The external kink mode in diverted tokamaks
- General Atomics, San Diego, CA (United States)
- Columbia Univ., New York, NY (United States)
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
- Consorzio RFX, Padova (Italy)
Here, an explanation is provided for the disruptive instability in diverted tokamaks when the safety factor at the 95% poloidal flux surface, q95, is driven below 2.0. The instability is a resistive kink counterpart to the current-driven ideal mode that traditionally explained the corresponding disruption in limited cross-sections when qedge, the safety factor at the outermost closed flux surface, lies just below a rational value. Experimentally, external kink modes are observed in limiter configurations as the current in a tokamak is ramped up and qedge decreases through successive rational surfaces. For qedge < 2, the instability is always encountered and is highly disruptive. However, diverted plasmas, in which qedge is formally infinite in the magnetohydrodynamic (MHD) model, have presented a longstanding difficulty since the theory would predict stability, yet, the disruptive limit occurs in practice when q95, reaches 2. It is shown from numerical calculations that a resistive kink mode is linearly destabilized by the rapidly increasing resistivity at the plasma edge when q95 < 2, but qedge >> 2. The resistive kink behaves much like the ideal kink with predominantly kink or interchange parity and no real sign of a tearing component. However, the growth rates scale with a fractional power of the resistivity near the q = 2 surface. The results have a direct bearing on the conventional edge cutoff procedures used in most ideal MHD codes, as well as implications for ITER and for future reactor options.
- Research Organization:
- General Atomics, San Diego, CA (United States)
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- FC02-04ER54698
- OSTI ID:
- 1354788
- Journal Information:
- Journal of Plasma Physics, Vol. 82, Issue 03; ISSN 0022-3778
- Publisher:
- Cambridge University PressCopyright Statement
- Country of Publication:
- United States
- Language:
- English
Kink instabilities of the post-disruption runaway electron beam at low safety factor
|
journal | March 2019 |
Similar Records
Feedback-assisted extension of the tokamak operating space to low safety factor
Resistive instabilities in negative central shear tokamaks with peaked pressure profiles