Improved sensing of vertical velocity for vertical position control using loop voltage signals on EAST

Vertical position control of shaped plasma is essential for all elongated tokamaks to achieve safe and stable plasma discharge. In EAST, the in-vessel coil (IC) is employed for fast control of plasma vertical motion which power supply capabilities limits the controllability of vertical instability. Besides, adequate signal-to-noise ratio of vertical position measurement can also realize an improvement in vertical control. In EAST, plasma vertical position Z_p is produced nearly by all integrated magnetics, fast Z error signal is obtained by filtering from Z_p. Then this fast Z error and its derivative term (dz/dt) are controlled by IC with a separate PD controller. However, it has a low signal-to-noise ratio which limits the gains that can be used successfully in the PD control loops. To provide less noisy dz/dt signal, the successful use of multiple up/down loop voltage pairs instead of derivative of z derived from magnetic probe signals will be discussed. Experiment show that even a single pair of loop voltage signal has better signal-to-noise ratio than differentiation of Z_p. In addition, flux loop pickup signals from the IC current are also measured with series of positive and negative IC current pulses.