Output power limits for pulsed relativistic microwave oscillators of different types

High-power microwave generation with high-current electron accelerators is studied theoretically. Output power limits are evaluated for most commonly used microwave devices, such as traveling-wave tubes, klystrons, magnetrons, and vircators.     

Thermal regimes of relativistic magnetron oscillators

Experiments with relativistic magnetron oscillators (RMOs) using high-current electron accelerators allowed a level of output power from several hundred megawatts to several gigawatts to be reached at an efficiency of 10–30%. The operation principle of RMOs is identical to that of classical devices, but the determination of working regimes and calculation of output characteristics require taking into account relativistic corrections. In addition, a high cathode-anode voltage and high current in RMOs lead to some specific effects related to the damage of anode units as a result of thermal shocks. RMO operation in a periodic pulsed (pulse train) regime, which is of the most practical interest, imposes stringent requirements on the working life of anode units. The principles of choice of the material and design for the anode units of RMOs are formulated. Calculations of the limiting thermal regimes of RMO operation depending on the electron beam power flux density and the off/duty ratio of heating pulses are presented. Methods of increasing the working life of anode units are proposed.     

Tunable microwave resonators and oscillators using magnetostatic waves

The status of magnetostatic wave (MSW) straight-edge resonators (SERs) and their applications in tunable oscillator circuits are reviewed. The resonators are based on magnetostatic waves propagating in high-Q cavities fabricated in thin ferrimagnetic yttrium iron garnet (YIG) films. The resonance frequency of these resonators can be tuned using a bias magnetic field. The theory of operation and design criteria for the straight-edge resonators are described with emphasis on the effect of the resonator parameters on the tuning range, power handling, and phase noise performance. The use of the SER as the frequency-selective element in oscillator circuits is reported. Examples of tunable oscillators are included.     

Mechanism for microwave pulse shortening in a relativistic BWT

It is shown that reduction in the pulse length in a relativistic BWT, is accompanied by the appearance of explosive-emission plasma on the surface of the ripples of the slow-wave system. Generation stops because electrons emitted by the plasma absorb the electromagnetic wave. This absorption is sharply enhanced by the presence of ions emitted from the plasma. Pis’ma Zh. Tekh. Fiz. 25, 27–36 (March 26, 1999)     

Periodic response and stability of hysteretic oscillators

Difficulties encountered in the study of the response of hysteretic systems under periodic force lie in the multivalued nature of the constitutive relationship. In this paper, some of these difficulties are circumvented by assuming an incremental formulation which results in an ordinary nonlinear problem with single-valued functions, though with an enlargement of the phase space. Consideration is given only to periodic oscillations that are found through the harmonic-balance method with many components; there thus ensues a system of algebraic equations that is solved numerically. Stability is studied by the linearized Poincar´ map determined via numerical integration. A simple hysteretic oscillator, that presents degrading and non-degrading behaviour, is considered. The results clearly show that the influence of higher harmonics is far from negligible. While non-degrading oscillators reveal stable behaviour over all the frequency range, in the degrading case there is instability that allows either saddle-node or Hopf bifurcation     

Self-modulated generation observed in a delayed feedback relativistic microwave gyrotron

Self-modulated generation regimes were studied in a delayed feedback relativistic gyrotron operating on the H ₀₁ mode with a central frequency of 9.2 GHz. At a fixed electron beam energy of 230 keV, an increase in the electron beam current from 10 to 45 A led to the transition from a stationary to periodic self-modulated generation regime. The modulation period was about 16 ns, while the relative amplitude of the modulation increased in proportion to the beam current, reaching up to 90%. The microwave pulse duration exceeded 6 μs at an average power of up to 1 MW. The experimental data obtained agree well with the results of simulation using the PIC code KARAT.     

Reduced transposed flicker noise in microwave oscillators using gaas-based feedforward amplifiers

Transposed flicker noise reduction and removal is demonstrated in 7.6 GHz microwave oscillators for offsets greater than 10 kHz. This is achieved by using a GaAs-based feedforward power amplifier as the oscillation-sustaining stage and incorporating a limiter and resonator elsewhere in the loop. 20 dB noise suppression is demonstrated at 12.5 kHz offset when the error correcting amplifier is switched on. Three oscillator pairs have been built. A transmission line feedback oscillator with a Qo of 180 and two sapphire-based, dielectric resonator oscillators (DROs) with a Qo of 44,500. The difference between the two DROs is a change in the limiter threshold power level of 10 dB. The phase noise rolls-off at (1/f)(2) for offsets greater than 10 kHz for the transmission line oscillator and is set by the thermal noise to within 0-1 dB of the theoretical minimum. The noise performance of the DROs is within 6-12 dB of the theory. Possible reasons for this discrepancy are presented.     

Spontaneous and stimulated emission in microwave oscillators with a virtual cathode

A one-dimensional model of electron beam emission with a virtual cathode is used to show that spontaneous emission occurs in a reditron while stimulated emission is observed in vircators and negative triodes. However, at a certain stage in the latter, the radiative instability is quenched as a result of the evolution of turbulence in the electron beam. The idea is therefore put forward that this quenching may be eliminated by specially shaping the leading edge of the high-voltage supply pulse to the diode of the microwave oscillator. Pis’ma Zh. Tekh. Fiz. 24, 41–46 (February 26, 1998)     

Synphase operation of nanosecond relativistic 37-GHz backward-wave oscillators without electrodynamic coupling

The possibility of in-phase excitation of two independent nanosecond-pulsed relativistic 37-GHz backward-wave oscillators (BWOs) with high-current electron beams has been studied. This regime can be achieved using BWO switching with a picosecond precision. It is shown that long-term (up to 100?C200 periods of the field) phase locking in each channel is stably reproduced from pulse to pulse, which ensures coherent summation of the output wave beams at a megawatt power.     

Microwave oscillators incorporating high performance distributed Bragg reflector microwave resonators

We previously demonstrated a new resonator device structure that achieves Q-factors well above those currently realisable. The new structure consists of a microwave cavity, where the enclosure walls consist of distributed Bragg reflectors (DBRs) in three dimensions, made of low-loss sapphire. Theoretical analysis has demonstrated that the resonator's performance is critically dependent upon accurate alignment of the DBR components, thereby maintaining the desired symmetry of the resonant structure. Breaking of the symmetry causes mixing of the high performance Bragg reflected mode with low-Q hybrid cavity modes. The fabrication tolerances required to achieve the expected resonator performance are met with a precise but simple alignment tool. A pair of these resonators have been built at 9.0 GHz, and have demonstrated unloaded Qs in excess of 700,000 at room temperature. These resonators are incorporated into simple two-port feedback oscillator circuits. Phase noise measurements were performed on the two free-running oscillators.     

High-efficiency subnanosecond microwave pulse generation in a relativistic backward wave tube

The regime of nonstationary oscillations with a short-time power burst, which is typical of the initial stage of a transient process developed when a beam current significantly exceeds the starting level, is studied in a relativistic backward wave tube (BWT) operating in the millimeter wavelength range. The results of numerical calculations allowed conditions to be established that provide for a nearly 90% efficiency of the power transfer from an electron beam with the parameters 300 keV, 2 kA, 1 ns to the microwave pulse with a duration of 8–10 periods of the microwave field oscillations. The experiments showed the possibility of generating such pulses with a width of 200–250 ps and a power of up to ∼400 MW at a central frequency of about 38 GHz.     

The effect of phase stabilization of microwave oscillations in nanosecond Gunn oscillators

The effect of the semiconductor structure of an oscillator diode on the phase stabilization of microwave oscillations in a nanosecond Gunn oscillator by using a modulating voltage pulse edge is investigated. Numerical simulation is employed to determine phase deviations depending on the scatter of pulseedge duration and pulse amplitude. The standard deviation of phase-delay time of microwave oscillations in the oscillator with regard to a constant level at the modulating pulse edge and the standard deviation of phase difference of microwave oscillations in two electrodynamically insulated oscillators connected in parallel to one and the same modulator have been measured.     

Statistical definition of locking bandwidth in an array of synchronised microwave oscillators

Arrays of N weakly coupled oscillators are considered in different configurations. The locking bandwidth for these arrays is defined statistically. Various factors affecting the locking bandwidth and the effect of the coupling network topology on locking bandwidth are studied by solving the dynamic equations of the array numerically. The analytical locking bandwidth for two configurations of arrays is computed and the results are compared with the numerical solution of dynamic equations.     

Industrial microwave plasma polymerization

The concept as well as the design of a high rate, large area plasma polymerization plant using a plasma generated by microwaves is introduced. Some properties of films deposited from different hydrocarbons are presented. They exhibit properties which are intermediate between polymeric films grown from hydrocarbons and amorphous carbon or i-C films. Detailed IR spectroscopic investigations of deposits from different monomers indicate differences in the layer structure which reflect the bonding structure of the corresponding monomers. X-ray photoelectron spectra reveal the presence of approximately 30% O at least in the monitored surface region of the samples. Since only residual traces of oxygen were present during polymerization we conclude that a marked absorption of oxygen by the sample takes place after preparation.     

Thermal stability of nanocrystalline diamond films grown by biased enhanced microwave plasma chemical vapor deposition

The thermal stability of nanocrystalline diamond (NCD) films grown on mirror-polished silicon substrates by biased enhanced microwave plasma chemical vapor deposition was investigated. Different pieces of a NCD sample were annealed for 1 h in an ambient argon atmosphere at 200, 400, 600, and 800 degrees C. The structural and mechanical properties of as-grown and annealed samples were assessed. The surface roughness and high hardness of the samples remained fairly constant with annealing temperature.     

Oxygen atom density in microwave oxygen plasma

We determined the density of neutral oxygen atoms in microwave plasma using a fiber-optics catalytic probe (FOCP). Plasma was created within a quartz tube with an outer diameter of 5 cm by a 2.45 GHz microwave generator with an output power up to 1000 W. The oxygen flow was varied between 4 and 20 l/h. The O-atom density was found to increase monotonically with the increasing discharge power, and it decreased with the increasing flow rate. The degree of dissociation of oxygen molecules in the plasma column depended largely on the flow rate. At the oxygen flow of 4 l/h, it was about 18% but it decreased to about 6% at the flow of 20 l/h.     

Frequency stability of high-finesse interferometers

The effects of temperature and pressure on the stability of a high-finesse interferometer are considered, and the design of a high-finesse interferometer that minimizes these effects is presented. The high-finesse interferometer has a free spectral range of 23,600 MHz, a finesse of greater than 30,000, and a measured stability of better than 7 MHz/h (0.3 mfringes/h).     

Optical emission patterns and microwave field distributions in a cylindrical microwave plasma source

A cylindrical high density (10¹² cm⁻³) large volume (32 cm in diameter and 50 cm in length) homogeneous argon plasma has been produced by a microwave with a frequency of 2.45 GHz and a power of 900 W without a magnetic field. The plasma source is based on a ring shaped rectangular waveguide with eight equally spaced slots in its inside wall. Several optical emission patterns are observed on different conditions and the microwave field is measured by a movable antenna, which showed a clear relationship between the optical emission patterns and the electron field distributions. A mode transition, from a TE_8j mode to a TE_16j mode, occurs when the gas pressure increases from 660 to 1000 Pa. And there is an optical emission pattern when the microwave power decreases from 900 to 300 W. All these phenomena are described in detail and analyzed according to the interactional theory of electrons in plasma with microwave.