Investigation of triggering mechanisms for internal transport barriers in Alcator C-Mod K. Zhurovich C. Fiore, D. Ernst, P. Bonoli, M. Greenwald, A. Hubbard, D. Mikkelsen*, E. Marmar, J. Rice MIT Plasma Science and Fusion Center *Princeton Plasma Physics Laboratory APS DPP Meeting Philadelphia, PA October 31, 2006

Motivation Core Edge Inward pinch Outward diffusion Background: Internal transport barriers (ITBs) can be routinely produced in C-Mod steady enhanced Dα (EDA) H-mode plasmas by applying ICRF at |r/a| ≥ 0.5 (off-axis heating) They are observed primarily in the electron particle channel and are marked by the steepening of the density and pressure profiles following the L-H transition Framework: During normal plasma operation inward neoclassical Ware pinch is balanced by the outward diffusion caused by the microturbulent modes, resulting in a flat density profile Core Edge Inward pinch Outward diffusion

Motivation Inward pinch Core Edge Background: Internal transport barriers (ITBs) can be routinely produced in C-Mod steady enhanced Dα (EDA) H-mode plasmas by applying ICRF at |r/a| ≥ 0.5 (off-axis heating). They are observed primarily in the electron particle channel and are marked by the steepening of the density and pressure profiles following the L-H transition. Framework: During normal plasma operation inward neoclassical Ware pinch is balanced by the outward diffusion caused by the microturbulent modes, resulting in a flat density profile Inward pinch Core Edge Outward diffusion Suppressing turbulent diffusion allows the pinch to overcome, resulting in a peaked density profile Longer modes (ITG) are responsible for transport Shifting the ICRF resonance outward flattens the temperature profile and decreases the mode’s drive

Plasma parameters (ITB vs. non-ITB) time (s) 6.3 T ITB line-integrated density (1020 m-2) density peaking RF power (MW) time (s) line-integrated density (1020 m-2) density peaking = RF power (MW) Magnetic field scan: shift the RF resonance location on shot-to-shot basis Plasma current adjusted proportionally to keep q95 constant Sharp threshold in BT consistent with previous observations

Pre-ITB electron temperature gradient non-ITB ITB Just inside ITB foot Near ITB foot location Temperature scale length is calculated from ECE measurements Averaging has been done over steady portions of the discharges (pre-ITB phase for ITB discharges) R/LT decreases as the ICRF resonance position is moved outward by raising the magnetic field This decrease is observed just inside the ITB foot location for ITB discharges

Change in electron temperature gradient time (s) R=0.83m R=0.78m ITB foot 70 MHz on-axis 80 MHz off-axis Dual frequency ICRF setup ITB develops during the off-axis heating phase Temperature measurements are done by high resolution (32 channels) ECE system Temperature scale length is derived from channels around the ITB location Profiles are shown at times corresponding to 100% on-axis heating, 50%-50% on- and off-axis, and 100% off-axis heating R/LT decreases in the region of ITB as the ICRF resonance moves off axis time (s) R (m) Te (keV) R/LT

Ion temperature profile measurements ITB non-ITB Ion temperature is measured by high resolution x-ray system (HIREX) Central ion temperature is derived from neutron rate measurements Ion temperature profile gets flatter as ICRF resonance is moved off axis

Ion temperature profile (TRANSP simulation) RF (x10) (Watts/cm3) ITB Ti is calculated by TRANSP to match the neutron rate (using feedback corrected multiplier on χneo to obtain χi) Ion temperature profile gets broader as ICRF resonance is move outward This trend is consistent with experimental observations

ITG growth rate profiles ITB non-ITB ITG/ETG growth rate profiles are calculated by linear gyrokinetic stability code GS2 based on TRANSP analysis No difference in ETG growth rates and spectra for ITB vs. non-ITB cases The region of stability for ITG modes gets wider as ICRF resonance is moved outward kρi spectra are similar for all runs and peak at ~0.3-0.4

Conclusions Experimental evidence: electron and ion temperature profiles get flatter as ICRF resonance location is shifted off-axis Ti profiles as calculated by TRANSP exhibit similar trend with the absolute deviation from the electron temperature being small Using TRANSP Ti profiles linear GS2 calculations show that region of stability to ITG modes gets wider as ICRF resonance is move outward Suppressing ITG turbulence can be a dominant factor in the triggering mechanisms for off-axis ICRF heated ITBs in C-Mod