Preliminary experimental measurements presented by G.L Schmidt of PPPL at the 2002 APS meeting, for deuterium pellets injected into the JET tokamak, were found to be in good agreement with scaling predictions from the pellet ablation model developed at GA. In this model, the large-R drift of pellet ablation material depends strongly on the initial pressure of the ionized portion of the cloud where the ablation flow is predominately parallel to the magnetic field, while moving as a whole along with the pellet. The higher the pressure the larger the drift distance in the large-R direction. Measurements of the cloud density using Stark broadening techniques for deuterium pellets injected into JET from the high-field-side show that the cloud density, and thus distance that the pellet penetrated into the plasma, increases with time. The measured densities fit quite well with the theoretical predictions for the initial cloud pressure and density n0 as a function of local plasma temperature T, density n and pellet radius, confirming the scaling n0 ~ nT1.5.The agreement can be seen at http://web.gat.com/theory/weekly_highlights/attachments/JET_cloud.pdf
A new two-dimensional linear ideal stability code, SCOTS, capable of handling the entire plasma, including open flux lines, was developed previously in collaboration with the University of Manchester Institute of Science and Technology and the Royal Observatory of Belgium. This code is now being used to study the eigenfunctions and map the ideal stability boundaries by varying the open flux and closed flux currents for helicity injected spheromaks and spherical tokamaks. For a given coaxial helicity drive, which drives the closed flux current, the stability boundary for the n=1 open flux kink mode determines the maximum closed flux current attainable in helicity injected experiments, since the instability is typically low amplitude and weakly nonlinear, and helicity injected discharges have previously been shown to remain in the vicinity of the n=1 linear stability boundary. The calculated n=1 stability boundary then determines the total toroidal current from coaxial helicity injection as a function of injected current. This provides an extremely useful tool for predicting the performance of helicity injection in experiments such as HIT-II, NSTX and SSPX. In particular, recent work to include open flux currents in reconstructed NSTX discharge equilibria will provide a good comparison to experiment. The code can also answer questions about the effect of open flux or divertor currents on stability in conventional tokamaks such as DIII-D and JET.
These highlights are reports of research work in progress and are accordingly subject to change or modification