[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] In tokamaks and stellarators - two leading types of devices used in fusion research - magnetic field lines trace out toroidal surfaces on which the plasma density and temperature are constant, but turbulent fluctuations carry energy across these surfaces to the wall, thus degrading the plasma confinement. Using petaflop-scale simulations, we calculate for the first time the pattern of turbulent structures forming on stellarator magnetic surfaces, and find striking differences relative to tokamaks. The observed sensitivity of the turbulence to the magnetic geometry suggests that there is room for further confinement improvement, in addition to measures already taken to minimise the laminar transport. With an eye towards fully optimised stellarators, we present a proof-of-principle configuration with substantially reduced turbulence compared to an existing design.

[en] Stellarators, the twisted siblings of the axisymmetric fusion experiments called tokamaks, have historically suffered from confining the heat of the plasma insufficiently compared with tokamaks and were therefore considered to be less promising candidates for a fusion reactor. This has changed, however, with the advent of stellarators in which the laminar transport is reduced to levels below that of tokamaks by shaping the magnetic field accordingly. As in tokamaks, the turbulent transport remains as the now dominant transport channel. Recent analytical theory suggests that the large configuration space of stellarators allows for an additional optimisation of the magnetic field to also reduce the turbulent transport. In this talk, the idea behind the turbulence optimisation is explained. We also present how an optimised equilibrium is obtained and how it might differ from the equilibrium field of an already existing device, and we compare experimental turbulence measurements in different configurations of the HSX stellarator in order to test the optimisation procedure.