JPS6340017B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a field emission type emitter protection circuit for protecting an emitter of a high acceleration voltage electron gun such as a field emission type electron gun or an ion gun from vacuum discharge.

The configuration of a conventional field emission type electron gun is shown in FIG. In the figure, reference numeral 1 represents an electron gun chamber kept in a vacuum, and the chamber includes an emitter 3 attached to a heating filament 2, an extraction electrode 4 for applying a strong electric field to the tip of the emitter 3, and an anode 5 kept at ground potential. Each of these electrodes is connected to a power supply case 10 by an insulated cable 6. Emitter 3 is installed in the power supply cabinet 10.
an accelerating power supply 7 for applying a high potential of about (-) + KV to the emitter 3, an extraction power supply 8 for providing a potential difference of about +KV for emitting a field between the emitter 3 and the extraction electrode 4, and a filament heating power supply 9. is provided.

In the above configuration, in normal operation, an extraction voltage of several +KV is applied between the emitter 3 and the extraction electrode 4 to perform field emission, and an electric field is applied between the emitter 3 and the anode (grounded electrode) 5. The device is operated by applying a voltage of (-) + KV as an accelerating voltage to accelerate the emitted electron beam.
On the other hand, for some reason during operation, for example, the electron gun chamber 1
This is a case in which vacuum dielectric breakdown occurs due to a decrease in the degree of vacuum, microprotrusions on each electrode, microparticles remaining in the electron gun chamber, etc. This vacuum breakdown is
Since the accelerating voltage (for example, 50 KV) is higher than the extraction voltage (for example, 5 KV), many occurrences occur between the extraction electrode 4 and the anode 5 to which a high voltage is applied. When vacuum dielectric breakdown occurs between the extraction electrode 4 and the anode 5 and a vacuum discharge occurs, the extraction electrode 4 momentarily becomes the ground potential, and since the extraction power source 8 has internal impedance, the difference between the emitter 3 and the extraction electrode 4 Then, a high voltage of several +KV, which is an accelerating voltage, is applied transiently, and an excessive voltage exceeding the allowable value is applied between the emitter 3 and the extraction electrode 4.
By the way, the radius of curvature of the tip of the emitter 3 is approximately
Since it is extremely small at 1000 Å, the emission current from the emitter 3 increases due to this excessive voltage, and the tip of the emitter 3 evaporates due to the Joule heat generation, causing an arc discharge and deforming the tip to a diameter of about 10 μm. , it becomes unusable as a field emission type emitter again.

The present invention has been made in view of these points, and includes an emitter, an extraction electrode for forming a strong electric field necessary for field emission of charged particles near the tip of the emitter, and an extraction electrode for forming a strong electric field necessary for field emission of charged particles near the tip of the emitter, and a In an apparatus having an accelerating electrode for accelerating charged particles, an extraction power source for supplying an extraction voltage between the emitter and the extraction electrode, and an acceleration power source for supplying the accelerating voltage between the emitter and the accelerating electrode, the acceleration By connecting a series circuit with a diode and a circuit in which a capacitor with a capacitance considerably smaller than the ground capacitance of the power supply is connected in parallel with a diode to the above-mentioned lead-out power supply, damage to the emitter can be prevented even in the case of vacuum discharge. This has been achieved to prevent this.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 is a schematic diagram showing an embodiment of the present invention. Components that are the same as those in FIG. 1 are given the same numbers and their explanations will be omitted. In the figure, a capacitor 11 and a resistor 13 are placed on the power supply cabinet 10, that is, on the high-voltage power supply side.
A series circuit consisting of a circuit in which these are connected in parallel and a diode 12 is connected in parallel to the extraction power source 8. As the capacitor 11, a capacitor having a considerably larger capacitance than the ground capacitance of the accelerating power source 7 is selected.

By the way, assuming an operating state in which an extraction voltage is applied between the emitter 3 and the extraction electrode 4 and an electron beam is generated, the capacitor 11 is connected to the diode 12.
Assume that the battery is charged to a voltage equal to the extraction voltage through the battery. Further, the resistor 13 makes the diode 12 conductive in the forward direction, and compensates for the delay in diode operation due to the accumulated carrier when an excessive voltage occurs and the charging current increases rapidly. capacitor 1
The terminal voltage of 1 is lowered by this resistor 13 to follow the same.

Here, the ground capacitance of the acceleration power source 7 is Cs,
The accelerating voltage is V, the capacitance of capacitor 11 is Co,
The operating voltage (drawing voltage) of the emitter is Ve,
Considering the case where a vacuum discharge occurs between the extraction electrode 4 and the anode 5, FIG. 2 can be replaced with an equivalent circuit as shown in FIG. 3. In the figure, during normal operation, that is, when no vacuum discharge occurs between the extraction electrode 4 and the anode, the switch SW is in the OFF state, and when a vacuum discharge occurs between the extraction electrode 4 and the anode, the switch SW is in the OFF state. can be considered as an ON state. Therefore, when no vacuum discharge is generated between the extraction electrode 4 and the anode, only the extraction voltage Ve is applied between the extraction electrode 4 and the emitter 3, but when no vacuum discharge is generated between the extraction electrode 4 and the anode. When the voltage is generated, the voltage between the extraction electrode 4 and the emitter 3 is an extraction voltage Ve, and a voltage obtained by dividing the acceleration voltage V by the ground capacitance Cs of the acceleration power source 7 and the capacitance Co of the capacitor 11. A voltage Vs (formula (1) below) corresponding to the sum is applied.

Vs=Cs/Co+CsV+Ve...(1) However, as mentioned above, the capacitance of capacitor 11
Since Co has a considerably larger capacity than the ground capacitance Cs of the accelerating power source 7 (Cs≪Co), it can be considered that Co+Cs≒Co in this equation, and the
Equation (1) can be replaced with the following equation.

Vs=Cs/CoV+Ve......(2) Here, for example, the acceleration voltage (V) is 50KV, the emitter operating voltage (extraction voltage) (Ve) is 3KV,
If the capacitance (Co) of the capacitor 11 is 0.1 μF and the ground capacitance (Cs) of the acceleration power source 7 is 100PF, then from the above equation (2), a voltage of Vs = 0.5KV + 3KV = 3.5KV is applied.

Therefore, even if a vacuum discharge occurs between the extraction electrode 4 and the anode 5, the voltage between the emitter 3 and the extraction electrode 4 will rise to about 3.5 KV in the above example, but it will not reach the level of destroying the emitter. Also, when the accelerating voltage is high, the capacitor capacity Co
A similar effect can be obtained by appropriately selecting . In this way, the emitter 3 and the extraction electrode 5 are connected during vacuum discharge between the extraction electrode 4 and the anode 5.
Although it is possible to suppress the application of excessive voltage between the emitter and the extractor, if the capacitor Co is made large, an insulator (which insulates the extractor from structures with an emitter potential other than the emitter between the emitter and extractor electrodes and mechanically holds the emitter) During creeping discharge (not shown) or vacuum discharge between the emitter potential structure and extraction electrode, the total charge accumulated in the capacitor Co contributes to the discharge, so the vacuum surrounding the emitter deteriorates and the field emission emitter The tip is surrounded by positive ions, and the electric field strength at the tip of the emitter increases, causing vacuum discharge and destruction of the emitter. Therefore, if the diode 12 is connected in series with the capacitor 11 as shown in FIG. It is no longer possible to flow into the electron gun chamber, and it is possible to suppress excessive voltage application between the emitter 3 and the extraction electrode 4 at the time of vacuum dielectric breakdown between the extraction electrode 4 and the anode 5, which is the object of the present invention. Furthermore, when a vacuum discharge occurs between the emitter 3 and the extraction electrode 4, damage to the emitter due to the vacuum discharge can be prevented without unduly worsening the vacuum.

The present invention is not limited to the above-described embodiments, but can also be applied to various ion sources in which, for example, an ionized substance is supplied to a tip or a sharpened emitter, and ions are generated by a strong electric field. circuit can be applied.

In the present invention, a series circuit consisting of a diode and a circuit in which a capacitor and a resistor with a capacitance considerably smaller than the ground capacitance of the accelerating power source are connected in parallel is connected to the lead power source in parallel, so that there is no connection between the lead electrode and the anode. Even if a vacuum discharge occurs, the accelerating voltage is not properly added to the extracting voltage between the emitter and the extracting electrode, and the accelerating voltage is applied by the ground capacitance of the accelerating power source and the capacitance of the capacitor. The voltage applied is only the same as the small voltage divided by the voltage applied.
Furthermore, the diode prevents the charge accumulated in the capacitor from flowing into the electron gun chamber, and the resistor compensates for the delay in the operation of the diode, making it possible to prevent damage to the emitter. .

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the configuration of a conventional field emission type electron gun, FIG. 2 is a configuration diagram showing an embodiment of the present invention, and FIG. 3 is an equivalent circuit diagram for explaining the present invention. . 1: Electron gun chamber, 2: Heating filament, 3:
Emitter, 4: Extraction electrode, 5: Anode, 6: Insulated cable, 7: Acceleration power supply, 8: Extraction power supply, 9:
Power supply for filament heating, 10: Power supply case, 11:
Capacitor, 12: Diode, 13: Resistor.

Claims (1)

[Claims] 1 An emitter, an extraction electrode for forming a strong electric field necessary for field emission of charged particles near the tip of the emitter, an accelerating electrode for accelerating the charged particles emitted from the emitter, and an emitter In a device having an extraction power supply that supplies an extraction voltage between extraction electrodes and an acceleration power supply that supplies an acceleration voltage between the emitter and the acceleration electrode, a capacitor having a capacitance considerably smaller than the ground capacitance of the acceleration power supply is provided. A field emission type charged particle generation circuit in which a series circuit of a resistor connected in parallel and a diode is connected in parallel to the above-mentioned extraction power source.