Investigation of triggering mechanisms for internal transport barriers in alcator c mod
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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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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

Philadelphia, PA

October 31, 2006


Motivation

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


Motivation1

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


Plasma parameters itb vs non itb

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


Pre itb electron temperature gradient

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


Change in electron temperature gradient

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


Ion temperature profile measurements

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


Ion temperature profile transp simulation

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


Itg growth rate profiles

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


Conclusions

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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