US3466534A - Method for determining filament condition by continuously evaluating current flow therethrough - Google Patents

Description

Sept. 9, 1969 METHOD FdR DETERMINING Filed Aug. 19, 1966 I I I L FILAMENT CURRENT/ LIFE I}; 90 '0 -3 g 2700| I 2700K g 80 END OF LIFE-- 3 2745K a v N 70 2 E 2940K\ o 60 z TIME (HOURS),

IOO I I EFFECT OF VACUUM ON LIFE \ONSTANT VOLTAGE Q5 2700K 2 ii -5 IO mm Hg. 3 O

O LU N 3 2 g 0 2 k TIME (HOURS) I 26 CONSTANT VOLTAGE I SUPPLY Q 1 I I6 INVENTOR. I E l KURT w WALLACE VOLTAG 32 34 /7 1/1 11/ SUPPLY gm; 36 BY ATTORNEY United States Patent 3 466,534 METHOD FOR DETERlVIINING FILAMENT CONDI- TION BY CONTINUOUSLY EVALUATING CUR- RENT FLOW THERETHROUGH Kurt F. Wallace, Redwood City, Calif., assignor to Ampex Corporation, Redwood City, Calif a corporation of California Filed Aug. 19, 1966, Ser. No. 573,670 Int. Cl. G01r 31/22 U.S. Cl. 324-20 1 Claim ABSTRACT OF THE DISCLOSURE The present invention relates to a method and apparatus for determining the end of life of a tungsten hairpin cathode such as used in demountable electron optical systems.

The conventional method of ascertaining the end of life of a tungsten hairpin cathode is to either wait for the filament to burn out or to have a time clock running and make an estimate of the end of life by going by previous experience. The main disadvantage of the time clock method of determining the filament end of life is that small variations in temperature of the filament and variations in pressures of the vacuum enclosure have a predominant eifect on the overall life of the filament, and thus the estimated life of a filament due to the particular operating conditions, may not correspond to the usual known life. For example, an error in temperature of 100 C. will cause a decrease in the life of a filament amounting to 40-50 hours at a particular operating temperature.

The present invention overcomes the above shortcomings of the prior art by providing a method and apparatus for increasing the life span of a filament, and for determining the true condition of the filament irrespective of its operating temperature and pressure.

Accordingly, it is an object of the present invention to provide a method and apparatus for increasing the life span of a cathode filament generally on the order of times over that of a filament operated under constant current conditions.

It is a further object of the present invention to provide a means for ascertaining the end of life of a tungsten hairpin cathode filament such as the type used in demountable electron optical systems.

It is another object of the present invention to provide a method and apparatus for determining the true condition of the filament during its operating life.

It is yet another object of the present invention to provide an apparatus for operating a filament under conditions of constant voltage while continuously monitoring the current flow therethrough to thus sense the condition of the filament.

It is still another object of the present invention to provide a method and apparatus for determining the true condition of a filament by watching the drop in current due to evaporation of the filament while maintaining a constant voltage thereacross.

These and other objects and advantages of the inven- Patented Sept. 9, 1969 ice tion will be apparent from the following specification, taken in conjunction with the drawings in which:

FIGURES 1 and 2 are graphs of results obtained in experimental tests, illustrating the normalized current change in filament in relation to operating life under various conditions of temperature and vacuum, respectively.

FIGURE 3 is a schematic diagram partially in block form showing an embodiment of the apparatus of the present invention.

It is well known in the art that the life of a filament is extended by operating the filament at constant temperature. In order to obtain such conditions of constant temperature, the temperature of the filament can be continuously monitored and fluctuations therein can be corrected by optimizing the electrical inputs thereto. However, such a process is impractical for various reasons; i.e., generally because the working space in which optical measurements of temperature can be made is inadequate. Therefore prior art systems generally operate the filament at a constant current in an attempt to approach constant temperature working conditions for the filament tip. However, when the filament is run at constant current, the temperature increases as the filament thins due to evaporation and the life-time of the filament is decreased due to the increased operating temperature.

Rather than attempting to maintain the temperature of the filament constant by maintaining the current constant, the present invention employs the unique concept of operating the cathode filament at a constant voltage, which provides a condition very nearly approximating the constant temperature conditions desired for filament operation. By operating the filament in accordance with the invention concepts it is found that cathode filament life has been increased by 10 times over the life of filaments operated under constant current conditions. In conjunction with operating the filament at constant voltage, the filament life may be further optimized by preselecting the length of the filament and the construction surrounding the filament to thus control the heat losses therefrom due to radiation and conduction.

Furthermore, as shown in FIGURE 1 by operating the filament at constant voltage and then monitoring the current, the life of the filament may be predicted. Thus, it has been found that when the normalized current shown in FIGURE 2 decreases to the order of the filament is nearing the end of its useful life. The decrease in normalized current shown in FIGURE 1 in essence corresponds to the decrease in the thickness of the filament as it evaporates, and thus the 20% decrease in normalized current corresponds essentially to the 10% loss of thickness of the filament. Accordingly, as taught iby the invention, the evaporation of the filament causes a drop in the filament current which in turn is 'a very good indication of the life of the filament. When approaching the arbitrary point, herein chosen by way of example only as 80% of the initial normalized current, the filament has reached approximately of its operating life irrespective of the temperature and pressure conditions under which it is operating.

Referring to FIGURE 2 it may be seen that the drop in filament current indicative of current life is also obtained When the filament is run under poor vacuum conditions. This may be seen by referring to the curve depicting the life test of a filament operated at the poor vacuum of 10' mm. of mercury. This curve shows, as does the other curves, that upon approaching 80% of the normalized current the filament is approaching the end of life, although it has run for only approximately 10 hours. Thus, even though the poor vacuum conditions may be unsuspected by the operator, the true condition of the filament, which would generally operate much longer than hours, is positively indicated. Accordingly, the present invention provides a filament life modulating scheme which is much preferred over the usual time clock scheme which does not indicate the vacuum history or temperature history of the filament.

Accordingly, referring now to FIGURE 3 there is depicted apparatus capable of providing a constant voltage across a cathode filament while constantly monitoring the current passed therethrough, to continuously determine the operating condition of the filament and thus indicate at any time the remaining life thereof. By way of example only, there is shown a portion of a conventional electron microscope gun 12 which includes an anode 14, a grid 16 and a cathode filament 18. The filament 18 is serially connected across a constant voltage supply 20 and a feedback path 22 is provided, whereby the voltage supply to the filament is regulated. A current meter 24 is connected in series with the filament 18 and is supplied with a shunting variable resistor 26 having a manually adjustable slider contact 38. At such time as a new cathode filament 18 is installed within the electron gun 12, the current meter 24 is adjusted to place a dial 30 thereof on the set position, :by adjusting the manually operable slider 28 of the shunt resistor 26. The electron gun 12 is maintained at the desired anode-cathode operating potentials by means of conventional voltage supply means 32, which is connected to one input lead to the cathode filament 18. A grid bias means 34 is connected from the same side of the cathode filament l8 and thence to the grid 16 within the electron gun 12, to provide the desired operating bias for the grid. In the system as shown, the anode 14 is connected to ground 36 and the voltage supply means 32 comprises accordingly a negative source of voltage.

By way of example only, the constant voltage supply 20 may be any voltage supply of the conventional type having an output of 23 amps and l.43 volts, which will thus supply a voltage to the filament 18 of a constant value within the range of from 1.4-1.7 volts. The current meter 24 is a conventional current meter reading up to, for example, 3 amps.

Although the present invention has been described herein with regards to a single embodiment it is to be understood that various modifications may be made thereto within the scope of the invention.

For example, the constant voltage supply 20 could be .4 replaced by a voltage supply which is not held constant, and a voltmeter could be placed across the latter supply, any fluctuations in voltage could then be compensated for by adjusting the value of a resistor or rheostat placed in series with the supply. Additionally, the current meter 24 could be set upon installation of a new filament 18 by an adjustable scale thereon, rather than by use of a fixed End of Life-Set scale and the adjustable shunting resistor 26 shown in FIGURE 3. Thus it is not desired to limit the invention except as defined by the following claim.

What is claimed is: 1. A method for continuously determining the condition of a replaceable tungsten hairpin cathode filament of the type used in demountable electron guns during the operation thereof comprising the steps of:

adjusting the initial current flow through the filament upon assembly thereof within the electron gun to provide an initial reference current value;

determining a selected value of monitored current with respect to the initial reference current value, such selected value being of the order of and indicative of the general useful lifespan of the filament;

continuously maintaining a constant operating voltage commensurate with the initial reference current value across the filament during the operation thereof; and

monitoring and indicating the current flow generated in the filament in response to the applied constant voltage until the indicated current value equals the selected value of 80% of the monitored current, to determine the degree of evaporation and thus the end-of-life of the filament and thereby indicate the need for replacement of the filament.

References Cited UNITED STATES PATENTS 2,082,036 6/1937 Stogoff 315-94 XR 3,005,949 10/1961 Oakes 324-20 3,040,244 6/1962 Owens 32424 RUDOLPH V. ROLINEC, Primary Examiner p E. L. STOLARUM, Assistant Examiner US. Cl. X.R. 324-62

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