The feedback phase instability in the HBT-EP tokamak

Observations of a performance limiting feedback phase instability in the HB T-EP tokamak are reported. The phase instability consists of a rapid growth of the phase difference between an min = 2/1 tearing mode and an external resonant magnetic perturbation. Observations of mode angular dynamics durin g phase instability test discharges show good agreement with theoretical es timates of the phase instability timescale. The phase instability limits fe edback performance in HBT-EP by decreasing the feedback loop's phase accura cy as gain increases.