JPS5935504B2 - electron gun - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron gun used in an electron beam generator such as an electron microscope.

As shown in FIG. 1, for example, an electron gun used in an electron microscope has a filament 1, a Wehnelt electrode 2, and an anode 3 as its main parts, and heating power is supplied from a filament heating device 4 to the filament 1 via a filament support 5. The filament is heated to a high temperature (for example, around 3000'C for a tungsten filament) by supplying a It emits line 7.

Note that 8 in the figure is a thermally and electrically insulating support for supporting the filament support 5.

The filament in such an electron gun is heated at a temperature at which the electron beam is emitted best (hereinafter referred to as the optimum temperature), for example, 3000°C for a tungsten filament.
It is heated up to the front and back.

However, if the filament is suddenly heated from a cold state to the optimum temperature or at a slow rate, the filament part will suddenly expand, causing distortion and fluctuations in the position of the tip. Needless to say, this has a significant adverse effect on the performance of the electron beam generator.

For this reason, conventionally, an operator carefully and slowly turns an external heir connected to a filament heating power source to slowly heat the filament to the optimum temperature.

However, such operations cause extreme trouble to the operator, and because of the troublesomeness, caution is sometimes neglected, leading to filament accidents.

The present invention has been made in view of these points, and it is an object of the present invention to provide a novel electron gun having a characteristic filament heating device.

FIG. 2 is an example of a filament heating device 4' showing the characteristic parts of the present invention. 9 is a voltage generating device such as a potentiometer to which a certain voltage (10 B volts) is applied, and an external heir (not shown) is shown in FIG. generates different voltages depending on the rotation of the

10 is a current control circuit that controls so that if the value of the current generated by the voltage generated by the voltage generator is above a certain set current value, the set current flows as an output, and if it is below, the current flows as an output. , for example pnp
It consists of a type transistor Tr, a resistor R1, and a Zener diode D1, and the set current is determined by the value of the resistor R1 inserted between the emitter and the base.

Reference numeral 11 denotes a voltage conversion circuit that gradually charges the output current of the current control circuit, and is composed of, for example, a capacitor C1.

T is a control transistor that supplies the output voltage of the voltage conversion circuit to the filament 1.

Note that R2 is a resistor provided between the voltage generator 9 and the current control circuit 100.

In an electron gun having such a filament heating device, the resistance value of the resistor R1 of the current control circuit 10 is set to R1, which has the same sign as R1, and the voltage across the resistor R1 is V.

In this case, the ratio of vo to R (Vo/R1=11) is set in advance to be equal to a value corresponding to the optimum temperature increase rate of the filament.

Therefore, you can freely turn the external legitimate child suddenly large, small suddenly, or thickly.
If the voltage generated by the voltage generation circuit 9 is eo and the output voltage of the voltage conversion circuit 12 is C2, then (
A current i proportional to the voltage difference el-02) flows.

At this time, if the current i is equal to or higher than the current 11 set by the current control circuit 10, the transistor Tr1 operates, and the difference between the two currents (i-il) is transferred from the emitter to the collector (ground).
The setting current 11 flows into the capacitor C1 forming the voltage conversion circuit 11.

The capacitor C1 is gradually charged by the set current 11 (.

That is, the voltage e2 of the capacitor C□ is e 2 = −
From the equations of Jl and dt1, as shown in A of FIG. 3, they gradually increase linearly with time t.

When the current i becomes smaller than the set current i, the transistor Tr turns off, so the voltage e2 increases according to the time constant (R1+R2)·C1 as shown in B in FIG.
As shown in C, it becomes stable when the voltage becomes equal to e1.

If the current i becomes smaller than the set current 11, the transistor Tr1 is of course off, and the voltage e2 increases from the beginning according to the time constant (R1+R2)·C1 until it reaches el.

Therefore, in any case, a heating current corresponding to the finally arbitrarily selected value of el is supplied to the filament 1 via the transistor Tr1, but the rate of increase of the heating current is set in advance. The optimum speed will not be exceeded.

Note that a switch is provided between the current control circuit 10 and the voltage generator 90 to operate in conjunction with the operation start mechanism of the high voltage generator for accelerating electrons from the filament.
If the voltage value of is semi-fixed to a certain value in advance,
Filament heating is automatically started by switching on the high voltage generator without touching the filament heating device 4' itself.

According to the present invention, when heating the filament to a desired temperature by rotating the external heir by an arbitrary amount, the rate of temperature rise of the filament will not exceed the upper limit even if the heir is rapidly rotated. Therefore, it can prevent filament accidents (based on careless operation).

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an electron gun, FIG. 2 is a diagram of an embodiment of a filament heating device which is a feature of the present invention, and FIG. 3 is a diagram for explaining the present invention in detail. 1: filament, 9: voltage generator, 10: current control circuit, 11: voltage conversion circuit.

Claims (1)

[Claims] 1 Consisting of a filament and a heating device for heating the filament, the heating device includes a power source for supplying a heating current to the filament, and a heating device corresponding to an indicated value of the heating current supplied from the power source to the filament. A control element for controlling a heating current supplied from the power supply to the filament to a value corresponding to a voltage value applied to the control element; Equipped with a capacitor for applying a charging voltage to the terminal, and a control circuit that compares the charging current supplied to the capacitor from the voltage generating means with a reference value and releases the charging current component exceeding the reference value to ground. An electron gun characterized by: