A large liquid wall in a fusion energy device

A large liquid wall in a fusion energy device can improve plasma performance. At last October’s American Physical Society division of plasma physics meeting in Albuquerque, New Mexico, Princeton Plasma Physics Laboratory researchers described how they tested this idea on a spherical torus, a rotund version of the well-known tokamak. The torus uses magnetic fields to confine hot plasma, and a portion of the outermost closed magnetic surface rests against a special high-temperature assembly called a limiter. At the bottom of their plasma vessel, the researchers placed a 2000-cm² stainless steel tray to act as a limiter. This limiter was an order of magnitude larger than in previous experiments, both at PPPL and elsewhere. The physicists then fired up the machine with the tray variously empty, containing solid lithium, and filled with liquid lithium. The liquid lithium performed best: It increased the efficiency of generating current within the plasma, did an excellent job of absorbing impurities, and minimized the “recycling” of ions that cool the plasma. Also, because the liquid surface both flows and can be continually replenished, it is not subject to the same degradation and damage that occur when neutrons bombard a solid metal wall. (APS meeting paper no. RI1.004; preprint available from R. Majeski, rmajeski@pppl.gov .)