Implementing phase feedback on the NSTX HHFW transmitters

A complete phase feedback system has been implemented for the high harmonic fast wave (HHFW) transmitters on the National Spherical Torus Experiment. This system allows the phase of the antenna loops to be defined regardless of plasma and transmitter interaction. The system consists of two multi-channel phase detectors (presented at the 19th SOFE), high-speed RF optical links, an operator control computer running a graphical interface and a new multiple-output variable-phase oscillator presented in an adjacent paper at this conference. This paper presents the overview and operational results of the phase feedback system that has been in service on NSTX since April 2002. To date, phase-stable operation has been achieved into all types of plasmas using both fixed and variable phase profiles during a shot.