Electron Gun Device Of Field Emission Type

Abstract

An electron gun device of the field emission type comprising an electron emitting cathode tip, a filament for heating the cathode tip, an anode, and a charged particle emitting electrode, in which charged particles are emitted from the electrode and bombard the anode surface to outgas the anode.

Description

BACKGROUND OF THE IVENTION

1. FIELD OF THE INVENTION

This invention relates to an improvement in the anode heating system of a field emission type electron gun device and more particularly to a field emission type electron gun device in which the anode is outgassed by a beam of charged particles.

2. DESCRIPTION OF THE PRIOR ART

A conventional electron gun device of the field emission type is shown in FIG. 1 in cross-sectional view. A flashing filament 1 is arranged to heat a cathode tip of a needle shape 2 for improving the electron emission efficiency. The cathode tip of needle shape 2 has a sharpened head and is fixed to the flashing filament 1. An anode is indicated by numeral 3. Electrons 4 are emitted from the top end of the cathode tip 2 by applying a high voltage between the cathode tip 2 and the anode 3 from a high voltage source (not shown). A part of the electrons 5 transmits through an aperture 6 of the anode 3 and is introduced into the lower part of the device, and will be referred to as effective electrons. A support 7 supports the flashing filament 1. An infrared lamp 8 is supported by a support 9 and heats the anode 3. The lamp 8 is supplied power from a source 10. It is required for emitting electrons from the cathode tip 2 that the inside of the device is maintained at an ultra high vacuum in the order of 1 .times. 10.sup. .sup.-10 Torr and that a high electric field in the order of 3 to 6 .times. 10.sup.7 V/cm is applied to the top end of the cathode tip 2. Among the electrons emitted from the tip 2 by said intense field, those except the effective electrons bombard the anode 3. If there are absorbed gas molecules on said anode, the absorbed gas molecules are released and ionized by said electrons 4. The ions thus generated are attractively accelerated to the cathode tip 2 by said intense field and strike the tip 2 with high speeds. Thus, the sharpened top end of the cathode tip is damaged and rounded by said ions. Thereby, the electric field around the top end is weakened and the electron emission efficiency decreases. To avoid the collison of such ions, an infrared lamp 8 has been conventionally provided near the anode 3 and preliminarily excited to heat the anode 3 to remove absorbed gas molecules. Alternatively, it has also been employed for heating the anode 3 that a current is directly allowed to flow through the anode 3. In these cases, an infrared lamp of large capacity or a large current is necessary for this heating since the heat capacity of a field emission type electron gun device is large.

As is described above, such methods have been employed in conventional field emission type electron gun device for preventing the damage of an electron source due to the collision of ions generated by the ionization of gas molecules absorbed on the anode, as exciting an infrared lamp provided near the anode or allowing a conduction current to flow through the anode itself so as to remove the absorbed gas molecules in the anode surface. But such conventional methods have a drawback in that an infrared lamp or a power source used for the above purpose should have a large capacity of 1 to 3 kw since the heat capacity of a field emission type electron gun device is large.

SUMMARY OF THE INVENTION

An object of this invention is to provide in an electron gun device of the field emission type including a cathode tip of needle shape, a filament for heating said cathode tip, an anode, and means for applying an electric voltage between said cathode tip and said anode to establish an electric field therebetween which causes electrons to be emitted from said cathode tip toward said anode, an improvement comprising means for emitting a beam of charged particles toward the surface of said anode, means for supporting said charged particle emission means, and power source means for supplying a heating current to said charged particle emission means.

According to an embodiment of this invention, there is provided a field emission type electron gun device of an excellent electron emission efficiency and stable operation in which the electron source is prevented from being damaged with only a small heating power by heating the anode with charged particles emitted from a charged particle emitting electrode. An embodiment comprises an electron source, an anode, and a charged particle emitting electrode, in which charged particles emitted from said charged particle emitting electrode are accelerated by the electric voltage applied between the anode and said electrode and heat said anode by the collision.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a conventional electron gun device of the field emission type.

FIG. 2 is a cross-sectional view of an embodiment of an electron gun device of the field emission type according to the invention.

FIG. 3 is a cross-sectional view of another embodiment of an electron gun device of the field emission type according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the invention will be described on the preferred embodiments referring to the accompanying drawings. An embodiment of a field emission type electron gun device according to the invention is shown in FIG. 2, in which in place of the infrared lamp 8 of the conventional device shown in FIG. 1 a hot cathode such as a thermal electron emitting filament 11 is mounted on a supporting member 9 opposing to the anode 3. A current is supplied from a souce 10 to said hot cathode to heat the cathode. At the same time, a negative voltage V is applied between the anode 3 and the hot cathode 11 by a d.c. source 12. Thus, the portion of the anode 3 which is to be bombarded by electrons 4 is locally heated by electrons emitted from said hot cathode 11 and accelerated by the voltage V to exhaust the gas absorbed on the anode 3.

FIG. 3 shows another embodiment in which a thermal electron emitting filament 11 is mounted unitarily to the same base 13 as the flashing filament 1 to enhance the replacement of the thermal electron emitting filament 11 in case of a cut of the filament 11. According to this method, a thermal electron emitting filament can be provided for each flashing filament or tip portion even in case of a field emission type electron gun device having a plurality of flashing filaments or tips. Thereby the aforementioned effects can also be obtained in such a case. In the above embodiments, a hot cathode is used as the charged particle emitting electrode, but an ion source may also be employed in place of said hot cathode to obtain similar effects.

As has been described above, according to the field emission type electron gun device of this invention, the anode surface can be effectively and locally heated by charged particles. Thereby, the total dimension of a device can be reduced and also gas molecules absorbed on the anode can be preliminarily and effectively removed. Therefore, the number of ions colliding with the tip decreases, hence the lifetime of the tip is elongated and simultaneously the stability of the field emission of electrons is improved.

Claims

We claim:

1. In an electron gun device of the field emission type including a cathode tip of a needle shape, a filament for heating said cathode tip, an anode, and means for applying an electric voltage between said cathode tip and said anode to establish an electric field therebetween which causes electrons to be emitted from said cathode tip toward said anode, the improvement comprising means for emitting a beam of charged particles toward the surface of said anode, means for supporting said charged particle emission means, and power source means for supplying a required power to said charged particle emitting means.

2. An electron gun device according to claim 1, wherein said charged particle emitting means includes an electrode for emitting electrons.

3. An electron gun device according to claim 2, further comprising a voltage source connected between said electrode and said anode for applying accelerating voltage for electrons emitted from said electrode therebetween.

4. An electron gun device according to claim 3, wherein said supporting means is so constructed as to support said electrode and said filament.