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Investigation of triggering mechanisms for internal transport barriers in Alcator C-Mod

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

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Investigation of triggering mechanisms for internal transport barriers in Alcator C-Mod

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  1. 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

  2. Motivation Inward pinch Core Edge 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

  3. Motivation 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

  4. Plasma parameters (ITB vs. non-ITB) 6.3 T ITB line-integrated density (1020 m-2) line-integrated density (1020 m-2) density peaking = density peaking RF power (MW) RF power (MW) time (s) time (s) 5.6 T non-ITB • 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

  5. Pre-ITB electron temperature gradient Near ITB foot location Just inside ITB foot ITB non-ITB • 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

  6. Change in electron temperature gradient 70 MHz on-axis 80 MHz off-axis R=0.78m Te (keV) ITB foot R=0.83m time (s) time (s) Te (keV) R/LT time (s) R (m) • 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

  7. Ion temperature profile measurements ITB non-ITB 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

  8. Ion temperature profile (TRANSP simulation) Te Ti RF (x10) (Watts/cm3) Te Ti RF (x10) (Watts/cm3) Te Ti RF (x10) (Watts/cm3) Te Ti RF (x10) (Watts/cm3) • 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 ITB

  9. ITG growth rate profiles non-ITB 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

  10. 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

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