Magnetic Diagnostics for Equilibrium Reconstructions in the Presence of Non-axisymmetric Eddy Current Distributions in Tokamaks

Laura Berzak, Princeton University

The Lithium Tokamak eXperiment (LTX)* is a modest-sized spherical tokamak (ST)(R_0 = 0.4m, a = 0.26m) designed to investigate the novel, low-recycling lithium wall operating regime for magnetically confined plasmas. LTX reaches this regime through a shell heated to 300 - 400 °C, internal to the vacuum vessel, and conformal to the plasma last closed flux surface. The shell is constructed in four electrically isolated segments from cm-thick copper explosively bonded to stainless steel. This structure is highly conductive and not axisymmetric. The three-dimensional nature of the shell causes the magnetic fields and eddy currents to be three-dimensional as well. The purpose of the shell is to provide a hot, uniform temperature first wall, which will be coated with a thin film of lithium. When the shell is coated, it provides a liquid lithium area of 5 m2. This area covers all of the plasma facing components and more than 85% of the last closed flux surface, allowing LTX to access very low recycling regimes. Magnetic diagnostics are designed to withstand incidental contact with liquid lithium as well as to survive shell operation up to 500 °C. Sensors are therefore fabricated from robust materials with heat and lithium resistance, such as yttria-stabilized magnesium oxide. The magnetic sensors have been tested during shell heating and during calibration shots, and they have performed as designed. In order to analyze the magnetic equilibrium in the presence of three-dimensional eddy currents, an extensive array of these unique magnetic diagnostics has been implemented. This array includes 76 B-dot probes (Mirnov coils), which are both single- and double-axis and are located in shell gaps, internal to the shell, and external to the shell. The array also includes 27 flux loops mounted on the shell and inside the centerstack as well as 2 plasma Rogowski coils, 1 vessel eddy current Rogowski coil, and a diamagnetic loop. This magnetics array is utilized to determine the effects of non-axisymmetric eddy currents, to calculate plasma equilibrium reconstructions, and to model the start-up phase of LTX. * Supported by US DOE contract #DE-AC02-09CH11466

Abstract Author(s): Laura Berzak, A.D. Jones, R. Kaita, T. Kozub, R. Majeski, J. Menard, L. Zakharov, LTX Team