Computer simulation of the backward-wave distributed-emission crossed-field amplifier

A computer simulation of the backward-wave distributed emission crossed-field amplifier has been achieved with a developmental two-dimensional time-dependent calculation. The model simultaneously treats the entire re-entrant electron stream, the vane RF circuit, the secondary-emitting cathode and the modulator. There is reasonable quantitative agreement with the measured magnetic field, anode voltage, current and RF power. An anomalous low-power mode, unexpected before these simulations, shows an irregular beam that is a possible cause of the low efficiency and the high cathode backbombardment that were measured.     

Efficiency of tapered backward-wave oscillators

The effect of a tapered circuit phase velocity on the efficiency of an O-type backward-wave-oscillator (BWO) is investigated. Circuit tapers based on a hard-kernel-bunch approximation are derived. It is shown that when such tapers are applied to realistic BWO's higher efficiencies result. An experimentalS-band BWO with a variable pitch helix designed on the basis of the hard-kernel-bunch theory was constructed. An efficiency improvement of 20 per cent to 80 per cent over a frequency range of 2 to 3.75 Gc has been obtained.     

Start-oscillation conditions in nonuniform backward-wave oscillators

The effects of nonuniformities in the circuit phase velocity and the beam potential on the start-oscillation conditions of an O-type backward-wave oscillator (BWO) are investigated. The nonuniformities include linear, quadratic, and exponential circuit phase velocity tapers and beam potential gradients. It is shown that for weak circuit phase velocity tapers or voltage gradients the start-oscillation current may be lower than that of a uniform tube; however, when the taper or gradient is made stronger than a particular value which depends on the operating parameters, a no-oscillation region results. The latter result indicates that such nonuniformities may be utilized to suppress backward-wave oscillations in forward-wave amplifiers.     

Start-oscillation conditions of tapered backward-wave oscillators

Coupled-mode theory is employed to describe the interaction between nonuniform electron beams and circuits. It is then applied to determine the start-oscillation conditions of a tapered backward-wave oscillator. It is shown that under high space-charge operating conditions where the coupling to the fast space-charge wave is negligible and for weak circuit tapers, an approximate closed form solution of the coupled-mode equations can be obtained through a Wentzel-Kramers-Brillouin-Jeffreys approximation. The start-oscillation conditions are then determined approximately from such a solution.     

Traveling-wave amplifiers and backward-wave oscillators for VHF

An important aspect of the design of traveling-wave amplifiers and backward-wave oscillators for frequencies below 500 mc is the problem of obtaining a tube of reasonably small physical dimensions. Hollow beams of greater perveance than is obtainable with solid beams offer one method of reducing the size of such tubes by permitting operation at a lower voltage and greater gain per wavelength, for a specified beam power, than is possible in a solid beam tube. Some aspects of the design of minimum size hollow-beam forward-wave amplifiers using single helix circuits and backward-wave oscillators using bifilar helix circuits are presented. Several tubes of these types for operation below 500 mc have been built. Amplifier bandwidths and oscillator tuning ranges in excess of four to one in frequency have been obtained experimentally. Amplifier efficiencies in excess of 20 per cent and oscillator efficiencies in excess of 10 per cent have been achieved.     

Influence of magnetic focusing fields and transverse electron motion on starting conditions for spurious oscillations in 0-type backward-wave oscillators

The ratio of the starting current for the first spurious mode of oscillation, in 0-type backward-wave oscillators, to the starting current for the main oscillation has been computed, taking into account the magnetic fields used for beam focusing as well as the transverse components of RF circuit fields. The results show an appreciable influence of both parameters on this ratio and partially explain the suppression of spurious outputs in backward-wave oscillators which can be achieved by deliberate misalignment of the tube in the magnetic field. For higher losses uniformly distributed over the RF circuit length, the ratio I's/I'odecreases; i.e., the tendency toward spurious oscillations increases. This effect may be understood from the coupled-mode theory, since for higher losses the interaction between the circuit wave and the slow space-charge wave decreases much more rapidly for the main mode than for the spurious one.     

Spurious oscillations in IMPATT oscillators

Incorrect circuit design of cavity-controlled IMPATT oscillators may cause low-frequency instabilities and frequency jumping, which are simply avoidable. A simple technique for identifying the avalanche resonance frequency of packaged IMPATT oscillators is also described.     

Design considerations for improved efficiency characteristics of M-type backward-wave oscillators

It has been observed that under certain conditions the electronic efficiency of M-type backward-wave oscillations (M-BWO's) can deteriorate appreciably. The deterioration in the efficiency of M-BWO's has been investigated from the viewpoint of cyclotron-wave interactions. It has been found that although the main cause of the deterioration is the interaction of the slow cyclotron wave with one of the forward circuit harmonic waves, other cyclotron-wave interactions can also result in reduction of efficiency. Design considerations of M-BWO's have been discussed that will avoid the simultaneous occurrence of any of the cyclotron-wave interactions within the useful frequency range of the oscillator. In this manner the useful bandwidth can also be maximized.     

The buildup of oscillations in an electron beam backward-wave oscillator

The natural modes of oscillation of an electron beam backward-wave oscillator are determined when the dc beam current exceeds the start oscillation value. The initial time-rate of buildup of these oscillations and the space variations of the beam and circuit excitations before nonlinearities set in are given.     

Buildup behaviour of IMPATT oscillators in linear and nonlinear ranges

The growth rate, frequency shift and starting amplitude of oscillations in IMPATT-diode oscillators are calculated in the linear and nonlinear range. It turns out that, with increasing r.f. amplitude, the frequency in such oscillators always decreases, and that this frequency shift cannot be compensated for by proper tuning of the load. The r.f. amplitude remains below the measurable level for a certain time depending on the oscillator tuning. This accounts for the observed delay of the onset of the r.f. pulse.     

A two wave theory of traveling-wave tubes and backward-wave oscillations

Starting from Pierce's small-signal TWT equations, a two-wave theory for complicated TWT's and BWO's is developed. Explicit gain expressions for TWT's with severs and velocity tapers are given. A detailed treatment is given of a BWA structure with a single-step velocity taper in the center. The BWO start oscillation current is calculated as a function of the taper strength. The effciency of a step velocity taper as a remedy for backward-wave oscillations in the higher passbands of a TWT is then analyzed. It is shown that a suitable taper can increase the start oscillation current by more than a factor of 10 without adverse effects on the TWT performance if the backward waye coupling impedance is low. A tube with a weak linear taper is analyzed by a perturbation technique and it is found that there is no significant difference between the start oscillation currents for tubes with weak linear or abrupt tapers of comparable strengths.     

Starting conditions for backward-wave oscillators with large loss and large space charge

Starting conditions of backward-wave oscillators are obtained both analytically and numerically for large loss and large space charge. Analytical results are obtained for zero loss and space charge up to infinity and for zero space charge and loss up to infinity. Computer results are carried out for circuit loss up to 225 dB and space charge up to QC = 25. An analytical equation is developed for computing starting conditions for large loss approaching infinite values and for large space charge approaching infinite values. Since the starting C is proportional to the loss per wavelength, large values of loss in a backward-wave oscillator can prevent oscillation from occurring unless the current can be made sufficiently large; i.e., the starting C is independent of length.     

Study of optical output couplers for submillimeter wavelength backward-wave oscillators (BWO's)

The machining of slow wave structures for high frequency BWO's is extremely difficult beyond l THz. Recently a microfabrication technique using photolithography and ion-beam assisted etching has been used to construct a prototype BWO operating at 200 to 265 GHz. The output coupler for such tubes remains a problem. Waveguides do not exist or are very lossy at the frequencies of interest (300 to 2000 GHz). This paper discusses several scaled experiments of optical output couplers for submillimeter BWO's. Various designs of planar antennas (Vivaldi horns) and lens-feed systems (hyperhemispherical lens) were constructed and tested between 20 and 100 GHz using a spectrum analyzer. The lens system was also tested at 337 GHz using a CO₂ pumped FIR laser.     

Bias-dependent oscillations in 10 ?m-long transferred-electron oscillators

Computer simulations show that a transferred-electron oscillator sustaining X band circuit-controlled relaxation oscillations will, in the same circuit, but at higher bias levels, shift to a space-charge dominated oscillation, where the terminal voltage always exceeds threshold.     

Formation and suppression of stationary and chaotic oscillations in a non-autonomous gyrotron backward-wave oscillator

Formation and suppression of chaotic oscillation modes in a non-autonomous gyrotron backward-wave oscillator is studied. It is shown that, in the considered system, developed chaotic oscillations and stationary single-frequency oscillations can be formed in a gyrotron tube upon variation in the frequency and power of an external signal. Domains in the plane of basic control parameters are found in which the considered oscillation modes of a non-autonomous gyrotron backward-wave oscillator are observed.     

Relaxation oscillations due to impact ionisation in epitaxial sheet-type Gunn oscillators

Relaxation oscillations have been observed in field-ionised epitaxial nGaAs samples on a semi-insulating substrate. The decay times of excess electrons and light output are reduced by a transverse magnetic field, indicating the presence of free holes driven towards the surface or the interface by the Suhl effect.     

Investigation of parasitic oscillations in IMPATT-diode oscillators by a simple locus chart

Using a simplified theory of parasitic oscillations in IMPATT-diode oscillators, a locus chart is constructed that facilitates the study of parasitic oscillations of the degenerate type. Results of experiments on a high-Q factor coaxial/waveguide circuit (Kurokawa circuit) are explained by this locus chart.     

Noise transport in the crossed-field diode

The present work treats the transport of noise, originating at a thermionic cathode, across a planar crossed-field diode. A multivelocity Monte Carlo analysis is used to simulate thermionic emission by using random numbers to generate electron emission times and velocities. Trajectories for many charges are followed through the diode space by means of a high-speed digital computer. Measurements performed on a planar triode model for the crossed-field diode show that the anode noise current increases rapidly near the critical magnetic field, at which the stream just grazes the anode. The theory agrees well with these experimental findings. The Monte Carlo study predicts the increase of both the normal anode velocity fluctuations and the spread of the velocity distribution with magnetic field. This is caused mainly by the "geometrical" spreading of the initial velocity distribution. The crossed-field potential minimum exhibits no instabilities in the planar model. The noise current smoothing at low frequencies in ordinary streams is predicted for moderate fields but disappears near the critical field.     

Reduction of noise in high-power crossed-field amplifiers

The letter describes a method of reducing the broadband noise of a high-power injected-beam crossed-field amplifier by means of a nonuniform magnetic field in the electron-gun region. The noise reduction achieved is some tens of decibels, and also results in improvements in other characteristics of the tube.     

Transformation equations of crossed-field drift regions

A general case of a noncentered beam in a uniform crossed-field drift region is treated to obtain transformation equations relating the ac beam quantities at the output plane to those at the input plane. The ac beam quantities considered are the surface charge density, beam displacement, and the longitudinal and tangential velocities.