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Author: John Finn
Requested Type: Poster
Submitted: 2016-02-12 15:04:12

Co-authors: D. Rhodes, M. Halfmoon, D. Brennan, A. Cole

Contact Info:
Los Alamos National Laboratory
PO Box 1663
Los Alamos, NM   87545

Abstract Text:
We focus on whether the double tearing mode dispersion relation captures the essence of modes in toroidal geometry with m, m+1, m-1 coupling and rotation shear. For double tearing modes, it is known that the coupling destabilizes the more unstable single tearing mode (upper) and tends to stabilize the more stable mode (lower). Also, the coupling is stronger for |m| small, when the mode rational surfaces are close, for single tearing modes with comparable growth rates, and for low rotation shear. The latter enters as the difference in rotation frequencies between the two surfaces. This condition holds strictly only when both tearing layers are in the viscoresistive (VR) regime, in which the modes have zero frequency. In other regimes such as the resistive inertial (RI) with the Glasser effect, the peak coupling is where the Doppler shifted real frequencies match. This is analogous to driving of tearing modes by error fields[1].

We have constructed a model similar to that used for the double tearing mode studies. This model consists of stepfunction profiles for current density and pressure, with a monotonically increasing q profile. Two poloidal harmonics, m and m+1 are included. The results show, using VR tearing layers, a regime in which the upper mode is destabilized and the lower mode is stabilized, as for double tearing modes. However, there is another regime in which the lower mode is also destabilized. This possibility indicates that a quickly growing m=1 mode might couple to and destabilize a slower growing (or stable) m=2 mode.

We have investigated these results as several parameters of the model are varied. We have also investigated the effects of a rotation difference between the plasma at the two tearing layers. Results comparing numerical studies with the PEST-III code are presented.

[1] J. Finn, A. Cole, and D. Brennan, "Error field penetration and locking to the backward propagating wave", Phys. Plasmas Letters, Vol. 22, 120701 (2015).