"The External Kink Mode in Diverted Tokamaks and the Role of q95"
Event details
| Date | 20.04.2015 |
| Hour | 10:30 › 11:30 |
| Speaker | Dr. A.D. Turnbull, General Atomics, San Diego, USA |
| Location |
PPB 019
|
| Category | Conferences - Seminars |
The current-driven external kink mode is predicted to be unstable in a tokamak when the edge safety factor at the plasma edge, qedge, lies just below a rational value n/m and the current density profile is sufficiently broad. In a cylindrical straight tokamak model, the corresponding unstable mode has toroidal mode number n and poloidal mode number m and is strongly peaked at the plasma edge. In a torus, the picture is essentially unchanged, except for additional coupling to neighbouring poloidal harmonics. The external kink modes have been observed experimentally as the current in a tokamak is ramped up and qedge decreases. In particular, for the case of m = 2 and n = 1 with qedge < 2, the instability is always encountered. However, for a diverted plasma, the edge q is infinite and one would naively expect stability to these current driven kink modes. For many decades, the limit has been observed instead when q at the 95% flux surface, q95, reaches 2.0; the limit q95 = 2, is still a hard limit and has only recently been overcome for short times by active magnetic stabilization. Ideal stability predictions indicate stability for that case despite the observation of gross instability in the absence of feedback stabilization. The conventional view is that the ideal kink mode becomes a tearing mode when the edge rational surface lies inside the finite resistivity plasma edge region. However, it is shown that instead a resistive kink mode is destabilized by the rapidly increased resistivity at the plasma edge. The resistive kink behaves much like the ideal kink with no sign of a tearing component.
Practical information
- Informed public
- Free
Organizer
- Prof. P. Ricci
Contact
- Prof. P. Ricci