Measurements of Ambipolar Diffusion in Helium

Microwave techniques are applied to the study of the diffusion of electrons and positive ions in helium. For the electron and ion densities realized experimentally, the diffusion takes place ambipolarly; that is, the flow of charged particles of both signs is equalized by the space charge field which they set up. At 1 mm Hg pressure and 0. 039-ev average energy, the measured value of the ambipolar diffusion coefficient, D₀, is 540 cm^2/sec. The measured variation of D₀ with gas density, energy of the particles, and diffusion container size and shape agrees closely with theory.