Stability of tokamak and RFP plasmas with an exended region of low magnetic shear
Event details
| Date | 17.08.2015 |
| Hour | 10:30 › 11:30 |
| Speaker | Daniele Brunetti (CRPP-EPFL) |
| Location |
PPB 019
|
| Category | Conferences - Seminars |
Magnetically confined plasmas, characterised by the safety factor q with a small or slightly inverted magnetic shear, have good confinement properties. Such plasmas typically have no internal transport barrier, operate with q95~4 and are good candidates for long pulse operation at high fusion yield in the reactor ITER. We present a study of the dynamics of hybrid plasmas, with weak or almost zero magnetic shear, in tokamak and Reversed Field Pinch (RFP) configurations, when q in the central region assumes values close to one (tokamaks) or to a rational number (tokamaks, RFPs), though the exact resonance is avoided.
We first focus our attention on tokamak and RFP equilibria with slightly reversed shear when an extremum in the safety factor is close to a low order rational. These equilibria are characterised by the possible presence of internal helical cores (with a symmetric plasma edge), which can be understood as the result of the nonlinear saturation of ideal MHD modes. The amplitude of large scale m=1 helical displacements is investigated by means of 3D equilibrium and nonlinear stability codes. The nonlinear amplitude of such saturated modes obtained with the stability code is compared both with the helical core structure resulting from equilibrium numerical calculations, and with analytic predictions which extend the nonlinear treatment of reversed q plasmas to arbitrary toroidal mode numbers.
Then we analyse (analytically and numerically) the stability of an initially axisymetric tokamak configuration when the safety factor is almost flat and very close to a rational value over a macroscopically extended region in the plasma centre. Such conditions typically occur either in hybrid scenarios or following reconnection of a global instability such as a sawtooth. A dispersion relation has been derived both for ideal and resistive modes with additional non-MHD effects, showing that the resistive sidebands coupled to a core kink-like mode exhibit extremely fast growth, though additional non-MHD effects tend to reduce the extreme growth rate. The existence of such modes has been confirmed numerically, where the sensitivity of the growth rate to changes in resistivity and two-fluid effects has been demonstrated.
We first focus our attention on tokamak and RFP equilibria with slightly reversed shear when an extremum in the safety factor is close to a low order rational. These equilibria are characterised by the possible presence of internal helical cores (with a symmetric plasma edge), which can be understood as the result of the nonlinear saturation of ideal MHD modes. The amplitude of large scale m=1 helical displacements is investigated by means of 3D equilibrium and nonlinear stability codes. The nonlinear amplitude of such saturated modes obtained with the stability code is compared both with the helical core structure resulting from equilibrium numerical calculations, and with analytic predictions which extend the nonlinear treatment of reversed q plasmas to arbitrary toroidal mode numbers.
Then we analyse (analytically and numerically) the stability of an initially axisymetric tokamak configuration when the safety factor is almost flat and very close to a rational value over a macroscopically extended region in the plasma centre. Such conditions typically occur either in hybrid scenarios or following reconnection of a global instability such as a sawtooth. A dispersion relation has been derived both for ideal and resistive modes with additional non-MHD effects, showing that the resistive sidebands coupled to a core kink-like mode exhibit extremely fast growth, though additional non-MHD effects tend to reduce the extreme growth rate. The existence of such modes has been confirmed numerically, where the sensitivity of the growth rate to changes in resistivity and two-fluid effects has been demonstrated.
Practical information
- Informed public
- Free
Organizer
- Prof. P. Ricci
Contact
- Prof. P. Ricci