JPS5913144B2 - field emission electron gun - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a field emission type electron gun whose emitter is prevented from being damaged.

Electron guns that utilize field emission can provide significantly higher brightness than thermionic emission types, and have recently been rapidly developed and put into practical use.

FIG. 1 shows a schematic diagram of the configuration of such an electron gun. The inside of a chamber 1 is evacuated to about 10-10 Torr by an exhaust device not shown, and the inside of the chamber has a tip with a diameter of 1.
An emitter 2 of approximately several hundred amperes is fixedly provided on a filament (not shown).

Reference numeral 3 denotes an extraction electrode, and a DC power source 4 is inserted between it and the emitter 1, and the electrode is kept at a positive potential with respect to the emitter 1.

5 is an accelerating electrode at ground potential, and the emitter is a high frequency oscillator 7. Step-up transformer8. A negative high voltage is applied from a DC high voltage generator 6 composed of a Cockroft circuit 9 and a filter circuit including a filter capacitor cf and a filter resistor Rf, and an accelerating electric field is formed between the Cockroft circuit 9 and the electrodes.

When the electric field strength at the tip of the emitter exceeds about 10 VA, field emission occurs.

While the field emission is occurring, the residual gas in the chamber 1 is ionized, and the ionized charged particles collide with the emitter 2, creating a crater-like roughness on the surface of the emitter tip.

Then, the electric field concentrates on the surface of the emitter tip, the emission current from this part increases, and the local nodule heat l2r (I = emission current of this part, r - resistance of the emitter local protrusion crater) increases to the emitter. When the evaporation temperature of
A phenomenon called mArc) occurs, resulting in damage.

In the conventional device as shown in FIG. 1, the emission current is detected as a voltage signal by an emission current detection resistor Re provided on the ground side of the DC high voltage generator, and is supplied to a comparator 10.

The comparator compares the voltage signal value with a reference voltage value from the reference power source ER, and connects the oscillator control circuit 1 such as a gate transistor.
1, the output of the high frequency oscillator 7 is controlled (cut off), and as a result, the voltage between the emitter 2 and the accelerating electrode 5 is lowered to prevent an increase in emission current.

However, the vacuum arc phenomenon occurs in several 100 m5 ec after the emission current starts to increase, and on the other hand, the above-mentioned emission current control system has a very slow response speed (over several 5 ecJJ), so it actually damages the emission. could not be prevented.

The present invention was made to solve these problems.
A novel field emission type electron gun is provided.

Now, one of the objects of the present invention is to stop the vacuum arc phenomenon.

The main cause of this vacuum arc phenomenon is that the energy stored in the filter capacitor Cf of the DC outage voltage generator 6 is released to the emitter 2 all at once, and this energy generates Joule heat in the rough parts of the emitter surface. It has been found that this is because the temperature is increased to reach the evaporation temperature of the emitter 2.

At the same time, it has also been found that energy released to the emitter due to distributed capacitance between the emitter and the DC high voltage generator is also a contributing factor.

The present invention relates to a field emission type electron gun that shuts out energy flowing from the filter capacitor to the emitter.

Figure 2 shows a field emission type electron gun showing an embodiment of the present invention.
The same numbers as those used in FIG. 1 indicate the same components.

In the figure, 12 is a comparator that connects the voltage signal detected by the emission current detection resistor R8 and the reference voltage source E.

Which voltages are compared, and when the detection signal exceeds the reference voltage, an activation signal is supplied to the relay drive circuit 13 that controls the operation of the relay circuit 14.

The reference voltage source E. The value of the reference voltage is set equal to the voltage value corresponding to the emission current when the Joule heat near the surface of the emitter tip approaches the evaporation temperature of the emitter.

15a is a conductor whose one end is connected between the DC high voltage generator 6 and the emitter 20, 15b is a conductor whose one end is connected to ground, and the other end of each wire is connected to the terminal T15.
B t Tls b, of which a high speed switch 15S is fixed to terminal T15b.

The switch 15S operates at high speed in conjunction with the operation of the relay circuit 14, and connects the terminals T□5a and T1. Tie b.

In such a device, the emission current is detected as a voltage signal by the emission current detection resistor R8, and is supplied to one input of the comparator 10 and the comparator 12.

The comparator 10 operates to cut off the high frequency oscillator 7 via the oscillator control circuit 11 when the voltage signal value is larger than the reference value as described above, and the comparator 12 operates to cut off the high frequency oscillator 7 between the voltage signal and the other input. The supplied reference voltage value is constantly compared, and if the voltage signal exceeds the reference voltage value, the comparator supplies a drive signal to the relay drive circuit 13 which activates the relay circuit 14.

The relay circuit 14 is activated, and the high-speed switch 15S connects the terminals T15b and T15a, so that the emitter and anode are short-circuited and the filter capacitor Cf
The electric charge stored in the DC high voltage generator 6 including
All charges stored in the distributed capacitance between the emitter and the DC high voltage generator are discharged to the ground through the switch 15S in a short period of time.

Therefore, the voltage applied between the emitter and the anode becomes zero, and the emitter vacuum arc phenomenon can be prevented.

The time it takes from the time when the comparator 12 emits a drive signal until discharge occurs is several m5 ec to several tens of m5 ec.

Furthermore, the number of relays used in the embodiment shown in Fig. 2 is at most 1.
It can withstand only about 0kV, but if you use a higher voltage, use a three-point gap 16 as shown in Figure 3.
is used.

The numbers used in FIG. 3 that are the same as those used in FIG. 2 are the same components.

In the figure, reference numeral 17 denotes a high-voltage pulse transformer which is excited by the charge stored in a capacitor 18 connected to the primary side, and generates a high-voltage pulse on the secondary side.

Thus, when the emission current increases beyond a certain reference value, the comparator 12 sends a drive signal to the high voltage pulse transformer control circuit 19 to activate the control circuit.

When the control circuit operates, the high voltage pulse transformer 17 is excited by the charge on the capacitor 18 and generates a high voltage pulse on the secondary side.

The pulse is generated by the trigger reed 16n of the three-point gap 16.
and the ground electrode 16a, a discharge occurs during this time and moves to an arc with the electrode 16e in contact with the emitter, causing a discharge in the DC high voltage generator 6 and the distributed capacitance between the emitter and the DC high voltage generator. The stored charge is transferred to the ground electrode 16a
is discharged to the earth through the

According to the present invention, when the emission current exceeds the reference value, the comparator operates and the charge stored in the DC high voltage generator including the filter capacitor is removed by the high speed switch or the three-point gap, and the emitter and DC high voltage Generator)
Since the charges stored in the distributed capacitance between the two can be discharged to the ground in a very short period of time, damage to the emitter due to vacuum arc can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a conventional field emission type electron gun, and FIGS. 2 and 3 are schematic diagrams of a field emission type electron gun each showing an embodiment of the present invention. 2...Emitter, 5...Acceleration electrode, 6
...DC high voltage generator, R8... Emission current detection resistor, 12... Comparator, 1
4...Relay circuit, 15S...Switch, 16...Three-point gap.

Claims (1)

[Claims] 1. An emitter, an extraction electrode placed opposite to the emitter, an acceleration electrode at ground potential for accelerating electrons emitted from the emitter, and at least an oscillation circuit. Emission current is detected in a field emission electron gun comprising a filter circuit including a booster circuit, a rectifier circuit, and a filter capacitor, and a DC high voltage generating means for applying a DC high voltage between the accelerating electrode and the emitter. means for comparing the detected emission current value with a reference current value; and means for short-circuiting the emitter and ground when the emission current value detected by the comparing means exceeds the reference current value. Features a field emission type electron gun.