Studies of Eda H-mode in Alcator C-mod

The advantages of EDA H-modes include good energy confinement and no impurity accumulation or large ELMs. In EDA, the edge pressure gradients are at or above the ideal ballooning limit but are not relaxed by type I ELMs; instead a continuous process would seem to be at work. This process is probably related to broadband and quasi-coherent fluctuations which are seen with reflectometry, probes, PCI and magnetic pick-up loops. The coherent component has a frequency on the order of 100 kHz in steady state but shifts to much higher frequencies as the plasma transitions to or from ELMfree. We note that the Doppler shift in the shear layer may make an important contribution to the lab frame frequency. The coherent feature is dominant in the PCI and probe measurements, which show a well defined spatial structure with 1 5 k cm θ − ≈ . Recent simulations of drift Alfven turbulence show a coherent surface mode arise as the pedestal pressure gradient approaches an MHD stability limit. TYPE I ELMy DISCHARGES ARE NOT SEEN IN C-MOD INSTEAD, A REGIME THAT WE HAVE CALLED EDA (ENHANCED Dα) IS OBSERVED • First signature enhanced level of light from neutral deuterium • Particle confinement much lower than ELM-free • No strong accumulation of impurities • Energy confinement slightly lower than ELM-free (10-20%) • No large discrete ELMs • So far, we don’t see type I ELMs, though pressure gradient challenges MHD ballooning stability limit • Te well above type III boundary • EDA regime shows continuous degradation of pedestal rather than relaxation oscillation EDA may be closely related to JET LPC H-mode Shares many characteristics with type II or other “small” ELM regimes (DIII-D, JT60-U, JFT-2M). EDA H-MODES DO NOT SHOW IMPURITY ACCUMULATION