JPH0138352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus magnet for generating a magnetic field for focusing an electron beam, for example in a television cathode ray tube.

Generally, as an electron beam focusing device for a television cathode ray tube, an electrostatic type device is used in which an electron beam is focused by an electron lens formed by an electrostatic field. On the other hand, for special tubes, such as X-ray tubes, magnetrons, traveling wave tubes, etc., a magnetic field system is used in which an electron beam is focused by a magnetic field type electron lens (hereinafter simply referred to as a magnetic field lens). However, recently, the application of the magnetic field method, which has superior resolution compared to the electrostatic method, has been considered for use in general television cathode ray tube devices.

Some magnetic field generating devices of this type use coils as electron beam focusing means, but those using permanent magnets are advantageous in terms of miniaturization and weight reduction of the device. However, if the intensity of the focusing magnetic field generated by the permanent magnet deviates from a predetermined value, the spot diameter of the electron beam will increase and the resolution will decrease, so the magnetic field intensity at the center of the magnet will be:
For example, strict accuracy of 450G±1% is required. Furthermore, if the magnetic field strength decreases due to changes in the ambient temperature of the magnetic field generator, the spot diameter of the electron beam increases, so if the ambient temperature is -10
Even when changing in the range of ~100°C, it is required that the variation in the strength of the central magnetic field is within ±1%. Moreover, from the viewpoint of resolution, a magnetic field lens with a focal length as short as possible is required.

As mentioned above, magnetic field type electron beam focusing devices are required to have strict characteristics, but conventional devices have not been able to obtain practically sufficient characteristics.

Accordingly, an object of the present invention is to provide a focus magnet that satisfies the above-mentioned various characteristics and can particularly shorten the focal length.

The present invention will be explained below with reference to the drawings.

FIG. 1 is a sectional view of a focus magnet according to an embodiment of the present invention.

2 is N in the axial direction, which is the traveling direction of the electron beam.
It is a ring-shaped permanent magnet having a pole and an S pole. 1
Reference numerals a and 1b denote disk-shaped yoke plates having a circular hole 11 formed in the center through which a neck portion of a cathode ray tube can be inserted, and are fixed to the axial end surface of the ring-shaped permanent magnet 2. 3a and 3b are ring-shaped permanent magnets having an N pole and an S pole in the axial direction, and have a shorter magnet length than the ring-shaped permanent magnet 2, that is, have a smaller magnetomotive force. The ring-shaped permanent magnets 3a and 3b are each attached to a yoke plate 1a so that magnetic poles of the same polarity as the ring-shaped permanent magnet 3 face each other.
and 1b. 4a and 4b are coils for magnetic field adjustment provided on the outer peripheries of the ring-shaped permanent magnets 3a and 3b, respectively, and may be omitted if there is no need for magnetic field adjustment.

As the above permanent magnet, in order to reduce fluctuations in magnetic field strength due to changes in ambient temperature, magnets with good temperature characteristics such as alnico magnets and rare earth cobalt magnets may be used, but these magnets are generally expensive. Therefore, it is advantageous to use ferrite magnets in terms of cost. However, since ferrite magnets do not have good temperature characteristics, it is preferable to fix a ring-shaped spacer 5 made of magnetic shunt steel to the outer periphery of the ring-shaped permanent magnet 2 in order to reduce fluctuations in magnetic field strength due to changes in ambient temperature.

In the focus magnet configured as described above, a leakage magnetic field is formed by the permanent magnet 2 from its north pole to the south pole via the yoke plates 1a and 1b, and this magnetic field creates the magnetic field necessary to focus the electron beam. occurs. Here, the N pole and S pole of the permanent magnet 2 are respectively the permanent magnet 3b.
Since it faces the north pole of the permanent magnet 3a and the south pole of the permanent magnet 3a, the width of the magnetic flux waveform in the axial direction is narrowed due to the same polar repulsion, and a magnetic field lens with a short focal length can be obtained. Note that if the magnetomotive force of the permanent magnets 3a and 3b is too strong, the magnetic flux waveform will be disturbed (the width of the waveform will become too narrow and the spherical aberration will increase), which will actually reduce the resolution.
The magnetomotive force of the magnet 3b needs to be smaller than the magnetomotive force of the permanent magnet 2.

Next, the magnetic flux density distribution in the axial direction when an anisotropic ferrite magnet is used as each permanent magnet in FIG. 1 is shown by a solid line in FIG. Further, the magnetic flux density distribution when the permanent magnets 3a and 3b are excluded from FIG. 1 is shown by a broken line in FIG. The magnetic flux density distribution in Figure 2 is based on point A in Figure 1, and from this point
The strength of the central magnetic field in the direction and the Y direction is measured. The aim of the focus magnet in this case was to have the magnetic flux density at point A of 370±5G and the widths W and W' of the magnetic flux waveforms both within 10 mm. According to the above, it can be seen that these characteristics can be satisfied.
Furthermore, in the above focus magnet, when the change in magnetic flux density at point A was measured when the ambient temperature was changed from -10 to 100°C, it was confirmed that the change was within 4%.

As described above, according to the present invention, since the width of the magnetic flux waveform is narrowed, the focal length of the magnetic lens is shortened, and a focus magnet with improved resolution can be obtained.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a focus magnet according to an embodiment of the present invention, and FIG. 2 is a diagram showing the magnetic flux density distribution in the axial direction of the focus magnet. 1a, 1b... Yoke plate, 2, 3a, 3
b...Ring-shaped permanent magnet, 4a, 4b...Coil, 5...Spacer.

Claims (1)

[Claims] 1. A ring-shaped permanent magnet magnetized in the axial direction is placed on both axial end surfaces of the ring-shaped permanent magnet, through a disk-shaped yoke plate having a circular hole in the center. A focus magnet characterized in that a ring-shaped permanent magnet having a small magnetic force is fixed so that magnetic poles of the same polarity as the magnetic poles of the ring-shaped permanent magnet face each other.