US3624390A

US3624390A - Specimen-heating means for electron beam irradiation apparatus

Info

Publication number: US3624390A
Application number: US74412A
Authority: US
Inventors: Tadao Watanabe
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1969-09-24
Filing date: 1970-09-22
Publication date: 1971-11-30
Anticipated expiration: 1988-11-30
Also published as: JPS491058B1

Abstract

A specimen-heating means for electron beam irradiation apparatus comprising a cylindrical mesh filament concentrically surrounding a specimen, a filament support concentric with said filament, a current source feeding said filament with a heating current, and means to take up thermions given off said filament and the heated specimen, whereby the specimen may be heated through the radiation of heat from said filament and through the bombardment of the specimen with thermions given off said filament.

Description

United States Patent Inventor Tadao Watanabe Katsuta, Japan Appl. No. 74,412 Filed Sept. 22, 1970 Patented Nov. 30, 1971 Assignee Hitachi, Ltd.

Chiyoda, Tokyo, Japan Priority Sept. 24, 1969 Japan 44/75249 SPECIMEN-HEATING MEANS FOR ELECTRON BEAM IRRADIATION APPARATUS 5 Claims, 6 Drawing Figs.

US. Cl 250/495 B, l3/3 l, 250/495 PE Int. Cl H01] 37/20 Field 01 Search l3/3l;

250/495 B, 49.5 A, 49.5 PE

[56] References Cited UNITED STATES PATENTS 3,171,958 3/l965 Coleman 250/495 B FOREIGN PATENTS l,l57,3l9 ll/l963 Germany 13/91 Primary Examiner-Anthony L Birch Attorney-Craig, Antonelli & Hill ABSTRACT: A specimen-heating means for electron beam irradiation apparatus comprising a cylindrical mesh filament concentrically surrounding a specimen, a filament support concentric with said filament, a current source feeding said filament with a heating current, and means to take up thermions given off said filament and the heated specimen, whereby the specimen may be heated through the radiation of heat from said filament and through the bombardment of the specimen with thermions given offsaid filament.

PATENTED nuvao I971 SHEET 1 BF 4 INVENTOR TA DAO WATANABE BY Cvoig, Anton Stewart 4 Ml ATTORNEYS PATENTEnuovsmsn 3,624,390

SHEET 2 UF 4 w) 37dfi09 OMB/-11 INVENTOR TADAO WATANABE Cwroig, nntoneul, Stewart 9 Hill ATTORNEYS mama] Huv30|97| 3624.390

I N VIZNTOR TADAO WATANABL m Gm), q t m, SLQMQFMI AU ATTORNEY5 SPECIMEN-HEATING MEANS FOR ELECTRON BEAM IRRADIATION APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to specimen-heating means for electron beam irradiation apparatus. More particularly, the invention concerns specimen-heating means adopted for use in such electron beam apparatus such as an electron microscope, or a scanning electron microscope for instance, where the specimen is bombarded with an electron beam and reflected electrons, secondary electrons, transmission electrons or specimen absorbed current is thus made available from the specimen and contains information about the specimen which is detected for analysis of the specimen.

2. Description of the Prior Art The electron beam irradiation apparatus of the kind mentioned above finds its use where the specimen to be analyzed is heated for continuously observing while changing states of the specimen at elevated temperatures as in the analysis of states of diffusion of different metals, crystalline structures of various crystals and so forth.

Heretofore, there have been proposed various specimen heating means adopted in this type of electron beam apparatus. These specimen-heating means, however, mostly provide heat to the specimen through thermal conduction from a heat source (a heater). Therefore, the temperature attainable with these heating means is about l,0OO to l,l() C. at most. Besides, the specimen and heat source should be disposed very close to each other to have an increased thermal efiiciency, so that the size of the specimen is limited to be relatively small.

Further, current through the heater sets up a magnetic field tending to deflect the electron beam directed toward the specimen. To avoid the generation of the magnetic field, a noninductive heater (consisting of a metal wire such as molybdenum wire wound in such a manner as to cancel out the magnetic field) is used. With this measure, however, the magnetic field cannot be completely eliminated, giving rise to the deflection problem, particularly when the energy of of the electron beam striking the specimen is low (in the scanning electron microscope the electron beam accelerating voltage is usually to kv.).

SUMMARY OF THE INVENTION An object of the invention, accordingly, is to provide a specimen-heating means for electron beam irradiation apparatus, which is capable of uniformly and promptly heating specimens oflarge size and large thermal capacity to high temperatures.

Another object of the invention is to provide a specimenheating means for electron beam irradiation apparatus, with which the specimen may be efficiently and promptly heated to high temperatures through the combination of heat radiation and electron bombardment, and the electron beam directed toward the specimen is free from deflection by the magnetic field set up by the heat source.

A further object of the invention is to provide a specimenheating means for electron beam irradiation apparatus, which includes a means to take up stray electrons or unnecessary thermions given off when the specimen is heated and enables the detection of reflected electrons, secondary electrons or specimen-absorbed current produced by the electron beam bombardment of the specimen in its heated state with reduced noise (background).

These and other objects of the invention will become more apparatus from the following description of the preferred embodiment of the invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional view illustrating an embodiment of the specimen-heating means according to the invention as applied to the electron probe microanalyzer.

FIG. 2 shows, in an enlarged scale, the mesh filament shown in FIGS. 1 and 5.

FIG. 3, FIG. 4a and FIG. 412 show the interrelation among the specimenheating time, specimen temperature and filament current.

FIG. 5 is a schematic longitudinal sectional view illustrating another embodiment of the specimen-heating means according to the invention as applied to the electron microscope.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a preferred embodiment of the specimen-heating means according to the invention as applied to the electron probe microanalyzer. Referring to the figure, reference numeral 1 designates an electron beam, which is generated, thinly focused or concentrated to l micron or less in diameter and accelerated toward a specimen 5 by an electron beam irradiation system (not shown) including an electron beam source and electron beam focusing and accelerating means. Numerals 2 and 3 designate objective lenses of an optical microscope individually formed with respective central electron beam passing apertures concentric with a portion of the surface of the specimen 5 to be analyzed. Numeral 4 designates the yoke of an electron lens to focus the electron beam. The electron lens includes upper and lower pole pieces 6 and 7. The electron beam bombardment of the specimen surface causes X-rays as indicated at 8 to be emitted from the specimen 5. The X-rays thus emitted are usually dispersed by a curved crystal, which is set to an appropriate angle to provide for the reflection of the characteristic radiation of a selected element, so that only the characteristic X-radiation carrying information about the specimen surface may be detected. The electron beam bombardment also makes available reflection electrons, secondary electrons, Auger electrons, etc., emanating from the specimen surface and containing information about the specimen surface, which may be simultaneously or selectively detected through appropriate detectors.

Numeral 9 designates a mesh filament to heat the specimen 5. It consists of heat-resisting thin metal wires, for instance thin tungsten wires, netted to form a mesh structure, as shown in FIG. 2. It concentrically surrounds the specimen 5 and is appropriately spaced therefrom. It is supported by a cylindrical filament support 10 concentrically disposed to surround it and also serving as a lead to cause heating current through it. Both the filament 9 and filament support 10 are made integral with each other and electrically insulated from the column of the electron probe microanalyzer through an insulating member or members 11 of such material as ceramics. Numeral 12 designates a radiation shield to provide for thermal shielding and a uniform temperature distribution within the specimen-heating means. It consists of a plurality of concentrically spaced cylindrical members made of molybdenum and concentrically surrounding the specimen 5. It is mounted on support poles 13 extending from the bottom of the column of the electron probe microanalyzer. The specimen 5 is accommodated in a specimen holder 14 secured through three legs 15 to a specimen support base 16. The specimen 5 (usually a metal specimen), specimen holder 14, legs 15 and specimen support base 16 are electrically connected to one another. They are fixed through thermal and

electric insulation bases

17 and 18 ofsuch material as ceramics to a movable specimen table 19, which is movable in the direction of axis of the electron beam and in two mutually perpendicular directions in a plane perpendicular to the electron beam axis. The movable specimen table 19 may be driven by a movable specimen table drive (not shown) outside the column of the electron probe analyzer. The specimen holder 14 is provided with a thermocouple 20 for temperature measurement connected through leads 21 across a thermometer 22. Numeral 23 designates a specimen-absorbed current meter for the measurement of the specimen-absorbed current. Numeral 25 designates a DC source to feed the filament 9 with heating current. In this embodiment, the source voltage is variable between and volts and a maximum current of 90 amperes can be supplied. Numeral 26 designates a bias DC source connected between a switch 27 and ground. When the switch pole is thrown to a switch contact 28, a positive voltage is applied to the filament support 10, so that thermions or stray electrons given off the filament 9 and constituting the so-called background may be taken up by the filament support 10, to be described hereinafter. On the other hand, when the switch pole of the switch 27 is thrown to a switch contact 29, a positive voltage is applied through the specimen support base 16, legs and specimen holder 14 to the specimen 5, so that the specimen may be heated by the electron bombardment thereof. In this embodiment. the voltage of the bias DC source is variable between 0 and 500 volts. Numeral 30 designates a protection shield to provide for the thermal shielding of the objective lenses 2 and 3 of the optical microscope and the electron beam irradiation system. it is cooled with liquid nitrogen 32 contained in a cooling tank 3]. The electron lens is cooled with water circulated through conduits 33. The column of the electron probe microanalyzer, within which the specimen-heating means described above is disposed, is evacuated and held under a degree of vacuum of about 10" to 10" Torr.

The operation and efiects of the embodiment of the above construction will now be described.

Firstly, for the continuous observation of changing states of the specimen at its elevated temperatures the specimen may be heated to a temperature of about l,000 to 1,100 C. through the heat radiation alone. At this time, the bias DC source 26 is held 0 The specimen is thus heated with DC current from specimen-heating DC source 25 passing through filament 9 and filament support 10. The specimen temperature attainable depends upon the current through the filament 9. With a filament current of 95 amperes the filament 9 may be heated to'about 2,000 K., and by the resultant heat radiation the temperature of the specimen 5 may be raised up to about i ,200" C.

By way of example, the specimen temperature (the reading of the thermocouple thermometer in mV) when heating the specimen through the heat radiation alone is correlated with the, time required for the specimen to reach it and the filament current, as shown in FIG. 3. In the FlG., the abscissa represents heating time and the ordinate represents specimen temperature,

The illustrated temperature curve shows the relation between heating time and specimen temperature in case of heating an iron specimen (l2 mm 20 mmh) by increasing the filament current F.C. step by step by 10 amperes for each step up to 70 amperes and thereafter appropriately adjusting it until the specimen temperature is increased to 1,000 C. The specimen temperature reaches l,000 C. with the filament current adjusted to 76 amperes minutes after the commencement of the heating. In the HQ, arrow E indicates a portion, for which the filament current is temporarily interrupted due to an accident in the source circuit.

In the electron beam irradiation apparatus such as an electron probe microanalyzer and scanning electron microscope the electron beam 1 is caused to scan the specimen surface perpendicular to the electron axis after a raster pattern provided by the two-dimensional deflection of the electron beam, while the specimen 5 is being heated, and the reflection electrons, secondary electrons emanating from the specimen surface or specimen-absorbed current may be detected for the display of the reflection electron image, secondary electron .image or specimen-absorbed current image on a cathode-ray tube.

In this operation, the specimen-absorbed current, that is, part of the electron beam absorbed by the specimen 5, is of the order of lXlO' ampere. The specimen 5 also absorbs electrons (of the order of I00 microamperes) given off the filament 9, thus disabling the display of the specimen-absorbed electron current image. Also, when the specimen temperature exceeds 1,000 C., thermions are emitted from the specimen. These undesired electrons, commonly termed stray electrons," constitute the background in the detection of reflection electrons, secondary electrons produced from the specimen and so forth by the electron beam bombardment, tending to decrease the precision of the specimen analysis. To eliminate these undesired stray electrons, a positive bias voltage from the bias DC source 26 is applied through the switch 27 and its contact 28 to the filament support 10. In this manner, the thermions given ofi the filament 9 and the specimen 5 will be taken up by the positively biased filament support 10.

In this embodiment, by arranging the filament 9 and filament support 10 such that the current from the DC source 25 passes through the filament 9 in one direction and through the filament support H) in the opposite direction it is possible to cancel the magnetic field set up by the filament current. Even if the magnetic field thus set up can not be completely canceled, the effect of the remaining magnetic field on the electron beam may be cancelled by arranging such that the electron beam passes through the center of the cylindrical filament 9.

In the conventional method of heating the specimen through the heat conduction, it has been difficult to eliminate the effects of magnetic fiux due to current through the specimen-heating holder as well as stray electrons, resulting in an inferior quality of the image of the specimen analyzed.

According to the invention, with the construction as described above the stray electrons may be prevented from entering into the specimen 5, so that a specimen image of excellent quality may be obtained from the specimen in the heated state.

Secondly, the specimen may be heated through the combination of heat radiation and electron bombardment to elevate the temperature of the specimen to above l,l00 C. With the preferred embodiment of the invention, it is possible to raise the specimen temperature up to about l,500 C. in this manner.

in this method, the specimen is heated through the heat radiation alone as described above until a specimen temperature of 1,000 to l,lO0 C. is reached, whereupon the switch 27 is switched over to the side of the contact 29 to apply a positive voltage of the bias DC source 26 to the specimen 5 via specimen support base 16, legs 15 and specimen holder 14 so as to additionally heat the specimen through the electron bombardment. When the specimen 5 is positively biased with respect to the filament 5, the thermions given off the filament 9 are accelerated toward the specimen 5 to bombard the same.

The specimen-absorbed electron current image, secondary electron image and reflection electron image cannot be displayed concurrently with the heating of the specimen through electron bombardment. In order to take pictures of the specimen image, heating through the electron bombardment should be interrupted (for about 1 minute) by switching the switch 27 over to the side of the contact 28, thereby applying a positive voltage to the filament support 10, so that the stray electrons may be taken up by the filament support to permit taking pictures of the specimen image displayed on the cathode-ray tube.

FIGS. 4a and 412 show the interrelation among the specimen temperature, heating time required to reach the corresponding temperature and bias voltage applied to the specimen for additionally heating the same through electron bombardment.

Referring to the NOS, for time range A the specimen, which has been heated through the heat radiation alone as described above up to l,l00 C. (with the final F.C. being adjusted to be equal to amperes), is additionally heated by successively increasing the bias voltage l-l.V. on the specimen step by step by volts for each step with a constant step interval of 1 minute from 100 to 300 volts, at a time 26 minutes after the commencement of heating the specimen (hereinafter referred to as p.h.c.), the specimen temperature reaches l,l30 C. At time 26 minutes p.h.c. the H.V. is reduced to zero with the F.C. held equal to 80 amperes. This condition is maintained until time 27.5 minutes p.h.c.

For time range B, the F.C. is increased to 85 amperes at time 27.5 minutes p.h.c. and the bias voltage l-I.V. is increased stepwise with a unit step of 100 volts up to 500 volts. At time 41.5 minutes p.h.c. the specimen temperature reaches l,320 C. In this time range, the specimen temperature changes between 1,] 10 and 1,320 C. during a period from time 27.5 minutes p.h.c. till time 41.5 minutes p.h.c.

For time range C, at time 41.5 minutes p.h.c. the bias voltage I-I.C. is reduced to zero with the F.C. held equal to 85 amperes. This condition is maintained until time 45.5 minutes p.h.c., whereupon the F.C. is increased to 90 amperes, and wherefrom the bias voltage I-l.V. is increased stepwise with a unit step of I volts up to 400 volts. At time 61 minutes p.h.c. the specimen temperature reaches 1,560 C. In this time range, the specimen temperature changes between 1,190 and l,560 C. during a period from time 45.5 minutes p.h.c. till time 6] minutes p.h.c.

For time range D, at time 61 minutes p.h.c. both H.V. and F.C. are reduced to zero. In this time range, the specimen temperature continues to be lowered.

This plot for the specimen temperature is obtained when heating the specimen through the combination of heat radiation and electron bombardment, for example. There may, of course, be conceived various other methods for heating the specimen than the first and second methods described above.

FIG. 2 shows the mesh filament 9 in detail. It consists of tungsten wires 90 of 0.18 mm. in diameter netted into the form of a mesh. The wires 90 are secured to one another by spot welding at their intersections 91. Thus, the filament 9 is mechanically stable and extremely strong in structure against thermal stresses and vibrations. It thus remains extremely strong even if it is heated. Also, as it is concentrically surrounding and appropriately spaced from the specimen 5, so that the specimen may be heated uniformly.

The protection shield 30 also serves to prevent the contamination of the objective lenses 2 and 3 of the optical microscope with gases (organic gases) evolved from the specimen 5 during the heating of the specimen. The gases evolved from the specimen 5 attach to the protection shield being cooled with liquid nitrogen 32. The protection shield 30 is formed with an aperture to permit the electron beam and an aperture to permit X-rays, secondary electrons, reflection electrons, etc. It may be removed away from the path of the electron beam if heating of the specimen is not required.

While in the preferred embodiment illustrated in FIG. 1 the DC source 25 and the bias DC source 26 are provided separately from each other, it is readily possible to distribute the required voltages and currents from a single source through an appropriate voltage-dividing means.

The preceding embodiment of the specimen-heating means according to the invention has been applied to the electron probe microanalyzer. The invention will now be described in connection with a specimen-heating means applied to the usual electron microscope with reference to FIG. 5.

Referring to the FIG., reference numeral 1 designates an electron beam, which is generated from an electron source and accelerated and focused through an electron beam irradiating system to strike a specimen 51 loaded in a specimen holder 50.

The specimen holder 50 is removably mounted in a heat conductive member 53 ofa movably specimen table 52, which is movable in a two-dimensional directions in a plane perpendicular to the axis of the electron beam. The heat conductive member 53 may be heated by a specimen-heating means, which has a cylindrical form with the cylindrical axis being coincident with the axis of the electron beam and is mounted on a heat insulating member 54 of such a material as a ceram- The specimen-heating means comprises a mesh filament 9 0 supported by u filament support 10 secured through an electric insulating member 11 to the heat insulating member 54 and a radiation shield I2 made of molybdenum to provide for thermal shielding and uniform temperature distribution within the specimen-heating means.

The mesh filament 9, filament support 10 and radiation shield 12 are disposed concentric with the axis of the electron beam 1. Numeral 55 designates an objective lens comprising an excitation coil 56, a yoke 57 and upper and

lower pole pieces

58 and 59. It is provided with a heat shield 60. In this embodiment, the cylindrical mesh filament 9 concentric with the electron beam axis provides the heat of radiation and bombarding electrons to the heat conductive member 53 to heat the same, thereby heating the specimen 51 loaded in the specimen holder 50 mounted in the heat conductive member 53.

The thickness of the specimen which may be observed by using the electron microscope is at most about 1 micron if the electron beam accelerating voltage is of the order of I00 kilovolts, and a specimen with a thickness of about 3 microns is observable with an electron beam accelerating voltage of the order of 1,000 kilovolts. Particularly in the field of metallography the aid of so-called high-voltage electron microscopy involving a high electron beam accelerating voltage of the order of 1,000 kilovolts is required for the observation of specimens in order to be able to observe the specimens in a state as close to the bulk state as possible without suffering from the restriction on the thickness of the specimen so as to obtain highly reliable results of analysis. In this respect, the specimen-heating means according to the invention is extremely effective in practice in that the specimen disposed at the center of the cylindrical mesh filament may be uniformly and promptly heated with excellent thermal efficiency through the combination of heat radiation and electron bombardment.

I claim:

1. A specimen-heating means for electron beam irradiation apparatus, comprising:

a cylindrical perforated filament disposed to concentrically surround a specimen to be analyzed at a predetermined distance from said specimen,

a filament support concentric with and electrically connected to said filament,

a multiwall radiation shield concentrically surrounding said filament, and

a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction.

2. A specimen-heating means for electron beam irradiation apparatus comprising:

a filament support concentric with and electrically connected to said filament,

a multiwall radiation shield concentrically surrounding said filament,

a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction and a means to apply a positive potential to said filament sup port.

3. A specimemheating means for electron beam irradiation apparatus comprising:

a filament support concentric with and electrically connected to said filament,

a multiwall radiation shield concentrically surrounding said filament,

a DC source feeding said filament with heating current. current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, and

a means to selectively apply a positive potential to either one of said filament and said filament support.

4. A scanning electron microscope comprising:

an electron source to generate electrons,

an electron lens system to focus electrons generated from said electron source into a thin electron beam and accelerate said electrons toward a specimen,

a cylindrical filament consisting of heat-resisting metal wires netted into a mesh form, said filament being disposed to concentrically surround said specimen at a predetermined distance from said specimen,

a filament support concentric with and electrically connected to said filament,

a multiwall radiation shield concentrically surrounding said filament,

a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction,

a means to selectively apply a positive potential to either one of said filament and said filament support, and

a heat shield member to ,thermally shield said electron lens system, said heat shield member being cooled with a coolant and formed with an aperture to permit the passage of the electron beam and an aperture to permit the passage of X-rays and rays of charged particles emanating from said specimen.

5. A specimen-heating means for an electron microscope comprising:

a heat conductive member accommodating a specimen holder loaded with a specimen to be analyzed, said heat conductive member being disposed concentric with an axis of an electron beam directed toward said specimen,

a cylindrical filament consisting of heat-resisting metal wires netted into a mesh form, said filament being disposed concentric with said heat conductive member,

a filament support disposed concentric with and electrically connected to said filament,

a multiwall radiation shield concentrically surrounding said filament,

a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, and

l 7C 1F

Claims

1. A specimen-heating means for electron beam irradiation apparatus, comprising: a cylindrical perforated filament disposed to concentrically surround a specimen to be analyzed at a predetermined distance from said specimen, a filament support concentric with and electrically Connected to said filament, a multiwall radiation shield concentrically surrounding said filament, and a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction.

2. A specimen-heating means for electron beam irradiation apparatus comprising: a cylindrical perforated filament disposed to concentrically surround a specimen to be analyzed at a predetermined distance from said specimen, a filament support concentric with and electrically connected to said filament, a multiwall radiation shield concentrically surrounding said filament, a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, and a means to apply a positive potential to said filament support.

3. A specimen-heating means for electron beam irradiation apparatus comprising: a cylindrical perforated filament disposed to concentrically surround a specimen to be analyzed at a predetermined distance from said specimen, a filament support concentric with and electrically connected to said filament, a multiwall radiation shield concentrically surrounding said filament, a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, and a means to selectively apply a positive potential to either one of said filament and said filament support.

4. A scanning electron microscope comprising: an electron source to generate electrons, an electron lens system to focus electrons generated from said electron source into a thin electron beam and accelerate said electrons toward a specimen, a cylindrical filament consisting of heat-resisting metal wires netted into a mesh form, said filament being disposed to concentrically surround said specimen at a predetermined distance from said specimen, a filament support concentric with and electrically connected to said filament, a multiwall radiation shield concentrically surrounding said filament, a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, a means to selectively apply a positive potential to either one of said filament and said filament support, and a heat shield member to thermally shield said electron lens system, said heat shield member being cooled with a coolant and formed with an aperture to permit the passage of the electron beam and an aperture to permit the passage of X-rays and rays of charged particles emanating from said specimen.

5. A specimen-heating means for an electron microscope comprising: a heat conductive member accommodating a specimen holder loaded with a specimen to be analyzed, said heat conductive member being disposed concentric with an axis of an electron beam directed toward said specimen, a cylindrical filament consisting of heat-resisting metal wires netted into a mesh form, said filament being disposed concentric with said heat conductive member, a filament support disposed concentric with and electrically connected to said filament, a multiwall radiation shield concentrically surrounding said filament, a DC source feeding said filament with heating current, current from said DC source being adapted to pass through said filament in one direction and through said filament support in the opposite direction, and a means to selectively apply a positive potential to either one of said filament and said filament support.