Homodyne reflectometer for neutral beam injection interlock on large helical device

Neutral beam injection (NBI) into low-density plasmas can cause serious damage to the vacuum vessel wall. It is necessary to stop the NBI when the plasma terminates. This needs a reliable density monitor for NBI interlock. A three-channel homodyne reflectometer, installed on a large helical device was used for an NBI interlock. Microwaves of 28.5, 34.9, and 40.2 GHz were injected with O mode polarization. At present, a simple homodyne detection scheme is used. The reflected signal consists of a dc component due to local and reflected power and an ac component due to fluctuations in the position of the cutoff layer. Since the change in dc signal was very small, the root mean square value of the ac signal was used as the interlock signal. At present the 34.9 GHz O mode channel, whose O mode cutoff density is 1.5×10^19 m^?3, is used for the interlock. The system has been working since the first NBI experiments on LHD in 1999