Conducting structures closely coupled to the plasma intensify the challenge of designing the magnetic fields necessary for successful plasma start-up. The need for close-fitting, conducting structures will continue to grow in importance, particularly as more experiments investigate alternative first walls and as experiments such as ITER incorporate large, highly conductive structures close to the plasma boundary.

The Lithium Tokamak eXperiment (LTX) is designed to investigate the novel, low-recycling lithium wall operating regime for magnetically confined plasmas. LTX reaches this regime through a heated copper shell coated with liquid lithium internal to the vacuum vessel and conformal to the plasma last-closed-flux surface. The three-dimensional nature of the shell causes the eddy currents and resultant magnetic fields to be three-dimensional as well.

An extensive array of unique magnetic diagnostics has been designed to yield data on non-axisymmetric, three-dimensional fields and implemented to quantify the temporal history and magnitude of fields due to the large eddy currents in the LTX shell. With data from the magnetic diagnostics, primary sources and paths of eddy currents have been elucidated. To further quantify eddy currents and their effect on in-vessel magnetic field, a two-dimensional electromagnetic code has been developed by utilizing data from the magnetic diagnostics. By developing an understanding of the effect of eddy currents on in-vessel fields through measurements and simulations, plasma start-up has been achieved in LTX. Highly-constrained plasma reconstructions have been calculated and used to predict the field evolution required for the desired discharge development.

To capture full three-dimensional effects, a three-dimensional electromagnetic code set is under development. Using this code set, a new calibration algorithm for determining the actual versus model locations of field coils and sensors has been developed, as well as a novel routine which mathematically calculates the poloidal field coil currents required for a desired plasma equilibrium. In addition, three-dimensional eddy currents in the shell and vacuum vessel have been simulated and confirm predictions from the two-dimensional code and interpretations of data from magnetic diagnostics.