## Columbia Non-Neutral Torus

The Columbia Non-neutral Torus (CNT) is a small stellarator at the Columbia Plasma Physics Laboratory (Columbia University) designed to conduct the first investigation of non-neutral plasmas confined on magnetic surfaces.

The CNT experiment probes the physics of non-neutral plasmas confined on magnetic surfaces..

The equilibrium equations for a pure electron plasma on magnetic surfaces were recently derived [Pedersen and Boozer, Phys. Rev. Lett. **88**, 205002, 2002]. The equilibrium is fundamentally different from pure electron equilibria in other confinement configurations, such as the Penning trap, and is also fundamentally different from equilibria of quasi-neutral plasmas on magnetic surfaces. This implies that new physics is involved. Finite temperature pure electron equilibria have been calculated in two dimensions, using a cylindrical approximation and elliptical magnetic surfaces [Pedersen, Phys. Plasmas **10**, p. 334 (2003)] and in three dimensions [Lefrancois et al., Phys. Plasmas 12, p. 072105)].

Equilibria are stable against low frequency perturbations, as long as these perturbations do not violate parallel force balance, particle and entropy conservation, and parallel temperature equilibration [A. H. Boozer, Phys. Plasmas **11** p. 4709 (2004)].

A stellarator configuration can confine plasmas with an arbitrary degree of neutralization, all the way from pure electron to quasi-neutral. This will allow studies of non-neutral ion-electron plasmas and may facilitate the creation of the first confined positron-electron plasmas [Pedersen et al., Journal of Physics B **36**, p. 1029 (2003)]. There are many issues in this area that are still largely unexplored.