Enhanced conversion of thermal electron Bernstein waves to the extraordinary electromagnetic mode on the National Spherical Torus Experiment

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.