Oblique-incidence millimetre-wave plasma diagnostics

A beam of millimetre-wave radiation entering a plasma obliquely may re-emerge by refraction just as radio waves are returned to Earth by the ionosphere. The separation D between the points of entry and re-emergence depends on the angle of incidence ¿i, and on the electron-density distribution in the plasma. If D is studied as a function of ¿i, the electron-density distribution can be deduced by a method which is explained in the paper. The method is based on ray theory, and the validity of ray theory in this special application is studied in some detail. It is assumed throughout that the plasma is (a) cold, (b) isotropic and (c) collision-free The method has been tested experimentally, and an example of an electron-density profile obtained by it is given.