LAUSR

Axisymmetric control of the vertical position of highly shaped tokamaks, such as ITER, requires active feedback. In ITER, this control will be lost during the period in which the plasma current decays following a thermal quench of a disruption; the decay is sufficiently rapid that the surrounding walls in ITER can be approximated as perfect conductors. When axisymmetric feedback control was required before the disruption, it is shown that the edge safety factor tends to drop as the plasma current decays following a thermal quench. When the q = 2 surface approaches the plasma edge, a strong nonaxisymmetric halo current is expected to arise, which could occur when the plasma current has decayed less than 20% from the predisruption magnitude.Axisymmetric control of the vertical position of highly shaped tokamaks, such as ITER, requires active feedback. In ITER, this control will be lost during the period in which the plasma current decays following a thermal quench of a disruption; the decay is sufficiently rapid that the surrounding walls in ITER can be approximated as perfect conductors. When axisymmetric feedback control was required before the disruption, it is shown that the edge safety factor tends to drop as the plasma current decays following a thermal quench. When the q = 2 surface approaches the plasma edge, a strong nonaxisymmetric halo current is expected to arise, which could occur when the plasma current has decayed less than 20% from the predisruption magnitude.