Stability Of Axisymmetric Modes Research Articles

Effect of liner non-uniformity on plasma instabilities in an inverseZ-pinch magnetized target fusion system: liner-on-plasma simulations and comparison with linear stability analysis

Two-dimensional (2D) magneto-hydrodynamic (MHD) liner-on-plasma computations have been performed to study the growth of instabilities in a magnetized target fusion system involving the cylindrical compression of an inverse Z-pinch target plasma by a metallic liner. The growth of modes in the plasma can be divided into two phases. During the first phase, the plasma continues to be Kadomtsev stable. The dominant mode in the liner instability is imposed upon the plasma in the form of a growing perturbation. This mode further transfers part of its energy to its harmonics. During the second phase, however, non-uniform implosion of the liner leads to axial variations in plasma quantities near the liner–plasma interface, such that certain regions of the plasma locally violate the Kadomtsev criteria. Further growth ofthe plasma modes is then due to plasma instability. The above numerical study has been complemented with a linear stability analysis for the plasma, the boundary conditions for this analysis being obtained from the liner-on-plasma simulation. The stability of axisymmetric modes in the first phase is found to satisfy the Kadomtsev condition Q0<1. Furthermore, the growth rates of these modes in the second phase are found to agree well with the predictions of the linear stability analysis. A linear stability analysis for m>1 modes, using equilibrium profiles from the 2D MHD study, shows that their growth rates can exceed those for m=0 by as much as an order of magnitude.

Stability of axisymmetric modes in plasma–vacuum equilibria

A stability criterion for axisymmetric modes of ideal MHD equilibria without wall stabilization is derived for linear profiles with vanishing current density at the edge. Using perturbation theory, the critical half-axis ratio of the elliptical cross-section is computed for finite aspect ratio.

On the stability of axisymmetric modes in straight tokamaks

Axisymmetric magnetohydrodynamic m=1 modes in plane plasma–vacuum equilibria, which reduce to rigid shifts in the cylindrically symmetric limit, are investigated. It is shown that in elliptically corrugated equilibria stability to a given shift depends crucially on the direction of the shift (leading to the well-known vertical-shift instability), whereas in triangular or even more highly corrugated equilibria the direction of the shift is of no importance. Additionally, stability of elliptically corrugated equilibria with vanishing total current (but nonvanishing current density) is investigated.

Divertor effects on the stability of axisymmetric modes and kink modes in a tokamak

Stability against axisymmetric modes and free boundary n = 1 kink modes in a nearly circular tokamak with a poloidal divertor is investigated by using the linear ideal MHD code ERATO-J. Perturbations localized near the plasma surface (divertor) are shown to reduce the growth rate of MHD modes. Although the local variation of the n-index is large because of the presence of an X-point, a wide stability window for axisymmetric modes similar to limiter configurations is obtained. The detailed characteristics of the axisymmetric stability depend on the position of the X-point in the poloidal plane and on the structure of the divertor coil system. The large shear near the plasma surface is effective in stabilizing free boundary n = 1 kink modes. This shear stabilization is sensitive to the details of the divertor geometry, such as the location and number of X-points, and to the plasma pressure. The dependence of the critical beta on the current profile is also discussed.

Stability of axisymmetric modes in JET

The JET experiment is designed to have an elongated plasma. This leads to the possibility of axisymmetric instability. The stability problem is complex because of the need to consider the discreteness of the coils and their circuit connections and to take account of the transformer core, the resistive vacuum vessel and the surrounding mechanical structure. Stability is calculated allowing for these features,and the dependence on plasma shape is investigated.

Axisymmetric MHD stability of elongated tokamaks

The stability of axisymmetric modes is studied by using the general ideal-MHD code ERATO. The parameters investigated are the triangularity and the elongation of the tokamak meridian cross-section and the distance between the plasma and an external conducting wall. Both aspect ratio, R/a = 3, and safety factor on the axis, q0 = 1, are kept fixed. The validity of simpler models is tested. Wall stabilization is found to be effective for axisymmetric modes.