Interferometric Measurement of Electron Density in Sceptre IV

Abstract

THE density of free electrons in a plasma may be determined from its refractive index. If electromagnetic waves are used, the frequency of which is greatly in excess of the plasma frequency and the electron cyclotron frequency, the refractive index is given by¹: µ_p is the refractive index, λ the wave-length (Å) and N_e the number of electrons per cm³. Optical interferometer techniques have previously been used to measure electron densities of about 10¹⁷ cm⁻³ in shock waves^1–3 and of the order of 10¹⁴ cm⁻³ in radio-frequency discharges⁴. This communication describes experiments to determine spatial and temporal variations in the electron density in a hydrogen discharge in Sceptre IV⁵ over the density range 10¹⁴–10¹⁶ cm⁻³. For the conditions used, the contributions of atoms and ions, both of hydrogen and impurities, to the refractive index can be neglected.