Plasma oscillations

This paper is a report on the investigations by the author and collaborators F. Berz, E. A. Ash, and D. Dracott at Imperial College. F. Berz has theoretically investigated wave propagation in a uniform plasma and found that even in the absence of collisions only damped waves can arise, because the fluctuating velocity distribution contains a term, overlooked by previous authors, which represents a flowing-apart of the electron density. The cutoff due to this effect alone is at about 1.15 of the Langmuir frequency, and the shortest wavelength at about 20 Debye lengths. Experimental investigations by E. A. Ash and D. Dracott extending over 5 years have at last elucidated the paradox of the existence of Maxwellian electron distributions in the positive column of arcs at low pressures. The interaction is not between electrons and electrons but between these and an oscillating boundary sheath. The sheath was explored by an electron beam probe and oscillations of about 100 mc observed under conditions when the plasma frequency in the arc was about 500 mc. Electrons diving into the boundary sheath spend about one cycle in it, during which time they can gain or lose energies of the order of several volts. Possible applications to radio astronomy are briefly suggested.