N.A. Nikolov

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.

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.

Numerical simulation of high-power virtual-cathode reflex triode driven by repetitive short pulse electron gun

A virtual-cathode reflex triode is investigated by numerical simulations. A trapezoidal in shape voltage pulse with an amplitude of 300 kV is applied to the solid cathode of the device to drive the cathode negative. The electron beam-to-microwave power conversion efficiency /spl epsiv/, calculated for the pulse flat top with a duration /spl tau//sub ft/=1.2 ns is approximately the same (about 1.5-2%) as well as for a long flat top (/spl tau//sub ft/=4 ns). The simulations show a 10-15% increase of /spl epsiv/ at /spl tau//sub ft/ shortening to 0.6 ns. However, this occurs when the anode mesh transparency is high (80-90%). Considerable enhancement of the efficiency (about four times) for /spl tau//sub ft/=0.6 ns has been calculated if the cathode side surface is brought near to the anode tube (from /spl ap/0.5% at cathode radius R/sub c/=1.6 cm to /spl ap/2% at R/sub c/=3.8 cm). The obtained results would find an application for the design of virtual-cathode reflex triode devices driven by a short pulse and high repetition rate electron gun.

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>>

Powerful second-harmonic gyrotron oscillator

Experimental results from a study of second-harmonic gyrotron oscillator are reported. A steady second-harmonic cavity emission has been observed at 28 GHz and waveguide mode TE/sub 02/. A peak power of approximately 30 MW has been reached with approximately 9% efficiency. For better understanding of the generation mechanism additional gyrotron experiments have been carried out with a similar cavity without a cutoff section at the exit, operating at the fundamental cyclotron harmonic. >