The theory of the instantaneous triple-probe method for direct-display of plasma parameters in low-density collisionless plasmas

An exact theory taking the ion temperature T1 into account is developed for a triple-probe in an orbital-motion-limited collisionless plasma, i.e. when charged particle mean free path>>Debye length>>probe radius. Formulae for determining electron temperature and electron density are given for both spherical and cylindrical probes. Analytical results show that the effect of T1 on measurements of plasma parameters is small when using a cylindrical probe and is negligible when compared to the errors obtained when using a spherical probe. The magnitude of the errors obtained in practical measurements are discussed. The possibility of ion temperature determination using this theory is also suggested. Experiments have been also done for confirmation.