JP2006286548A - Ion current measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ion current measuring apparatus capable of measuring ions with energy of about several keV, suppressing an applied voltage to a small level and being miniaturized. <P>SOLUTION: A particle orbit deflection portion is formed on the side of an electrode plate between an electrode plate 2 and a collector 3, a electrostatic shield grid 5 for forming an ion capturing portion is provided on the side of the collector 3, and means for applying a positive voltage to the electrode plate 2 and a negative voltage to an electrostatic shield means 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ion current measuring device, and more particularly to an ion current measuring device used in a mirror type open-ended plasma confinement device.

  When considering an economical fusion reactor, it is necessary to further improve the confinement performance by further reducing the edge loss of the plasma from the mirror magnetic field. A tandem mirror system that confines plasma with a potential wall has been devised as a measure for improving this confinement. That is, when the mirror magnetic fields are linearly arranged and high-temperature and high-density plasma is generated in the mirror portions at both ends, a positive potential higher than that of the central mirror portion is formed. The plasma at the central mirror is confined by this potential wall. This principle of potential confinement was demonstrated in the world's first tandem mirror device, Gamma 6, and the confinement performance was improved.

  In the tandem mirror type plasma confinement device gamma 10, in addition to plasma confinement by a magnetic field, plasma confinement potential is formed by electron cyclotron heating (ECH), thereby significantly improving the plasma confinement performance. Recently, by controlling the radial power distribution of the ECH, taking advantage of the greatest advantage of the mirror device that can easily control the potential distribution and the radial electric field, the disturbed vortex-like structure in the plasma generates high potential and strong electric field shear. The inventors have pioneered the world that it is controlled by the formation of dE (r) / dr. In addition, even in different confinement methods such as tokamak and helical, it has been found that the confinement effect by these potential / electric fields is universal and essential for plasma improvement. Research has become a universal and urgent research subject that needs to be elucidated beyond the equipment format including the International Thermonuclear Experimental Reactor.

  In order to strictly evaluate ion confinement performance due to potential generation, an ion current amount measuring device capable of strictly measuring the ion current amount is required. The inventors of the present invention have two parallel plate structures as an ion current amount measuring device capable of strictly measuring the ion current amount, and by applying a positive voltage to one of them and deflecting the incident electrons and ions in the electric field, Proposed ion current meter that separates ions, collects ions in the collector, and is equipped with an irregular electric field correction plate, and measures ions from several tens eV to several thousand eV while suppressing the influence of incident electrons, etc. Made possible.

  However, this ion current measuring instrument has a depth of about 100 mm and exceeds the depth (50 mm) of the measuring instrument array for the purpose of installing a plurality of measuring instruments inside. There is a demand for downsizing.

In promoting miniaturization,
(1) The depth of the measuring instrument is 45 mm or less in order to install it at an angle that is perpendicular to the lines of magnetic force. (2) The measurable ion energy range is equivalent to the above-mentioned ion current measuring instrument. (3) It is required to suppress the applied voltage to several kV.

  An object of the present invention is to provide an ion current amount measuring device that can measure plasma ions having energy of about several keV in view of such a point, can suppress an applied voltage, and can be downsized.

The present invention is an ion current measuring device for measuring an ion current of an incident plasma incident between an electrode plate and a collector,
An ion current amount measuring device comprising: an electrostatic shielding means for forming a particle trajectory deflecting portion on the electrode plate side in the space between the electrode plate and the collector and forming an ion collecting portion on the collector side; provide.

  According to the present invention, by providing an electrostatic shielding means (for example, an electrostatic shielding grid) between the electrode plate and the collector, the electrode plate side is used as the particle trajectory deflecting unit, and the collector side is used as the ion collecting unit. It is a simple structure that enables accurate ion measurement that suppresses the influence of incident electrons and secondary electrons, and it is possible to optimize the potential distribution and collector structure. A quantity measuring instrument can be provided.

  Before describing an embodiment of the present invention, a tandem mirror type device gamma 10 (hereinafter referred to as a gamma 10 device) in which the present embodiment is applied as one application example will be described. FIG. 1 shows a tandem mirror device gamma 10 which is an open-ended plasma confinement device. In FIG. 1, a tandem mirror type device gamma 10 that is an open-ended plasma confinement device generates five mirror magnetic fields by coils arranged on an axis to confine plasma inside a magnetic flux tube, and plug barrier portions at both ends. Plasma confinement by magnetic field and electric field is realized by injecting neutral particle beam and electron microwave installed in mirror magnetic field and heating plasma to form thermal barrier potential and ion confinement potential. Has improved.

Next, the device end of the tandem mirror type device gamma 10 will be described with reference to FIG.
At the end of the gamma 10 device, there is a plasma that overcomes the magnetic and electric fields of the plasma confinement and flows out along the magnetic field lines. By separating the ions and electrons in this edge loss plasma and measuring the time variation of the current amount and energy spectrum of the edge loss ions, the edge loss ion temperature, ion confinement potential, and plasma confinement time are derived, depending on the potential. The plasma confinement performance can be evaluated.

  The edge loss plasma flows out spatially and the symmetry of the plasma is related to the plasma confinement evaluation. Therefore, several proposed measuring instruments are installed in the plasma radial direction so that the spatial distribution can be measured. Since the end of the device is spatially limited, the installation space for the measuring instrument is in a duct with a depth of 5 cm.

  Prior to the present invention, an example of an end loss ion current amount absolute value measuring instrument which is a measuring instrument designed and developed for proof of principle will be described with reference to FIG. This measuring instrument uses the “secondary electron self-recovery function using an external magnetic field (plasma confinement magnetic field)” which is one of the basic principles including the present invention.

  An electric field is formed between an electrode plate placed in parallel to the incident plasma and the collector, and only ions are incident on the collector by orbital deflection of ions and electrons by the electric field. In order to prevent the scattering of low energy ions due to the irregular electric field generated in the vicinity of the plasma inlet, an irregular electric field correction plate is installed at the upper part of the plasma inlet, enabling low energy ion measurement of several tens of eV.

  By installing the collector in parallel with the external magnetic field, secondary electrons generated from the collector with the incidence of ions rotate around the lines of magnetic force due to the Lorentz force due to the external magnetic field and are collected again by the collector. This secondary electron self-recovery function eliminates the need for a secondary electron repeller grid that was installed in conventional detectors. In addition, the grazing incidence ion energy spectrum measuring instrument transmits five grids before the ions are incident on the collector for energy analysis. For this reason, conversion to the ion current at the plasma inlet is necessary, but since this measuring instrument does not use a grid at all, the current measured at the collector is equal to the ion current at the plasma inlet, and the measurement efficiency 100% measuring instrument.

  However, since ion measurement with energy of several keV is required, the depth of the measuring instrument is 8.2 cm, and further downsizing is necessary for installation in the duct of the edge loss ion measuring instrument array.

The embodiment of the present application is an ion current measuring device for measuring an ion current of an incident plasma incident between an electrode plate and a collector,
In the space between the electrode plate and the collector, a particle trajectory deflecting portion is formed on the electrode plate side, and an electrostatic shielding means for forming an ion collecting portion on the collector side is provided,
The apparatus has means for applying a positive voltage to the electrode plate and a negative voltage to the electrostatic shielding means, and collects secondary electrons generated from the collector by an external magnetic field parallel to the incident ions. An ion current meter is configured.

  The electrostatic shielding means is formed from an electrostatic shielding grid provided between the electrode plate and the collector, and a holding box that holds the electrostatic shielding grid and serves as an ion collector. .

The collector is formed in a box shape or an L shape.
The collector is formed in a box shape or an L shape and is accommodated in the holding box.

  The electrostatic shielding grid is formed by arranging wires arranged in parallel to the lines of magnetic force parallel to the direction of the incident plasma at equal intervals.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 4 shows a schematic configuration of the ion current measuring instrument 100 which is an embodiment of the present invention, including a partial cross-sectional portion. In the present application, the ion current measuring device is used in a concept including a charged particle current measuring device.

  In FIG. 4, an ion current measuring device 100 includes a collimator 1 that forms part of an incident plasma sending unit, an electrode plate 2, a collector 3, and an electrostatic shielding means 4 that are close to the collimator 1. A shielding grid 5 is provided. The electrostatic shielding means 4 includes an electrostatic shielding grid 5 disposed in a space between the electrode plate 2 and the collector 3 and a box-shaped holding box 6 that holds the electrostatic shielding grid 5.

  The collector 3 is formed in a box shape, is housed inside the holding box 6, and has an open top surface.

FIG. 5 shows details of the electrostatic shielding grid 5.
The electrostatic shielding grid 5 is provided with 50 μm wires 11 parallel to the incident direction of plasma at intervals of 3 mm. For this reason, the influence of the grid shielding effect due to the difference in the incident trajectory due to the energy difference of the incident particles is eliminated, and the grid transmittance of the particles becomes constant for all the incident particle energies. The geometrical transmittance of the electrostatic shielding grid 5 is as high as 98.3%, and the electrostatic shielding grid 5 through which incident particles are transmitted from the plasma entrance 12 of the collimator 1 to the collector 3 is one electrostatic shielding grid. Therefore, the current amount of ions and electrons at the plasma entrance is determined only by the "single grid geometric transmittance" with a structure that allows easy analysis of transmittance where multiple grids overlap, allowing accurate current amount evaluation. .

  In FIG. 4, the electrode plate 2 and the electrostatic shielding grid 5 are installed in parallel with respect to the incident plasma, and an electric field is formed between them to deflect the trajectory of ions and electrons. When a positive voltage is applied to the electrode plate, ions are deflected, and when a negative voltage is applied, electrons are deflected to the grid side.

  In order to reduce the size of the ion current measuring instrument, the distance until the incident charged particles are deflected is shortened to increase the deflection electric field. On the other hand, since many pieces are installed in a duct having a depth of 5 cm, the applied voltage is set to several kV or less in order to suppress discharge from the electrode plate 2. On the other hand, when the ions are collected, if the electric field at the front surface of the collector 3 becomes strong, the effect of secondary electron emission by the electric field becomes stronger than the secondary electron self-collecting function by the external magnetic field, and the secondary electrons are not self-collected. . Further, the secondary electron emission ratio is increased by the deflection electric field. In order to solve these problems, "orbital deflection of charged particles by electric field" and "secondary electron self-recovery function that can return itself to collector 3 by Larmor motion using an external magnetic field (plasma confinement magnetic field)" An electrostatic shielding grid 5 was provided.

  By installing the electrostatic shielding grid 5 between the electrode plate 2 and the collector 3, the space inside the measuring instrument, that is, the space between the electrode plate 2 and the collector 3, is divided into two regions, a particle trajectory deflecting unit and a particle collecting unit. It is divided into. By reducing the distance between the electrode plate 2 and the electrostatic shielding grid 5, it is possible to generate a high electric field even when a low voltage is applied. On the other hand, the particle collection unit is shielded from the deflection electric field by the electrostatic shielding grid 5, and the electric field in front of the collector becomes weak, so that the secondary electron self-recovery function is restored. For example, a voltage of 1.5 kV is applied to the electrode plate 2, and a voltage of −0.1 kV is applied to the electrostatic shielding grid 5.

  The electrostatic shielding means 4 is box-shaped, and a box-shaped collector 3 is installed therein. Since the electrostatic shielding grid 5 forms an electric field, the collector shape can be optimized for efficient collection of charged particles.

  FIG. 6 shows a potential distribution when a voltage optimized in the case of ion measurement is applied. FIG. 7 shows the potential distribution when the electrostatic shielding grid 5 is not provided for comparison. The dimensions were also entered in the figure.

  By applying a voltage of 0.1 kV to the electrostatic shielding grid 5, the potential in the vicinity of the plasma entrance becomes 0 kV. Therefore, in this embodiment, in order to correct the irregular electric field generated in the vicinity of the plasma entrance, the above-described end is used. The irregular electric field correction plate required for the absolute measuring device of the loss ion current is no longer necessary.

  As described above, by installing the electrostatic shielding grid 5, the inside of the measuring instrument is separated into the particle trajectory deflecting unit and the particle collecting unit. A deflection electric field of the particle trajectory is formed between the electrode plate 2 and the electrostatic shielding grid 5. Therefore, the change in the collector shape in the particle collecting portion does not affect the orbital deflection electric field at all. The role of the orbital deflection electric field is not to drop particles into the collector 3, but to drop into the particle collecting section. In contrast, the role of the collector 3 is to reliably take in the particles incident on the particle collecting portion, and the collector shape can be optimized.

  When a negative voltage of several hundred volts is applied to the electrostatic shielding grid 5, the potential distribution in the vicinity of the entrance is applied to the electrostatic shielding grid 5 as shown in FIG. The position can be moved near the entrance. Actually, the electric field configuration is optimized by adjusting the distance from the entrance 12 to the electrode plate 2, but it is no longer the electric field configuration that prevents the incidence of particles. You can see from the figure.

  As described above, by applying a negative voltage to the electrostatic shielding grid 5 and adjusting the position of the electrode plate, a structure that does not require the installation of an irregular electric field correction plate having a depth of several tens of mm can be achieved.

  Next, details of the shape of the collector 3 will be described. As described above, the shape of the collector 3 can be changed without considering the influence on the electric field for deflecting the particle trajectory. However, the collector shape is as simple as possible and reliably collects particles so that the simulation and processing are not complicated. I made it.

  Now, the particles to be collected by the collector 3 are incident ions and secondary electrons from the collector 3. Further, as particles that should not be collected, incident electrons, secondary electrons from the electrostatic shielding grid 5, and recoil electrons due to incident electrons hitting the electrode plate can be considered.

Consider these in two cases: a direction parallel to the magnetic field and a direction perpendicular to the magnetic field.
First, as for the parallel direction, among the ions to be collected, high-energy ions of keV or higher reach the collector 3 with almost no influence of the confined magnetic field. On the other hand, ions of several hundreds eV are rotated by the influence of the applied voltage to the electrostatic shielding grid 5 and the confinement magnetic field, depending on the incident position, which is excessively deflected by the deflecting electric field and strongly deflected to the collector entrance side. It can be divided into those that flow behind the instrument while exercising.

  Therefore, it is desirable to adopt a “U-shaped” shape in order to reliably collect such variously behaving particles.

  Next, in the vertical direction, mainly relevant particles are electrons that rotate under the influence of a magnetic field. Of these, incident electrons are deflected to the electrode plate side, so there is no need to think about them. Also, secondary electrons from the electrostatic shielding grid 5 are emitted because the area of the electrostatic shielding grid 5 is very small. Since the rotation radius of secondary electrons is about several millimeters, the influence can be ignored if the depth is designed to be several tens of millimeters. The remaining electrons are secondary electrons from the collector 3 and recoil electrons from the electrode plate. However, since the rotation directions of the two are opposite to each other, the secondary electrons on the side where the secondary electrons from the collector 3 gather are collected. The collector 3 may be erected so that the electrons are easily collected, and the collector may not be provided on the side where the recoil electrons are collected. Therefore, the “L-shaped” structure is adopted in this direction.

  As for the depth, when −0.1 kV of ions and secondary electrons from the collector 3 are applied to the electrostatic shielding grid 5, 11 mm which can be reliably collected is found by orbital calculation. It became a 11 mm “duster type”. Since the final collector 3 is open in this way, an exhaust effect can be expected.

  When 1.5 kV was applied to the electrode plate 2 and −0.1 kV was applied to the electrostatic shielding grid 5, the trajectory calculation result of incident ions having energies of 50 eV, 100 eV, 1 keV, 3 keV, and 5 keV was obtained. According to the trajectory calculation results, it was found that low-energy ions could be measured without an irregular electric field correction plate. Moreover, the measurement performance of the energy range (mainly 1-2 keV) sufficient for the measurement of edge loss ion is also satisfy | filled.

と与えられる。 The ions that end up almost horizontally with the magnetic field lines while wrapping around the magnetic field lines have an incident angle (pitch angle) with respect to the ion current measuring instrument 100. The maximum incident angle θ _max is determined by using gamma 10 throat part magnetic field intensity B _max = 30.13 kG and magnetic field intensity B _detector = 0.08 kG at the measuring device installation position,

And given.

  When the pitch angle is 3 degrees, all of the ions whose positions where the particles are incident on the collector 3 are shifted from front to back and from side to side can be guided to the collector 3, and the pitch angle deviation does not affect the ion measurement. The shape of the collector 3 may be any configuration as long as incident ions easily collide as can be seen from FIG. The calculation of the trajectory of incident electrons was performed under the conditions of an electrode plate of 1.5 kV, an electrostatic shielding grid of 0.1 kV, and a magnetic field strength of 80 Gauss. Each orbit is of incident electrons with energy of 0.1 keV, 1 keV, 5 keV. Since a voltage of 1.5 kV is applied to a distance of 10 mm, a strong electric field is formed, and incident electrons are subjected to a strong electric field deflection and are incident on the electrode plate 2. Further, electrons with energy exceeding 5 keV flow to the back of the measuring instrument.

  From the result of this orbit calculation, incident electrons do not enter the collector 3. Many of the electrons that can be considered as electrons existing inside the measuring instrument are incident electrons and secondary electrons, but in this embodiment, incident electrons collide with the electrode plate 2. It is also necessary to consider secondary electrons. The trajectory calculation results of recoil electrons and secondary electrons when incident electrons (3 keV) collide with the electrode plate 2 will be described.

  Here, it has been found that low energy secondary electrons with a high emission ratio of about 100 eV have a very small radius of rotation, so that electrons up to about 1 keV are reliably recovered. Electrons with energy exceeding 1 keV, which has a low emission ratio, are either recovered again by the electrode plate or guided outside the measuring instrument from the open region for releasing recoil electrons provided in the ion collector. .

  As described above, according to the ion current measuring instrument 100 according to the present embodiment, the simple structure in which one electrostatic shielding grid is sandwiched between the electrode plate installed in parallel with the incident plasma and the box-shaped collector. Therefore, accurate ion measurement is possible while suppressing the influence of incident electrons and secondary electrons.

  The electrostatic shielding grid 5 divides the generated electric field inside the measuring instrument into two parts (the particle orbit deflecting part on the electrode plate side and the ion collecting part on the other side), and the functions are clearly separated to reduce the voltage and size. Realized.

  First, in the particle trajectory deflecting unit, (1) electrons and ions are completely separated by generating an electric field perpendicular to the incident plasma, and only ions are selectively guided to the ion collecting unit. At this time, (2) by reducing the distance of the deflection electric field generated between the electrode plate and the grid, a strong electric field can be generated even at a low voltage.

  On the other hand, in the ion collector, (3) generated between the electrostatic shielding grid (-0.1 kV) and the collector (GND) by the Larmor rotation motion using the plasma confinement magnetic field outside the measuring instrument parallel to the collector plate. By adding the effect of the suppressed electron emission electric field, it is possible to suppress the rotation radius of the secondary electrons emitted from the collector to a smaller value and cause the collector to recover again, thereby completely suppressing the noise caused by the secondary electrons. At this time, (4) since an external magnetic field is used, the size of the measuring instrument can be reduced. (5) Since the electric field of the ion collector is independent from the particle orbital deflection electric field, the collector can be shaped like a box. In addition, (6) a negative voltage is applied to the electrostatic shielding grid. Along with the application, a potential distribution that hardly disturbs charged particles in the electric field can be formed in the region immediately after the plasma entrance, so that ions in a wide energy region (100 eV to 5 keV) including a relatively low energy region can be measured. Yes.

The figure which shows schematic structure of the tandem mirror type apparatus gamma 10 which is an open end type plasma confinement apparatus. The figure which shows the apparatus edge part of the tandem mirror type | mold apparatus gamma10. The figure which shows schematic structure of an end loss ion current amount absolute value measuring device. The figure which shows the structure of the ion current amount measuring device which is an Example of this invention. The figure which shows the detail of an electrostatic shielding grid. The figure which shows the electric potential distribution which generate | occur | produces inside an ion current amount measuring device when an electrostatic shielding grid is installed. The figure which shows electric potential distribution when not providing an electrostatic shielding grid.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Collimator, 2 ... Electrode plate, 3 ... Collector, 4 ... Electrostatic shielding means (apparatus), 5 ... Electrostatic shielding grid, 6 ... Holding box, 11 ... Wire, 12 ... Plasma inlet, 100 ... Ion current amount Measuring instrument.

Claims (6)

In the ion current measuring device that measures the ion current of the incident plasma incident between the electrode plate and the collector,
An ion current measuring device characterized in that in the space between the electrode plate and the collector, a particle orbital deflection unit is formed on the electrode plate side, and electrostatic shielding means is formed on the collector side to form an ion collection unit.   2. The secondary electron generated from the collector according to claim 1, further comprising means for applying a positive voltage to the electrode plate and a negative voltage to the electrostatic shielding means, and recovering secondary electrons generated from the collector by an external magnetic field parallel to the incident ions. An ion current measuring instrument characterized by that.   2. The electrostatic shielding means according to claim 1, wherein the electrostatic shielding means is an electrostatic shielding grid provided between the electrode plate and the collector, and a box shape that holds the electrostatic shielding grid, and serves as an ion collector. An ion current measuring instrument characterized by comprising a box.   2. The ion current measuring device according to claim 1, wherein the collector is formed in a box shape or an L shape.   4. The ion current measuring device according to claim 3, wherein the collector is formed in a box shape or an L shape and is housed in the holding box. 6. The ion current measuring device according to claim 5, wherein the electrostatic shielding grid is formed by arranging wires arranged in parallel to the lines of magnetic force parallel to the direction of the incident plasma at equal intervals.

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