Cyclotron wave electrostatic amplifier

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cyclotron wave electrostatic amplifier

Vladimir A. Vanke

Faculty of Physics, Moscow State University

Hiroshi Matsumoto, Naoki Shinohara

Radio Atmospheric Science Center, Kyoto University, Japan



Physics principles of a new type of microwave input amplifiers are described. Cyclotron wave electrostatic amplifier (CWESA) has a low noise level, broad band, switchable gain, super high self-protection against microwave overloads, rapid recovery and small DC consumption. CWESAs are widely used in Russian pulse Doppler radars and other systems.



1. INTRODUCTION

Development of modern microwave communication, radio, radar and Microwave Power Transmission Systems (MPTS) facilities places additional stringent requirements upon the microwave input amplifiers.

Together with improving the sensitivity of the input amplifier and reducing its intrinsic noises, the problems put to the forefront include the improving of the linearity of amplitude and phase-frequency characteristics over a wide dynamic range and stability to powerful pulsed and continuous overloads in the input signal level, the protection of subsequent receiver stages in case of such overloads and the reduction of the time needed for restoration of the serviceability after overloads.

Realization of such requirements on the basis of solid-state microwave amplifiers is a rather complicated problem, which necessitates the creation of special efficient and high-speed devices for protection of the input amplifiers and subsequent receiver stages.

In Russia active work is being carried on to create input microwave amplifiers (practically unknown today in the West) based on cyclotron waves -- Cyclotron Wave Parametric Amplifiers (CWPA) [1-4] and Cyclotron Wave Electrostatic Amplifiers (CWESA) [5-9] complying with the present-day requirements [10-39].

The operation of these devices is based on the principles of transverse grouping of the electron beam in the longitudinal magnetic field. In contrast to conventional longitudinal grouping of electrons into dense bunches, this principle employs the Lorenz force as an elastic force and leads to spatial distortion of the electric beam without electron bunches being formed (Fig. 1).

In this way it is possible to considerably overcome the fundamental restrictions which are characteristic of longitudinal grouping devices (both vacuum and solid-state ones) and are associated with non-linear influence of the space charge fields upon the process of the input signal amplification, thereby laying the basis for developing new input amplifiers with improved characteristics.

The oldest Russian microwave organization (The ISTOK State Research & Production Corporation) and Moscow State University are currently developing input amplifiers based on using cyclotron waves -- Cyclotron Wave Electrostatic Amplifiers (CWESA).

The CWESA has an unique complex of characteristics: a low level of intrinsic noises (100-200 K and lower), a remarkable linearity of the amplitude and phase-frequency characteristics over a wide dynamic range (W and higher), a capability of sustain high overloads in the input signal (up to 200-500kW in the pulse), a highly efficient protection of subsequent receiver cascades under overloads (60-120dB), a short time needed for restoration after overloads (2-20ns), which explain its wide application in various communication, radar and navigation systems.

The capability of CWESA to operate without any additional protection when it is connected directly to the receiving-transmitting antenna may be especially useful when employed in cases, where a great number of microwave systems are deployed and hence powerful overloads in the signal channel may arise.

2. GENERAL CWESA DESCRIPTION

2.1 The Principle of Operation of CWESA

cyclotron electrostatic amplifier microwave

The principle of operation of CWESA and the main elements of its structure are illustrated in Fig. 2.

· The electron gun

forms a circular or ribbon electron beam (150-300 , 10-50V) passing through the series of the input coupler (the input resonator), the region of electrostatic gain and the output coupler (the output resonator) to the collector.

· The drift region

contains the adiabatically decreasing magnetic field making it possible to reduce the equivalent noise temperature of the fast (1+) and slow (1-) cyclotron waves [5]:

, (1)

where is the cathode temperature.

In the CWESA construction this is realized with the help of specially designed thermo-cathodes which allow an additional magnet (see Fig. 2) to be placed near the emitting surface of the electron gun cathode.

The focusing magnetic system creates an uniform longitudinal magnetic field corresponding to the resonance value of the cyclotron frequency in these regions is the charge-to-electron mass ratio i.e. in the input resonator, in the amplification region and in the output resonator.

· The input coupler

provides an efficient interaction of the electron beam with the uniform transverse electric field in the gap between the resonator pads. When the cyclotron resonance conditions are realized, the efficient energy exchange with the electron beam can take place [1,4]. Under the action of the transverse electric field there appear cyclotron rotations of the electrons in the electron beam, to be exact, a fast cyclotron wave is excited, whose amplitude is proportional to the input signal level. At the same time noises of the fast cyclotron wave within the operating frequency band can be removed (extracted) almost completely from the electron beam into the external load.

of the CWESA contains a plane periodic structure (Fig. 2, Fig. 4) connected with the dc sources , and creating a spatial periodic electrostatic field in the interaction channel. Such plane periodic electric field contains a quadruple component spatially twisted along the z-axis. The electrons moving along such a system are now affected by the electric field alternating in time, and if the frequency of such a field is close to the double cyclotron frequency:

(2)

( is the longitudinal velocity of the electrons in the amplification region created by the “synchronism” potential , is the periodic structure lag) the resonance interaction takes place and the tangential force of such quadruple component of the field will accelerate or decelerate cyclotron rotation of each electron in dependence on its initial phase. The amplitudes of both the fast and slow cyclotron waves will be increased.

The gain of the CWESA can vary in a wide range as the “amplification” potential varies.

The essential feature of signal amplification in the CWESA is the active coupling between the fast and slow cyclotron waves. Noises of the slow cyclotron wave are practically not removed from the beam in the input coupler and, due to the active coupling with the fast cyclotron wave, are transferred into this wave and are therefore present at the amplifier output. Thus, to realize a low level of intrinsic noises in the CWESA it is necessary to take additional steps to reduce noises of the slow cyclotron wave in the electron beam, i.e. in the drift region with the divergent magnetic field (Fig. 2).

· The output coupler

is used to extract the fast cyclotron wave energy from the electron beam.

2.2 Main Advantages of CWESA

Operation of Cyclotron Wave Electrostatic Amplifiers is based on an essentially different method of grouping of the electron beam. Due to this fact, the CWESA possesses a unique combination of advantages of highly sensitive input amplifiers with improved linearity of the amplitude-phase characteristics with the ability to sustain considerable overloads in the input signal and protect the subsequent receiver stages.

Let us consider in brief some fundamental advantages of CWESA:

· Low level of intrinsic noises.

An additional magnet placed in the near cathode surface allows the CWESA noise level to be reduced considerably. As a result the CWESA noise factor is determined mainly by the cathode temperature and the ratio of the magnetic fields and :

(3)

where is the cathode temperature on Kelvin's scale, .

At and , the CWESA noise factor does not exceed 0.8-1.2 dB.

· Linearity of amplitude characteristic

The principle of transverse grouping of the electron beam imposes no physical restrictions on the amplitude of the signal carried by the fast cyclotron wave. Therefore the output amplitude characteristic of CWESA has a perfect linearity throughout the dynamic range.

· Broad dynamic range

The amplitude of the signal carried by the fast cyclotron wave is directly proportional to the radius of cyclotron rotation of the electron beam. Therefore the CWESA dynamic range is limited by the value of the input signal which causes the electron beam to deposit on the output resonator pads, i.e. when the double radius of cyclotron rotation of the electrons (plus the electron beam cross-section diameter) becomes greater than the gap between the output resonator bars.

The typical level of the maximum input signal of CWESA (at linear amplification) reaches (may be done up to by additional device development and/or voltage control).

· Linearity of phase-frequency characteristics

The Lorenz force used as an elastic force in transverse grouping of the electron beam in CWESA is always proportional to the amplitude of the signal at the fast cyclotron wave. Moreover, phase velocities of cyclotron waves are determined by the external parameters only (the value of the focusing magnetic field, the accelerating potential) and are independent of the internal parameters of the beams (the reduced plasma frequency, transverse dimensions, etc.). In addition, the amplification principle of CWESA is independent of the operation frequency.

Ultimately, all these factors lead to an improved linearity of the phase-frequency characteristics of CWESA.

· Protection and stability to high power pulsed and continuous overloads



When a powerful input signal (for example, that of an active or passive disturbance) impinges on the CWESA input, the amplitude of the fast cyclotron wave and, hence, the electron rotation radius increases so rapidly that the electron beam is absent in the second resonator (see Fig. 5).

In this case the electron beam will be totally intercepted by the input resonator pads and will change (increase) the value of VSWR (from 1.05-1.2 up to 20-30 and higher, see Fig. 9 also). Thus, the power input signal will be reflected from the CWESA input and will not damage the amplifier. In this case the electron beam does not enter the input resonator, which provides a reliable protection of the subsequent receiver stages against high power overloads in the input signal levels.

The electron beam interception by the resonator pads is not dangerous also owing low values of the beam current and the accelerating voltage (usually about 150-300m A and not higher than 50V). On removing the overload, the serviceability of CWESA is restored in a short period of time (usually 2-20 ns).



This unique property of CWESA allows the applications of this device without any additional protection when it is directly connected to the receiving-transmitting antenna circuits of radar and radio-navigation systems.

2.3 Structure and Typical Parameters of CWESA

· Structure

One of CWESA versions is shown in Fig. 6. CWESA has a compact magnetic system based on Sa-Co permanent magnets. The input signal enters the CWESA through the waveguide flange, which is directly connected with the receiving-transmitting antenna circuit. The input waveguide section is calculated for the transmitter power level.

· Typical parameters of CWESA

Typical parameters of CWESA are summarized in the following table:

Frequency range, GHz. 0.7-11

Operation frequency range, % 5-10

Noise factor, dB 0.8-4 *

Gain, dB 20-25

Dynamic range (input signal), ? W 10 **

Amplitude-phase noise

at the frequency 1 kHz, dB/Hz < -120

Third order intercept point, dBm 18-20

Allowed input microwave power level

pulsed, kW up to 200-500

average, kW up to 2-5

Protection of subsequent stages, dB 60-120

Leakage power peak, mW < 0.1

Restoration time after microwave

overload, ns 2-20 ***

DC power consumption, W 1-2,5

Weight, kg 2-5

__________________________

*) Depends on operational frequency.

**) Additionally: +20 dB of dynamic range extension by CWESA voltage control.

***) Depends on input impact pulse intensity and its form.

Fig. 7-10 illustrate the main characteristics of CWESA in graphics form.

Equivalent nose temperature as a function of operational frequency is shown in Fig. 7.

Fig. 8 illustrates the third-order intercept point, i.e. degree of linearity of amplitude-frequency characteristics.

Voltage Standing Wave Ratio of the input of the CWESA is very low at the small signals (amplification regime) and became very high when the beam is intercepted by input resonator bars at high level of input signal (protection regime) - Fig. 9.



The time dependence of the protection regime is shown in Fig. 10. Only a few percents of impact pulse power are accepted by input cavity. No electron beam in the output cavity and saturation (isolation) may reach -120 dB or even more.



Fig. 10 Diagram illustrating the process of speed recovery after pulse overload

The level of the leakage power peak is restricted by the gaps of input and output resonator bars and does not exceed 1 mW in principle. The restoration time (up to full sensitivity) is 10-20 ns or less.



3. CONCLUSION

CWESA is one of the results of long-term academical and industrial research of Russia in the field of non-traditional microwave electronics based on electron beam transverse waves using [33, 36-39].

CWESA are designed for use as input amplifiers of super high quality radars, navigation communication systems. The CWESA possesses the properties and functions of a low noise and broad band input amplifier with switchable gain and a high degree of linearity and those of a self-protecting device with very small leakage spikes and rapid recovery after powerful microwave overloads.



4. APPENDIX

Cyclotron Wave Protectors (CWP)

If a powerful microwave signal (disturbance interference) impinges on the CWP input, the electron beam is deposited on the input resonator pads. As a result of mismatch between the resonator and the input stage this signal is reflected from the resonator and The unique self-protecting property of CWESA and its capability of protecting of subsequent receiver stages against powerful microwave overloads can be used to design on the basis of CWESA some simple special protective devices - Cyclotron Wave Protectors (CWP), whose principle of operation (protection) is illustrated in Fig. 11. When a powerful microwave signal (disturbance interference) impinges on the CWP input, the electron beam is deposited on the input resonator pads. As a result of mismatch between the resonator and input stage this signal is reflected from the resonator and reradiated.

Since the diameters of the openings and of the drift channel between the CWP resonators can be small enough, and the connection by the electron beam can be disrupted, a powerful microwave signal does not enter the output resonator and, hence, does not appear at the output of the protecting device.

Cyclotron waves protectors are likely to become promising devices for application at the short-wave range of the cm-wave band and in the mm-wave band. Fig. 12 illustrates the CW Protectors developed recently by Istok Corp. at the frequencies of 9 and 35 GHz.



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