Turbulent electric fields during strong beam-plasma interactions: experiment and theory

Observations of satellites of forbidden helium lines have been used to diagnose very strong electric fields. These fields, of 10 kV/cm and higher, apparently derive from powerful instabilities generated by passing a 10-kA, 800-kV electron beam through a background helium plasma of density in the 10¹³/cm³ range. A probability distribution exp(-E²) describes the fields, with at least 1% of the plasma volume experiencing E²≥ nT. There is strong evidence that turbulent fields are not smoothly distributed through the beam-plasma volume. These results can be described either by a model invoking strong wave correlations over ranges exceeding a Debye length, or by a picture requiring that strong cavitons interact to form a statistical ensemble on times shorter than the average caviton decay time. The experiments also yield spatial measures of 〈E〉 as a function of r and z in a cylindrical geometry. Results for a numerically integrated model have been compared with direct observations of E(r,z,t), using beam voltage and current as input and requiring no fitting parameters. A detailed picture of the strongly interacting regime of plasma turbulence has emerged. Such turbulence could provide a powerful means to stop electron-neutralized ion beams that penetrate plasmas.