No abstract

The Herlofson paradox is exemplified by a capacitor containing cold, collisionless, inhomogeneous plasma as the dielectric: its response to a sinusoidal driving signal can exhibit continuous energy absorption, even though the system is lossless. The underlying mechanism has been explained generally by Barston in terms of the transient response of the system. In this paper, we confirm Barston 's conclusions by examining in detail several analytically tractable cases of delta-function and sinusoidal excitation, and consider the effects of collisions and non-zero electron temperature in determining the steady state fields and dissipation. Energy absorption without dissipation in plasmas is analogous to that occurring after application of a signal to a network of lossless resonant circuits. This analogy is pursued, and extended to cover Landau damping in a warm homogeneous plasma, in which the resonating elements are the electron streams making up the velocity distribution. Some of the practical consequences of resonant absorption are discussed, together with a number of paradoxical plasma phenomena which can also be elucidated by considering a superposition of normal modes rather than a single Fourier component.

This is the first of a series of three papers which examine the excitation of low‐frequency plasma waves arising from a current source moving through a cold, magnetized plasma. In particular, the effects of motion of the source upon the radiation pattern are studied. In the present paper the general theory of wave excitation by a moving source is developed and specific approximations are made which are valid for frequencies much below the ion‐cyclotron frequency. The amplitude and pattern of shear‐Alvén waves excited by a current source are studied for sources whose size in the direction of motion is large in comparison to the size in other directions and also for sources whose size is large in the direction of current flow (also in comparison to the size of the source in other directions). It is shown that the radiation pattern of the shear waves arises naturally as a consequence of the motion of the source and that any modulation of the source current has little effect upon the waves, other than to reduce their amplitude.

A theoretical study has been made of the electromagnetic radiation arising from pulsed electron beams. The study assumes an electron beam which has a well‐organized spatial structure determined by a fixed trajectory in a magnetic field and on/off pulsing governed by the electron source. From this model the electromagnetic radiation is determined by adding coherently the radiation from each individual electron in the helical stream. The radiation per unit frequency interval is determined, as well as the radiation per unit solid angle, as a function of both propagation and ray angles, electron beam pulse width and separation, total number of pulses, and beam current. As expected for a coherent process, it is found that the radiated power varies at the square of the beam current. The relatively high efficiency of the beam in producing electromagnetic radiation is illustrated by consideration, among others, of a 1‐keV, 100‐mA beam used in recent experiments on the space shuttle. For these parameters the total radiated power per steradian is calculated at selected angles to be greater than 1% of the total beam power carried as electron kinetic energy. These results provide a useful theoretical basis for planning future electron beam experiments in space plasmas.