Measurements of core lithium concentration in a Li-conditioned tokamak with carbon walls

The National Spherical Torus Experiment (NSTX Ono et al 2000 Nucl. Fusion 40 557) is exploring the use of lithium as candidate plasma-facing material to handle the large power flux to the wall of fusion devices. This paper reports on the measurements of lithium concentration in the plasma core during the 2010 NSTX experimental campaign, during which 1.3 kg of lithium was evaporated into the NSTX vessel. It is shown that lithium does not accumulate in significant amounts inside the plasma, resulting in an upper bound for the measured lithium concentration that is well below 0.1% of the electron density for a broad range of experimental conditions. Carbon, which constitutes the primary material of the NSTX inner wall, remains the dominant plasma impurity even after large amounts of lithium, of the order of hundreds of milligrams, are evaporated into the vacuum vessel.