Search

The conditions for the generation of runaway electrons in an air gap are compared at different degrees of inhomogeneity of the electric field distribution provided by varying the opening angle of the conical cathode: in the range 40 ° – 120 ° in experiments and 0 ° – 180 ° in calculations. It is demonstrated that, in a weakly inhomogeneous electric field (according to the proposed classification, this corresponds to cones with angles greater than the Taylor angle of 98.6 °), the runaway condition has a local character. The transition of free electrons into the runaway mode is determined by the local distribution of the electric field near their starting point—the tip of the cone. The local electric field strength must exceed a threshold value comparable to the strength critical for the runaway of electrons in a uniform field. In a strongly inhomogeneous field (cones with angles less than 98.6 °), this condition is not sufficient for electrons to run away throughout the gap. Electrons accelerating in the near-cathode region may begin to slow down in a weak field at a distance from the cathode. In this case, the runaway condition becomes nonlocal. It is determined by the dynamics of electrons in the entire gap, primarily in the near-anode region, and reduces to the requirement that the potential difference applied to the gap exceeds a certain threshold value. [ABSTRACT FROM AUTHOR]

This paper presents the results of research, development, and testing of magnetically insulated air diodes with replaceable graphite and stainless-steel tubular and coaxial cathodes of various configurations capable of generating directed bunches of runaway electrons. At the anode, the bunches have cross sections shaped as circles or rings with an outer diameter of 1–2 cm. The durations of the bunches, which carry currents of a few to tens of amperes, range from tens of picoseconds to 100 ps, and their charges range from tenths of a nanocoulomb to a few nanocoulombs. The kinetic energy of the bunch electrons at the peak of the current pulse is typically of the order of 150 keV. The bunch parameters are set (and varied) by varying the amplitude and duration of the subnanosecond high-voltage pulse driving the diode; they depend on the cathode material and on the strength and profile of the applied external magnetic field. The bunches, retaining their cross-sectional structure, are brought out from the diode, along the field lines, through a thin foil or mesh anode into the open space with a quasi-uniform magnetic field between two Helmholtz coils. In this space, the samples to be irradiated with electrons, similarly to objects exposed to radiation in various experiments and technological applications, can be placed. [ABSTRACT FROM AUTHOR]

We present an analysis of the characteristics of the noise from a magnetized high-current electron beam at a nanosecond-long interval since the explosive electron emission onset on the graphite cathode by applying accelerating pulses with an amplitude of about −300 kV and a varying subnanosecond rise time. The registration bandwidth of electromagnetic noise and the beam current modulation was as high as 59 GHz. A leading wideband electromagnetic signal linked with a short beam current front was recorded. Subsequent noise, including that with discriminated spectral maxima, can be qualified as spontaneous cyclotron radiation of moderately relativistic electrons rotating in a longitudinal magnetic field. [ABSTRACT FROM AUTHOR]

This paper presents new experimental data that enable the observed processes in nonlinear ferrite lines to be related with the theoretical positions of the spin waves of the magnetization precession under the conditions of a high-power microwave. Such an approach has not been considered in earlier discussions on the subject and can contribute to the theory of the generation of oscillations in a gyromagnetic ferrite medium. These new aspects were used to design a new type of generator. The specific feature is the presence of regularity in the ferrite lines in the generation mode of microwave oscillations. The repeating regularity enabled the implementation of multistage pulse shape converters capable of operating in two modes and providing extreme parameters of the output pulses. Two variants of multistage converters of nanosecond high-voltage pulse shapes with a duration of ∼4 ns at a half-height and with an amplitude of −500 kV were designed and tested. The assembly of the converters and the driving generator are described in the stationary setup. In the first case, the rise time shortened to ∼45 ps, and the amplitude increased to −850 kV due to the sharpening of a pulse and the formation of a shock wave by the cascade of three nonlinear transmission lines. A record rate for the increase of the leading peak voltage of ∼15.5 MV/ns was reached. In the second case, the new approach for the generation of a sequence of subnanosecond pulses was presented and tested, and each pulse of the previous stage was modulated by the next stage doubling the number of pulses while conserving a deeper modulation. As a result, at the top of the incident pulse, a sequence of subnanosecond peaks with a large modulation depth (∼70%) was formed when the maximum voltage amplitude reached −700 kV. The results of the emission of such pulses are also presented. [ABSTRACT FROM AUTHOR]

We investigate in detail the method of coherent summation of multiple sub-gigawatt Ka-band backward-wave oscillators (BWOs) based on their phase-imposed excitation by an incoming short seed electromagnetic pulse. For theoretical analysis, we use particle-in-cell simulations as well as a basic model that describes both spontaneous and stimulated Cherenkov emission of an electron beam moving in corrugated waveguides. In the scope of the model, the influence of the electron beam front edge length, the phase and the power of the seed pulse, and other conditions important for the performed experiments is studied. The predictions of the model are compared to the experiments in which both superradiant and longer-pulse BWOs are explored. Despite sub-nanosecond duration of the seed pulse, phase stability throughout the entire generated pulse with up to nanosecond duration is demonstrated. [ABSTRACT FROM AUTHOR]