Development of a folded waveguide antenna for ICRF heating in the large helical device

We report results of folded waveguide antenna experiments with high power, up to 1 MW, which include the dependence of plasma loading resistance on the plasma density and how much r.f. power can be delivered. We propose its application to the large helical device, which is a heliotron-torsatron device scheduled to produce its first plasma in 1998. The folded waveguide antenna launches an ion Bernstein wave to heat electrons via the Landau damping process. Preliminary experiments show a sharply defined wavenumber spectrum along the magnetic field line, which suggests core plasma electron heating. Direct electron heating is superior to fast wave minority heating because it avoids the orbit losses of deeply trapped high energy ions.