in heating the cathode. Normally this is not an objection if power is readily available, but if the equipment is mobile such as pack carried radio units power is not easily available. Therefore steps must be taken to conserve power.

Transistor radio units are used to conserve power but transistors due to their inherent properties are only useful in receivers and limited power transmitters. Electron tubes with heated cathodes are the only known signal amplifiers that can be used in high power transmitters. Normally to conserve power one would turn off the equipment since keeping the cathode warm consumes a major portion of the available power. Then there would be a time lapse between turning on the equipment and readiness of operation. This time lapse is longer in transmitters with higher power tubes than in transmitters with lower power tubes.

Although directly heated cathodes have a shorter time lapse than indirectly heated cathodes, directly heated cathodes have disadvantages which interfere with proper operation of high power equipment. Some of the disadvantages of directly heated cathodes are that they are not uni-potential, they tend to produce an A.-C. hum if they are heated by alternating current, they have limited emission surfaces, so they have low rate of emission per watt of heating power. Also, since directly heated cathodes are preferably made from very thin wire or filaments they are mechanically unstable in that any jarring of the equipment will change the close spacing between the cathode and grid. Filaments are usually made of a brittle refractory metal such as tungsten and will break under repeated vibrations. Indirectly heated cathodes overcome the above disadvantages of directly heated cathodes, but as stated they have the dis-advantage of a relatively long warm up period between turning on and readiness for operation of the equipment.

It has been suggested in the prior art thatthe disadvantage of indirectly heated cathodes can be overcome by initially passing sufficiently large currents through the cathode body surface to bring it rapidly up to emission temperature by resistanceheating. The cathode is then automatically disconnected from the power supply and is heated by radiation from the filament. Since the total resistance across the cathode body is low, a large current, and therefore a large power supply is required to rapidly bring the cathode structure up to operating temperature.

It is an object of the present invention to provide an improved fast heating cathode using a moderate power supply and to provide a method of operating the same.

It is another object of the present invention to provide a fast heating cathode in which. the cathode is heated initially by electron bombardment.

It is a further object of the present invention to provide an indirectly heated cathode structure for electron tubes which includes an emissive filament and an emissive cathode structure.

It is still a further object of the present invention to ice provide an electron tube power supply which rapidly heats an indirectly heated cathode of a multi-electrode tube to operating temperature.

It is yet another object of this invention to provide an electron tube power supply which rapidly brings a watt plate dissipation tube to operating temperature in less than one second.

These and other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawmg.

Referring to the drawing:

FIGURE 1 is a view, partly in section, showing an indirectly heated cathode structure;

FIGURE 2 shows a circuit suitable for operating an electron tube in accordance with the present invention; and

FIGURE 3 is a graph showing the power input to the filament, the cathode bombardment power, and the plate current for a tube in accordance with the present invention.

In FIGURE 1, there is illustrated a cathode assembly. The cathode assembly includes a cup-shaped cathode 11 which is held by a cathode support 12. The outside surface of the cathode 11 is coated with an emissive material, for example, an oxide coating 13. Disposed Within the cathode cup there is a filament structure designated generally by the reference number '14. The filament indirectly heats the cathode 11 to cause emission from the oxide coating 13. The filament structure illustrated includes an axial post 16 which supports one end of a helically wound filament -17 and a second post 18 supports the other end of the filament. Filament current is applied to the filament through the

posts

16 and 18.

In accordance with the present invention, the filament '17 is made of refractory material such as thoriated tungsten or tungsten which thermionically emits electrons when it reaches a predetermined temperature. A carburized coating 19 is applied to the filament 17 to im prove its electron emission qualities.

Operation of the cathode assembly illustrated in FIG- URE 1 is as follows: Suitable voltage is applied between the

posts

16 and 18 to cause current to flow in the filament 17. The filament 17 rapidly reaches a temperature sufficient to cause thermionic emission. A voltage applied between the filament 17 and the cathode cup 11 accelerates the emitted electrons from the filament 17 to the inner surface of the cathode cup. The electrons strike or bombard such inner surface releasing their kinetic energy and causing rapid heating of the cathode. Suitable circuitry is provided for removing the voltage applied between the cathode and filament after the cathode reaches electron emission temperature. Thermal radiation from the filament 17 then maintains the cathode at operating temperature and the tube operates in a conventional manner.

In essence, the filament and cathode cup form a diode in which the cathode cup serves as an anode and is bombarded by electrons emitted by the filament.

Referring to FIGURE 2, a circuit suitable for programming the application of power to the cathode assembly is illustrated. A D.-C. voltage source V connected to

terminals

21 and 22, charges a capacitor 23 through a dropping resistor 24. The capacitor 23 is connected between the filament 17 and the cathode 11. The filament may be heated from an A.-C. power source V applied across the

primary terminals

26 and 27 of a transformer 28. Initially, a switch 29 in the transformer is in a position as indicated whereby a relatively high voltage is applied by the secondary of the transformer to the filament 17 to rapidly heat the same. When the filament reaches a temperature at which it Patented Aug. 15, 1961 starts to emit, the energy stored by the capacitor 23 causes an avalanche of these emitted electrons from the filament to bombard the cathode. The electron flow discharges the capacitor and rapidly heats the cathode. After the filament begins to emit, the switch 29 is programmed by suitable circuitry (not shown) to switch to its alternate position, applying three volts to the filament and thus conserving power but maintaining the cathode at operating temperature.

The capacitor 23 must store a given amount of energy so that electron bombardment of the cathode will raise the cathode to emission temperature. To rapidly heat the cathode to emission temperature, it is preferable to have a high D.-C. voltage across the capacitor 23. Since electrical energy is equal to volts times amperes times seconds (E=VIT) and the emission rate (I) for the filament tends to increase with the voltage, a higher voltage (V) provides the required heating energy (E) in a lesser period of time (T).

The resistor 24 should have a sufiicient value so that a small current flow will produce a large voltage drop thereacross to prevent excessive heating of the cathode by the electron bombardment after the capacitance is discharged. Also, the resistor 24 should have a value so that the capacitor will charge at a rate that approximates the rate at which the cathode loses heat during a cooling cycle. This automatically prevents excessive supply of energy to the cathode when it has not completely cooled before being re-energized.

If the filament happens to emit excessive electrons during normal tube operation, overheating the cathode because resistor 24 has too low a value which was determined by the rate of cooling of the cathode, the series circuit comprising the capacitor, filament and cathode may be opened by a switch 31.. One should understand that the invention may operate without the need of switch 31. The

switches

29 and 31 may be the contacts of a relay or other suitable control devices in a control or programming circuit (not shown). Other circuits can be employed for carrying out the operations just as described. However, the circuit shown carries out the required programming with simplicity.

FIGURE 3 is a graph showing filament power, cathode bombardment power, and plate current as a function of time. The curves were obtained by replacing the conventional cathode structure in an Eitel-McCullough vacuum tube, known by manufacturing specifications as the 4X150A, with a cathode structure of the type described including a carbonized thoriated tungsten filament. The capacitor 23 was 260 microfarads, and the resistor 24 was 50,000 ohms. Switch 31 was not required in this circuit. The curve shown in FIGURE 3 shows the programming of the circuit.

A voltage (V of approximately 800 volts was applied through the resistor 24 across the capacitor 23 and between the filament 17 and the cathode 11. A voltage (V of approximately 110 volts was applied across the primary of the transformer 28. At the time corresponding to time zero on the graph of FIGURE 3 the switch 29 was thrown to the position shown, applying approximately 6 volts to the filament. Since six volts is in excess of the voltage required to heat the filament, the filament was rapidly heated to a temperature higher than its operating temperature. At the end of .25 second, and before deleterious results could occur to the filament, the voltage was reduced to 3 volts by throwing the switch 29 to its alternate position. Referring to the graph, it will be seen that before the end of .25 second the filament has already begun to emit electrons to the cathode, causing the cathode bombardment power to increase, as indicated. The electron emission of the filament increased rapidly to a temperature limited value, and at the end of .35 second the cathode bombardment power reached a maximum value of over 200 watts. It

will be seen that the cathode will start to emit electrons to the plate at the end of .3 second. The cathode bombardment power immediately begins to decrease from its maximum value as the capacitor 23 is discharged by the current flow between the filament and the cathode reducing the voltage between the filament and the cathode. The break in the cathode bombardment power occurring at the end of approximately .8 second indicates that the filament has cooled from the high temperature imparted by the inital 6 volt pulse so that it is no longer operating temperature limited but instead the emission of the filament is decreasing not only as a function of the decreasing voltage of the capacitor 23 but as a function of temperature of the filament. It will be seen from the graph that the cathode emission or plate current increases steadily after .3 second to full value of approximately 175 milliamperes in less than one second. The 4X150A tube, as manufactured by Eitel-McCullough to meet Government specification MIL-E-l/ G, dated April 15, 1959, requires a minimum of 30 seconds to heat the cathode to operating temperature.

This it is seen that an improved cathode assembly and method of operating the same is provided using a moderate power supply. The novel cathode assembly permits turning off of the equipment during stand-by periods and yet the equipment is in readiness for almost instantaneous use upon turning on the power. The novel circuit allows a high rate of energy to be obtained from a moderate power source to rapidly heat a cathode. From simple mathematical calculations, one sees that the power source V delivers energy at a maximum rate of about 12 watts. The electrical energy from the power source is stored in the capacitor 23 or other convenient means. When the capacitor discharges it delivers energy at a rate of in excess of 200 watts for a short period as illustrated by the cathode bombardment power curve.

We claim:

1. In combination an electron tube comprising an emissive filament, an emissive cathode, and an anode, means for heating said filament to a high enough temperature to cause it to emit electrons, and means for applying a time decreasing voltage between said filament and said cathode to cause time decreasing electron bombardment of said cathode.

2. In combination an electron tube comprising an emissive filament, an emissive cathode, and an anode, means for heating said filament to a temperature high enough to cause it to emit electrons, means for applying a time decreasing voltage between said filament and said cathod to cause time decreasing electron bombardment of said cathode, and means for decreasing the temperature of said filament to a temperature which will maintain said cathode at emissive temperature.

3. In combination an electron tube comprising an emissive filament, an emissive cathode, and an anode, means for supplying relatively large electrical power to said filament whereby the filament rapidly reaches electron emission temperature, means for applying a relatively high voltage between said filament and said cathode to cause electron bombardment of said cathode, means for reducing said voltage simultaneously with the occurrence of said electron bombardment, and means for reducing electrical power applied to said filament to a value merely suflicient to maintain said cathode at emissive temperature, the values to which said voltage and electrical power are reduced being such that the filament does not bombard the cathode with electrons.

4. In combination an electron tube comprising an emissive filament, an emissive cathode, an anode, a D.-C. voltage source electrically connected between said cathode and said filament through a resistor whereby said cathode has a positive potential with respect to said filament, a capacitor electrically connected between said cathode and said filament, and said resistor being serially connected between said capacitor and said voltage source.

5. In combination an electron tube comprising an emissive filament, an emissive cathode, an anode, a D.-C. voltage source electrically connected between said cathode and said filament through a resistor whereby said cathode has a positive potential with respect to said filament, a capacitor electrically connected between said cathode and said filament, said resistor being serially connected between said capacitor and said voltage source, and means for applying a high power for a predetermined time to said filament for rapidly heating said filament to emission temperature.

6. An electron tube comprising a filament, a cathode and an anode, said filament being non-emissive when heated with a normal operating current to a normal operating temperature, said filament being emissive when heated with a higher current than said normal current to a higher temperature than normal temperature, and said filament maintaining said cathode at electron emissive temperature when said filament is at normal operating temperature.

7. In combination an electron tube comprising an emissive filament, an emissive cathode which has a given cooling rate, an anode, a D.-C. voltage source electrically connected between said cathode and said filament whereby said cathode has a positive potential with respect to said filament, a capacitor electrically connected between said cathode and said filament, and resistor means between said D.-C. voltage source and said capacitor for controlling the capacitors charging rate to correspond to said cooling rate of said cathode.

8. In combination an electron tube comprising an emissive filament, an emissive cathode, and an anode, means for storing electrical energy, means for heating said filament to a high enough temperature to cause it to emit electrons, and means for applying said stored electrical energy between said filament and said cathode whereby electrons emitted from said filament are accelerated towards said cathode to cause heating of said cathode and dissipation of said stored electrical energy, and means for decreasing the temperature of said filament to a temperature which will maintain the cathode at emission temperature by'radiation rather than by electron bombardment.

10. In combination an electron tube comprising an emissive filament, an emissive cathode having a given cool ing rate, and an anode, means for storing electrical energy at a rate substantially equal to said cooling rate, means for heating said filament to a high enough temperature to cause it to emit electrons, and means for applying said stored electrical energy between said filament and said cathode whereby electrons emitted from said filament are accelerated towards said cathode causing rapid heating of said cathode by electron bombardment and rapid discharge of said stored electrical energy.

References Cited in the file of this patent UNITED STATES PATENTS 1,787,300 Alexanderson Dec. 30, 1930 1,929,369 Found Oct. 3, 1933 2,509,053 Calbick May 23, 1950 2,552,047 Kurshan May 8, 1951 2,774,916 Katz Dec. 18,