[en] Torsatron and stellarator plasma confinement devices rely on magnetic surface mapping to determine the critical vacuum magnetic field structure. A recently developed method employing an emissive filament offers some advantages over the traditional technique of mapping with a directed electron beam. On the Auburn Torsatron a comparative study between the emissive filament and directed electron beam techniques has been conducted. The parameters varied in the comparative study are: filament geometry, emission current, bias voltage, background gas pressure, and magnetic field strength. This comparative study indicates that the emissive filament technique is reliable over a broad and easily accessible range of parameters. We have also measured the spatial distribution of electrons on a given magnetic surface. As an application of the emissive filament technique, the optimization of the magnetic surfaces on the Auburn Torsatron is shown

[en] A university-scale concept exploration experiment is proposed to investigate the improved confinement properties of the quasi-toroidal stellarator. The experiment would investigate three issues germane to a larger proof-of-principle experiment: first, the improved neoclassical confinement through measurement of the ion temperature dependence on axisymmetry; second, the impact of internal currents on the stability of this configuration; and third, the effect of the reduced viscosity on plasma rotation and the formation of transport barriers

[en] Two experiments conducted on the Compact Auburn Torsatron will be described. In the first, the effect of overlapping magnetic islands on pure electron transport has been studied. The experimental results have been compared with stochastic field transport theory. Fair agreement between experiment and theory is found. Of particular interest is the good confinement seen at low collisionality with unperturbed magnetic surfaces. A second set of experiments has investigated the dependence of drift orbit island size on various perturbations. These perturbations include magnetic and electric error fields. The size of these error fields can be controlled through the use of auxiliary trim coils. The experimental results are compared with theory and good agreement is found. 5 refs

[en] An overview is presented of an experimental program of magnetic field line mapping on the research-grade Compact Auburn Torsatron (CAT). The vacuum magnetic flux surfaces of the CAT device have been experimentally mapped in a variety of magnetic configurations. The results are compared with an extensive computer model in order to validate the coil design. In initial field mapping experiments, an up-down symmetry was identified in the vacuum magnetic surfaces, and was corrected with the use of a radial trim field. Magnetic islands are observed and their size has been reduced, also through the use of auxiliary trim coils. The Compact Auburn Torsatron is equipped with two pairs of large Helmholtz coils producing mutually orthogonal magnetic fields in the horizontal plane, and two pairs of helical saddle coils wound directly on the toroidal vacuum vessel. These trim coils are used to control the size and phase of the t = 1/2 magnetic island. Through a systematic variation of trim field components, the authors demonstrate a reduction of the inherent t = 1/2 magnetic island size by a factor of three. The technique is applicable to correcting small error fields in larger helical confinement devices. The measurements of island size are compared with measurements of magnetic field line rotation within the island, and are found to be in good agreement with first-order perturbation theory

[en] Detailed knowledge of the experimental plasma equilibrium is becoming as important for stellarators as it is for tokamaks, but reconstruction of magnetic flux surfaces from magnetic measurements is more difficult in the three-dimensional geometry of stellarators than in axisymmetric configurations. We report the first reconstructions of experimental stellarator equilibria with the V3FIT[1] code. We also introduce the signal effectiveness, and describe initial studies of density-driven disruption in current-carrying stellarator plasmas. At low vacuum transform, disruptions can be triggered at a plasma density comparable to the Greenwald limit, but disruption signatures disappear as the vacuum rotational transform is raised above 0.1 (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Magnetic surface optimization studies have been conducted on the Auburn Torsatron. Winding errors in the original helical field coil resulted in a displacement and an up-down asymmetry in the magnetic surfaces. The displacement was initially corrected by unbalancing the outer Helmholtz coils. Computer modelling showed that the observed displacement and asymmetry are expected for a helical field coil winding error having a cos (θ) modulation. A helical correction coil with the appropriate modulation was designed and installed. Magnetic surface mappings with and without the helical correction coil were conducted. With the correction coil, there was a significant improvement in surface symmetry. The experimental results are in good agreement with the predictions of the computer modelling. (author). 8 refs, 6 figs, 1 tab

[en] Torsatron and stellarator plasma confinement devices rely on magnetic surface mapping to determine the critical vacuum magnetic field structure. A recently developed method employing an emissive filament offers some advantages over the traditional technique of mapping with a directed electron beam. On the Auburn torsatron a comparative study between the emissive filament and directed electron beam techniques has been conducted. The parameters varied in the comparative study are filament geometry, emission current, bias voltage, background gas pressure, and magnetic field strength. This comparative study indicates that the emissive filament technique is reliable over a broad and easily accessible range of parameters. We have also measured the spatial distribution of electrons on a given magnetic surface. As an application of the emissive filament technique, the optimization of the magnetic surfaces on the Auburn torsatron is shown

[en] The presence of large magnetic islands associated with rational magnetic surfaces in toroidal fusion devices can lead to a reduction of the confinement time of plasmas in such devices. In stellarators and torsatrons, the island size is usually minimized by careful design and coil construction within close tolerances in order to reduce the effect of error fields. In addition, magnetic islands resulting from unavoidable error fields can be reduced or eliminated through the use of auxiliary trim coils. The Compact Auburn Torsatron has been equipped with two pairs of large Helmholtz coils producing mutually orthogonal magnetic fields in the horizontal plane. These trim coils are used to control the size and phase of the t=1/2 magnetic island. A reduction of the inherent t=1/2 magnetic island size by a factor of three is achieved through a systematic variation of both horizontal trim field components. The minimization of the island size occurs simultaneously with the largest poloidal rotation rate of the island as a function of the correction coil current. (author). 8 refs, 7 figs

[en] In stellarator-type magnetic confinement devices (of which the torsatron is one), the magnetic field is produced entirely by external, current-carrying coils. Two methods for mapping magnetic surfaces in the Auburn Torsatron were tested and compared, both of which involve the use of highly transparent screens. The first method consists of coating the screen with a phosphor that emits light when struck by electrons emitted by an electron gun. A pattern representative of a magnetic surface is formed on the screen, and this pattern is recorded photographically. The second method uses an uncoated screen to collect electrons emitted from an emissive probe, which is scanned over a poloidal cross section of the torus. Under certain conditions, the collected current is a constant over a particular magnetic surface so that a contour plot of the current versus position is equivalent to a plot of the magnetic surfaces. Parametric studies of the two methods are presented, and the effectiveness of each technique is discussed

[en] In toroidal plasma confinement devices, the highly energetic fusion products will have trajectories defining drift surfaces that deviate considerably from the magnetic surfaces. An understanding of these drift surfaces is important for efficient operation of a fusion reactor. By appropriate choice of electron energy on the Compact Auburn Torsatron [Fusion Technol. 18, 281 (1990)], normalized drift surface shifts similar to those for fusion products in a reactor can be studied. As a benchmark, a set of experiments have been conducted to study the axis position and central rotational transform associated with varying vertical fields. Corresponding experiments have been conducted which measure the axis position and central transform associated with the drift surfaces of highly energetic electrons. These experiments were compared and a theoretical model incorporating the spatial nonuniformity of the applied vertical magnetic field was developed. The experimental results agreed well with this theory