EP0422451A1 - Electron tube - Google Patents

Abstract

In an electron tube having a powerfully emitting cathode (1), an anode (4) and at least one autonomous first grid (2) between the cathode (1) and the anode (4), the cathode (1) has powerfully emitting and at most weakly emitting regions (7.1, 7.2, 7.3 and 8 respectively). The powerfully emitting regions (7.1, 7.2, 7.3) are arranged behind openings (6) of the first grid (2), and the at most weakly emitting regions (8) correspond to a shadow cast by the first grid (2), which is adjacent to the cathode (1), onto said cathode (1).

Description

Technical field

The invention relates to an electron tube with a cathode, an anode and at least one independent first grid between cathode and anode.

State of the art

Electron tubes of the type mentioned have long been known and are commercially available. The cathode of such an electron tube is completely covered with osmium in order to be able to achieve higher current densities. The osmium has the task of reducing the work function of the electrons.

An osmium-coated dispenser cathode is e.g. from patent EP-0 156 454 or from Technical Bulletin # 116, "Osmium coated dispenser cathodes - 'M' Type", Spectra-Mat Inc., 1240 Highway 1, Watsonville, California CA 95076, USA.

Other publications dealing with layers of high emissivity for cathodes are:
- Japan J. Appl. Phys. 1, Regul. Paper short notes (Japan), Vol 27 No. 8, pp. 1411-14, August 1988, S. Yamamoto, S. Taguchi, I. Watanabe, S. Sasaki. To increase the Electron emission from an impregnated cathode covered with WSc₂O₃, a monoatomic surface layer consisting of Ba, Sc and O is deposited.
- "High current density cathodes - an update", M. Feinleib, MC Green, IEDM 84, pp. 314-317. In order to reduce the interfering effect of the interdiffusion and to stabilize the emission, a tungsten matrix cathode is covered with two layers. An Os or Os / Ru layer serves as the first layer and a W / Os alloy serves as the second layer.
- "Modern dispenser cathodes", JL Cronin, IEE Proc. Vol. 128, Pt. 1, No. 1, 19-31, February 1981. This describes, inter alia, an M-type dispenser cathode (consisting of porous tungsten and impregnated with barium calcium aluminate), which is covered with a thin layer of Os / Ru or Os / Ir.
- "Long Life High Reliability Iridium-coated M-Type Dispenser Cathode", Ioshiaki Ouchi, Sakae Kimura, Toshiharu Higuchi, Kasou Kobayashi, Toshiba Review No. 157, spring 1987, pp. 25-29. Impregnated dispenser cathodes, which are coated with Os, an Os / Ru alloy or Ir (so-called M-type cathodes), have a longer service life than S-type cathodes without surface metallization because of their approximately 100 ° lower working temperature.
- "Performance analysis of three different M-type dispenser cathodes", B. Latini, P. Cristini, I. Fragala, G. Marletta, Int. Conf. on Microwave Tubes in Systems, Problems and Prospects (Conf. Publ. No. 241), Issue 22-23 October 1984, pp. 35-41. Various M-type cathodes were examined for their emissivity and compared with S-type cathodes. At 1300 K there was an increase in emissivity by a factor of 4.2 for an Os (80%) / Ru (20%) coating, and an increase of a factor of 3 for an Os (50%) / W (50%) coating and with an Ir (100%) coating an increase by a factor of 1.6.
- "Dispenser Cathodes: The Current State of the Technology", LR Falce, IEDM - 83, pp. 448-451. This article provides an overview of the common types of cathodes.

A problem with known electron tubes of high current density is the lattice load due to electron bombardment. It leads to an undesirable thermal emission of the lattice, which results from cathode material being deposited on the lattice during operation and thereby reducing the work function of the electrons.

Presentation of the invention

The object of the invention is to provide an electron tube of the type mentioned at the outset, which has a low lattice load even at high currents.

According to the invention, the solution is that the cathode has both strongly emitting and at most weakly emitting regions, the strongly emitting regions being arranged behind openings in the first grid and the at most weakly emitting regions corresponding to a shadow cast by the first grid adjacent to the cathode onto the cathode .

The advantage of the arrangement according to the invention lies in the fact that the stream of charged particles is only generated where it also comes into play. It is thus focused on the openings in the grille. The preferably cylindrical cathode ensures the necessary stability and precision with the usually very small distances between the first grid and the cathode.

In the case of particularly fine grid structures, a sheet metal grid is preferably used. Such grids are easier to manufacture and are mechanically less sensitive than wire grids.

Graphite grids are particularly advantageous because they suffer only very low temperature-related mechanical deformations even when subjected to a greater current load. Grid currents occur in an electron tube with a cathode structured according to the invention, which lead to temperature increases. If the grid is now very fine and close to the cathode, deformations of the grid can severely disrupt the geometric alignment in the tube.

The preferred graphite grids include, in particular, pyrographite and electrographite grids.

However, the invention is also suitable in connection with conventional wire grids.

In a simple embodiment, the openings in the grid are diamond-shaped or rectangular. The strongly emitting areas also have a regular, lattice-like shape.

A tetrode with a cathode designed according to the invention can switch high currents without showing the high load on the control grid which is present in the prior art.

In order to further reduce the emission in the at most weakly emitting areas, the cathode there can preferably be coated with a layer, e.g. from Mo / Ru, passivated.

In the strongly emitting areas, the cathode is preferably coated with a material from the following group: Sc₂O₃ / W, Os / Ir, Os / Ru, Os / W, Ir, Os, Os + W / Os, Os / Ru + W / Os. Os, Os / Ru, Os / W are particularly preferred because of the strong increase in emissivity. The double layers of Os + W / Os, Os / Ru + W / Os also represent a special group, because of their properties that prevent interdiffusion.

It corresponds to a preferred embodiment to use a so-called matrix or dispenser cathode, which is known for its current capacity, as the cathode.

Further advantageous embodiments result from the description and the patent claims.

Brief description of the drawings

• Fig. 1 einen Längsschnitt durch eine erfindungsgemässe Tetrode;
• Fig. 2 einen Querschnitt durch eine erfindungsgemässe Tetrode; und
• Fig. 3 eine Draufsicht auf die Kathode der Tetrode.

The invention is to be explained in more detail below on the basis of exemplary embodiments and in connection with the drawings. Show it:

• 1 shows a longitudinal section through a tetrode according to the invention;
• 2 shows a cross section through a tetrode according to the invention; and
• Fig. 3 is a plan view of the cathode of the tetrode.

The reference symbols used in the drawings and their meaning are summarized in the list of designations. In principle, the same parts are provided with the same reference symbols in the figures.

Ways of Carrying Out the Invention

1 shows a tetrode in longitudinal section according to a preferred embodiment of the invention.

A cylindrical cathode 1, a first grid 2 adjacent to the cathode 1, a second grid 3 and a cylindrical anode 4 are arranged coaxially with a common axis 5.

The first grid 2 is e.g. a sheet metal grid with openings 6. The second grid is designed accordingly.

According to a preferred embodiment, the cathode is constructed in the manner of a matrix cathode, as is e.g. is known from the cited patent specification EP-0 157 454. A porous tungsten matrix is impregnated with a barium-containing substance.

In order to reduce the grid load according to the invention, the cathode 1 is selectively covered with a strongly emitting layer. It is preferably an osmium-containing, i.e. osmated layer. In the sense of the invention, strongly emitting regions 7.1, 7.2, 7.3 are located behind the openings 6 of the first grating 2. In between, one or more regions 8, which emit at most weakly, are provided. The at most weakly emitting regions 8 correspond to an imaginary shadow cast by the grid 2 on the cathode 1.

The situation mentioned is illustrated in FIG. 2. It shows a cross section through the tetrode of FIG. 1. The (imaginary) shadow is created by projecting the first grating 2 in the radial direction (with respect to the axis 5). The strongly emitting areas 7.1, 7.4, 7.7 are separated by the at most weakly emitting area 8.

3 finally shows a top view of the cathode 1 when the openings of the grid 2 are rectangular. The strongly emitting areas 7.1, 7.2, ..., 7.9 are in this case rectangles which form a regular pattern in two directions perpendicular to one another. The at most weakly emitting region 8 separates the strongly emitting regions 7.1, 7.2, ..., 7.9 in the same way as the grating 2 and the openings 8.

In principle, the shape of the openings is of subordinate importance. In practice, however, are usually regular Grid used, for example diamond-shaped. have rectangular openings.

The advantage of the invention is that the flow of charged particles, which undesirably strikes the first grating 2, is largely prevented. Rather, the emitted electrons are guided very specifically through the openings 6 of the grid. As a result, the grid no longer heats up so much and it can no longer interfere with the electron tube in the manner of a parasitic cathode.

The osmized areas usually have two to three times higher emissivity than the non-osmotic areas. If the ratio is to be increased further, the non-osmized areas are preferably additionally passivated with an emission-inhibiting layer. A layer consisting essentially of Mo and Ru (Mo / Ru layer) is particularly suitable for this. In such a case, the areas in the shadow of the first grating not only emit weakly, but essentially not at all.

A cathode according to the invention can be produced using known means. Especially in the case of dispenser cathodes, which were previously used in a final process step e.g. have been osmized over the entire area, it means only a small change to cover the cathode with a suitable mask before osmosis, so that an osmium layer structured according to the invention is formed.

On the other hand, it is also possible in an additional step to provide a finished cathode with a passivating layer according to the invention which, as described, essentially corresponds to the shadow cast by the first grid.

What has been said so far in connection with Os of course applies mutatis mutandis to the preferred layers of Sc₂O₃ / W, Os / Ir, Os / Ru, Os / W, Ir, Os, Os + W / Os, Os / Ru + W / Os. In principle, however, all those material layers are suitable which result in a reduction in the work function of the electrons.

The grid can also be designed as a graphite grid (e.g. pyrographite, electrographite). Such grids endure higher current loads and thus allow the electron tube to be operated at higher current densities as a whole.

The invention is in no way limited to tetrodes. It is used wherever at least one independent grid is arranged in front of a cathode. Accordingly, the invention can be used with triodes as well as with pentodes and even more complex electron tubes.

It is also obvious that cylindrical symmetry is not a mandatory requirement, but only a preferred feature of the invention. Accordingly, the invention is also suitable for flat structures.

In summary, it can be said that the invention provides electron tubes with large currents and at the same time a low lattice load.

LIST OF DESIGNATIONS

1 - cathode; 2 - first grid; 3 - second grid; 4 - anode; 5 - axis; 6 - openings; 7.1, ..., 7.9 - strongly emitting areas; 8 - at most weakly emitting areas.

Claims (10)

1. electron tube with a) a cathode, b) an anode and c) at least one independent first grid between cathode and anode,
characterized in that d) the cathode has both strongly emitting and at most weakly emitting regions, the strongly emitting regions being arranged behind openings in the first grid and the at most weakly emitting regions corresponding to a shadow cast by the first grid adjacent to the cathode onto the cathode 2. Electron tube according to claim 1, characterized in that the cathode in the strongly emitting areas a layer made of a material from the group Sc₂O₃-W, Os-Ir, Os-Ru, Ir, Os, W-Os, Os + W-Os , Os-Ru + W-Os. 3 electron tube according to claim 1, characterized in that the cathode is cylindrical and that the cathode, grid and anode are arranged cylindrically symmetrically. 4. Electron tube according to claim 1, characterized in that the independent grid is a sheet metal grid. 5. Electron tube according to claim 1, characterized in that the independent grid is a wire grid. 6. Electron tube according to claim 1, characterized in that the independent grid is a graphite grid. 7. Electron tube according to claim 1, characterized in that the openings of the first grid are diamond-shaped or rectangular. 8. Electron tube according to claim 1, characterized in that exactly two grids are provided between the cathode and anode in the sense of a tetrode. 9. Electron tube according to claim 1, characterized in that the cathode is covered with a passivating layer in the at most weakly emitting areas. 10. Electron tube according to claim 1, characterized in that the cathode is an oxide cathode, in particular a matrix cathode.

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