This invention relates generally to microwave, electromagnetic radiation and more particularly to a method and apparatus for producing controlled, coherent microwave radiation from a warm, uniform plasma at approximately twice the electron plasma frequency.
As was pointed out in our U.S. Pat. No. 3,944,946 issued on 16 Mar. 1976, and an article in THE PHYSICS OF FLUIDS Vol. 19, No. 3, of March 1976 pages 464-478 by Balram Prasad, an electron beam propagating through a plasma generates quasi-stable longitudinal electrostatic (plasma) waves peaked at electron plasma frequency via the two-stream instability mechanism. These plasma waves are scattered by both mass (ion density) and charge (electron density) inhomogeneities or fluctuations (at ion and electron plasma frequencies, respectively) which are always present in a warm plasma.
Scattering by mass density fluctuations of the incident or "pump" electrostatic waves builds up a coherent background field of scattered plasma waves via the Rayleigh scattering process. Consequently, the charge density fluctuations are coherently enhanced.
Scattering of pump waves by the coherently enhanced charge density fluctuations (Raman scattering) results in the conversion of both the electrostatic pump waves and the charge density fluctuations into coherent transverse electromagnetic waves. The frequency of this transverse radiation is the sum of the frequencies of the pump waves and the Rayleigh scattered background waves, i.e., approximately twice the electron plasma frequency.
In general, the pump waves propagate in the direction of the electron beam which produces them, much as the wake behind a speeding boat in water. The Rayleigh scattered electrostatic waves have a spatial distribution which is an asymmetric dipole elongated in the direction of the beam. That is, the spatial distribution has two asymmetric lobes, a small backwardly directed lobe and a large elongated forwardly lobe. The maxima of these lobes lie along the direction in which the pump waves are propagating. Since only near "head-on" collisions between pump waves and Rayleigh scattered waves result in Raman emission, it is the interaction of pump waves with the small backwardly directed lobe of the coherent background of Rayleigh scattered waves that is responsible for the Raman emission of transverse electromagnetic waves in a single electron beam-plasma system. The much stronger forwardly directed lobe makes little or no contribution to the Raman emission process.
It is herein proposed to launch two oppositely directed electron beams into a warm plasma such that the transverse electromagnetic power output will be 10 to 100 times the amount obtainable using a single electron beam-plasma system.
With two oppositely directed electron beams, there will be two counter-streaming sets of pump waves and two sets of coherent background charge density fluctuations. Thus, if one set of pump waves is right-directed and the other is left-directed, not only do the right-directed pump waves interact with the right-directed coherent background wave field but also with the left-directed coherent background wave field. The same occurs for the left-directed pump waves which also interact with both sets of coherent background wave fields. It follows, therefore, that there are four distinct interaction processes leading to the Raman emission of transverse electromagnetic waves in a two opposing beam-plasma, whereas there is only one for a single beam-plasma system. Although the input power is doubled (i.e., two beams instead of one), the output power (transverse electromagnetic waves) is at least 10 times greater. Hence, the efficiency of the two opposing beam-plasma system is at least 5 times that of a single beam-plasma system. Moreover, since the forwardly directed lobe of the Rayleigh scattered wave field becomes increasingly stronger than the backward scattered lobe as the phase velocity of the electrostatic waves decrease, their contribution to the Raman process becomes far greater than that of the backwardly directed lobe.