Confirmation of a large density variation along the magnetic axis of the Columbia Non-neutral Torus

Significant variations in the density and potential along the axis of a pure electron plasma in the Columbia Non-neutral Torus (CNT) stellarator have now been measured. Large variations along the magnetic field are predicted by three-dimensional equilibrium reconstructions of CNT plasmas and by simple electrostatic and geometric arguments [Lefrancois and Pedersen, Phys. Plasmas 13, 120702 (2006)]. The density variation, naxis,φ=0° naxis,φ=90°, is measured directly for several different plasma equilibria, and has a median value of 7.8, consistent with the predicted density variation of 4.4, because the error bars are large. The associated variation in potential predicted from the Boltzmann relation, eΔ Te =ln(4.4)=1.5, was also measured experimentally. The median measured, eΔ Te, was 1.1, which is of the predicted sign and in rough agreement with the measurements, but smaller than predicted. The difference is statistically significant, but might be related to the imperfect numerical modeling of the complicated electrostatic boundary conditions in CNT. The measured variations reconfirm that the Debye lengths of these plasmas are small.