Method for manufacturing a directly heated thermionic cathode

Abstract

FIELD: cathode productionSUBSTANCE: method for manufacturing a directly heated thermionic cathode for vacuum tube devices, in particular for scanning electron microscopes. The cathode is made in the form of a U-shaped arc, at the top of which a jumper is formed between the legs of the arc, with a cross-sectional area several tens of times smaller than the cross-sectional area of the legs of the arc. The cathode material used is a foil of thickness d made of refractory metal. Moreover, the spatial position of the legs of the U-shaped arc is fixed by placing them in grooves of depth <d / 2 of two plates made of dielectric material fastened together, and the formation of a jumper between the legs of the U-shaped arc of length l≈d and height Δ≈ is carried out using the method of micro-sized laser milling by through-piercing the foil with a pulse packet in the amount of 15–50 pieces with a duration of 300–400 ns, a repetition frequency of 30–50 kHz in a packet with an average power of 40–80 W, a packet repetition period of 2.5 ms and a step of movement of the pulse packets along the cutting surface of 15 μm.EFFECT: increase in the reliability of the cathode design and simplification of the cathode manufacturing process.1 cl, 6 dwg

Description

[0001] The invention relates to electronic engineering, namely to a method for manufacturing a directly heated thermionic cathode for vacuum tube devices, in particular for scanning electron microscopes.

[0002] Methods for producing a directly heated thermionic cathode are known, in which an electron emitter in the form of a tip 1-2 mm long is made of tungsten wire with a pointed tip [Patents GB 15270022 A, JP 2012174691 (A)]. The base of the tip is attached to a holder in the form of a U-shaped arc at its bend. The legs of the arc are welded to metal holders in the form of wire segments with a diameter of ~1 mm, which in turn are fixed in holes in a disk made of a dielectric material and connected to a voltage source. When an electric current flows, the tungsten wire arc and the tip with a pointed tip are heated by Joule heating to a temperature sufficient to initiate thermionic emission.

[0003] A method is known [Patent JP 2009295376] for manufacturing a directly heated thermionic cathode, in which, in order to increase thermionic emission, a single crystal of lanthanum hexaboride with a low electron work function in the form of a rectangular parallelepiped measuring 0.2 mm × 0.4 mm × 0.5 mm was placed at the top of a pointed tip. One of the faces of the parallelepiped with the orientation <100> , facing the vacuum, was processed to a spherical surface with a radius of curvature of ~15 µm.

[0004] A drawback of the proposed technical solutions is that not only the tip, which is the source of the "useful" electrons, but also the arc carrying them along its entire length, all the way to the attachment point on the holders, heats up to the onset of thermionic emission. Electrons emitted by the arc in all directions ionize the residual gas in the device. The ions are adsorbed on cooler components of the device's electron-optical system, including the dielectric insulator, which can lead to ohmic leaks during operation.

[0005] The closest technical solution is a method for fabricating a directly heated thermionic cathode from tungsten wire or lanthanum hexaboride (LaB6) in the form of a U-shaped arc. At the apex of the U-shaped arc, a limited surface area is formed, representing a bridge between the arc legs, with a cross-sectional diameter several dozen times smaller than the cross-sectional diameter of the arc legs. The arc legs are attached to holders made of wire pieces inserted into holes in a ceramic disk and connected to a filament voltage source.

[0006] According to the Joule-Lenz equation, when an electric current flows, the smaller the cross-sectional diameter of a conductor, the greater the thermal energy released. Therefore, the bridge at the apex of a U-shaped arc, with a cross-sectional diameter several dozen times smaller than the diameter of the legs, heats up to the onset temperature of thermionic emission, while the legs do not reach this temperature and do not emit thermionic current. This technical solution eliminates the main drawback of similar designs—the presence of "unwanted" electron flow from the arc legs.

[0007] A drawback of the prototype is the difficulty of fabricating a localized section at the apex of the U-shaped arc from metal wire in the form of a bridge with small, difficult-to-control dimensions. During the process of attaching the arc legs to the holders, the bridge between the legs deforms, which can lead to mechanical failure.

[0008] The technical result of the present invention is to simplify the manufacturing technology, increase the reliability of the design, and reduce the time and cost of manufacturing the cathode.

[0009] The technical result is achieved in that a directly heated thermionic cathode in the form of a U-shaped arc with a local section at the top with a small cross-sectional area in the form of a bridge with a large radius of curvature or in the form of a hollow tip with a small radius of curvature of the emitting surface is formed from a refractory metal foil by the method of micro-sized laser milling with experimentally selected laser radiation parameters after fixing the legs approximately at the middle of their length in the grooves of the ceramic plates.

[0010] The bridge between the legs may have a different shape and curvature of the electron-emitting surface, depending on the operating mode of the cathode. In the thermionic emission mode, without the Schottky effect, the radius of curvature of the emitting surface facing the anode may be large. When operating with the Schottky effect and a reduced work function of the cathode material due to the external electric field strength, the bridge between the legs is formed in the form of a point with a radius of curvature of the emitting surface amounting to units of micrometers (M.I. Elinson, G.F. Vasiliev. Field Emission / / State Publishing House of Physics and Mathematics Literature. Moscow. 1958. - 272 p.).

[0011] The proposed invention is explained by drawings.

[0012] Fig. 1 shows a cathode blank in the form of a U-shaped arc with undivided legs.

[0013] Fig. 2 shows a bushing made of dielectric material with grooves.

[0014] Fig. 3 shows a cathode blank with legs fixed in the grooves of two bushings made of dielectric material.

[0015] Fig. 4 shows a cathode with a large radius of curvature of the bridge between the legs.

[0016] Fig. 5 shows a cathode with a small radius of curvature of the bridge in the form of a hollow point with the cavity size increasing from the top.

[0017] Fig. 6 shows a cathode with a small radius of curvature of the tip in the form of a point, placed in the aperture of the diaphragm.

[0018] Where:

[0019] 1 - the top of the U-shaped arc;

[0020] 2 - arc legs;

[0021] 3 - grooves for placing the arc legs;

[0022] 4 - local area at the top of a U-shaped arc in the form of a jumper;

[0023] 5 - a local area at the top of a U-shaped arc in the form of a hollow point;

[0024] 6 - a cavity expanding from the top of the tip;

[0025] 7 - diaphragm hole;

[0026] 8 - diaphragm with hole.

[0027] The sequence of technological operations in the implementation of the proposed method is as follows. A cathode blank in the form of a U-shaped arc with an apex (1) and legs (2) unseparated at the base (Fig. 1) is cut out from refractory metal foil of thickness d. Grooves (3) for the legs (2) of the cathode with a depth less than d / 2 (Fig. 2) are formed in bushings made of a dielectric material. The legs (2) of the U-shaped arc are fixed in the grooves (3) between two bushings (Fig. 3) and the base of the legs is cut off (Fig. 3). At the apex of the U-shaped arc (1), a jumper (4) is formed between the legs (2) of the arc with a large radius of curvature of the emitting surface (Fig. 4) or with a small radius of curvature in the form of a hollow point (5) of height H and a cavity (6) expanding from the apex with a size A (Fig. 5). When the cathode operates in Schottky mode, the top of the tip (5) is located in the opening (7) of the diaphragm (8) (Fig. 6).

[0028] The bridge between the legs is limited on the outside by the surface of the cathode apex with a large or small radius of curvature, and on the inside by the surface of the cavity expanding from the apex. The length of the bridge is comparable to the foil thickness l≈d. The height of the bridge is also approximately comparable to the foil thickness Δ≈d to ensure the required strength and service life of the cathode, taking into account the evaporation of the material in the operating mode of the cathode at temperatures of 1200÷1400°C. The height of the tip is selected based on the possibility of placing the apex of the tip (5) in the hole (7) of the diaphragm (8) when operating in the Schottky mode and creating the required gap between the tip and the diaphragm, ensuring resistance to electrical breakdown (Fig. 6). As a rule, the value H = 400÷500 μm is sufficient.

[0029] Laser radiation parameters, primarily the average power generated on the material surface, are most critical when forming a localized emitting region at the apex of a U-shaped arc in the form of a bridge with a small cross-sectional diameter. High power can cause the bridge to melt, while low power will not provide the necessary temperature to melt the material.

[0030] Laser milling was performed on a BY-100GS laser system based on a JPT YDFLP-E-100-M7-MR ytterbium-doped pulsed fiber laser with a wavelength of 1064 nm in pulsed mode, applying a pulse train of a specific duration and repetition rate to the surface. The average radiation power for through-piercing the foil increases with the higher the melting temperature of the material. For standard values ​​of 15-50 pulses in a train and a single pulse duration of 300-400 ns, an average radiation power sufficient for through-piercing 100 µm-thick foil was sequentially selected in a pulse repetition rate range of 30-50 kHz. For refractory materials such as hafnium, molybdenum and tungsten with melting points of 2230°C, 2620°C and 3420°C, respectively, the average pulse power in a packet for through-foil piercing is in the range of 40-80 W.To reduce the average temperature of the forming bridge, the pulse train repetition period was ~2.5 ms. The train of successive laser pulses moved along the length of the cut in 15-µm increments. During the piercing process, the bridge temperature did not exceed the hafnium recrystallization temperature of 760°C and was monitored using a Keller disappearing-filament pyrometer. To prevent material oxidation, the laser milling process took place in a closed, oxygen-free chamber with a nitrogen flow blowing through the processing area.

Claims

A method for manufacturing a directly heated thermionic cathode, including its manufacture in the form of a U-shaped arc, at the top of which a limited surface area is formed, which is a bridge between the legs of the arc, with a cross-sectional area several tens of times smaller than the cross-sectional area of ​​the legs of the arc, characterized in that a foil of thickness d made of refractory metal is used as the cathode material, the spatial position of the legs of the U-shaped arc is fixed by placing them in grooves of depth <d / 2 скрепленных между собой двух пластин из диэлектрического материала, а формирование перемычки между ножками U-образной дуги длиной l≈d и высотой Δ≈d осуществляют методом микроразмерного лазерного фрезерования путем сквозной прошивки фольги пачкой импульсов в количестве 15-50 штук длительностью 300-400 нс, частотой следования 30-50 кГц в пачке со средней мощностью 40-80 Вт, периодом следования пачек 2,5 ms and a step of moving pulse packets along the cutting surface of 15 µm.,

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