Correction of Field Errors in the Compact Toroidal Hybrid Torsatron Experiment
Author: Stephen Knowlton
Submitted: 2005-12-20 14:07:12
Co-authors: S. Knowlton, C. Montgomery, J. Peterson, R. Kelly, J. Hanson, and J. Munoz
206 Allison Laboratory
Auburn, AL 36849
Studies on the Compact Toroidal Hybrid (CTH) experiment are pursuing issues of equilibrium and stability in current-carrying, low-aspect ratio helical plasmas as a concept-exploration element in the US Compact Stellarator Program. The program also that also includes transport and configuration optimization experiments on the HSX, NCSX, and QPS stellarators. While compact stellarators offer potential economic advantages in comparison with helical devices with more conventional larger aspect ratios, it should be demonstrated that these 3-D configurations indeed achieve good flux surfaces, i.e. provide adequate confinement and stability, in the outer region of the plasma in the presence of finite plasma current, beta, and small magnetic error fields in order to gain the advantages of low aspect ratio. Field-mapping studies are underway on the recently-completed CTH (R0 = 0.75 m, aVessel= 0.3 m, B ≤ 0.5 T) to measure the experimentally-achieved magnetic flux surfaces in low aspect ratio configurations. The CTH device has a nominal plasma aspect ratio Ap ≥ 4 with a moderately-sheared rotational transform that can be strongly varied with an auxiliary toroidal field coils as well as with inductive plasma current. A set of 15 independently-controlled error correction coils is used to minimize magnetic islands that may appear on rational surfaces. At a low value of rotational transform (iota ≤ 0.2 with no major rational surfaces present in the plasma), the vacuum configuration achieved is close to the design. The large-volume plasma limited by the wall, as expected. When the vacuum rotational transform is raised, small island chains are observed at the surfaces corresponding to iota = 1/3 and 1/2. Experiments are being performed to control the size of these islands with the flexible error correction coil set. The studies will be extended to experiments in current-driven plasmas in which the rotational transform will depend on the current profile, and in which the response to applied 3-D fields may be significantly modified by the plasma itself.
This work supported by US DoE Grant DE-FG02-00ER54610