Results from the triton electron beam-plasma interaction experiment

A 1 MeV, 40 – 80 kA, 60 nsec electron beam was injected into a 2 × 10¹⁵ cm⁻³ density, cool (Te = Ti = 3 eV), 4 m long theta pinch plasma. Local time dependent magnetic probe measurements were made across a plasma diameter, including within the beam channel, and measurements of the plasma electron density and velocity distribution function within the beam channel were made by Thomson scattering before, during and after beam injection. With a beam cross-sectional area of 40 cm², the beam to plasma density ratio was < 10⁻³ and the interaction was weak, with beam energy coupling to the plasma of ≲ 1% per meter. Nevertheless, the observed heating rate was too large to be explained on the basis of classical collisional processes. Scattering and magnetic diagnostics gave heating rates and coupling efficiencies which disagree by as much as a factor of two. Net current density within the beam-plasma interaction region was higher than predicted by return current theory.