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The problem of control of the vertical instability is formulated for a massless filamentary plasma. It is found that the system can be stabilised up to a critical decay index, which is predominantly a function of the geometry of the passive stabilising shell. A second, smaller critical index, which is a function of the geometry of the control coils, determines the limit of stability in the absence of derivative gain in the control circuit. The system is also studied numerically in order to incorporate the non-linear effects of power supply dynamics. The power supply bandwidth requirement is determined by the open-loop growth rate of the instability. The system is studied for a number of control coil options which are available on the DIII-D tokamak. It is found that many of the coils will not provide adequate stabilisation and that the use of inboard coils is advantageous in stabilising the system up to the critical index. Experiments carried out on DIII-D confirm the appropriateness of the model. Using the results of the model study, DIII-D plasmas were stabilised with decay indices up to 98% of the critical index. Measurement of the plasma vertical position is also discussed.

Additional information

Authors: LAZARUS E A, General Atomics Inc., San Diego, Ca. (US);LISTER J B, Centre de Recherches en Physique des Plasmas, École Polytechnique Fédérale de Lausanne (CH);NEILSON G H, Oak Ridge National Laboratory, Oak Ridge, Tennessee (US)
Bibliographic Reference: Report: LRP 376/89 EN (1989)
Availability: Available from Confédération Suisse, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, 21 avenue des Bains, 1007 Lausanne (CH)
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