Particle Methods for Plasma Diagnostics

Abstract

A basic advantage of the method considered here, as compared with electric probes, is the fact that an ion beam of low intensity creates a very small perturbation of the plasma. A beam of charged particles that passes through a plasma can be used to investigate both constant and time-varying electric fields. The frequency and amplitude of high-frequency oscillations can be determined directly from an analysis of the energy distribution or the spatial localization of a beam that is transmitted through the plasma at different instants of time. The method by which the experiment is carried out depends very much on the nature of the problem, on the configuration of the magnetic field, and on other experimental conditions, so that it is extremely difficult to formulate general principles for the use of a beam of charged particles. We shall limit ourselves to the analysis of several particular cases. We first consider experiments on the determination of the azimuthal and radial components of the electric field that arises in an axially symmetric magnetic-mirror device when it is filled with plasma. The beam of electrons moves along the lines of force of the magnetic field in the absence of plasma; in the presence of an electric field these electrons experience a drift in the crossed fields. By locating an electron gun in the region of one of the mirrors and a fluorescent screen at the other it is possible to obtain visual or photographic indications of the displacement of the position of the beam due to the components of the electric field which are perpendicular to the lines of force of the magnetic field. The system essentially comprises an oscilloscope with the role of the deflection plates being played by the stationary and high-frequency electric fields that arise in the plasma. In the absence of the plasma the beam of electrons moves along the lines of force of the magnetic field and produces a luminous dot on the screen. The screen is protected by a thin aluminum foil from the incident light and is also furnished with a system of grids, which prevent slow charged particles from striking the screen.