Abstract
A fourfold increase in the conversion of thermal electron Bernstein waves ~EBW! to the extraordinary mode (X mode! was measured when the density scale length (Ln) was progressively shortened by a local boron nitride limiter in the scrape-off of an Ohmically heated National Spherical Torus Experiment plasma @M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference ~IAEA, Vienna, 1999!, Vol. 3, p. 1135#. The maximum conversion efficiency approached 50% when Ln was reduced to 0.7 cm, in agreement with theoretical predictions that used locally measured Ln . Calculations indicate that it is possible to establish Ln ,0.3 cm with a local limiter, a value predicted to attain ;100% EBW conversion to the X mode in support of proposed EBW heating and current drive scenarios. � 2003 American Institute of Physics. @DOI: 10.1063/1.1564081#
