JPH021014Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a device for detecting a break in a heating filament used in a thermionic emission type electron gun.

[Prior Art] As electron beam sources used in electron microscopes, those that utilize thermionic electrons generated when a tungsten filament is heated have been widely used. As this tungsten filament is used for a long time, it becomes thinner and eventually breaks, so the electron beam may disappear while using the microscope.
If an electron beam is not generated when the device is started, it is necessary to investigate whether the cause is a break in the filament. In this case, it is difficult to break the vacuum in the electron gun chamber and directly check whether the filament is broken, so we operate the filament heating control circuit installed on the ground potential side. I was checking for filament breakage.

FIG. 1 is a schematic diagram showing the main parts of a filament breakage detection device in a conventional electron gun used in an electron microscope or the like. In the figure, 1 indicates a filament housed in an electron gun chamber 2 together with various electrodes, both ends of which are connected to the secondary winding of an isolation transformer T2 via a cable, and a negative DC high voltage power supply 3.
is held at a high negative potential by The primary winding of the isolation transformer T2 is connected to the secondary winding of the isolation transformer T1, and the output of the high frequency oscillator 5 is supplied to the primary winding of the transformer T1 via the power amplifier 4. The output of the power amplifier 4 is continuously variably adjusted from zero to a predetermined intensity by rotating the knob of the potentiometer 6. 7
is an ammeter for displaying the current supplied to the primary winding of the transformer T1;
The current supplied from the amplifier 4 to the primary winding of the transformer T1 can be monitored by the ammeter 7. Curve A in Figure 2 shows the relationship between the amount of rotation of the knob of potentiometer 6 and the value displayed on ammeter 7 when filament 1 is broken, and the same applies when filament 1 is not broken. The relationship is shown by curve B in FIG. The difference between curves A and B corresponds to the current supplied to the filament. Therefore, by recording the characteristics represented by curves A and B in advance and determining which curve the current value indicated by the ammeter 7 is closer to when the knob of the potentiometer 6 is rotated. Therefore, it is possible to detect a filament breakage.

[Problem to be solved by the invention] However, as the voltage of the negative DC high voltage power supply 3 applied to the electron gun increases, the number of stages of isolation transformers connecting the filament 1 and the power amplifier 4 increases. The relationship between curves A and B shown in the figure changes as shown by curves A' and B' shown in FIG. This is because the power consumed by the filament 1 is relatively small while the power consumed by the large number of isolation transformers T1 to Tn is large. In such cases, the curve
Since the difference between A' and B' is small, it is difficult to confirm filament breakage with the conventional device described above, resulting in low reliability.

The purpose of the present invention is to solve these problems and provide a filament breakage detection device for an electron gun that can easily detect filament breakage without breaking the vacuum even in an electron gun with a high acceleration voltage. There is.

[Means for Solving the Problems] Therefore, the present invention includes a heating filament that is incorporated in the electron gun chamber and to which high DC voltage is applied, and a heating filament that is connected to the heating filament through multiple stages of insulation transformers from the first stage to the final stage. turning on the connection between a heating power supply supplying alternating current, a display means for displaying power supplied to the primary winding of the first stage transformer, and the secondary winding of the last stage transformer and the filament; It is characterized by providing a switch for turning off the switch and a control means for switching and controlling the switch from the ground potential side.

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 4 is a schematic diagram showing an embodiment of the present invention, and the same reference numerals as in FIG. 1 represent the same components. In the device shown in FIG. 4, since the voltage of the DC high voltage power supply 3 is high, the number of stages of isolation transformers T1 to Tn is increased. Unlike the device shown in Fig. 1, a switch 8 is provided to turn on and off the connection between the secondary winding of the final stage transformer Tn and the filament 1, and this switch 8 is controlled by switching from the ground potential side. A switching control means 9 is provided for this purpose.

In order to detect a break in the filament 1 using the apparatus shown in FIG. 4, first, the knob of the potentiometer 6 is rotated and fixed near the normal operating state. Next, the changeover control means 9 is operated to repeatedly turn the changeover switch 8 on and off. At this time, the display on the ammeter 7 is observed, and if the displayed value fluctuates, it can be determined that the filament 1 has not broken, and if the displayed value does not fluctuate, it can be determined that the filament 1 has broken. According to such a filament breakage detection device in an electron gun, the curves A′,
The relationship shown in B′ holds, so the curve
Even if the difference between A' and B' is small, the difference can be recognized as a change in the displayed value, so filament breakage can be detected easily and with high reliability.

[Effects of the invention] As described above, according to the invention, filament breakage can be easily detected even with an electron gun with a high acceleration voltage. Maintenance of the device becomes easier.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a conventional electron gun, FIGS. 2 and 3 are schematic diagrams for explaining the operation of the device shown in FIG. 1, and FIG. 4 is a schematic diagram showing an embodiment of the device of the present invention. . 1... filament, 2... electron gun chamber, 3...
DC high voltage power supply, T1, T2~Tn...Isolation transformer, 4...Power amplifier, 5...High frequency oscillator, 6...Potentiometer, 7...Ammeter, 8
...On/off switch, 9...Switching control means.

Claims (1)

[Scope of utility model registration request] A heating filament that is incorporated in the electron gun chamber and to which high DC voltage is applied; a heating power supply that supplies alternating current to the heating filament via multiple stages of insulation transformers from the first stage to the final stage; a display means for displaying power supplied to the primary winding; a switch for turning on and off the connection between the secondary winding of the final stage transformer and the filament; A filament breakage detection device for an electron gun, characterized in that it is provided with a control means for switching control.