RU2316844C1 - Method and field-emission lamp for controlling field emission current of lamp - Google Patents

Abstract

FIELD: electronics, vacuum triodes which enable switching heavy currents at low voltages and use field-emission cathodes.

SUBSTANCE: proposed method for controlling field-emission current of lamp that can be used in elements of functional electronics, such as amplifiers, oscillators, short high-voltage pulse generators, and the like involves variation of field strength about cathode by varying cathode-to-anode distance. Field-emission lamp incorporating anode and field-emission cathode spaced short distance apart has its cathode made in the form of multipoint electron source which is essentially surface with regular micron projections whose specific ohmic resistance is higher by a few orders of magnitude than specific resistance of metals; cathode is installed on component that changes geometric dimensions under impact of energy supplied. Input control circuits and output circuits of field-emission lamp are electrically insulated.

EFFECT: enhanced reliability of lamp control, enhanced power, voltage, and current gain, enlarged range of admissible voltages, currents, and power capacities.

5 cl, 10 dwg

Description

The invention relates to the field of electronics, namely to vacuum triodes, which allow switching high currents with low voltages and using field (cold) cathodes. It can be used in elements of functional electronics: amplifiers, generators, shapers of short high-voltage pulses, etc. Elements of functional electronics based on this device will find application in household appliances, automotive, plasma chemistry, energy, aviation, rocket technology, etc.

A known method of controlling field emission current of a lamp using a control grid. Known vacuum triode [1], controlled on the basis of this method, which contains an anode, a micro-tip field emission cathode and a control grid. The use of a control grid for switching field emission has such disadvantages as spurious grid current, the inability to use a blocking voltage, which leads to spurious emission of electrons from the surface of the grid.

In the field emission triode [2], a cathode made in the form of a nanocrystalline diamond emitter having an emission threshold of the order of 2-6 V / μm is used, as a result of which a blocking voltage is obtained which is whiter than an order of magnitude less than in [1]; less and unlocking voltage. Therefore, the stray current of field emission between the grid and the anode at a blocking voltage is significantly reduced. The interception of electrons at the unlocking voltage is also reduced. It should be noted that conventional electronic lamps can operate in amplifying mode at negative voltages on the control grid; at the same time, their input resistance is very large - of the order of hundreds of ohms or more. Field emission lamps are locked at negative voltages. In the amplifying mode, field emission lamps have a positive potential on the control grid and, therefore, have a grid current. Therefore, their input impedance is less than that of a conventional lamp, which is a drawback. Another drawback of field-emission lamps is that the distance between the cathode and the control grid is orders of magnitude smaller than that of conventional lamps at comparable voltages, and their input capacitance is much larger, which degrades their frequency properties. The disadvantage is the lack of galvanic isolation between the input and output, which leads to spurious feedback.

These shortcomings complicate the management of the field emission lamp and reduce the reliability of its operation, increase the input power and, accordingly, reduce the efficiency.

The problem solved by the invention is the creation of a method for controlling the field emission current of a lamp and a lamp, combining the advantages of both a transistor and a traditional electronic lamp, namely: high input and low output impedance, wide frequency range, high gain, wide power range , a wide range of voltages and currents, lack of glow, small dimensions.

The technical result consists in increasing the reliability of controlling the operation of the lamp due to the galvanic isolation of the input control circuits and output circuits of the field emission lamp, increasing the gain in power, voltage and current, increasing the allowable range of voltages, currents and powers.

This result is achieved by the fact that in the proposed method for controlling the field emission current of the lamp, the field value near the cathode is changed by changing the distance between the cathode and the anode.

The indicated result is also achieved by the fact that in a field emission lamp containing an anode and field emission cathode located at a small distance relative to each other, the field emission cathode is made in the form of a multi-tip electron source, which is a surface with regular micron protrusions having a resistivity of several orders of magnitude higher resistivity of metals, and is mounted on an element that changes geometric dimensions under the action of the supplied energy.

Such an element may be a piezoelectric element, electrostrictive or magnetostrictive element.

The lamp current control is based on a change in the distance between the cathode and the anode, which is carried out by means of the inverse piezoelectric effect, electrostriction, or magnetostriction. In accordance with the functional purpose of the lamp, the initial distance between the anode and cathode, the material and geometry of the striction element, the resistivity and the work function of the micron protrusions change. The energy source is an industrial network of 220 ... 380 V of both alternating and direct current. Possible operation of the proposed device at a lower and higher supply voltage.

Given the exponential dependence of the cathode field emission current on the field, and hence on the distance between the cathode and the anode, striction control can be very effective. Figure 1, 2, 3, 4 shows the dependence of the field emission current of the lamp on the distance between the anode and cathode at a voltage of 300 V and 30,000 V and two values of the work function - 1 eV and 4.8 eV, calculated by the formula:

where E is the field between the cathode and the anode in V / cm,

U _pit is the voltage between the anode and cathode,

d is the distance between the cathode and the anode in cm,

φ is the work function in eV.

Figure 1 shows the dependence of the current density of the field emission of the cathode with a work function of 1 eV on the distance of the anode-cathode at a supply voltage of 300 V, the cathode area is 1 cm ² .

Figure 2 shows the dependence of the current density of field emission cathode with a work function of 4.8 eV from the anode-cathode distances at a supply voltage of 300 V, the cathode area of 1 cm ^2.

Figure 3 shows the dependence of the current density of the field emission of the cathode with the work function of 1 eV on the distance of the anode-cathode at a supply voltage of 30,000 V, the cathode area is 1 cm ² .

Figure 4 shows the dependence of the current density of field emission cathode with a work function of 4.8 eV from the anode-cathode distances at a supply voltage of 30,000 V, cathode area of 1 cm ^2.

The calculation of the curves in Figs. 1, ..., 4 was carried out at a field enlargement coefficient at the tips of the protrusions μ = 1.5. Such a low increase in the field at the tips occurs with a regular structure of the protrusions due to the influence of mutual screening [3, 4]. This is also facilitated by the distance between the cathode and the anode, comparable with the distance between the protrusions [4].

[4]. Расчетная рабочая плотность тока

.The high resistivity and relatively low current density of the protrusions works as feedback, protecting the lamp from explosive emissions. It is known that explosive emission (and, accordingly, breakdown) occurs at a current density

[four]. Estimated Operating Current Density

.

If the calculated working conditions for any protrusion determined by expression (1) are violated, for example, during chemisorption or increased fluctuations in ion bombardment, the protrusion melts, and the tip grows under the influence of the field. In this case, the current density through the protrusion increases sharply. As a result, the voltage drop across the protrusion increases, and in the vacuum gap, the voltage drop for a given protrusion decreases, which, when the current density depends on the field exponentially, leads to stabilization of the current density for this protrusion. From this condition, we find the resistivity and, accordingly, the protrusion material in accordance with the formula:

Here U _pit is the cathode-anode voltage, ρ is the protrusion resistivity, h is the protrusion height, S is the cathode area, j is the density of the emission current. Protrusions with high resistivity are created on the conductive substrate by thermal diffusion and subsequent electroerosive or laser treatment. The resistance of all the protrusions is made up of their parallel connection, which, with their micron thickness and amount of 10 ^5-6 per cm ^2, is expressed by a value of several orders of magnitude less than 1 Ohm.

Figure 5 shows the current-voltage characteristic of a lamp with a field-emission cathode, in which the work function is 2.49 eV, the cathode area is 1 cm ² , the field magnification factor on the tips is 1.5, and the operating point is 80 A and 410 V.

Figure 6 shows the current-voltage characteristic of a lamp with a field-emission cathode, in which the work function is 2.49 eV, the cathode area is 1 cm ² , the field magnification factor on the tips is 1.5, and the operating point is 7.5 A and 1500 V.

From the characteristics it can be seen that both current and voltage can be significantly changed with a linearity of several percent.

In nonlinear mode, it is efficient to use a lamp to form shorter pulse fronts.

7, 8, 9, 10 schematically shows field emission lamps having different elements, changing their geometric dimensions under the action of the supplied energy. The area of the field emission cathode is 1 cm ² .

7 shows a diagram of a broadband field emission lamp with piezoelectric elements or electrostrictive control elements.

The lamp contains a metal housing 1 having a deformable conductive part 2 with a field emission cathode 3 fixed to it. Insulators 6, 7 are pressed into the housing to isolate the anode 4 and the control electrode 5; the anode is located opposite the cathode, a supply voltage is applied between them. The insulation provides the necessary vacuum between the cathode and the anode. The control electrode is located between the piezoelectric elements 8 and is separated from the housing 1 by an insulator 7. The piezoelectric elements are clamped by a plug 9. The field emission cathode 3 is made in the form of a multi-pointed source of electrons. It is a surface with regular micron protrusions having a resistivity calculated by the formula (2) and which is 2-4 orders of magnitude higher than the resistivity of metals. The surface is mounted on a deformable conductive part 2 of the housing 1, which can change its geometric position.

The number of piezoelectric elements 8 and their thickness are determined by allowable breakdown fields, frequency properties, allowable control voltages and lamp supply voltage.

The lamp operates as follows. An input control signal is supplied to the control electrode 5 relative to the grounded housing 1, which changes the geometric dimensions of the piezoelectric elements 8 in accordance with the expression

Here l is the length of the piezoelectric elements,

d ₃₃ - buzzer module for longitudinal piezoelectric effect,

E _piezo - the magnitude of the field inside the piezoelectric element.

Given that Δl << l, the field inside the piezoelectric element is determined by the input voltage.

As a result of changing the geometric dimensions of the piezoelectric elements, depending on the magnitude of the input voltage, the deformable part 2 of the housing 1 will shift, changing the distance between the cathode 3 and the anode 4. In accordance with formula (1), the current through the lamp will change. The operating point A (Fig. 5, 6) on the load line for a given power supply is determined by the initial distance between the cathode 3 and the anode 4. From Fig. 5, 6 it is seen that, provided that the changes in the distance between the cathode and the anode are small compared to the original the distance between them, the relationship between the output current and the input voltage is close to linear.

Lamps on piezoelectric elements or striction elements are characterized by a large gain in current and power.

The width of the strip is determined by the piezoelectric elements 8. As a rule, they are linear, i.e. reproduce the shape of the control electrical signal. However, when solving some problems, it is more profitable to use electrostrictive elements in which the dependence of the change in geometric dimensions is proportional to the square of the field.

On Fig depicts a resonant field emission lamp.

The structural difference of this lamp is the presence of an additional structural element - a concentrator 10, calculated by the standard method and located between the deformable conductive part 2 of the housing 1 and the piezoelectric elements 8.

The lamp operates at a resonant frequency, which is determined by the geometric dimensions of the piezoelectric elements 8 and the hub 10. A variable frequency voltage (equal to or close to the resonant voltage) is applied to the lamp input (control electrode 5 relative to the housing 1) depending on the quality factor of the system of piezoelectric elements 8 and the hub 10 The deformable conductive part 2 of the housing 1 under the action of the concentrator 10 will oscillate with the frequency supplied to the input, approaching and removing the cathode 3 to the anode 4. In the resonant mode, the movements of the cathode 3 are maximum that will allow a non-linear mode to form steep edges of the output pulses.

It can be seen from formula (1) that the resonant current in the linear mode will pass through the lamp, i.e. at small displacements, or pulsed current in a nonlinear mode, i.e. with large movements of the control electrode. The resonant lamp has a large gain in current, voltage and power.

Figure 9 shows a magnetostrictive lamp in which a magnetostrictive element is used instead of an electrostrictive element or a piezoelectric element. Lamps on magnetostrictive elements have a large voltage gain.

Structurally, the lamp comprises a metal housing 1 having a deformable part 2 with a field emission cathode 3 fixed to it. Insulators 6, 7 are pressed into the housing 1 to isolate the anode 4 and the control electrode 5; the anode 4 is located opposite the cathode 3. Between the cathode 3 and the anode 4, a supply voltage is applied. The insulator 6 provides the necessary vacuum between the cathode 3 and the anode 4. The control electrode 5 is connected to the turns 8 of the control coil 9, in which there is a magnetostrictive element 10. One of the control electrodes can be grounded. The coil 9 with the magnetostrictive element 10 is clamped by a plug 11.

The lamp operates as follows. An input voltage is applied to the control electrodes 5 of the control coil 9. The current flowing through it creates a magnetic field, under the influence of which the dimensions of the magnetostrictive element 10 increase, displacing the deformable part 2 of the housing 1. Thereby, the distance between the cathode 3 and the anode 4 changes, causing a current change according to (1).

To work with voltages of tens and hundreds of kilovolts, it is proposed to use several distances (gaps) between the cathode and anode.

The series connection of several gaps, for example two (Fig. 10), on the deformable part 2 of the housing 1, will allow, without affecting the lamp parameters, to increase the voltage.

The lamp of FIG. 10 with a large number of gaps contains a housing 1 into which insulators 2, 3 are pressed. Anode 4 is mounted in the insulator 2. Co-located with the anode is a field emission cathode 5, which is part of the housing 1 and on which a multi-point source of electrons 6 is located. a control electrode 7 is fixed to the insulator 3, located between the piezoelectric elements 8. The deformable part 9 of the housing 1 is insulated by the insulator 10 from the conductive substrate 11 parallel to the cathode 5, on which it is created by diffusion followed by a laser or EDM otkoy auxiliary field emission cathode 12, which is a mnogoostriyny electron source 4 disposed parallel to the anode.

The hub 13 located between the deformable part 9 of the housing 1 and the piezoelectric element 8 serves to increase the vibrations of the deformable part 9 of the housing 1 when a resonance occurs between the frequency of the input voltage and the resonant frequency of the hub 13 and the piezoelectric elements 8. Piezoelectric elements 8, the concentrator 13 are pressed against the deformable part 9 of the housing 1 plug 14. The supply voltage is supplied to the anode 4 relative to the field emission cathode 5.

The lamp works as follows. Upon receipt of the input voltage to the control electrode 7, the emerging electric field changes the geometric dimensions of the piezoelectric elements 8. The distance between the field emission cathode 5, auxiliary cathode 12 and anode 4 changes, which changes the current in accordance with expression (1). In this case, the distance (gap) between the field emission cathode 5 and the anode 4 is two times larger, which allows the voltage between them to be doubled so that the field emission field remains the same. The hub 13 allows, at a frequency of the input voltage, which coincides with the resonant frequency of the piezoelectric elements and the concentrator, to increase the oscillation amplitude of the auxiliary field emission cathode 12 and, therefore, according to (1), increase the current through the lamp. The conductive substrate 11 is an additional anode for the cathode 5 and provides galvanic communication with the field emission cathode 12. The number of gaps and, accordingly, the supply voltage can be increased by increasing the number of auxiliary field emission cathodes and additional anodes that are introduced through the insulator. The plug 14 provides the initial deformation of the deformable part 9.

The input characteristics of the lamps are determined by the characteristics of the elements, which change their size under the action of the supplied energy (field). Estimates show that the current gain of the lamp at the resonant frequency can be equal to 10 ⁶ and 100 for voltage.

In the case of a magnetostrictive element with a constant bias at the resonant frequency, the voltage gain can be more than 10 ⁴ .

Oxides or carbides of elements, for example nickel, are used as a multi-edge source of electrons of the field emission cathode. For the anode, for example, copper, tungsten or other materials are used, depending on the purpose of the lamp. Piezoelectric elements, for example, can be industrially produced grades PKV-460. The magnetostrictive element may be, for example, nickel.

Thus, the proposed method of controlling the field emission current of the lamp has the following advantages:

- allows to increase the reliability of lamp operation control due to galvanic isolation between the cathode and anode;

- allows you to increase the gain in power, voltage and current;

- allows you to increase the input impedance and reduce the output;

- allows you to expand the ranges of operating voltages, currents, capacities;

- allows you to combine the advantages of lamps and transistors in a lamp;

- it allows you to purchase due to the above properties that can not give separately lamps or transistors, for example, to obtain an output voltage of tens of kilovolts at a supply voltage of 380 volts.

Information sources

1. I. Brodie, P. R. Schwoebel, Proceeding of the IEEE, 1994, v. 82, n. 7, p. 1006.

2. RF patent №2161840, cl. H01J 21/20, cl. H01J 21/12, 2001.

3. I.N. Slivkov, V.I. Mikhailov, N.I. Sidorov, A.I. Nastyukha. Electrical breakdown and discharge in a vacuum, Atomizdat, 1966.

4. Non-heated cathodes. Edited by Professor M.I. Elinson. Moscow: Soviet Radio, 1974.

Claims (5)

1. The method of controlling the field emission current of the lamp, which consists in changing the field near the cathode, characterized in that the field value near the cathode is changed by changing the distance between the cathode and the anode. 2. A field emission lamp containing an anode, a field emission cathode located with a gap relative to each other, characterized in that the field emission cathode is made in the form of a multi-pointed electron source, which is a surface with regular micron protrusions having a resistivity of several orders of magnitude higher than the resistivity of metals , and mounted on an element that changes the geometric dimensions under the action of the supplied energy. 3. The field emission lamp according to claim 2, characterized in that the element with the ability to change geometric dimensions under the action of the supplied energy is a piezoelectric element. 4. The field emission lamp according to claim 2, characterized in that the element with the ability to change geometric dimensions under the action of the supplied energy is an electrostrictive element. 5. The field emission lamp according to claim 2, characterized in that the element with the ability to change geometric dimensions under the action of the supplied energy is a magnetostrictive element.

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