Konstantin G. Kostov

Examining by the Rayleigh-Fourier method the cylindrical waveguide with axially rippled wall

Axially corrugated cylindrical waveguides with wall radius described by R/sub 0/(1+/spl epsi/cos2/spl pi/z/L), where R/sub 0/ is the average radius of the periodically rippled wall with period L and amplitude /spl epsi/, have been largely used as slow-wave structures in high-power microwave generators operating in axisymmetric transverse magnetic (TM) modes. On the basis of a wave formulation whereby the TM eigenmodes are represented by a Fourier-Bessel expansion of space harmonics, this paper investigates the electrodynamic properties of such structures by deriving a dispersion equation through which the relationship between eigenfrequencies and corrugation geometry is explored. Accordingly, it is found that for L/R/sub 0//spl ges/1 a stopband always exists at any value of /spl epsi/; the condition L/R/sub 0/=1 gives the widest first stopband with the band narrowing as the ratio L/R/sub 0/ increases. For L/R/sub 0/=0.5 the stopband sharply reduces and becomes vanishingly small when /spl epsi/<0.10. Illustrative example of such properties is given on considering a corrugated structure with L/R/sub 0/=1,R/sub 0/=2.2 cm, and /spl epsi/=0.1, which yields a stopband of 1.5-GHz width with the central frequency at 8.4 GHz; it is shown that in a ten-period corrugated guide, the attenuation coefficient reaches 165 dB/m, which makes such structures useful as an RF filter or a Bragg reflector. It is also discussed that by varying L/R/sub 0/ and /spl epsi/ we can find a variety of mode patterns that arise from the combination of surface and volume modes; this fact can be used for obtaining a particular electromagnetic field configuration to favor energy extraction from a resonant cavity.

A proposed 4 GHz, 60 kW transit-time oscillator operating at 18 kV beam voltage

A high-power transit-time oscillator operating at 4.0 GHz in the cylindrical-cavity TM/sub 010/ mode is proposed. Without requiring an externally applied magnetic field, the oscillator comprises a diode electron gun and a cylindrical cavity that are combined into a single unit that makes for a compact, lightweight device. The transit-time effect, which underlies the operation principle of the oscillator, is examined through a small-signal analysis from which a relation embodying the cavity length, resonant frequency, and cathode voltage is derived. Proper DC voltages of 18 kV are applied to the diode gun which runs in the space-charge-limited regime and produces a 27 A-current hollow electron beam. Output power is extracted axially from the system by aperture coupling the cavity to an external waveguide where outgoing travelling waves with 60 kW peak power are observed by means of 2.5 D particle-in-cell computer simulations.

Experimental study of virtual cathode oscillator in uniform magnetic field

High-frequency high-power microwave radiations by a virtual cathode oscillator have been obtained. We performed experiments using 350 kV. 7 kA, 30 ns electron beam. A virtual cathode forms when the magnetized relativistic electron beam is injected into circular waveguide. The microwave generation exists in wide range of magnetic field magnitudes (2 kGs-40 kGs). The measured peak microwave power is about 15 MW at frequency region of 11-17 GHz. The radiation frequency has not depend on the magnetic field intensity. A nonsymmetric TM mode is observed. The sources of the microwave emission are both the reflexing electrons and the oscillating virtual cathode.

A 5.7-GHz, 100-kW microwave source based on the monotron concept

A conceptual transit-electron tube based on the monotron effect is described. The device comprises a temperature-limited diode gun operating at 33-kV cathode voltage and 40-A beam current. As indicated by two-and-a-half-dimensional (2.5-D) particle-in-cell simulations, a 5.7-GHz single frequency radiation has been generated at the peak-power output level of 100 kW. The self-induced RF voltage across the diode gap produces a density modulation on the beam electrons which translates into a negative electronic admittance at proper applied dc voltages. Assuming a strictly monoenergetic beam, the transit-time effect-underlying the operation principle of the device-is examined by means of a small-signal analysis from which an analytic expression for the starting current is derived.

Power and efficiency enhancement of reflex-triode virtual-cathode oscillator operating at short pulse

A reflex-triode virtual-cathode oscillator operating at short-pulse duration is numerically investigated. On the time scale of a few nanoseconds, it turns out to be possible to decrease the gap between the cylindrical surface of the cathode and the anode tube wall before vacuum breakdown occurs. Simulations are performed at fixed cathode radius of 3.4 cm for two anode models, namely, thin-foil and anode-mesh models. Both models lead to a significant enhancement (/spl ap/2-5 times) of the total output microwave power upon reducing the anode-cathode radial gap from 2.2 to 1.0 cm for the input voltage range of 300-700 kV. For the same reduction in the radial gap, a considerable enhancement in the electron-beam-to-microwave-power conversion efficiency is obtained-approximately 1.5 times at lower input voltage (300 and 400 kV), using the thin-foil model and about 1.5-2 times at higher voltages (500 and 600 kV), when the anode-mesh model is considered. The results from the present numerical investigation can be applied in the design of high-growth rate high-power microwave (HPM) sources such as short-pulse small-size axial vircators, operating at single-pulse or pulsed-repetition mode. These vircators have a potential to be more efficient and to generate considerably higher output microwave power in comparison with long-pulse vircators.

Numerical study of relativistic electron-beam electrostatic pumping by anode aperture

Quality and pumping of an electron beam by means of the inhomogeneous electrostatic field at an anode aperture are investigated using numerical simulation. A specially developed relativistic particle‐in‐cell code, described here, is employed. The longitudinal velocity spread and the pitch ratio of the beam electrons are derived for different aperture radii and cathode‐aperture distances. The presented results could be practically useful for operation of high‐power cyclotron resonance masers with a sharpened cathode and an anode aperture. The beam pumping in such devices is typically achieved by more complicated systems (pumping magnet, adiabatically changing magnetic field, etc.). The numerical simulations show a significant pumping effect coming from the anode aperture itself.

Cyclotron autoresonance maser in the millimeter region

Results from a cyclotron autoresonance maser experiment are reported. Two maxima of the output microwave power (8 and 10 MW) at a wavelength of 5 and 5.5 mm, respectively, have been observed. The nonadiabatic beam pumping is achieved by transit of the electrons through an external magnetic‐field local inhomogeneity due to the presence of a copper aperture.

Doppler-shifted cyclotron resonance maser driven by an electrostatic pumped beam

A Doppler‐shifted cyclotron resonance maser experiment using a 500 keV, 30 nsec high‐current electron beam emitted by a Blumlein pulse accelerator has produced approximately 20 and 8 MW at frequencies of 13 and 37 GHz in TE01 and TE03 waveguide modes, respectively. The beam has been electrostatically pumped by means of a strongly inhomogeneous electrostatic field created in the diode region. The experimental results are in a good agreement with the numerical calculations of a cold system electrostatic field and with simulation studies of beam propagation. The transverse electron velocity has been estimated on the basis of the performed computations and experimental measurements.

Generation of TE12 mode from cyclotron resonance maser driven by nonadiabatic electrostatically pumped electron beam

Intense emission of the TE12 mode has been detected in an experiment based on the cyclotron resonance maser mechanism with nonadiabatic electrostatic pumping of an electron beam. The pitch‐ratio α of beam electrons has been found experimentally and estimated from a computer simulation. Both results are in good agreement with each other.

High-power second-harmonic gyrotron oscillator

Experimental results from a study of a nonadiabatic pumped second-harmonic gyrotron oscillator are reported. A steady second-harmonic cavity emission has been observed at 28 GHz and for the TE/sub 02/ waveguide mode. A peak power of approximately 25 MW has been reached with approximately 9% efficiency. The magnetic field in the pumping section region is modeled, and beam pumping is simulated.<<ETX>>