Internal Current-Driven Kink Modes in Line Tied Screw Pinch.

In recent theoretical studies of external kink mode stability in line-tied geometry, a current channel was assumed to be isolated from a cylindrical conducting wall by a vacuum gap [1,2]. In a more general model [3], the current channel was surrounded by a low density external plasma yielding the no-wall marginal stability condition q$_{b }$=1 where b is the radius of the external plasma. Recent experimental results from the Rotating Wall Machine (RWM) show that during the initial ramp-up period of the plasma current, the m=1 mode becomes unstable before q$_{b }$ falls below unity at the plasma boundary. The instability has an internal character corresponding to the appearance of the resonance surface q(r$_{s})$ = 1 in the plasma region. Theoretical analysis based on an energy principle calculation for internal kink modes in line-tied geometry show that the line tying effects are strong enough to stabilize the ideal internal kink mode. This discrepancy motivates our interest in studying non-ideal effects in line tied geometry and non-MHD sheath mechanisms that can reduce line-tying stabilization and explain the experimental observations. [1] D.D.Ryutov, R.H.Cohen, L.D.Pearlstein, Phys. Plasmas, v.11, No 10, 4740 (2004). [2] C.C.Hegna, Phys. Plasmas, v.11, No 9, 4230 (2004).[3] V.V.Mirnov, C.B.Forest, C.C.Hegna, Bull. of the APS, v.49, No 8, p.231 (2004).