Investigation of ELM magnetic precursors in NSTX discharges with and without lithium wall coatings

The evolution of ELM magnetic precursors in a series of NSTX discharges without lithium and with increasing lithium deposition [1, 2] are examined. Data from the high-n Mirnov array was used to estimate the toroidal mode number (n) of the precursors. ELMs were observed to have n=1 and/or n=2 magnetic precursors with some delayed modes in the range from n=3 to n=6, which persist as the lithium coating is increased and ELMs become partially suppressed. The D-alpha signal of a few ELMs is preceded by a slow growing plateau period which appear to be dominated by n= 3 to n=6 modes, however, n= 1 and/or n=2 modes appear as precursors to the main ELM peak. The observed n=1 precursors may be evidence of SOL currents in NSTX, similar to those observed in DIII-D [3]. [1] R. Maingi, et al., 36th Eur. Phys. Soc. Conf. on Plasma Physics (Sofia, Bulgaria), P2.175. [2] R. Maingi, et al., Phys. Rev. Lett. (2009) at press. [3] H. Takahashi, et al., Nucl. Fusion 44 (2004) 1075.