The standard high-energy Born-Bethe approximation for the stopping power of electrons in a neutral or weakly ionized gas is not adequate for modeling medium-Z pellets, dust, or gas sources, since the velocity of the incident plasma electrons in the intermediate energy range 50 < E < 5000 eV is not necessarily large compared with that of bound atomic electrons. A modified Bethe-like expression for the stopping power was previously proposed by others to describe the energy loss of electrons. Convenient analytical expressions have now been derived for the effective number of atomic electrons Z* and the mean excitation energy, or I* value, key parameters in the modified Bethe-like expression. The modification replaces the quantum oscillator strength distribution with the Lindhard and Sharff equivalent local plasma dispersion approximation (LPDA) and uses the Thomas-Fermi atom model for the atomic “electron plasma” cloud. E and Z enter the expressions for Z* and I* only in the combination P = E/Z4/3. When P < 30 eV, Z* and I* become noticeably lower than Z and the standard literature I values, respectively. The new analytical expressions provide a rapid means of calculating the lifetime and penetration distance for solid neon and argon pellets used for inducing radiation-driven plasma quenches on DIII-D. The incorporation of the modified Bethe expression in the Z > 1 transonic flow ablation theory seems to afford a means of qualitatively explaining aspects of argon pellet injection experiments on DIII-D.
In collaboration with Allan Reiman (PPPL) a DIII-D experiment was proposed at the recent DIII-D Research Opportunities Forum aimed at collecting sufficient data, taking advantage of the new 3D diagnostics that have been installed on DIII-D, to measure the plasma response to non-axisymmetric fields from the DIII-D I –coils. In a continuation of the work completed for the 2012 Fusion Energy Science Theory Milestone, a new national collaborative effort has been set up to utilize linearized and extended MHD codes, as well as a number of 3D stellarator codes, to predict the plasma response and verify the predictions against the data. The collaboration involves a total of 11 different codes with participation from 6 different institutions and the proposed experiment is being designed to accommodate the restrictions on each of the codes. It is hoped that the experimental data will enable discrimination between the detailed code predictions from different physics models in the different codes. In particular, the effort is expected to resolve the previously identified discrepancy in the internal response predictions between VMEC and the linear MHD response from MARS-F, IPEC, and M3D-C1 (see previous Highlights from June 14 2013 at Theory Weekly Highlights for June 2013, April 26, 2013 at Theory Weekly Highlights for April 2013, March 15, 2013 at Theory Weekly Highlights for March 2013, and February 17, 2012 at Theory Weekly Highlights for February 2012).
In an effort to strengthen the GA integrated modeling capabilities with respect to ITER, the OMFIT framework has been extended to enable compatibility with the Integrated Tokamak Modeling (ITM) framework developed by the ITM Task Force (ITM-TF) in Europe. A prototype ITER Integrated Modeling Framework and its associated data model are being developed by ITER based on the ITM framework experience. The ability to set up and execute a simulation and access the ITM data with OMFIT was demonstrated. In principle, all the codes adhering with the ITM-TF standards could be executed from within OMFIT. The OMFIT framework then provides an interface between the software developed at GA and the ITM framework. Release of a prototype ITER integrated modeling framework for all members of the ITER party to test is expected in the next 18 months.
These highlights are reports of research work in progress and are accordingly subject to change or modification