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Merging spheromaks to access large s FRC dynamics
| Author: | Cothran C. D. |
| Coauthor: | J. Fung, M. Brown (Swarthmore), M. Schaffer (GA), E. Belova (PPPL) |
| Institution : | Swarthmore College |
| Abstract text: | Using the traditional theta-pinch formation method, it has to date been difficult to produce a field reversed configuration (FRC) that does not have strong kinetic effects (low s). Research at the Swarthmore Spheromak Experiment (SSX) uses an alternate formation method, counter-helicity spheromak merging, to produce weakly collisional, MHD-like (large s) FRCs. These experiments are well diagnosed, with up to 128 simultaneous internal measurements of vector-B for determination of the global magnetic structure and n=0,1,2 toroidal mode amplitudes. The results show that the configuration evolves consistent with the picture of two reconnecting spheromaks joined by an FRC-like (purely poloidal) structure at the midplane. Despite the complete reconnection of the poloidal flux (3-4 mWb), some toroidal fields of the two initial spheromaks remain (in an axially antisymmetric pattern). The configuration therefore never achieves the purely poloidal structure expected of a true FRC; however, this may be a virtue, as the resulting magnetic shear could be stabilizing. It is observed that the non-axisymmetric toroidal mode amplitudes decrease during reconnection, then grow once poloidal reconnection is complete. The n=1 (tilt) amplitude appears to grow linearly at this point, at 6-8 times slower than the MHD tilt growth rate. Recent axisymmetric and 3D nonlinear resistive MHD numerical simulations (see E. Belova, this conference) have satisfactorily reproduced these characteristics for values of resistivity and viscosity consistent with experimental conditions. Impurity ion doppler spectroscopy will be used in future experiments to explore the role of toroidal flows, generated during reconnection, on the global stability of the configuration.
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