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The role of fluctuations on energy confinement in the SSPX spheromak
| Author: | McLean H.S. |
| Coauthor: | D.N. Hill, E.B. Hooper, R.D. Wood, R.H. Bulmer, B.I. Cohen, C.T. Holcomb, J. Moller, B.W. Stallard, |
| Institution : | LLNL |
| Abstract text: | We have made considerable progress on SSPX in understanding how magnetic fluctuations relate to energy confinement and, to a lesser extent, magnetic field generation. Electron thermal diffusivity (computed from Te profile measurements, edge magnetic probes, and the CORSICA MHD equilibrium model code) has been shown to correlate with fluctuations: the n=1 during build-up, and higher order modes later in the discharge. Slowly decaying discharges show a progression of toroidal/poloidal modes n/m evolving from 1/2 to 2/4 to 3/5 to 2/3, all agreeing with the evolution of equilibrium model q-profiles. Steep gradients in Te are measured at resonant surfaces. To further study flux surface quality, we carried out experiments using an external magnetic coil to create a static n=1 perturbation. Detailed MHD modeling predicts that the required mode amplitude necessary to cause island formation and stochasticity is larger than that measured supporting the existence of closed flux surfaces. 3D MHD modeling with the NIMROD code has begun showing improved quantitative agreement with experiment, including peaked electron temperature profiles, the faster decay of higher order magnetic modes when edge current is maintained, and closed flux surfaces. The dominant n=1 mode present during spheromak formation is also reproduced by NIMROD modeling, but the details of helicity transport and the reconnection physics involved in formation require further study. Our new understanding has led to improved performance now limited by the size of the energy storage capacitor bank. Plans are underway on the experiment to extend the discharge time to further explore confinement and magnetic field generation.
Work performed under the auspices of the U. S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
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