Development of low-energy beams for fueling the central cell of a tandem mirror

In this article we describe the theoretical motivation and experimental development of low-energy (2 keV) neutral beams for fueling the central cell of the TMX-U thermal-barrier tandem mirror experiment at Lawrence Livermore National Laboratory. Theoretical estimates are given for the beam current required to build up a low-density plasma (nc=0.5–2.5×1012 cm−3) to a final-state high-density plasma (nc=1×1013 cm−3). The design of low-divergence (±3°) beam modules is described together with test stand measurements of beam-neutral current, divergence, and species fractions. Experimental results are given for injection of 45 aA of low-energy neutral-beam current into the central cell of TMX-U.