Electrodynamics of helical plasma structures

Electrodynamic properties of a hybrid slowing down structure consisting of co-axial beam and plasma were investigated theoretically. Dispersion relations for axial-symmetric waves in this system were found in hydrodynamic approximation. It was shown that in presence of a plasma column the phase velocity increases compared with that in case of a spiral in vacuum. Analytical expressions for increments of excited fields for beams of small and large density were obtained. At a sufficiently large beam density not only dispersion of the system changes, but the mechanism of vibration excitation changes too. Namely, with the beam density increase, splitting of its dispersion curve leads to disappearance of Cherenkov instability and appearance of anomal Doppler effect instability. Field topography for spiral-plasma system was investigated analytically and experimentally. 3 refs.; 11 figs.; 1 table. (author)