High decay index plasmas in DIII-D
This paper describes experiments to study the vertical position problem in highly elongated plasmas in the DIII-D tokamak, and stabilisation of the high decay indices. The starting point for the model of the vertical stability dynamics is a rigid massless plasma which experiences a vertical force. The vacuum vessel provides a restoring force via the radial field resulting from the image currents induced in it. The vessel currents are decomposed into orthogonal modes, of which the first antisymmetric mode dominates. Active coils were added to the model in terms of their mutual inductance to the vessel current modes and their radial field at the plasma axis. This leads to a low order dynamical system model, 2nd order when only one active pair of poroidal field control coils is considered. Out of this formulation, came the concept of hybrid vertical control. The main, slower radial field was provided by an outboard coil and a faster bu weaker radial field was provided by inboard coils, allowing the control of plasmas up to decay indices close to the ideal MHD positional stability limit with reduced power requirements. These data show that the predictions of a simple vertical control model can lead to significant improvements in the achieved decay index, by providing a simple clear picture of the controlled system. Such an improvement was in addition obtained with a small amount of experimental data, contrary to the habitual trial and error optimisation.
Bibliographic Reference: Paper presented: 16th European Conference on Controlled Fusion and Plasma Physics, Venezia (IT), March 13-17, 1989
Availability: Available from (1) as Paper EN 34430 ORA
Record Number: 198910540 / Last updated on: 1994-12-01
Original language: en
Available languages: en