During tokamak experimental operation, events which rapidly terminate the plasma discharge occasionally occurs. These disruptions are of great concern because The complete and rapid loss of thermal and magnetic energy can result in destruction of the material wall. For proposed next step experiments such as the International Thermonuclear Experimental Reactor (ITER), the stored energy will be approximately 100 times greater than present day devices, which increases the engineering challenges. Exacerbating the difficulties, the disruption phenomena is often highly non-axisymmetric [1] resulting in localized deposition of the heat loads. Achieving a greater understanding of disruption mechanisms and how the heat flux gets deposited on the wall is necessary for advancing the tokamak as a reactor concept. In this set of simulations, a particular disruption of a DIII-D tokamak plasma is studied to understand both the linear and nonlinear mechanisms leading to a disruption, and to understand what physics is needed to adequately model the disruption mechanisms.
The particular discharge studied is of discharge #87009 which studied a rapid loss of confinement while the plasma was being heated as shown below.
As seen in the top figure, the plasma has 16 MW of applied beam power being applied. This increases the internal energy of the plasma as seen by the increase in the normalized beta shown in the figure. At approximately 1680 msec, the plasma observed an abrupt loss of stored energy and plasma current. Many features of the onset of this disruption were explained using a combination of analytic and linear numerical simulations [1,2] which models the disruption as an ideal mode being driven through the ideal stability boundary. The success of the onset mechanism makes this an attractive case to study nonlinearly.
References
[1] J.D. Callen et.al., Phys. Plas. 6, 2963 (1999)
[2] A.D. Turnbull et.al. In the Proceeding of the 18th Fusion Energy Conference, IAEA, Sorrento, Italy (2000)