Abstract. Argon seeding of ELMy H-mode with the outer strike point on the divertor dome has enabled extension to parameters approaching ITER requirements ( HH98(y, 2) 1; ne 0.8nGW, the Greenwald density limit; and Prad/Pheat 0.8).The maximum divertor heat flux due to ELMs decreased by a factor of 10 in plasmas with high triangularity (delta=0.5) and medium-delta (0.36),dome-top configurations. A possible reason for the change in ELM behavior in high-delta discharges without Ar might be access to second stability for ideal ballooning modes. Improved core tau at high ne with Ar has been attributed to an increase in pedestal Ti and profile stiffness inside the pedestal. To understand the improved with Ar, GS2-code linear microinstability analyses have been done. For the dome-top case the maximum growth rate, gam(max), for the ITG/CTEM vanishes at r/a 0.63, and the Ti gradient increases inside this point. The ITG reappears when the Ar is reduced. Further GS2 calculations will study the sensitivity of gam(max) to impurity mass, n(I), dilution, Ti/Te, etc. FULL-code calculations, including rotation, will further elucidate the reduced transport.