The Columbia Non-neutral Torus ͑CNT͒ ͓T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgreen, and X. Sarasola, Fusion Sci. Technol. 50, 372 ͑2006͔͒ is a stellarator used to study non-neutral plasmas confined on magnetic surfaces. A detailed experimental study of confinement of pure electron plasmas in CNT is described here. Electrons are introduced into the magnetic surfaces by placing a biased thermionic emitter on the magnetic axis. As reported previously, the insulated rods holding this and other emitter filaments contribute to the radial transport by charging up negatively and creating E ϫ B convective transport cells. A model for the rod-driven transport is presented and compared to the measured transport rates under a number of different conditions, finding good agreement. Neutrals also drive transport, and by varying the neutral pressure in the experiment, the effects of rod-driven and neutral-driven transport are separated. The neutral-driven electron loss rate scales linearly with neutral pressure. The neutral driven transport, presumably caused by electron-neutral collisions, is much greater than theoretical estimates for neoclassical diffusion in a classical stellarator with strong radial electric fields. In fact the confinement time is on the order of the electron-neutral collision time. Ion accumulation, electron attachment, and other effects are considered, but do not explain the observed transport rates.

Abstract. Sources of error fields were indirectly inferred in a stellarator by reconciling computed and numerical flux surfaces. Sources considered so far include the displacements and tilts of the four circular coils featured in the simple CNT stellarator. The flux surfaces were measured by means of an electron beam and fluorescent rod, and were computed by means of a Biot-Savart fieldline tracing code. If the ideal coil locations and orientations are used in the computation, agreement with measurements is poor. Discrepancies are ascribed to errors in the positioning and orientation of the in-vessel interlocked coils. To that end, an iterative numerical method was developed. A Newton-Raphson algorithm searches for the coils' displacements and tilts that minimize the discrepancy between the measured and computed flux surfaces. This method was verified by misplacing and tilting the coils in a numerical model of CNT, calculating the flux surfaces that they generated, and testing the algorithm's ability to deduce the coils' displacements and tilts. Subsequently, the numerical method was applied to the experimental data, arriving at a set of coil displacements whose resulting field errors exhibited significantly improved agreement with the experimental results.

Visualizations of magnetic surfaces are a valuable diagnostic in the Columbia Non-neutral Torus (CNT). The CNT is a compact stellarator, which is currently being used to study non-neutral plasmas confined on magnetic surfaces. The full 3-D shapes of magnetic surfaces created by CNT's simple four circular coil geometry are readily visualized by using an electron beam and neutral gas. These visualizations are useful for probe alignment and the confirmation of the magnetic surface topology, and they were necessary for the recent installation of a conducting boundary conforming to the last closed magnetic surface. Index Terms-Magnetic surfaces, non-neutral, stellarator, visualization.A S A STELLARATOR, the Columbia Non-neutral Torus (CNT) offers unique benefits for the study of non-neutral plasmas [1]. Plasmas of various degrees of neutrality can be studied because no external electric field is required for confinement. Steady-state low-density plasmas can be confined without internal currents. Long confinement times are expected as a result of the large-space charge electric field [2].Magnetic surfaces in CNT are created using a simple four circular coil design. Thus, design, optimization, construction, and installation were relatively fast and inexpensive [1]. Currently, the confinement [2], stability [3], and equilibrium [4] of pure electron plasmas are being studied in CNT.Pure electron plasmas in CNT are usually studied with pressures in the 10 −9 torr range to minimize the effect of electronneutral collisions. However, by backfilling to a pressure above 10 −5 torr and emitting a beam of electrons from an electron gun, it is possible to visualize the complete 3-D shape of the magnetic surfaces (Fig. 1). The electron gun is a heated-biased filament surrounded by a grounded metal cap with a hole in the end. The emitted electrons are accelerated out of the cap and travel along the magnetic surface with enough energy to excite the neutral background gas, which then emits visible light. Brightness increases with background pressure at the cost of reduced electron-beam-path length. Changing backfill gas results primarily in different color visualizations, although inert gases also allow for longer filament life.Before magnetic surface topology was confirmed [1] by using the electron-beam fluorescent-rod technique [5], this visualization method demonstrated CNT's magnetic surfaces.