|poster.pdf||2006-03-21 16:02:28||Christop Cothran|
Spectroscopic flow and ion temperature studies of high s FRCs
Author: Christop D. Cothran
Submitted: 2005-12-21 23:10:10
Co-authors: J. Fung, M. Brown, M. Schaffer, E. Belova
500 College Avenue
Swarthmore, PA 19081
The Swarthmore Spheromak Experiment (SSX) produces FRCs at large s by merging counter-helicity spheromaks within a cylindrical flux conserver. Past results have shown that the toroidal fields in each spheromak do not annihilate even after poloidal flux has completely reconnected. A radially directed current density must exist at the midplane, crossing the axial poloidal field; the resulting sheared toroidal JxB must be balanced either inertially or viscously, in either case implying a non-trivial fluid velocity in the resulting FRC. Significant reconnection driven flows and ion heating are also anticipated during merging.
Flow and ion temperature studies of FRCs at SSX are ongoing. The primary diagnostic is an instrument for fast (MHD timescale) high resolution ion doppler spectroscopy (IDS). Bi-directional jets are observed during the reconnection phase of the FRC formation. The IDS data show that the selected emission line (CIII 229.7nm) bifurcates intermittently into two well separated gaussians (each with 15-20 eV thermal width). The separation of the peaks corresponds to 0.4vA. This is greater than expected from a resistive MHD simulation, but comparable to reconnection outflows when the Hall term is included. At present, further analysis of the IDS data is necessary to validate the model for midplane JxB driven flows (described above).
Helium glow discharge cleaning capability has recently been added to SSX, and operation at 4 times lower density (2-3x10^14/cm3) is now possible. The (impurity) ion temperature increases by approximately the same factor (up to 60-80 eV), possibly indicating that beta remains constant. The kinetic parameter is smaller (s=5) for these low density FRCs, so the ideal tilt may be slower or inhibited; internal magnetic probe studies are planned.