[en] The current density in the vicinity of a rational surface of a force-free magnetic field subjected to an ideal perturbation is shown to be the sum of both a smooth and a delta-function distribution, which give comparable currents. The maximum perturbation to the smooth current density is comparable to a typical equilibrium current density and the width of the layer in which the current flows is shown to be proportional to the perturbation amplitude. In the standard linearized theory, the plasma displacement has an unphysical jump across the rational surface, but the full theory gives a continuous displacement. A resolution of the paradox of a jump in the displacement is required for interpreting perturbed tokamak equilibria.

[en] The Columbia Non-neutral Torus started operation in November 2004. CNT is an ultralow aspect ratio stellarator designed to study the physics of non-neutral plasmas confined on magnetic surfaces. It is created from a unique and simple coil set consisting of two pairs of planar circular coils. Hence, its coil set is simpler than that of any other stellarator, or any tokamak. The first results from CNT include detailed magnetic surface mappings, and initial pure electron plasma experiments. The magnetic surface mapping experiments confirm the existence of large high quality magnetic surfaces with last closed flux surface aspect ratios as low as <1.9. These experiments were performed at magnetic field strengths up to 0.1 Tesla, and are in very good agreement with the numerical calculations. This makes CNT by far the lowest aspect ratio stellarator ever built. A significant, but smaller, volume of good magnetic surfaces is found even at very low magnetic fields, B=3 milliTesla. Detailed field line mapping results will be presented. A stationary electron emitter has been inserted into the confinement region to create pure electron plasmas. Measurements show that up to 10¹¹ electrons fill the volume of the magnetic surfaces, and that the electron confinement time can be more than 10 milliseconds, despite the modest magnetic field strength (B<0.1 T), the lack of quasi-symmetry, and the presence of a macroscopic material object in the plasma (the emitter rod). Since the estimated drift escape time is less than 1 msec, the much longer confinement time is experimental evidence that an equilibrium exists for a pure electron plasma in a stellarator, as predicted from theory. The confinement time is observed to decrease with increasing neutral pressure, and decreasing magnetic field strength. We will report on these first experiments, and discuss the results of upcoming experiments that will provide more detailed information

[en] The Columbia Nonneutral Torus is a new stellarator experiment being built at Columbia University, New York, to study the confinement of nonneutral and electron-positron plasmas. It will be a two-period, ultralow aspect ratio classical stellarator configuration created from four circular coils. The theory of the confinement and transport of pure electron plasmas on magnetic surfaces is reviewed. The guiding principles behind the experimental design are presented, together with the actual experimental design configuration

[en] It is demonstrated that there exists a plausible evolution of the discharge from the vacuum state to the desired high beta state with the self-consistent bootstrap current profile. The discharge evolution preserves stability and has adequate quasi axisymmetry along this trajectory. The study takes advantage of the quasi-axisymmetric nature of the device to model the evolution of flux and energy in two dimensions. The plasma confinement is modeled to be consistent with empirical scaling. The ohmic circuit, the plasma density, and the timing of the neutral beam heating control the poloidal flux evolution. The resulting pressure and current density profiles are then used in a three-dimensional optimization to find the desired sequence of equilibria. In order to obtain this sequence, active control of the helical and poloidal fields is required. These results are consistent with the planned power systems for the magnets