1. Field of the Invention
The present invention relates to an electron tube device, more particularly to an electron tube device mounted with a cold cathode having an electron gun which uses a cold cathode provided with an array of field emitters as an election source, and a method of impressing voltages on electrodes of the electron tube device.
2. Description of the Related Art
Collision of positive ions with a cold cathode is one of the reasons for degradation of the cold cathode of an electron tube which uses a cold cathode as an electron source. Positive ions are generated when beam collide with an electrode such as a collector electrode or an accelerating electrode having electric potential higher than that of the emitters or the residual gas in an electron tube. Since generated positive ions tend to proceed in the direction of lower electric potential, some of the ions proceed toward the cold cathode. When these positive ions collide with a cold cathode emitter, the emitter is deformed. The beam current from the cold cathode is highly sensitive to deformation of the shape of the emitter and easily changed by its influence. The degradation of characteristics of a cold cathode caused by collision of positive ions is remarkably larger than in a hot cathode. Therefore, in an electron tube having a cold cathode as an electron source, degradation of characteristics proceeds rapidly
In order to prevent the degradation characteristic in a cold cathode, for example, as disclosed in Japanese Patent Laid-open No. 63489/97, an electron tube mounted with a cold cathode of this kind has hitherto been provided with a mechanism which prevents the degradation of the cold cathode caused by collision against the cold cathode by positive ions generated on the collector electrode side.
FIGS. 1A and 1B show an example of a structure of an electron tube mounted with a cold cathode disclosed in Japanese Patent Laid-open No. 63489/97. Around cold cathode 11 for emitting electron beam e, there is provided Wehnelt electrode 12, with accelerating electrode 13, ion trap electrode 14 and collector electrode 15 also provided. In cold cathode 11, for example, a part of which is shown in an enlarged view in FIG. 1B, a number of needle-shaped emitters 22 are regularly disposed on the surface of silicon substrate 21, and gate electrodes 24 are disposed each having gate hole 23 which is disposed in front of and near the top of the emitter 22 corresponding to each emitter. Gate electrode 24 is composed of a metallic thin film and disposed on substrate 21 through insulation layer 25. When the electron tube is operated, as shown in FIG. 1A, a control voltage in a range of 0.+-. several volts is applied from gate power supply 31 to gate electrode 24 against cold cathode 11. Further, a negative voltage of several hundred V is given to Wehnelt electrode 12 from Wehnelt power supply 32, and a positive accelerating voltage of several kV is impressed on accelerating electrode 13 from power supply 33. Further, a negative voltage of several hundred V against collector electrode 15 is applied from power supply 35 to ion trap electrode 14.
The operation of the electron tube device mounted with the cold cathode will next be described. By controlling gate electrode 24 to the proper electric potential, electrons are emitted from the top of each emitter 22 and radiated in the direction of collector electrode 15 passing through each corresponding gate hole 23 with the acceleration potential generated by accelerating electrode 13. At this time, positive ions generated in collector electrode 15 have a tendency to proceed in the direction of a cathode of low electric potential (direction of ion trap electrode 14). However, since the electric potential of accelerating electrode 13 is sufficiently high, positive ions are repelled by means of the electric potential of the accelerating electrode 13 and acquired by ion trap electrode 14. Therefore, positive ions can hardly reach cold cathode 11 and hence deterioration of the cold cathode can be prevented.
Further, in Japanese Patent Laid-open No. 192638/95, there are disclosed conditions that prevent deterioration of a cathode caused by the collision of positive ions against the cathode in a traveling wave tube device which is one of electron tube devices mounted with cold cathodes, the positive ions being generated in a slow wave circuit or the collector electrode side of the traveling wave tube device.
FIG. 2 shows an example of a structure of the traveling wave tube disclosed in Japanese Patent Laid-open No. 192638/95. A traveling wave tube is an electron tube which amplifies a microwave by utilizing the interaction between the electron beam (e) and the microwave, and has slow wave circuit 2 which makes the electron beam and the microwave interact between an electron gun and a collector electrode (not shown). The electron gun includes cathode 10, Wehnelt electrode 12, accelerating electrode 13 and ion barrier electrode 16. If a beam current is denoted as Io (A), a beam radius ro (m), the inside diameter of ion barrier electrode 16 rib (m), electric potential of slow wave circuit 2 Vo (V), the inside diameter rib and the electric potential Vib of ion barrier electrode 16 are determined so that they can satisfy the following relationship. ##EQU1##
According to the present invention, the ion barrier electrode can prevent ions from reaching the cathode by always forming a surface of high electric potential which can prevent the generation of positive ions to caused in a slow wave circuit or the collector electrode side, that is, a barrier. The patent has no description with reference to a cold cathode, but it is also applicable to a traveling wave tube mounted with a cold cathode.
In this way, a mechanism is proposed which can prevent the deterioration of the characteristics of a cathode caused by collision of a cathode with positive ions generated in a collector electrode or a slow wave circuit other than an electron gun.
In an electron gun using a hot cathode as an electron source, the maximum emission current to be obtained from the electron gun is determined by the Langmuir-Child law. In other words, according to the Langmuir-Child law, the maximum emission current is determined by the product of a coefficient inevitably determined by the electron gun structure (hereinafter called a perveance) and 3/2 power of the accelerating electrode voltage.
On the other hand, in the electron gun using a cold cathode as the electron source, the emission current is necessarily determined by the gate electrode impressed voltage and does not satisfy the above Langmuir-Child law. Consequently, when a cold cathode is used as the electron source, a beam current in excess of the product of an electron gun perveance determined by the structure of the electron gun and 3/2 power of the accelerating electrode voltage can be removed from the cathode.
In this case, when the beam current emits electrons from the cathode under an operating condition exceeding the operating conditions of a space charge restriction region indicated by the product of the perveance of the electron gun and 3/2 power of the accelerating electrode voltage, there is a problem that electric charges in express of the electric charges allowed by the electron gun structure will exist in the space in the vicinity of the cathode. Particularly, an array of cold cathodes composed of two or more emitters form a domain where electron density becomes high within the region in which electrons emitted from neighboring emitters interact. That is, in a cold cathode composed of a single emitter, the beam current receives only space charge restrictions formed in the extreme vicinity of the emitter surface. Electrons which override the space charge restrictions in the vicinity of the emitter surf ace fly under the control of the electron lens system. On the other hand, in the cold cathode comprising an array of field emitters which can supply a large current, electrons overriding the space charge restrictions in the vicinity of said emitter surface are next subject to space charge restrictions from electrons emitted from the neighboring emitter, being accordingly subjected to restrictions related to lateral divergence. Therefore, when the electron beams are considered as a whole, charges are accumulated in the region in which electrons emitted from neighboring emitters on the cathode surface interact, then beam transmission is rapidly deteriorated from the effects by the electric field formed by these excessive charges, that is, the electron beam diverges. At this time, motion energy of the electron is almost 0 eV for the hot cathode, but is about several tens eV for the cold cathode because electrons are accelerated by the gate electrode impressed voltage. A part of these dispersed electrons become uncontrollable and collide with the accelerating electrode and a helix disposed to it in the traveling wave tube. When electrons collide with the accelerating electrode, positive ions or gas are generated from the accelerating electrode. When the beam strikes the gas generated from the accelerating electrode, positive ions are generated. These positive ions collide with the cold cathode and cause deformation of the emitter. In this way, in the pro art electron tube device mounted with the cold cathode, the emission characteristics of the cold cathode are deteriorated.
In the prior art, there has been a method for preventing positive ions generated by the collector from colliding with the cathode by providing an ion barrier electrode. However, since there has been no definite limitation in the relationship between the beam current and the accelerating electrode voltage, no applicable means has been presented for preventing positive ions from being generated between the cathode and the accelerating electrode, and hence design procedures for a constantly stable action have never been realized.