Die Beeinflussung des Ionisationsgrades durch Diffusions­ und Massenströme im Plasma

Abstract

Abstract A stationary, magnetically stabilized helium arc plasma is taken as an example to show that ionization degrees in such plasmas with large electron temperature and electron density gradients may not be calculated from static ionization relations valid for homogeneous plasmas (e. g. the Saha equation). Instead, ionization degrees in inhomogeneous plasmas must be determined from continuity equations of the form div (nΖυΖ)nΖ = JΖ-1 nΖ-1 -RΖnΖ nz where JΖ-1 and nΖ-1 are the ionization rate and the density of the Z -1 times charged ions, and RΖ , nΖ and υΖ are the recombination rate, the density and the centre-of-mass velocity of the Z times charged ions. Static ionization formulae may not be applied to inhomogeneous plasmas because the finite relaxation times for the attainment of static ionization equilibria result in these ionization equilibria being displaced by the motion of the ions parallel to the direction of electron temperature and density gradients. This is proved by means of two independent operations: firstly, the velocities of the helium ions, these being governed primarily by ambipolar diffusion, are calculated with the spectroscopically measured state variables from the momentum equations of the plasma; secondly, the velocities of the doubly charged ions are determined by integrating the continuity equation for these ions, use again being made of the measured state variables and of the ionization and recombination rates J1 and R2 of the helium ions that were calculated for this special plasma. The agreement between the independently obtained velocity values proves that the degree of ionization of the helium ions in this inhomogeneous plasma is described not by a static ionization formula, but by the continuity equations for these ions. Furthermore, it is shown that the ionization "equilibria" between singly charged and neutral helium particles and between three times (CIV) and four times (CY) charged carbon particles are not determined by means of static ionization formulae either. The influence of this hitherto disregarded effect on the spectroscopic determination of the electron temperature in the plasma discussed is illustrated in a diagram.