US3928783A - Thermionic cathode heated by electron bombardment - Google Patents

Thermionic cathode heated by electron bombardment Download PDF

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Publication number
US3928783A
US3928783A US423107A US42310773A US3928783A US 3928783 A US3928783 A US 3928783A US 423107 A US423107 A US 423107A US 42310773 A US42310773 A US 42310773A US 3928783 A US3928783 A US 3928783A
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Prior art keywords
cathode
cylinder
electron
coil
layer
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US423107A
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English (en)
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Shigeru Hosoki
Michio Ohtsuka
Satoru Fukuhara
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Hitachi Ltd
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Hitachi Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/20—Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment

Definitions

  • ABSTRACT A structure wherein a cathode, which is made of a material, such as lanthanum hexa-borides, prone to react with metals at high temperatures and having a high electron emissivity, is held by a supporter which is made of an electrically insulating material. Concentric metal cylinders are arranged at the outer circumference of the supporter to surround a part of the cathode and a heating coil is arranged in the interstice between the metal cylinders. An electron-emissive metal oxide layer is formed on the inside surface of the inner metal cylinderf' Thermions created from the oxide layer strike the-cathode, and the cathode is heated by the heat of the-electron bombardment.
  • a cathode which is made of a material, such as lanthanum hexa-borides, prone to react with metals at high temperatures and having a high electron emissivity
  • borides such as lanthanum hexa-borides (LaB and yttrium hexa-borides (YB have a small work function, and are suitable as cathode materials.
  • LaB and yttrium hexa-borides have a small work function, and are suitable as cathode materials.
  • YB yttrium hexa-borides
  • the direct heating type requires high power.
  • Indirect heating type is, therefore, effective in reducing power comsumption to as low a value as possible.
  • FIG. 1 is a schematic diagram showing an example of a prior-art cathode heating device, which is constructed such that a heating coil 2, made of tungsten or the like, is held in a space surrounding cathode 1, made of lanthanum hexa-boride (LaB and heating power is supplied from a power source 3 to the coil 2. Between the cathode l and the heating coil 2, an accelerating power source 4 is connected for electron bombardment.
  • a heating coil 2 made of tungsten or the like
  • the cathode l when the coil 2 is heated, the cathode l is heated by the radiant heat. Simultaneously therewith, thermions emitted from the coil 2 are drawn to the cathode l by the voltage of the accelerating power source 4, and the cathode l is heated by heat which is generated by the electron bombardment.
  • the heating coil 2 In order to increase the temperature .of the cathode l to working temperature the heating coil 2 must be heated to a temperature of at least about 2,5 00-2,800C when employing a tungsten wire.
  • heat losses due to thermal conduction from the leads at both ends of the coil 2 to the'exterior and the heat loss due to the thermal radiation from the. coil 2 become so great as not to be negligible.
  • FIG. 2 shows a sketch for roughly estimating the heat losses due to thermal conduction and due to thermal radiation.
  • leads 5 of stainless steel are connected to both ends of the heating coil 2 of tungsten'in order to diminish the loss due to the thermal conduction.
  • T denote the temperature of the central part of the heating coil 2 which is uniform
  • T denote the temperature of the point of contact between the coil 2 and the lead 5
  • T denote.
  • the temperature of the end of the lead 5 remote from the coil 2.lt is assumed that the diameter ofthe coil 2 is 0.02 cm, that the length of the coil extended in a straight line is 2.0 cm, that the sectional area of the lead 5 is 4.45 X cm, that T 2,800C, that T I,500C and that T, l,000 CI
  • a heat shielding plate is provided around the coil 2
  • the temperature of the heat shielding plate is assumed to be I,Q00C.
  • the radiation heat loss and the condition heat loss can be respectively calculated to'be approximately tO W and W.
  • An object of the present invention is to provide a device for heating a cathode made of a material such as lanthanum hexa-borides (LaB in which the heat loss by the thermal radiation is small and the heating efficiency is high.
  • LaB lanthanum hexa-borides
  • the present invention includes a heating coil provided within an indirect heating case and an electron-emissive metal oxide layer formed on the surface of the indirect heating case facing to a cathode.
  • FIG. 1 is a schematic view depicting a prior-art heating arrangement
  • FIG. 2 is a schematic view for calculating losses due to thermal radiation and thermal conduction froma heating coil
  • FIG. 3 is a constructional view of an embodiment of the present invention.
  • FIG. 4 is a schematic view for calculating thermal conduction
  • FIG. 5 is a structural view of a cathode
  • FIG. 6 is a constructional view showing another embodiment of the present invention.
  • FIG. 7 is a sectional view of a portion of still another embodiment of the present invention.
  • FIGS. 8a to 8d are sectional views of supporters for use in the present invention.
  • FIG. 9 is a structural view in the case where the heating device of the present invention is applied to an actual cathode.
  • a boride material such as lanthanum hexa-boride (LaB and yttrium hexa-boride (YB is positioned in a hollow core portion of a supporter 6 made of a high temperature-resistant material, such as boron nitride (BN), which is electrically insulating.
  • concentric cylinder bodies 10a and 10b made of nickel or the like.
  • the end parts of the cylinders on one side are connected by a metal sheet 100.
  • an electron-emissive wall made of a coating 9 which is made of an electron-emissive metal oxide such as barium oxide (BaO), calcium oxide (CaO) and strontium oxide (SrO).
  • An electrode 7 of graphite or the like is mounted on one end of the cathode l, and is connected through a lead 8 to the positive terminal of an electron accelerating power source 4 for electron bombardment.
  • the negative terminal of the power source 4 is connected to the cylinder a.
  • a heating coil 2 of tungsten or the like insulated by an alumina coating layer is arranged between the concentric cylinders 10a and 10b, and is supplied with heating power from a power source 3.
  • the cylinders 10a and 10b are also heated.
  • the oxide .layer 9 is subjected to the indirect heating.
  • the oxide layer 9 is heated to approximately 800C, thermions are emitted. Since the cathode l is applied with a positive potential through the electrode 7, which is made of graphite or the like material and does not readily react with the cathode at high temperatures the thermions emitted from the oxide layer 9 are attracted toward the cathode l and impinge thereon.
  • the cathode l is then heated to approximately l,300-I,800C by the heat of the electron bombardment.
  • the cylinder 10b Since, in this case, the cylinder 10b has a temperature lower than the cathode 1, heat losses due to thermal conduction and thermal radiation from the cathode l are mostly fedback to the cylinder 10b as is apparent from the construction shown in the figure. Accordingly, once the cylinders 10a and 10b have been heated, the power required for maintaining them at 800C may be very slight. Heat losses due to thermal radiation from the cylinder 10a are extremely small, because the temperature of this cylinder is as low as 800C.
  • the cathode 1 is not heated directly by the radiant heat of the heating coil 2, but is heated by the electron bombardment heat in such way that the metal cylinders 10a and 10b are heated and that thermions emitted from the oxide layer 9 formed on the inside surface of the cylinder 10b are accelerated and impinge upon the cathode 1, so that the cathode 1 can be heated to a desired high temperature in the state in which the metal cylinders 10a and 1012 are at a temperature lower than that of the cathode 1.
  • the heat loss components are those due to thermal conduction from the cathode 1 through the supporter 6 to the cylinder 10b and these directly escaping due to thermal radiation from the cathode 1. Since both these heat loss components are fed-back to the cylinder 10b surrounding the cathode, there is essentially no heatloss, and a highly efficient heating can be effected.
  • the material of the cylinders 10a and 10b is not restricted to nickel, but may be any metal having deoxidizing properties at a high temperature of about 800C.
  • the coating layer is not restricted to electron-emissive metal oxide layer 9 but a layer of a sintered body or an impregnation layer of a porous metal may be adopted, insofar as it has an electron emissivity equivalent to that of the coating layer 9.
  • the improved device has a few problems.
  • One of them is that as the loss heat from the cathode I is effectively fed-back to the cylinder 10b, the temperature of the metal oxide layer 9 increases more than is necessary, with the result that the deterioration of the oxide layer 9 is hastened.
  • the excessive increase in the temperature of the oxide layer 9 in FIG. 3 is attributable to the fact that the thermal feedback from the cathode I is too great.
  • the causes for the thermal feedback are l the thermal radiation from the cathode l and 2 the thermal conduction through the supporter 6. Rough estimates will be hereunder explained for both mechanisms of thermal feedback.
  • the desired temperatures are approximately l,500C for the cathode I made of LaB or the like and approximately 800C for the oxide layer 9. For the sake of simplicity, therefore, those quantities of heat flow, for the respective heat transfer mechanisms when the temperature difference between the specified values is assumed may be determined.
  • FIG. 4 indicates the dimensions necessary for such calculations.
  • T and T are the temperatures of the cathode l and the oxide layer 9 respectively.
  • 6 is the emissivity
  • 0' is Stephen-Boltzmanns constant
  • K the coefficient of heat transfer of the supporter 6.
  • alumina' has a coefficient of heat transfer of;I( 0.07- Joule/ sec cm C (at 800C), which is about one order smaller than the coefficient of heat transfer of boron nitride
  • the use of alumina is preferable to boron nitride.
  • the value 700C has been observed as the temperature difference between the cathode 1 and the oxide layer 9, and the life of the oxide layer 9 has been extended.
  • the desired temperature difference can be established by appropriately determining the dimensions 11,, d 1 and l or by making the areas and shapes of the contact surfaces between the cathode 1 and the supporter 6 and between the supporter 6 and the cylinder 10b different.
  • the power consumption of the cathode can thus be minimized by selecting the insulating material of the supporter 6 in dependence on the shape and dimensions of the cathode l with reference to equations (1) and (2) for Qc and Qr respectively.
  • the cathode 1 is heated to a temperature (for example, 2000C) considerably higher than the usual working temperature, the temperature of the contact part between the supporter 6 and the cathode l increases, and both members chemically react with each other in some cases.
  • a layer 11 made of graphite powder or a sintered body thereof may be formed between the cathode 1 and the supporter 6, as shown in FIG. 5.
  • Another problem of the embodiment in FIG..3 is that when the cathode l is used at high temperatures for a long period of time, a thin film which is electrically conductive is formed on the surface of the supporter 6 of the high temperature-resistant insulating material by the vaporization of the cathode material such as LaB resulting in a degradation of the insulation between the cathodel and the oxide layer 9.
  • an evaporation preventing plate 12 may be provided in proximity to the supporter 6 as illustrated in FIG. 6. Since the evaporation preventing plate 12 functions as a mask for the vaporization of the cathode material and prevents the material from adhering to the surface of the supporter 6, there is good electrical insulation between the cathode 1 and the oxide layer 9 for a long period of time.
  • FIG. 9 shows an overall concrete structure of the heating device of the present invention.
  • the aperture of the metal sheet for connecting the end parts of the cylindrical bodies 10a and 10b is made-smaller than the inside diameter of the cylindrical oxide layer 9. This serves to prevent thermions for the electron bombardment, emitted from the oxide layer 9, from being mixed into the thermions which are emitted from the cathode 1 towards the opening portion of a Wehnelt electrode 16 or a grid electrode.
  • the heating coil 2 has power supplied thereto through lead wires from a power source (not shown) which is disposed outside a cathode base 14 of glass or the like.
  • the cylinder 10a is connected to electrode terminals by lead wires 13.
  • a thermionic cathode structure comprising:
  • a cathode made of a material having a high electron emissivity
  • a supporter made of an electrically-insulating and high temperature-resistant material for supporting a part of said cathode
  • a first cylinder held in contact with said supporter and arranged so as to surround a part of said cathode
  • a cathode heating coil arranged in an interstitial space between said first and second cylinders
  • flange means extending from said first cylinder toward said cathode past said oxide layer, for preventing electrons emitted from said oxide layer from being mixed with electrons emitted from a part of said cathode;
  • a power source connected to said coil for supplying heating power to said coil
  • a power source connected between said cathode and said first cylinder in order to cause electrons, emit ted from said oxide layer, to strike against said cathode.
  • said oxide layer is made of at least one member selected from the group consisting of barium oxide, strontium oxide and calcium oxide.
  • a heating coil disposed adjacent said cathode
  • said means comprises a first cylinder supported by said insulating body and surrounding said cathode and a layer of electron-emissive material coated on the interior surface of said fist cylinder facing said cathode;
  • a flange extending from said first cylinder toward said cathode past said layer of electron-emissive material, said electron-emissive layer being confined between said flange and said insulating body.
  • said means comprises a wall made of an electron-emissive material which, when heated by said coil, emits thermions which impinge upon said cathode to heat said cathode.
  • said means further includes a second cylinder surrounding said coil, with said coil disposed between said second cylinder and said first cylinder.
  • said means further includes a first power source for applying a potential between said first cylinder and said cathode and a second power source for applying a heating current to said heating coil.
  • said layer of electron-emissive material is made of a 8 material selected from the group consisting of a metal oxide, a sintered body, and a porous metal-impregnated layer.
  • said metal oxide is made of at least one material selected from the group consisting of barium oxide, strontium oxide and calcium oxide.

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Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4054946A (en) * 1976-09-28 1977-10-18 Bell Telephone Laboratories, Incorporated Electron source of a single crystal of lanthanum hexaboride emitting surface of (110) crystal plane
US4055780A (en) * 1975-04-10 1977-10-25 National Institute For Researches In Inorganic Materials Thermionic emission cathode having a tip of a single crystal of lanthanum hexaboride
US4115720A (en) * 1977-03-31 1978-09-19 Rca Corporation Device having thermionic cathode heated by field-emitted electrons
US4137476A (en) * 1977-05-18 1979-01-30 Denki Kagaku Kogyo Kabushiki Kaisha Thermionic cathode
US4258283A (en) * 1978-08-31 1981-03-24 Balzers Aktiengesellschaft Fur Hochvakuumtechnik Und Dunne Schichten Cathode for electron emission
US4288717A (en) * 1979-11-06 1981-09-08 Denki Kagaku Kogyo Kabushiki Kaisha Thermionic cathode apparatus
US4297615A (en) * 1979-03-19 1981-10-27 The Regents Of The University Of California High current density cathode structure
US4333035A (en) * 1979-05-01 1982-06-01 Woodland International Corporation Areal array of tubular electron sources
US4438557A (en) * 1979-05-01 1984-03-27 Woodland International Corporation Method of using an areal array of tubular electron sources
US4560907A (en) * 1982-06-25 1985-12-24 Hitachi, Ltd. Ion source
GB2338825A (en) * 1998-06-24 1999-12-29 Advantest Corp An electron gun
US20150187541A1 (en) * 2013-12-30 2015-07-02 Mapper Lithography Ip B.V Cathode arrangement, electron gun, and lithography system comprising such electron gun
WO2019172433A1 (en) * 2018-03-09 2019-09-12 Atonarp Inc. Device including an ionizer
US20210050174A1 (en) * 2018-03-23 2021-02-18 Freemelt Ab Cathode assembly for electron gun
US11094493B2 (en) * 2019-08-01 2021-08-17 Lockheed Martin Corporation Emitter structures for enhanced thermionic emission
EP3886137A1 (en) * 2020-03-24 2021-09-29 FEI Company Charged particle beam source
CN119008352A (zh) * 2024-10-22 2024-11-22 中国人民解放军空军工程大学 一种电子枪阴极的电子轰击加热装置

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS49127562A (enrdf_load_stackoverflow) * 1973-04-06 1974-12-06

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US1210678A (en) * 1915-05-19 1917-01-02 Western Electric Co Thermionic amplifier.
US2159824A (en) * 1936-12-01 1939-05-23 Hans J Spanner Discharge device
US2386790A (en) * 1944-11-29 1945-10-16 Sylvania Electric Prod Electron gun and the like
US2561768A (en) * 1950-04-10 1951-07-24 Zenith Radio Corp Thermionic cathode activation
US2585582A (en) * 1949-07-07 1952-02-12 Bell Telephone Labor Inc Electron gun
US3333138A (en) * 1965-01-11 1967-07-25 Rauland Corp Support assembly for a low-wattage cathode
US3369145A (en) * 1965-04-09 1968-02-13 Wagner Electric Corp Thermionic emissive cathode
US3440475A (en) * 1967-04-11 1969-04-22 Lokomotivbau Elektrotech Lanthanum hexaboride cathode system for an electron beam generator
US3474281A (en) * 1965-12-23 1969-10-21 Siemens Ag Electron beam production system for electronic discharge
US3621324A (en) * 1968-11-05 1971-11-16 Westinghouse Electric Corp High-power cathode
US3727093A (en) * 1971-01-20 1973-04-10 Westinghouse Electric Corp Electron beam apparatus

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1210678A (en) * 1915-05-19 1917-01-02 Western Electric Co Thermionic amplifier.
US2159824A (en) * 1936-12-01 1939-05-23 Hans J Spanner Discharge device
US2386790A (en) * 1944-11-29 1945-10-16 Sylvania Electric Prod Electron gun and the like
US2585582A (en) * 1949-07-07 1952-02-12 Bell Telephone Labor Inc Electron gun
US2561768A (en) * 1950-04-10 1951-07-24 Zenith Radio Corp Thermionic cathode activation
US3333138A (en) * 1965-01-11 1967-07-25 Rauland Corp Support assembly for a low-wattage cathode
US3369145A (en) * 1965-04-09 1968-02-13 Wagner Electric Corp Thermionic emissive cathode
US3474281A (en) * 1965-12-23 1969-10-21 Siemens Ag Electron beam production system for electronic discharge
US3440475A (en) * 1967-04-11 1969-04-22 Lokomotivbau Elektrotech Lanthanum hexaboride cathode system for an electron beam generator
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Cited By (27)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4055780A (en) * 1975-04-10 1977-10-25 National Institute For Researches In Inorganic Materials Thermionic emission cathode having a tip of a single crystal of lanthanum hexaboride
US4054946A (en) * 1976-09-28 1977-10-18 Bell Telephone Laboratories, Incorporated Electron source of a single crystal of lanthanum hexaboride emitting surface of (110) crystal plane
US4115720A (en) * 1977-03-31 1978-09-19 Rca Corporation Device having thermionic cathode heated by field-emitted electrons
US4137476A (en) * 1977-05-18 1979-01-30 Denki Kagaku Kogyo Kabushiki Kaisha Thermionic cathode
US4258283A (en) * 1978-08-31 1981-03-24 Balzers Aktiengesellschaft Fur Hochvakuumtechnik Und Dunne Schichten Cathode for electron emission
US4297615A (en) * 1979-03-19 1981-10-27 The Regents Of The University Of California High current density cathode structure
US4333035A (en) * 1979-05-01 1982-06-01 Woodland International Corporation Areal array of tubular electron sources
US4438557A (en) * 1979-05-01 1984-03-27 Woodland International Corporation Method of using an areal array of tubular electron sources
US4288717A (en) * 1979-11-06 1981-09-08 Denki Kagaku Kogyo Kabushiki Kaisha Thermionic cathode apparatus
US4560907A (en) * 1982-06-25 1985-12-24 Hitachi, Ltd. Ion source
GB2338825A (en) * 1998-06-24 1999-12-29 Advantest Corp An electron gun
GB2338825B (en) * 1998-06-24 2001-03-28 Advantest Corp Method for extending life of an electron gun
US20150187541A1 (en) * 2013-12-30 2015-07-02 Mapper Lithography Ip B.V Cathode arrangement, electron gun, and lithography system comprising such electron gun
US9466453B2 (en) 2013-12-30 2016-10-11 Mapper Lithography Ip B.V. Cathode arrangement, electron gun, and lithography system comprising such electron gun
US20160314935A1 (en) * 2013-12-30 2016-10-27 Mapper Lithography Ip B.V. Focusing electrode for cathode arrangement, electron gun, and lithography system comprising such electron gun
US10622188B2 (en) * 2013-12-30 2020-04-14 Asml Netherlands B.V. Focusing electrode for cathode arrangement, electron gun, and lithography system comprising such electron gun
US9455112B2 (en) * 2013-12-30 2016-09-27 Mapper Lithography Ip B.V. Cathode arrangement, electron gun, and lithography system comprising such electron gun
WO2019172433A1 (en) * 2018-03-09 2019-09-12 Atonarp Inc. Device including an ionizer
US12062518B2 (en) * 2018-03-23 2024-08-13 Freemelt Ab Cathode assembly for electron gun
US20210050174A1 (en) * 2018-03-23 2021-02-18 Freemelt Ab Cathode assembly for electron gun
US11094493B2 (en) * 2019-08-01 2021-08-17 Lockheed Martin Corporation Emitter structures for enhanced thermionic emission
JP2021153051A (ja) * 2020-03-24 2021-09-30 エフ イー アイ カンパニFei Company 荷電粒子ビーム源
US20210305006A1 (en) * 2020-03-24 2021-09-30 Fei Company Charged particle beam source
KR20210119320A (ko) * 2020-03-24 2021-10-05 에프이아이 컴파니 하전 입자 빔 소스
US11380511B2 (en) * 2020-03-24 2022-07-05 Fei Company Charged particle beam source
EP3886137A1 (en) * 2020-03-24 2021-09-29 FEI Company Charged particle beam source
CN119008352A (zh) * 2024-10-22 2024-11-22 中国人民解放军空军工程大学 一种电子枪阴极的电子轰击加热装置

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JPS4979768A (enrdf_load_stackoverflow) 1974-08-01

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