Whistler wave emission from a modulated electron beam injected in a cylindrical duct with enhanced plasma density

Abstract

A study is made of the electromagnetic radiation from a thin modulated electron beam of finite length injected parallel to an external magnetic field in a magnetic-field-aligned cylindrical density duct immersed in a uniform ambient magnetoplasma. A rigorous solution for the beam-excited field is obtained and the power lost by the beam in the presence of the duct is examined. Although a general theory is developed, attention is focused on ducts with enhanced density and modulation frequencies belonging to the whistler band. Expressions for the time-averaged power emitted from the beam are derived as a function of the beam and duct parameters. A significant difference between the features of whistler wave emission in the presence of a duct and in the case of a homogeneous unbounded plasma is revealed. Estimates of the power emitted in the whistler band are given for representative examples of artificial density ducts in the laboratory and space plasmas. It is shown that the power lost by the beam in the presence of an enhanced-density duct can increase noticeably due to Čerenkov excitation of guided whistler modes.