Initial comparison of HIT-SI measurements to NIMROD calculations
Author: R. G. O'Neill
Requested Type: Consider for Invited
Submitted: 2006-12-12 18:25:07
University of Washington
AERB rm. 120, box 352250
Seattle, WA 98195
A key scientific goal in the HIT-SI program is to compare the experimental current drive results to the predictions of NIMROD resistive MHD calculations. The NIMROD calculations require the resistivity, density, injector current, injector flux, and injector oscillation frequency as input. This requires measurement of the experimental electron temperature and density so that the calculations can be run at experimentally relevant parameters. The electron temperature is measured with a Langmuir double probe mounted on an actuator which is swept with a continuous triangle wave through the discharge to provide time resolved data. The density is measured by a combination of Langmuir probing and FIR interferometry. The FIR instrument uses two optically pumped difluoromethane gas lasers to produce a heterodyne beat of up to 2 MHz, an improvement over the previous 250 kHz system. The increased frequency is required to track density fluctuations on HIT-SI. The experimentally measured ratio of toroidal current to injector current is compared to that predicted by the NIMROD resistive MHD model at comparable parameters. Initial data indicates poor agreement between the experimental and computational data, with the experimental data showing much greater toroidal currents than predicted. When the computational model is run at very low values of resistivity, and high values of injector lambda (well outside of the range of experimental values), the calculation shows significant toroidal current formation as seen in the experiment. In these cases, the shape of the calculated B field profile is in good agreement with the experimental B profile shape as measured by an internal magnetic probe array.