Klystron is an electrovacuum device in which the conversion of a constant stream of electrons into a variable occurs by modulating the velocities of the electrons by the microwave electric field (when they pass through the gap of the volume resonator ) and then grouping the electrons into bunches (due to the difference in their velocities) in the drift space free of Microwave fields.
Content
- 1 Classification
- 2 History
- 3 Span Klystrons
- 3.1 Device and principle of operation
- 3.2 Parameters and characteristics
- 3.2.1 Efficiency
- 4 Multi-cavity klystron
- 4.1 Device and principle of operation
- 4.2 Parameters and characteristics
- 4.2.1 Efficiency
- 5 Reflective Klystron
- 5.1 Device and principle of operation
- 5.2 Parameters and characteristics
- 5.2.1 Efficiency
- 5.2.2 Frequency tuning range
- 6 Application
- 7 See also
- 8 Notes
- 9 Literature
Classification
Klystrons are divided into 2 classes: transient and reflective .
In a transit klystron, electrons sequentially fly through the gaps of volume resonators. In the simplest case of resonators 2: input and output. A further development of the flying klystrons is the cascade multi-resonator klystrons, which have one or more intermediate resonators located between the input and output resonators.
The reflective klystron uses a single resonator through which the electron stream passes twice, reflected from a special electrode - a reflector.
History
The first designs of flying klystrons were proposed and implemented in 1938 by American engineers [1] .
A reflective klystron was developed in 1940 by N. D. Devyatkov , E. N. Daniltsev, I. V. Piskunov and independently V. F. Kovalenko .
Span Klystrons
The principle of operation of the flying klystron (PC) is based on the use of inertia of electrons of an extended rectilinear electron flow. The PC is used as a power amplifier, frequency shift converter and frequency multiplier. PC frequency range from 200 MHz to 100 GHz, output power from 1 W to 1 MW in continuous mode and up to 100 MW in pulse mode. PC is the most powerful microwave amplifier.
Device and principle of operation
The klystron has two volume resonators with capacitive grid gaps. The first resonator is called the input, or modulator; the second is a weekend. The space between them is called the space of drift or grouping.
Electrons emitted by the cathode are accelerated by a constant voltage the second electrode and fall into the narrow grid gap of the first resonator, in which there is a longitudinal microwave field. This field periodically accelerates and slows down the electrons, modulating the electron flow in speed. Moving further in the drift space, the electrons gradually form clumps due to the fact that fast electrons catch up with slow ones. This density-modulated electron beam enters the second resonator and creates in it an induced current of the same frequency as the frequency of the input modulating field. As a result, a high-frequency electric field appears between the resonator grids, which begins to interact with the electron flux. The necessary parameters of the klystron are selected in such a way that the electric field of the second resonator slows down the clumps of electron density and accelerates its rarefaction. As a result, on average, over a period of one field oscillation, a greater number of electrons are inhibited than accelerated. The kinetic energy of the electrons is converted into the energy of the microwave oscillations of the electromagnetic field of the second resonator, and the electrons, having passed through the resonator, settle on the collector, scattering the rest of the kinetic energy in the form of heat.
Parameters and characteristics
Efficiency
The klystron efficiency is usually understood as the electronic efficiency :
i.e. power ratio given by the electron beam to the microwave field in the output resonator at the nth harmonic to the supplied power
Solving the problem of inducing power in the load of the output resonator from the general principles of inducing current by the electron beam, we can obtain that maximum , and therefore the maximum efficiency is determined by the maximum of the Bessel function :
Where - Bessel function of the first kind of the n- th order, - harmonic number, - the so-called grouping parameter .
The table shows the maximum electronic efficiency of the two-cavity klystron and the optimal value of the grouping parameter for various harmonics.
| one | 58.2 | 1.84 |
| 2 | 48.7 | 1,53 |
| 3 | 43,4 | 1.40 |
| 8 | 32,0 | 1.22 |
| 16 | 26.0 | 1.13 |
If you decrease the parameter , for example, by reducing the amplitude of the input signal or by increasing the amplitude of the accelerating voltage, the electron beam will be ungrouped . Efficiency and power output are reduced. The same thing happens in a regrouped thread.
The real efficiency of the transit two-cavity klystron, taking into account losses in the oscillatory system, on the resonator grids and other factors, is much less and does not exceed 20%.
Multi-cavity klystron
Device and principle of operation
In multi-cavity klystrons, additional unloaded resonators are placed between the input and output resonators. As an example, explaining the features of their work, it is enough to consider a three-cavity span klystron.
Assume that the intermediate resonator is precisely tuned to the frequency of the input signal. As in the two-cavity klystron, in the input resonator the electrons are modulated by speed and then grouped in the first drift space. If the input receives a weak input signal, then the modulation of the electron beam will be negligible. In this case, the induced current in the second resonator will also be small. However, since the unloaded intermediate resonator is a high-Q system, even with a small amplitude of convection current, the voltage created on its grids will be large. This is greatly favored by the fact that the second resonator is not connected with an external load. The total active conductivity of the second resonator is determined only by the losses in the resonator itself and the electronic load of the gate.
In steady state, the current and voltage in the second resonator have the same frequency as the frequency of the input signal. The voltage induced on the second resonator causes a strong modulation of the electron velocity and a strong grouping of the electron flux in the second drift space. As a result, the distribution of electrons in their density clusters will be determined by the second resonator and the dependence of the convection current in the third resonator will be approximately the same as in the two-cavity klystron formed by the second and third resonators, but with a modulating voltage much higher than the modulating voltage of the first resonator. In this case, the gain will increase significantly, since the grouping of electrons is carried out at a significantly lower amplitude of the input signal supplied to the first resonator. Similar processes occur in each intermediate cavity of a multi-cavity klystron.
The principle of operation of the device can be clearly demonstrated by the example of a sufficiently long busy section of the road equipped with traffic lights. Despite the fact that cars have different speeds of acceleration and deceleration during acceleration and deceleration (an analog of the distribution of electron velocities) in the areas following the traffic lights, the traffic will be quite clearly modulated with a frequency equal to the switching frequency of the traffic signals (analog of the resonator), and this modulation will be maintained at a certain distance from traffic lights. If all the traffic lights work in concert (the Green Wave system), then for some length of the road the average speeds of the cars will equalize and the modulation of the flow will be maintained throughout its length. Even if at the initial section of the road traffic light regulation does not apply to all cars (some of them drive at unregulated intersections), which is analogous to a weak signal at the input of the first klystron resonator, a synchronization of speeds will occur in a relatively small section.
From a physical point of view, increasing the gain of a multi-cavity klystron is achieved not by increasing the efficiency and output power, but by reducing the signal power required at the input of the amplifier to control the electron beam.
Parameters and characteristics
Efficiency
In the ideal case considered above (when the second resonator is precisely tuned to the input signal frequency), the maximum output power and electronic efficiency remain the same as in the two-cavity klystron, that is, the maximum value of the efficiency is 58%, since the maximum value of the amplitude of the first harmonic remains the same convection current in the last cavity.
To increase the efficiency in multi-cavity klystrons, the detonation of intermediate resonators is performed, where the voltage created by the induced current is high (usually the penultimate resonator). At the same time, a decrease in the output power and gain of the klystron that occurs during the detuning of the resonators is compensated by an increase in the number of resonators. (The gain is approximately equal to db where - the number of resonators.) Theoretical calculations show that in this case the electronic efficiency can be increased to 75% and the operating frequency band expanded to several percent. In practice, four to six resonator klystrons are usually used.
Reflective Klystron
Device and principle of operation
Reflective klystrons are designed to generate microwave power of low power.
Reflective klystron has one resonator, twice penetrated by the electron beam. The return of electrons is carried out using a reflector located at a negative constant potential with respect to the cathode. Thus, the resonator plays the role of a grouper in the first passage of electrons and the role of the output circuit in the second passage. The gap between the resonator and the reflector plays the role of a drift space, where the modulation of the electron beam in velocity passes into the modulation in density.
In order for the klystron to generate microwave oscillations, it is necessary that the electron flow bunches formed during the first passage through the resonator pass through the resonator during reverse motion at those moments when it has a braking high-frequency electric field.
Parameters and characteristics
Efficiency
The electronic efficiency of reflective klystrons is lower than that of transit klystrons, and its realistically achievable value does not exceed several percent.
Frequency Tuning Range
Within each generation zone, electronic frequency tuning is possible. In practice, it is carried out by changing the voltage on the reflector, since the current in the reflector circuit is zero and the generation frequency is controlled without power consumption.
The range of electronic frequency tuning in reflective klystrons usually does not exceed 0.5% of the average frequency value.
Mechanical frequency tuning is also possible. It is carried out by changing the frequency of the resonator. There are two types of mechanical adjustment: inductive and capacitive. The first is carried out by means of adjusting screws and pistons. The second occurs when the second resonator grid is seated on an elastic membrane, acting on which, the gap between the resonator grids can be changed. The mechanical tuning range is approximately 25% of the average frequency, which is significantly larger than the electronic tuning range. But at the same time, the speed of adjustment is low.
Application
Span klystrons are the basis of all powerful microwave transmitters of coherent radio systems , where stability and spectral purity of highly stable hydrogen frequency standards are realized. In particular, in the output stages of the world's most powerful radars for the study of asteroids and comets (radar telescopes, planetary and asteroid radars), which are located in the observatories Arecibo ( Puerto Rico ), Goldstone ( California ) and Evpatoria ( Crimea ), it is used span water-cooled klystrons.
Reflective klystrons are used in various equipment as low-power generators. Owing to their low efficiency, they are not used to obtain large capacities and are used as local oscillators of microwave receivers, in measuring equipment, and in low-power transmitters. Their main advantages are structural simplicity and electronic frequency tuning. Reflective klystrons have high reliability and do not require the use of a focusing system .
Currently, in those applications where high resistance to ionizing radiation is not required, reflective klystrons are replaced by microwave semiconductor generators.
See also
- Magnetron
- Optical Klystron
Notes
- ↑ Kuleshov, 2008 , p. 314.
Literature
- Kuleshov V.N., Udalov N.N., Bogachev V.M. and others. Generation of oscillations and the formation of radio signals. - M .: MPEI, 2008 .-- 416 p. - ISBN 978-5-383-00224-7 .