I-Mode regime with an edge energy transport barrier but no particle barrier in Alcator C-Mod

A regime of operation has been investigated on C-Mod which features a strong edge thermal transport barrier and H-mode-like energy confinement, but with little or no particle barrier. There is generally no increase in density or impurities or sudden drop in D$_{\alpha}$; impurity particle confinement is at L-mode levels. This ``I-mode'' regime is normally obtained by operating with the ion grad-B drift away from the active X-point and with ICRF heating. While the name was originally an abbreviation for ``Improved L-mode'' [1], it is now clear that this is a high confinement regime, with H$_{98(y,2)}$ up to 1.2 achieved. I-mode discharges have now been obtained over a wide parameter range, B=3-6 T, I$_{p}$=0.7-1.3 MA, q$_{95}$=2.5-5, and maintained in steady state for many $\tau _{E}$. Most discharges are ELM-free; small ELMs are in some cases triggered by large sawtooth heat pulses. I-modes are obtained at powers comparable to the L-H threshold for density barrier formation. This is 1.5-3 times above that in the favorable configuration, up to 6 MW, and scales quite differently, in particular increasing at low q$_{95}$. The I-mode regime is of considerable interest for transport barrier studies since it separates particle and energy transport channels. An edge E$_{r}$ well develops [2], and in many cases a clear bifurcation from L-mode edge temperatures is seen; pedestals up to 1 keV and edge $\nu $* as low as 0.15 have been obtained. Changes in edge turbulence are observed as the T barrier forms. Broadband fluctuations in the 50-200 kHz band decrease, while a broad peak at higher frequencies appears. This new mode apparently helps to regulate particle transport. \\[4pt] [1] F. Ryter et al, Plasma Phys. Control. Fusion \textbf{40} 725 (1998) \\[0pt] [2] R. McDermott et al, Phys. Plasmas \textbf{16} 056103 (2009)