JPS61145429A - Measuring element for high speed ionization vacuum gauge - Google Patents

Abstract

PURPOSE:To obtain a measuring element normally operated under the ferromagnetic field of a nuclear fusion reactor, receiving the reduced effect of a secondary electron and capable of measuring the rapid change in gas pressure, by arranging the electrodes of the measuring element in a concentric circular form. CONSTITUTION:A needle electrode 3 is attached to the center of concentric electrodes and a reticulated filament 1 is attached outside a grid 2 in a concentric form. Positive potential is applied to the grid 2 and negative potential to the anode 3. The thermoelectron emitted from the filament 1 is accelerated by the positive potential of the grid 2 and ionizes a gaseous molecule in the space between the filament 1 and the grid 2. The ionized ion passes through the mesh of the grid 2 and is accelerated by the negative potential of the anode 3 and collected by the anode 3 to be converted to an ion current. By this mechanism, the area of the anode 3' can be reduced to a large extent and concentric electrode arrangement can be held and, therefore, a measuring element can be normally operated even under a ferromagnetic field of which the direction changes timewise.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a high-speed ionization vacuum gauge, and particularly to an ionization vacuum gauge used under a strong magnetic field in a nuclear fusion device or the like.

[Background of the invention]

In order to generate plasma in a nuclear fusion device, gas is injected at a constant pressure prior to discharge.
Since plasma is generated in a short time of several milliseconds to several milliseconds, the injection gas pressure at that time also changes greatly in a short time. Plasma instability also occurs during the discharge, often resulting in the plasma contacting limiters and walls and desorbing large amounts of gas from them. This process also occurs in a short time of about several milliseconds. Accurately measuring these changes in gas pressure over a short period of time will help elucidate plasma behavior.
It is also important in performing control to maintain plasma stably. For this reason, a vacuum gauge that can measure gas pressure changes at high speed is required.

The gas pressure to be measured is lXl0-”P, ~lXl0-
"Since the Pl range, an ionization vacuum gauge is most suitable as a vacuum gauge. An ionization vacuum gauge ionizes the gas in a vacuum container and measures the degree of vacuum from the ion current. The measurement time of the probe itself is determined by the gas ionization time, which is less than 1 μs, making it suitable for high-speed vacuum gauges, but the actual measurement time is determined by the conductance between the vacuum vessel and the probe. Therefore, in order to measure gas pressure changes in a vacuum vessel at high speed, the probe must be attached as close to the vacuum vessel as possible.On the other hand, fusion devices use a strong magnetic field, and the plasma itself is affected by the plasma current flowing within it. generates a magnetic field, so the magnitude and direction of the magnetic field are not constant and change over time.Since the probe of the ionization vacuum gauge must be installed near the plasma, the magnitude and direction, especially the direction, change over time. It must operate normally under a strong magnetic field.Under a strong magnetic field, the orbits of the electrons that ionize the gas and the ions generated as a result of ionization are different from those in the absence of a magnetic field.As a result, the size and direction change. To operate normally even under strong magnetic fields,
Conventionally, as discussed by Maehashi et al. in JAERI-M6712 (1976) entitled "JFT-2 High-speed ionization vacuum gauge for measuring the density of neutral molecules around plasma," the anode and grid of the probe.

Electrodes such as filaments are arranged concentrically.

Figure ls2 shows the structure of the probe. In FIG. 2, each electrode of the probe is attached to seven flanges 4 via insulators 5 made of ceramic or the like. The filament 1 is placed at ground potential and heated by electricity. A positive potential is applied to the grid 2, accelerating the hot electrons emitted from the filament 1. Accelerated thermionic electrons collide with gas molecules in the vacuum container,
ionize.

Since a negative potential is applied to the anode 3, the ionized ions gather at the anode 9 and form an ionic current. filament 1
The electrons emitted from the grid 2 and accelerated by the anode 3
Because it is decelerated by the negative potential of , it does not reach the anode. The number of ions generated is proportional to the gas pressure and the number of thermionic electrons, so if the thermionic current from the filament is kept constant,
Gas pressure can be measured from the value of ion current.

The grid 2 has a mesh shape to allow ions to pass through. In high-speed ionization vacuum gauges, gas pressure changes must be measured at high speed, so the gauge is usually shaped like a mesh to maximize the inductance between filament 1 and grid 2, which is the ionization region of gas molecules. do.

By arranging the electrodes concentrically in this manner, gas pressure can be measured even under a strong magnetic field whose direction changes over time. On the other hand, plasma emits various kinds of visible light, ultraviolet rays, and X-rays. When these ultraviolet rays and X-rays collide with each electrode of the probe, secondary electrons are generated. In an ionization vacuum gauge, the electron current is large at 1ff)A to 10mA, but the ion current is small at about 1μA to 10μA. Therefore, when secondary electrons are emitted from the anode 3, an apparently large amount of ion current flows, making it impossible to accurately measure the gas pressure. Although the amount of secondary electrons generated is proportional to the area of the anode 30, the ionic current itself is not significantly related to the area. For this reason, it is possible to make the anode 3 needle-shaped like the so-called B-A gauge, which is a measuring point for ultra-high vacuum measurements, but then it will not have a concentric electrode structure, so it will not work normally under a strong magnetic field. It stops working. Therefore, in order to avoid the influence of secondary electrons, the structure shown in Figure 2 has conventionally been used, and the sensor head is surrounded by a shield to prevent ultraviolet rays and X-rays from reaching the anode. However, in this case, since it is surrounded by a shield, it becomes difficult for the gas molecules themselves to reach the probe, resulting in a problem that the conductance becomes small and it becomes difficult to measure rapid changes in gas pressure.

As mentioned above, in conventional probes, in order to operate normally under strong magnetic fields, the electrodes are arranged concentrically around the filament l, and as a result, the area of the anode increases, resulting in In order to avoid an apparent increase in ion current, the probe is surrounded by a shield, which reduces conductance and makes it impossible to measure rapid changes in gas pressure.

[Purpose of the invention]

An object of the present invention is to provide a measuring head for a high-speed ionization vacuum gauge that operates normally under a strong magnetic field, is less affected by secondary electrons, and can measure rapid changes in gas pressure.

[Summary of the invention]

The present invention has a structure in which the electrodes of the probe are arranged concentrically,
A needle-shaped anode is attached to the center of the concentric circles, and a cylindrical mesh-like filament is attached to the outside of the grid to reduce the influence of secondary electrons and maintain a concentric structure.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention. In FIG. 1, a needle-shaped anode 3 is attached to the center of the concentric electrodes, and a mesh-like filament 1 is attached concentrically to the outside of the grid 2. The filament 1 is heated with electricity at a contact potential,
A positive potential is applied to the grid 2 and a negative potential is applied to the anode 3. Thermionic electrons emitted from filament 1 are accelerated by the positive potential of grid 2, and
ionizes gas molecules in the space between. The ionized ions pass through the mesh of the grid 2, are accelerated by the negative potential of the anode 3, and are collected at the anode to form an ionic current. By arranging the electrodes in this manner, the area of the anode 3 can be greatly reduced, and a concentric arrangement of the electrodes can be maintained. Therefore, it can operate normally even under strong magnetic fields whose direction changes over time, and can greatly reduce the amount of secondary electrons emitted from the anode due to ultraviolet rays and X-rays emitted from the plasma, making it possible to measure ion currents. The error can be made so small that it can be ignored. Therefore, there is no need to surround the probe to shield it from ultraviolet rays and X-rays, so it is possible to measure rapid changes in gas pressure even though the probe has a large conductance.

〔Effect of the invention〕

According to the present invention, the area of the anode can be reduced while keeping the electrode structure concentric, so it can operate normally even under a strong magnetic field whose direction changes, and the amount of secondary electrons emitted from the anode is small. Since the measurement error of the ion current can be reduced and the conductance of one probe increases, there is an effect that rapid changes in gas pressure can be measured.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a structural diagram of a conventionally used probe. 1...Filament, 2°...Grid, 3...
Anode, 4Fig.

Claims (1)

[Claims] 1. In an ionization vacuum gauge probe with coaxial electrodes,
A measuring head for a high-speed ionization vacuum gauge characterized by a filament in the form of a coaxial cylindrical mesh.