US3197634A - Delayed warm-up filament power supply for mass spectrometer with emission current regulation - Google Patents

Description

y 1965 R. WATTERS 3,197,634

DELAYED WARM-UP FILAMENT POWER SUPPLY FOR MASS SPECTROMETER WITH EMISSION CURRENT REGULATION Filed Oct. 30, 1963 [/3 [Q Electron Collector 3 H Electric Power Z -l flegu/afor and Ourpul Mass Spectrograph 2 Pane! Source 4 9 Filament Auxiliary J: 8 l6 6 7 PowerSaurce T /5 30 7'0 Filament I TOE/ec/r0n I Collector I c 0 l 50 I 5/ I 3 J 3 F 'g a I S lnvenfor Robe/l L. Walters N Time, in. His Attorney.

United States Patent DELAYED WARP/LU? FEAMENT PGWER SUPPLY The present invention relates to protective means for thermionic electron-emissive filaments, and more particularly pertains to means for increasing the useable lifetime of such filaments.

The mass spectrometer has proved to be a valuable analytical tool in many fields of scientific endeavor. In the mass spectrometer a small thermionic electron-emissive filament is used to provide a source of electrons which ionize particles in the electron stream. The ionized, or charged, particles are thereafter accelerated under the infiuence of an electricfield into a magnetic field where the particles follow a curved trajectory. The curvature of the trajectory depends primarily upon the velocity of the particle, the magnetic field strength, and the means of the particle. For a given field strength and velocity, heavier particles curve less than lighter particles. Hence, the particles are separated according to their masses. Suitable indicating means are used to measure the relative number of each weight of particle present, and this is related to the atomic structure of the particles. Thus, the mass spectrometer is used to analyze the elements and compounds present in the vicinity of the filament.

In the past, numerous difficulties have been encoun tered in mass spectrometers because of early filament failure. For this reason the devices are frequently supplied with dual filaments which may be used successively. Still, the total filament life is short, the cost of replacement high and the time required for replacement long. For example, the normal time required for filament replacement is in excess of three weeks, which is a relatively long time to have expensive equipment inoperative. There is a definite need for means to provide longer life for the filaments in mass spectrometers.

Accordingly, it is a primary object of this invention to provide means for increasing the lifetime of filaments in mass spectrometers.

it is another object of this invention to provide increased lifetime for the filament in mass spectrometers by means independent of operator error.

Briefly, I discovered that the reduced lifetime of filaments in mass spectrometers is due in large part to the high magnitude of filament current demanded by the emission current regulator when the mass spectrometer is initially energized. Accordingly, with the present invention, I provide an emission current regulated power supply for the filament which delays the application of full power to the filament. for a predetermined time during which a gradual increase in power occurs. The time delay is accomplished, preferably, with a source of gradually increasing voltage which is used to supplement the electron collector input signal to the control element of the filament power supply. After a predetermined time lapse, the auxiliary voltage stabilizes at a constant value, in order to minimize any disturbance of the emission current control function, and more particularly to avoid adversely affecting the rapid response time thereof.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be under- 3,197,634 Patented July 27, 1965 stood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a block diagram which shows the system organization of the emission current regulated power supply of my invention;

FIGURE 2 is a schematic circuit diagram of a specific emission current regulated power supply, which I prefer to use, in accordance with the present invention; and,

FIGURE 3 is a graph of filament current versus time using the power supply of FIGURE 2.

It is known that the thermionic, electron emissive filaments of mass spectrographs normally have a relatively short useful life. It is also known that the expense of replacing the filament in mass spectrograph tubes is approximately /5 the cost of a new tube, for the smaller models, and that ordinarily such replacement may not be effected in the laboratory where such equipments are used, necessitating shipment to a repair facility. In most cases, the cost of mass spectrograph devices is so high that a standby, or substitute, cannot be justified. Accordingly, premature filament failure results in high replacement expense and a long time delay during which the equipment is inoperative.

it is known that the useful lifetime of relatively long, thin electron emissive filaments oftentimes can be extended by providing a slow warm-up or reduced voltage operation during initial energization. The reason for this is that the mechanical shock of the initial inrushing current is frequently the cause of a mechanical failure or fracture, of the filament. The initial current surge in tungsten, for example, is sometimes as high as 15 times the normal current at the high operating temperature. A common example of such mechanical failure is to be found with the incandescent filament of the ordinary light bulb. Such light bulbs fail, almost without exception, during initial energization thereof and very rarely expire during sustained operation.

Although it was known that current surge could create mechanical problems in filaments having a relatively long length relative to their cross sections, it was not considered, prior to my discovery thereof, that initial current inrush caused the reduced useful life in the filaments of mass spectrometers. One reason for this is that ion gauges, which utilize similar emission current regulated filament supplies, provide long useful lives with relatively long thin filaments. The filaments of mass spec trometers. are relatively short and have thick cross sections which would be thought normally to minimize breakage by mechanical stress due to current surge.

In the light of my discovery that reduced filament power during the warm-up period increases mass spectrometer filament lifetime, it appears that filament failure previously was caused by a lack of thermal equilibrium throughout the filament during the warm-up period. Thus, the relatively large electrically conductive means, which are normally connected to the ends of the low voltage, high current filament, provided a high capacity eat sink which maintained the ends of the filament much cooler than the center thereof. Thus, when the emission current regulator sensed the reduced emission, which normally occurs during warm-up, full filament power was applied to the filament and the energy thereof dissipated primarily in the central portion of the filament. In this way, the central portion of the filament was reduced in cross section by loss of material therefrom as a consequence of the high energy dissipated therein. Thereafter, the central portion of the filament exhibited a higher resistance than the end portions, resulting in a further unbalance of power dissipation even after warmup had been completed. Thus, even when a relatively small reduction in cross section occurred during warm up, the deleterious effect was compounded during normal operation.

It should be understood that'the foregoing explanation isofiiered as the most likely cause of prior filament failures and the present invention is not dependent upon the correctness thereof. The significant fact is my discovery that gradually increased power during warm-up markedly increases the useful lifetime of the high current, low voltage filaments used in mass spectrometers.

FIGURE 1 illustrates, by a block diagram, an emission current regulated power supply in accordance with the present invention. The power supply includes an electric power regulator 1 which has a source of electric power and control means. Control terminal 2 provides means for supplying a power input to regulator 1. Preferably, regulator 1 is responsive to variations in the magnitude of electric power supplied to control terminal 2 to provide a variation in the same direction in the magnitude of electric power available at the regulator output means, conveniently shown as terminals 3 and Regulator 1 may be any of a plurality of current controlling means including a magnetic amplifier, vacuum tube or transistor power amplifier, and a thyratron or silicon controlled rectifier power controlling circuit. I prefer to use a transistor series regulator using a power transistor because such devices are particularly well suited for low voltage, high current regulation as required for use with mass spectrometer filaments.

Electrically conductive means, which may conveniently be provided by high current capacity braided copper straps, are shown schematically as conductors 5 and 6. Conductors 5 and 6 electrically connect regulator output terminals 3 and 4 to the terminals '7 and 8, respectively, of thermionic, electron-emissive filament 9. Filament 9, of mass spectrograph 1Q, normally has a length which is in the same order of magnitude as its cross-sectional di mension. This configuration is required to provide a concentrated source of electrons and carries with it the additional requirement that the "filament be a low voltage high current device. Filament 9 provides a quantity of electrons by thermionic emission which varies in mag-- nitude in the same direction as variations in the magnitude of power delivered from output terminals 3 and 4, as is well-known in the art.

Spectrograph 10 includes an electron collector 11 which receives substantially all of the electrons emitted by filament 9. Collector 11 is electrically connected to collector terminal 12 externally of the envelope of spectrograph 10. Conductive means 13,'which may be, for example, a copper wire, electrically connect collector electrode 12 to control terminal 2 of regulator 1.

In accordance with the present invention an auxiliary source of electric power 14 is provided having output 15 thereof connected, by conductive means 15 to control terminal 2 of regulator 1. Source 14 provides a power 'output at terminal 15 which gradually increases between the time of energization thereof and a predetermined time at which full output power capability is reached. The output power increases gradually during the delay period, providing an output not unlike that provided by what is known in the art as a ramp generator, owing to the shape of the voltage waveform. At the end of the delay period, when the predetermined time has expired, the power output from source 14 is essentially constant.

Operation of the system of FIGURE .1 is as follows. When the system is initially energized, there is no power output from source 14 to control terminal 2. Also, no electrons are being emitted from filament 9 to be collected by electron collector 11 and, consequently, no power flows from terminal 12 to control terminal 2. Thus,

regulator. 1 is supplying no power to filament 9. There after, an increasing magnitude of power is supplied by source 14 to control terminal 2. In response to the input power to control terminal 2, regulator ll generates an output between terminals 3 and 4 which energizes filament 9. Thus, the filament power is applied gradually until a predetermined time after which the output from source 14 is essentially constant. Control is then relinquished to electron collector 11. The power flowing in the circuit including collector 11 is arranged to have a much lesser influence on regulator 1 than the power supplied by source 14, and the power in the electron collector circuit serves to regulate the emission from filament 9.

The power in the electron collector circuit is arranged to provide a variation in the opposite direction from the power supplied by source 1.4. This may be accomplished, conveniently, by providing a source 14 having a positive polarity output since, then, an increased quantity of electrons to collector 11 results in a less positive potential applied to control terminal 2. Thus, as the emission from filament 9 increases, the control power to terminal 2, and regulator 1, decreases, reducing the power to and emission from filament 9. In this way, the emission current is regulated, or maintained substantially constant. The power supplied to control terminal 2 by electron collector 11 may be considered as providing a small modulation of the power supplied to control terminal 2 by source 14.

FIGURE 2 is a schematic circuit diagram of an emission current regulated power supply which I prefer to use in the practice of my invention. The electric power regulator 1 is a series transistor regulator. The power source for regulator 1 includes a transformer 20, full- :ave rectifying bridge 21 electrically connected thereto, and current filtering means provided by resistor 22 and capacitor 23.

Regulation is provided by control of power transistors 25 which have, preferably, load balancing resistors such as resistors 26 and 27 connected in series in their emitter circuit paths. Of course, one transistor will suffice in many lower power applications and others, requiring higher power, may advantageously include more than two.

Transistor 28, which is electrically connected in series in the base circuits of transistors 24 and 25 controls the impedance between the principal electrodes (collectors and emitters) of the power transistors. The circuit connection is such that as the base electrode 29 of transistor 28 is caused to assume a more positive potential, transistor 28 and transistors 24 and 25 conduct more heavily providing an increased power output at regulator output terminals 30 and 31.

Auxiliary power source 14 includes a tranfsormer 32 connected to full-wave bridge circuit 33 to provide direct current conversion. Capacitor 36 and resistor 37 serve as a filter. Resistor 38 provides a discharge path for the delay means to be presently described. In accordance with the present invention the power supplied to output terminals 34 and 35 of source 14 is caused to increase gradually.

Delay in power supplied by output terminals 34 and 35 is secured by a delay network connected to the base electrode 39 of transistor 40, which has its principal electrodes connected in series with the output from terminal 34. The delay is achieved by the relatively long time required for large capacitors 41 and 42 to charge through resistors 43 and 44, which have a relatively high value of resistance. A resistor 45 connects one principal electrode of transistor 40 to output terminal 34. A zener diode 46 and a capacitor 47 are connected in parallel and shunting output terminals 34 and 35. Zener diode 46 provides a substantially constant voltage output from terminals A and B after the initial delay has occurred and capacitor 47 reduces the high frequency noise voltage normally generated in zener diodes. In order to effect initial control of the regulator by auxiliary source 14, positive terminal 35 of source 14 is connected to base 29 of transistor 28 and terminal 34 is connected to serially disposed resistors 50 and 51, which provide a return path to the emitter of transistor 28. In addition, terminal 34 is connected through a direct current power supply, generally designated at 52, to the electron collector of the mass spectrograph. Thus, the electron collector output signal is in series with the auxiliary source in the circuit of FIGURE 2, whereas it was in parallel therewith in the diagram of FIGURE 1. Power supply 52 could be replaced, conveniently, by a battery. Supply 52 serves to maintain the electron collector at a sufficiently high potential to effect collection of electrons thereon.

Operation of the circuit of FIGURE 2 is similar to that previously described in connection with FIGURE 1 and, for the sake of brevity, will be only briefly treated herein. When the system is energized by connecting a source of electrical energy to transformer input terminals 53 and 54, initially negligible current flows to the filament of the mass spectrometer through terminals 30 and 31. This is because no power is initially supplied from terminals 34 and 35 of source 14 or from the electron collector to base 29 of transistor 28. Therefore, base 29 of transistor 28 continues to assume a potential equal to that of its emitter and no current flows between the principal electrodes of transistor 28. Accordingly, no current flows in the base circuits of power transistors 24 and 25 and the resistance between their respective principal electrodes is high enough to permit only a negligible flow of current through terminals and 31.

Thereafter, the slowly rising positive potential supplied to base 29 of transistor 23 causes increasing conduction in this transistor and in power transistors 24 and 25, resulting in a gradually increasing power supplied to the filament through terminals 30 and 31. At a predetermined time, primazily determined by the value of the resistors and capacitors in the time delay networks of source 14, the voltage between terminals 34 and 35 ceases to increase and assumes a constant magnitude determined by zener diode 4-6. Thus, the filament power is gradually increased over a prolonged time, efiecting protection of and longer life for the filament of the mass spectrometer.

After the predetermined time has elapsed, terminals 34 and 34 provide a circuit element substantially identical to a constant voltage, low impedance battery. Thus, power in the electron collector circuit is readily transmitted to base 29 of transistor 28 through terminals 34 and 35. It is of particular significance to note that the low alternating current impedance between terminals 34 and 35 ensures that control of filament power in response to rapid variations in electron collector power can be effected. In other words, the time response of the emission current regulated power supply is not deleteriously affected after the predetermined warm-up time has been consumed.

A particularly desirable emission current regulated power supply in accordance with my invention as shown in FIGURE 2 utilized the following specific components, which are given merely to aid those skilled in the art in the practice of my invention:

Regulator 1 Transformer 2% Secondary Winding, 8 volts R.M.S. 6 amperes. Rectifying bridge 21-4 1N1349A diodes.

Transistors 2d and 25 Type 2Nl550. Transistor 28 Type 2N498. Resistor 22 0.3 ohm.

Resistor 26 0.3 ohm.

Resistor 2,7 0.3 ohm.

Resistor 50 10,000 ohms. Resistor 51 3,000ohms. Capacitor 23 4,000 microf-arads.

6. Auxiliary source 14 Transformer 32 Secondary winding, 12% volts R.M.S. milliamperes. Rectifying bridge 33-4 1Nl696 diodes.

Transistor 40 Type 2N43. Resistor 37 100 ohms. Resistor 38 1,000 ohms. Resistor 43 15,000 ohms. Resistor 4d 15,000 ohms. Resistor 45 ohms. Capacitor 36 2,000 microfarads. Capacitor 41 500 microfarads. Capacitor 42 500 microfarads. Capacitor 47 20 microfarads. Zener diode 46 1Nl523.

Power supply 52 70 volts l milliampere DC. output between terminals C and D.

FIGURE 3 is a graph showing filament current in amperes versus time in seconds using the specific embodiment detailed above to supply a mass spectrometer filament rated at two volts and siX amperes. While mass spectrometer filaments of this type were previously attaining useful lifetimes of approximately one hour with six stops and starts, the filament energized in accordance with the present invention is yet in use after 50 hours of operation which included 30 tops and starts.

While only certain preferred features of the invention have been shown by way of specific illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An emission current regulated power supply for the filament of a mass spectrometer having a thermionic electron emissive filament and an electron collecting electrode, said power supply comprising:

(a) an electric power regulator including a source of electric power and having electric power output means and control means, said regulator being responsive to variations in the magnitude of electric power supplied to said control means to provide a variation in the same direction in the magnitude of electric power delivered by said output means;

(b) electrically conductive means connecting said filament to said electric power output means for energization thereby, said filament providing a quantity of electrons which varies in magnitude in the same direction as variations in the magnitude of power from said output means;

(c) an auxiliary source of electric power having output means, said auxiliary source providing a delay between the time of energization thereof and a predetermined time at which full output power capability is reached, said output power capability increasing gradually to full output power capability and thereafter being essentially constant; and

(d) conductive means connecting said electron collecting electrode and the output means of said auxiliary source to said control means, said auxiliary source being arranged to provide an electric power input to said control means which efii'ects a gradual increase in the power supplied to said filament until said predetermined time elapses and thereafter provide a constant magnitude power input to said control means, said electron collecting electrode being arranged to provide variations in the magnitude of power to said control means which are in the opposite direction to variations in the quantity of electrons emitted by said filament and received by said ca era (a) a series transistor regulator connected to a source of unidirectional c rrent and having electric current output means and control means, said regulator being responsive to variations in the magnitude of electric voltage supplied to said control means to provide a variation in the same direction in the magnitude of electric current delivered by said output means; (1)) electrically conductive means connecting said fila- 7 electron collecting electrode to effect substantially constant emission current from said filament after said predetermined time has elapsed.

2. An emission current regulated power supply'for the filament of a mass spectrometer having a thermionic electron emissive filament and an electron collecting electrode, said power supply comprising:

(a) an electric power regulator connected to a source of electric power and having electric power output of electrons emitted by said filament and received by said electron collecting electrode to effect subment to said electric current output means for enermeans and control means, said regulator being re- 10 gizatiou thereby, said filament providing a quantity sponsive to variations in the magnitude or" electric of electrons which varies in magnitude in the same voltage supplied to said control means to provide a direction as variations in the magnitude of current variation in the same direction in the magnitude of fromsaid output means; electric current delivered by said output means; (0) an auxiliary source of electric power having out- (b) electrically conductive means connecting said filaput means shunted by a zener diode, said auxiliary ment to said electric power output means for encrso rce providing a delay between the time of energization thereby, said filament providing a quantity grzation thereof and a predetermined time at which of electrons which varies in magnitude in the same full output voltage is reached, said output voltage direction as variations in the magnitude of current increasing gradually to full output voltage as deterfrom said output means; by said zener diode and thereafter being es- (c) an auxiliary source or" electric power having outsentially constant; and

put means, said auxiliary source providing a delay (d) conductive means connecting said electron colbetween the time of energization thereof and a prelecting electrode in series circuit with a constant voltdetermined time at which full output voltage is source, in series circuit with the output means reached, said output voltage increasing gradually to of said auxiliary source and to said control means, full output voltage and thereafter being essentially said auxiliary source being arranged to provide an constant; and electric voltage to said control means which effects Conductive 1621115 n ting said electron collecta gradual increase in the power supplied to said filaing electrode in series circuit with a constant vol age ment until said pr ned time elapses and theresource, in series circuit with the output means of 39 after provide a constant magnitude input voltage to Said auXiliaiy $011K: and to Said means, i said control means, said electron collecting electrode auxiliary source being arranged to provide an elecbeing arranged to provide variations in the magnitric voltage to said control means which effects a tude of power to said control means which are in the gradual increase in the current supplied to said filaopposite direction to variations in the quantity of ment until said predetermined time elapses and thereelectrons emitted by said filament and received by after Provide a Constant magniilldfi input Voltagfi t0 said electron collecting electrode to effect substansaid control means, said electron collecting electrode tially o t t emission current from aid fil t being arranged to provide variations in the magniafter said predetermined time has elapsed. tude of voltage to said control means which are in the opposite direction to variations in the quantity 40 References Cited by the Examiner UNITED STATES PATENTS stantially constant emission current from said filagizg g 1 a ment after said p ed t rmm fi has elapse 2,942,123 6/60 Schuh 307 88,5

3. An emission current regulated power supply for the filament of a mass spectrometer having a thermionic electron emission filament and an electron collecting electrode, said power supply comprising:

RALPH G. NELSGN, Prirrzary Examiner.

Claims (1)

1. AN EMISSION CURRENT REGULATED POWER SUPPLY FOR THE FILAMENT OF A MASS SPECTROMETER HAVING A THERMIONIC ELECTRON EMISSIVE FILAMENT AND AN ELECTRON COLLECTING ELECTRODE, SAID POWER SUPPLY COMPRISING: (A) AN ELECTRIC POWER REGULATOR INCLUDING A SOURCE OF ELECTRIC POWER AND HAVING ELECTRIC POWER OUTPUT MEANS AND CONTROL MEANS, REGULATOR BEING RESPONSIVE TO VARIATIONS IN THE MAGNITUDE OF ELECTRIC POWER SUPPLIED TO SAID CONTROL MEANS TO PROVIDE A VARIATION IN THE SAME DIRECTION IN THE MAGNITUDE OF ELECTRIC POWER DELIVERED BY SAID OUTPUT MEANS; (B) ELECTRICALLY CONDUCTIVE MEANS CONNECTING SAID FILAMENT TO SAID ELECTRIC POWER MEANS FOR ENERGIZATION THEREBY, SAID FILAMENT PROVIDING A QUANTITY OF ELECTRONS WHICH VARIES IN MAGNUTUDE IN THE SAME DIRECTION AS VARIATIONS IN THE MAGNIDUDE OF POWER FROM SAID OUTPUT MEANS; (C) AN AUXIALLY SOURCE OF ELECTRIC POWER HAVING OUTPUT MEANS, SAID AUXILIARY SOURCE PROVIDING A DELAY BETWEEN THE TIME OF ENERGIZATION THEREOF AND A PREDETERMINED TIME AT WHICH FULL OUTPUT POWER CAPABILITY IS REACHED, SAID OUTPUT POWER CAPABILITY INCREASING GRADUALLY TO FULL OUTPUT POWER CAPABILITY AND THEREAFTER BEING ESSENTIALLY CONSTANT; AND (D) CONDUCTIVE MEANS CONNECTING SAID ELECTRON COLLECTING ELECTRODE AND THE OUTPUT MEANS OF SAID AUXILIARY SOURCE TO SAID CONTROL MEANS, SAID AUXILIARY SOURCE BEING ARRANGED TO PROVIDE AN ELECTRIC POWER INPUT TO SAID CONTROL MEANS WHICH EFFECTS A GRADUAL INCREASE IN THE POWER SUPPLIED TO SAID FILAMENT UNTIL SAID PREDETERMINED TIME ELAPSES AND THEREAFTER PROVIDE A CONSTANT MAGNITUDE POWER INPUT TO SAID CONTROL MEANS, SAID ELECTRON COLLECTING ELECTRODE BEING ARRANGED TO PROVIDE VARIATIONS IN THE MAGNITUDE OF POWER TO SAID CONTROL MEANS WHICH ARE IN THE OPPOSITE DIRECTION TO VARIATIONS IN THE QUANTITY OF ELECTRONS EMITTED BY SAID FILAMENT AND RECEIVED BY SAID ELECTRON COLLECTING ELECTRODE TO EFFECT SUBSTANTIALLY CONSTANT EMISSION CURRENT FROM SAID FILAMENT AFTER SAID PREDETERMINED TIME HAS ELAPSED.

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