Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT).

Cold pure electron plasmas confined in Penning-Malmberg traps with mirror fields are known to exhibit density variations along field lines, such that the density is roughly proportional to the magnetic field strength, n∼B. The Columbia Nonneutral Torus (CNT) is the first stellarator designed to study pure electron plasmas, and exhibits substantial mirroring, with Bmax≈1.8Bmin. However, results of a three-dimensional equilibrium solver, presented in this Letter, predict a factor of 5.3 increase in density from the minimum-field cross section to the maximum-field cross section along the magnetic axis, for a 1.5 cm Debye length plasma (a≈15 cm for CNT). In this Letter, it is shown that the density variation of electron plasmas in mirror traps can be significantly enhanced in a device that has a cross section that varies from cylinder-like to slab-like, such as the CNT. A simple analytic expression is derived that describes the axial density variation in such a device, and it is found to agree well with the computational predictions for CNT. [ABSTRACT FROM AUTHOR]