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 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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 ~

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