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

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

• Background
• Internal transport barriers (ITBs) can be
  routinely produced in C-Mod steady enhanced Da
  (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 Da
  (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 modes
  drive

4
Plasma parameters (ITB vs. non-ITB)
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

• 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

• 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

• 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)
ITB

• Ti is calculated by TRANSP to match the neutron
  rate (using feedback corrected multiplier on ?neo
  to obtain ?i)
• Ion temperature profile gets broader as ICRF
  resonance is move outward
• This trend is consistent with experimental
  observations

9
ITG growth rate profiles

• 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
• k?i 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