US2355191A

US2355191A - Power supply for electron microscopes

Info

Publication number: US2355191A
Application number: US440771A
Authority: US
Inventors: Arthur W Vance
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-11-15
Filing date: 1942-04-28
Publication date: 1944-08-08
Anticipated expiration: 1961-08-08
Also published as: US2419428A

Description

Aug. 8, 1944. A. w. VANCE 2,355,191 7 POWER SUPPLIES FOR ELECTRON MICRQSCOPES Original Filed Nov. 15, 1940 3 Sheets-Sheet l OIP/VEB ffcf/fzse 1-74 75,6

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Zhwentor attorney Aug. 8, 1944. A. w. VANCE 2,355,191

ROWER SUPPLIES FOR ELECTRON MICROSCOPES Uriginal Filed Nov. 15, 1940 3 Sheets-Sheet 5 raca/mmufa arma-- 67 VAV VAVAV V V V V 97 Mama J;

l'lt 50/3 1): w B 1 I @& 7 w I 1%? fAAAAAA attorney Patented Aug. 8, 1944 POWER SUPPLY FOR ELECTRON MICROSCOPES Arthur Radio a of Delaware 3., assignor to Original application November 15, 1940, Serial No.

365,750, now vember 24,

Patent No. 2,302,900, dated No- 1942. Divided and this application April 28, 1942, Serial No. 440,771

7 Claims.

The subject invention relates to power supply systems for electronic devices and has for its principal object the provision of an improved method and means for producing a highly stabilized source of voltage for use with the electron microscope whereby greatly improved performance may be obtained.

This application is a division of applicants copending U. S. application, Serial No. 365,750, filed November-15, 1940; Patent No; 2,302,900,

grantedNovember 24, 1942,-entitled Power sup-- plies for electron microscopes."

The development .of theelectron microscope has presented new problem in the design of high voltage power supplies-and, in the design of current regulating systems for the various electromagnetic coils which constitute the magnetic lens system of the microscope. In order to produce a beam of high velocity electrons, an electron gun is utilized which is energized with a direct current voltage which may be as high as 100 kilovolts. The current load of the high voltage source is of the order of 1 milliampere, and the problem, therefore, is one of voltage stability.

, The magnetic lenses require currents of the order of. 400 milliamperes which likewise must be maintained constant over short periods.

The stability of the -voltage supply system for an electron microscope is measured over the period of time required to properly expose a photographic plate used to record the images. In practice, the longest exposure is approximately 30 seconds. Consequently, the tern must not vary appreciably within this period, since slight variations in the electron speed or in the focus of the magnetic fields will defocus the electron image and distort the photograph.

It is also necessary to consider the short-time constancy of the power supply system since this limits the resolution of the microscope. Theoretical calculations based on a resolving limit of 10 Angstrom .units indicate that the most critical current must not vary more than .004 of one percent and that the high voltage should not vary more than'.006 of one percent.

It is well known that the electron beam must be thoroughly shielded from all external fields. The difficulty of shielding magnetic fields produced by 60 cycle currents has heretofore necessitated the use of direct current to supply power to the filament of the high velocity electron gun. Since the cathode is normally operated at a high negative potential, with the anode grounded, the filament supply is necessarily at a high voltage voltage supply sysof this invention to eliminate the necessity of supplying direct current for the electron gun by energizing the filament with radio'frequency currents. A still further object of this invention is to provide an improved high voltage rectifier circuit having a feedback stabilizer which permits the required high voltage to be obtained with less equipment and fewer parts than heretofore had been required, thus greatly reducing the cost and size of the power supply system. Still further objects of the present invention are to provide an improved automatic relay and protective circuit; to provide an improved oscillator for supplying the radio frequenpy currents utilized in energizing the high voltage rectifier, and to provide improved current regulators for use in conjunction with the microscope lenses.

Briefly, these objects are accomplished by employing a source of radio frequency currents coupled to a voltage doubling rectifier circuit in which the filament of one of the rectifier tubes is energized with radio frequency currents, thus facilitating filtering problems, deriving a potential corresponding in amplitude to the rectified output, and utilizing this potential to control the output of the oscillator supplying, the high frequency currents to the rectifying system.

- This invention will be better understood from the following description when-considered in connection with the accompanying drawings in which Figure 1 is a block diagram of the essential elements of a complete power supply system for an electron microscope; Figure 2 is an equivalent circuit diagram illustrating the operation of the-high voltage rectifier arid filter system; Figure 3 is a simplified circuit 'jdiagram of the system including the high voltage rectifier; Figure 4 is the circuit diagram of the driver oscillator which supplies high voltage radio frequency The power supply system Referring to Fig. 1, a complete power supply system for an electron microscope is illustrated in block diagram form. Power is derived from a 60 cycle power line 9 and applied to a number of regulated power units which energize various components of the system' and which are dewith respect to ground. It'is a further object as signed to have output voltages which are determined by their intended use. The regulated power supply ll energizes a pair of low frequency oscillators'l3 and i which are utilized, respectively, to energize the filament of the electron microscope and the filament of one of the high voltage rectifiers. The high frequency-high voltage rectifier and filter circuits are represented by the block I! and will be described in detail hereinafter. Another power unit l9, which may be unregulated, energizes a driver oscillator 2| which generates high voltage radio frequency currents for the high voltage rectifier. A modulating amplifier 23 is energized through a connection 22 by the unregulated power unit Hand is also supplied with a voltage over a lead 25 which is proportional to the rectified output voltage. This voltage is compared to that supplied to the modulated amplifier by a standard battery 21, in a circuit which will be described in detail hereinafter, to produce an output control voltage which is. utilized to control the amplitude of oscillation of the driver oscillator 2|. This control connection is illustrated by lead 29. The amplifier is also connected by a lead 20 to a regulated power source 3| which supplies a negative voltage of the order of 350 volts, the purpose of which is to be described later.

Another regulated power unit 33 supplies a direct voltage of the order of +400 volts to the driver oscillator 2| and to the modulating amplifier 23 through a lead 35. In order to achieve the highest degree of accuracy, the modulating amplifier 23 and, in addition, a current regulator 31 for the objective coil are supplied with a regulated current through connection 24 from a source 39 which is used to energize the filaments of certain of the tubes of these devices. Regulation in the current regulator 33 is achieved by utilizing the -350 volts obtained from the regulated power supply 3| by a connection 32 as a standard reference voltage, in a manner which will be described hereinafter. The projection coil and the condenser coil 4| and 43 are enersized by

current regulators

45 and 41, which derive power from

power supplies

42 and 44, respectively, both regulators having a connection over conductor 32 to the regulated power supply iii. A particular advantage of this system of interconnection. is that one power supply which is carefully regulated controls the output voltages of the other power supply units, and control by a regulated or standard voltage is eflectuated in a much more economical manner than by any other method.

The high voltage rectifier Referring now to Fig. 2, 'a circuit diagram of the high voltage rectifier system is shown. Since an extremely high voltage is produced, the high voltage equipment has been mounted in an oil tank 5| which not only protects the user from harm, but also decreases the danger of accidental fiashover or short circuits. The driver oscillator 2| is connected to a series resonant circuit comprising an inductance Lo and a capacitance Co, the latter representing the eflective lumped capacity between the point 54 and ground due to the primary of transformer T2, and the rectifier tubes Vl'and V2 in series with the capacitor CI. The cathode of the first rectifier Vl is grounded,

while its anode is connected to the inductor Lo,

through the coupling capacitor Cl. and al's'o'the cathode of the second rectifier V2. The anode of the second rectifier is connected to the cathode of the high velocity electron gun of the mi- 15 cluding a coupling'transformer T2 having an un-- tuned secondary and a tuned primary.

Radio frequency actuation of the high voltage rectifier has a number of advantages over the use of a conventional 60 cycle alternating power source for the production of high voltages of great constancy at low current drains- For example, much smaller filter condensers are required for a given ripple output. This means that the stored energy in the filter circuit is very much less than that of the usual 60 cycle "brute force filter system. The importance of this is that in case of a flash-over in the high voltage circuit, due to a loss of vacuum in the microscope, for example, the stored energy is not suflicient to burn up the equipment, as was the case in the earlier microscopes. Since resonant circuits are used, the ripple fed through the rectifier capacitor is sinusoidal and consequently can be resonated out to a large degree. Furthermore, by using low loss coils in the resonant circuit, an extremely high impedance may be realizel which occupies but a small space and is extremely light in weight as compared to an equivalent impedance at the conventional power frequency. Also the exciting power required by such a coil is greatly reduced.

The use of radio frequency high voltage power supply also improves the operation of the output control system. When the control is operated on the low voltage input side of the rectifier, as is desirable, the speed of control is limited by the frequency of the supply. Consequently, this limitation is negligible where the supply voltage.

is a radio frequency voltage.

cathode or filament of the high velocity electron gun' has two important advantages. In the first place, the microscope may be readily shielded from the stray high frequency fields which are produced, and this result is aided by the fact that no 60 cycle high voltage transformers are required which have very large external fields. In the second place, the use of radio frequency eliminates the necessity for employing bulky storage batteries or their equivalent.

The operating frequency for the rectifier system is not critical. The problem is essentially that of obtaining the required output voltage with a minimum exciting power. This, 'of course, requires obtaining the maximum resonant impedance of the high voltage coil. The

-' resonant impedance is given by the formula:

where C is the distributed capacity of the coil and Br. and Re are the effective series A. C. resistances of L and C, respectively. In the frequency region from approximately 20 kc. up to several hundred k0,, the maximum Q possible in a coil of given volume is more or less independent of frequency; thus, the coil should, in general, operate below 50 kc.

The rectifier circuit is seen to be of the type wherein the inverse voltage of the half wave rectifier VI is rectified by V2, thereby charging the output capacitor C2 to a voltage nearly equal to twice the peak voltage which appears across the inductance Lo. 'It is to be noted that one side of the input circuit is grounded and that no primary winding is required on the high voltage coil.

The filament transformer T2 has a low interwinding capacity which is preferably of low power factor. The primary and secondary winding must be spaced sufficiently to withstand the peak voltage output, and, as a result, a large amount of energy must be stored in the tuned primary in order that the secondary may absorb the power necessary to excite the filament. Preferably, the tuned primary of transformer T2 constitutes the tank circuit of the driving filament oscillator. The size of the secondary is selected to match the impedance of the filament load and to provide the desired voltage. The filament current may be controlled conveniently by varying the frequency of the oscillator l5.

Fig. 3 shows the rectifier circuits connected to the other elements of thepower supply system. It will be noted that the negative voltage for the electron gun cathode obtained from the output of rectifier V2 is applied toa center tapped resistor 51 connected across the secondary of the transformer T3 which supplies radio frequency current to the filament of the electron gun 59. The filament, as a whole, is 60 kilovolts below ground potential and must therefore be carefully insulated. The radio frequency current supplied by the filament oscillator I3 is applied to the filament through a coaxial cable 6|.

It will be noted that the filter capacitor C2, .005 microfarad, for example, is connected to ground through a; parallel connected coil L5 and condenser CB. The values of these reactors are such that at the operating radio frequency, effective inductive reactance of L5 and C6 resonates with capacitor C2 and forms an effective ground for ripple voltages appearing in the output circuit. At a higher frequency, for instance, midway between the fundamental and second harmonic L5 and CB become parallel resonant, presenting a high impedance, but as there is no ripple at this frequency anyway, no harm results. At higher frequencies including the second harmonic, the shunt capacitor C6 in series with the output capacitor C2 provides an efiective ground. A spark gap 63 is connected across the shunt reactors L5 and C6 in order to discharge voltage surges without causing a breakdown of the elements.

High voltage regulator ground and connected through an isolating re-,

sistor to an adjustable tap on a standard battery 69, the negative terminal of which is grounded. The standard voltage produced by the battery 69 is therefore compared to the divided voltage appearing across resistor 61, so that no input voltage is applied to the amplifier 23 when the divided voltage obtained from the high voltage output is exactly equal to that of the standard battery. By varying the taps on the standard battery 69, the output voltage may be controlled, for example, in 5 k. v. steps from 30 to k. v.

The details of the driver oscillator which supplies high voltage for the rectifier tubes VI and V2 is shown in Fig. 4. Since the resonant frequency of the series resonant inductor L0 and Co included within the oil tank 5| varies considerably with temperature, a master oscillator-power amplifier is impractical without automatic frequency control to keep it at resonance. It is therefore proposed to utilize a self-oscillating circuit whose frequency is determined wholly by the resonant frequency of the load Lo, Co. This oscillator must also be capable of modulation over a considerable range in order to provide control over the output voltage. The circuit utilized comprises a two-stage oscillator including tubes V3 and V4, the output of the latter being coupled to the input of the former by a conventional impedance coupling system, tube V6 being shunt fed through inductor LB and coupled to the resonant circuit load through a small series resistance R: Inductor L4 is resohated at the operating. frequency by a capacitor C5. A feedback voltage is obtained by the drop across resistor R, the terminals of which are connected to the primary of a shielded transformer II, the secondary of which is coupled to the input of the first oscillator tube V3. The transformer 1| preferably has a broad frequency response, this. being accomplished, for example, by a damping resistor and capacitor 13 and 15. The feedback gain is sufficient only to sustain oscillation at or very near the resonant frequency of the load Lo, .00.

The amplitude of oscillation is controlled by varying the screen grid potential of the output tube V6. This is illustrated in Fig. 4 by the potentiometer connection shown in dotted lines, the potentiometer representing the control bias derived from the D. C. modulating amplifier 23.

I The details of .this connection are illustrated in Fig. 5. The tank circuit L4, C5 has a high L/C ratio and it therefore exerts only slight control over the frequency of oscillation. Its principal function is to maintain reasonably sinusoidalvoltage conditions in the plate of V4.

Referring to Fig. 5, a unique D. C. amplifier is illustrated suitable for use in applying the small variable D. C. voltage of the control system to the driver oscillator to effectuate control of the amplitude of oscillation. Resistor 61 corresponds to the similarly numbered resistor in the voltage divider circuit of Fig. 3. At any instance, the potential of the lead 22 connecting this resistor to the control grid of the first amplifier tube I'I is equal to the sum of the negative divided voltage and the voltage due to the standard battery 69$ The latter battery is preferably tapped in steps and constitutes the main control of the high potential output.

The filament of the first amplifier tube 11 is connected to a regulated current source, which may be of the type, illustrated in Fig. 8 and hereinafter described, in order to assure conwhich is connected to a terminal :1. A'gas'fllled is now made.

Transformer stant electron emission. Screen'grid potential is obtained from a voltage divider '|98I connected to .a suitable sourcelof positive potential which is'applied to terminal 93. Plate;voltage is obtained from ,a resistor I95 connected between the plate and the terminal 99. The plate is connected-to the controlgrid or the second amplifier tube 85 through a parallel connected resistor" and capacitor. A phase control network comprising resistor 91 and capacitor- 99 is connected in shunt with the plate ofthe amplifier tube 11. 1 Screen grid potential for the second amplifier ,tube 85 is -derived through a dropping resistor 95- from a suitable sourcepf positive potential.

regulator tube 991s utilized to control the screen grid potential. Theanode of theoutput tube 95 is connected through an' anode' load resistor III! to the positive supply terminal 83 and also through an: isolating resistor I93 to the screen gridfof 'the second oscillator tube Vl, which is shown in Fig. 4. J

vIt will be noted that theplate voltage of the-first amplifier tube r1 isjimpressed on the grid of the secondamplifier tube 95 through the coupling resistor 81. Since the cathodes of the two amresistor I23, 'the energizing coil of an overload I relay I and a resistor 121-. A neon regulator tube I29 and a'pair of condensers -I3I and-III "are connected between the output terminals '01 the meter A and the Junction points of the resistors H1, H9 and Ill, respectively. The output terminal otthe meter A isalso connected to groundthrough a limiting neon tube I95.

The overload relay circuit includes a switch I99 which, in its normal position,makes contact to a grounded terminal I ll. When actuated by an overload current, the relay armature makes contact to a terminal I which is connected. to a source of negative voltage, folexample, the 350 volts provided by the power supply 3|.

'The armature is connected to the grid return of the oscillatortubes of the driver oscillator asv illustrated in detail in- Fig. 4. The armature is also connected through a push button I" to the relay coil through a current limiting resistor I49. The same armature is also connected to v a voltage divider I5I 1 across a. portion of which a plifier tubes are both, operated at ground potential, 'it-is necessary-to overcome the effect of the plate voltage on the grid of the second amplifier, This is accomplished by connecting a source of "negative voltage, ,for example, 350

neon indicator tube I53 is connected.

'A current surge, caused by an are or ,breakdown-of a high voltage terminal, flows through 'the limiting'resistors H1, H9 andIlI and charges capacitors I and I33. In addition, the voltage across the output meter A is limited by the volts derived from the voltage supply 3| i1lustrated in Fig. 1, through an isolating resistor IOI to-the control grid of the output tube. This has an effect similarto that of the usual series buck- 'ing battery in D. C. amplifiers; but has the advantagev that one terminal of the source of negati've, voltage isoperated .at ground potential,

' which is not true in the conventional case.

' It has also been found that it may be desirable to drive the screen grid of the oscillator tube more negative than is ,possible by direct control from the amplifier tube 95. This isaccomplished its oscillation.

neon tube I29: The meter-is therefore protected from damage. The sustained overload flows through the actuating coil of the relay I25 and .connects the armature .to the negative potential source, thusv removing the normal-ground connection and applying a high negative potential to the driver oscillator suilicient to stop Since the driver'oscillator provides the high voltage for the high voltage rectifier. system, it will be apparent that the relay irhmediatelyshuts oil the high. voltage supply.

-At the same time, the relay connects a holding circuit through the-push button I41 so that current 'fiows through a resistor I55 and through therelay to hold it in its closed position. It will by connecting the 350 volt source to the lead Hi9 through -a-resistor -III, thus reducing the average potential of I the screen' and making it possible to'drive the screen more negative when the output tube 95 is drawingmaximum plate current, I

' Protective circuit g It has been iound that, due to the high voltage used, there'is some danger. of'arcing at the high voltage terminals. 'This may occur within the vacuum chamber or-the microscope, due to a failureof the vacuum, for example, or it may occur by reason of a failure of the insulation of the high voltage condensers.

Referring to Fig. 3, it will be noted-that the groundreturn of thehigh voltage supp .llows through the output current meter A. Inorder .to protect the ,meter from danger, a protective circuit 82 has-beenprovided ,which absorbs sudden'current surges and limits" the. current to substantially the normal. value. In order-to" protect the apparatus from sustained overloads, a cutoutoroverload relay-G0 has been provided. The

details of the protective circuit and overload reremain in this position until the circuit is broken by operating the push button I". :If the fault has cleared, theove'rload relay will open, restoring the high voltage to the rectifiers. If the fault has not cleared, the high voltage willnot remain on, indicating that the circuit must be checked. When the .overload relay is actuated,

3 .6 neon'output indicator I5'3.will light, thus providing a visual indication to the operator that the high voltage has been cut-oil.

Low voltage or curfentregulators A current. regulator of the typepreferred foruse with ,the projection'and objective coils of the microscope is illustrated in Fig. 7. A conventionlay are illustrated in Fig. to which reference numbered transformer or Fig. 3 which. supplies 'I I5 corresponds ,to the similarly resistor III.

It will be observed thatthe cathode of the trial rectifier I51 and filter I59 supply a high voltage as the plate of a triode. ISI, the cathode of which-is serially connected to ground through theprojection or objective coil l6! and current con-.

trol resistors I and I51, the former being adjustable. The voltage dropacross the latter resistors is compared-to the voltage of a standard battery I99 and applied to the grid of a regulating, tube III, the plate of which is energized by a suitable source of positive potential connected to an input terminal I13 through a plate resistor I". Theplate of this control tube is connectedto the grid of the ode Iii is positive with respect to ground. HOW-\ ever, the plate of tube "I cannot become sumtriode Iil through a coupling ciently less positive than the cathode of tube |6l to provide the necessary bias for the latter tube under conditions of low output current. In order to supply the proper voltage to the grid of the tube I6], it is connected through an isolating resistor 11.9 to the source of regulated negative voltage, as in the case of the d. c, amplifier described above.

A current regulator of somewhat simpler form which does not require a standard battery is illustrated in Fig. 8. This amplifier is preferably used for the condenser coil of the microscope and ,may also be used to regulate the filament current of the d. c. amplifier and the control tube I'll of the current regulator illustrated above in Fig. 'l. The unit'may also be considered as a voltage regulator since the voltage across a constant impedance is constant when the current through it is constant. Consequently, the same control circuit is employed, for example, in regulating the output of the regulated 800 volt power supply ll.

. As before, the conventional rectifier and filter 151 and I59 are connected to the plate of a limiting trlode IS], the cathode of which is connected to ground through the electron microscope condenser coils or the regulated filament, as the case may be, and through the control and adjusting resistors I65 and IE1. The voltage drop across the latter resistors is compared to a voltage derived from the regulated 350 volts obtained from the powen supply 3|, thus eliminating the standard batteries I69 utilized in the preceding regulator. A particular advantage of this method is that the standard voltage source is operated with one terminal grounded, which is not possible where standard batteries are used in a series circuit. Since the cathode of the triode I6! is positive with respect to ground, as noted above, it may not be necessary to apply the auxiliary negative potential to the grid of this tube so that in this case the plate of the control tube I'll is connected directly to the grid of the trlode IB I. It will be apparent that in both cases, changes in the current through the coil and control resistors produces a variable voltage drop across the resistors which is applied to the control tube ill to vary the amplitude 'of the current in a direction tending to compensate for the change.

The measured stability of the power supply system herein described has been found to be well in excess of that required for stability and definition of an electron microscope. Several Previously known microscopes could not be treated in this manner and generally had to be taken apart and repaired after an internal fiashover due to the severity of the discharge from the filter.

I claim as my invention:

1. In a device of the character described, a source of radio frequency oscillations, resonant means for deriving a voltage from said source, a voltage doubling rectifier including a pair of thermionic tubes having cathode and anode electrodes coupled to said resonant means, and a second source of radio frequency coupled to the cathode of one of said tubes for energizing said tween said cathode and said anode, said source including a radio frequency oscillator, resonant means in circuit with said oscillator for establishing thereacross a radio frequency voltage, a

pair of rectifiers having anode and thermionic cathode electrodes, the first of said rectifiers being connected in series circuit with said resonant means for rectifying voltage peaks of one polarity, the second of said rectifiers being connected in circuit with said first rectifier for rectifying the inverse voltage peaks, and a second source of radio frequency oscfllations for energizing the cathode of said second rectifier.

4. In an electron microscope having a cathode and an anode, a source or high potential direct voltage for establishing a potential difference between said cathode and anode,- said source including a radio frequency oscillator having a resonant circuit for establishing a radio frequency voltage, means for rectifying and filtering said voltage, said filtering means including a filter capacitor of low capacity whereby adequate filtering is provided with a minimum of stored ener y.

hundred photomicrographs have been success:

fully taken with no indication that the results have beenlimited by variations of the power supply. This is in distinct contrast with microscopes of the prior art in which a large percentage of the photomicrographs is spoiled by reason of changes in the power supply voltage, The system is so stable that the microscope may be reset to pre'--.:

vious conditions without observing the electron image or the focusing. Exposures thus made have good resolution. In addition, it is possible to make wide variations of the image intensity ,without changing the focus.

A particularly severe test which was successfully passed-by the electron microscope operated .in conjunction with the power supply and control system of the present invention is that of reducing thevacuum during operation until an internal arc takes place,- causing the overload relay to actuate, then restoring the vacuum,.and applying the high voltage to obtain the original picture exactly in focus without readjustment.

and a shunt-connected capacitor and inductor serially connected in circuit with said filter capacitor, the inductive reactance of said shuntconnected capacitor and. inductor being series resonant with said filter capacitor at the frequency of oscillation of said oscillator.

5. In an electron microscope having a cathode and an anode, a source of high potential direct voltage for establishing a potential differencebetween said cathode and anode, said source including a radio frequency oscillator having a resonant circuit for establishing a radio frequency voltage, means for rectifying and filtering said voltage, said filtering means including a filter capacitor of low capacity whereby adequate filtering is provided with a minimum of stored energy, and a parallel-connected capacitor and inductor serially connected in circuit with said filter capacitor, the inductive reactance of said parallel-connected capacitor and inductor being series resonant with said filter capacitor at the ground ed, means for energizing said cathode, said means comprising a radio frequency oscillator coupled to said cathode'throug h a coupling transformer having primary and secondary windings, and means for applying said high potential to d s c ry winding to thereby establish the negative potential of said cathode.

- '7 In apparatus-of the type described in claim 3, control means for automatically. interrupting said high voltage comprising-an overload relay in said high voltage circuit, and means responsive to the operation 01' said relay for applying a bias voltage to said oscillator to prevent oscillations.

.. ARTHUR VANCE.