Analysis of Edge Current Effects on DIII-D Using Zeeman Polarimetry
Abstract
The large pedestal pressures observed in DIII-D lead to sizeable local pressure gradients which can produce significant edge bootstrap current. Reducing the local shear can self-consistently alter the pressure gradient MHD stability limit. We are studying the effects of currents in this region by using a high spatial resolution (dR ∼ 5 mm, dψ ∼ 015) 32-channel Zeeman polarimetry diagnostic to measure the local magnetic field evolution. Close spacing of the diagnostic sightlines should allow us to infer edge current behavior via Ampere's law or through comparison with magnetic reconstructions for a wide variety of DIII-D magnetic geometries. Measurements of the magnetic pitch angle (γ = B_pol/B_tor) for a variety of DIII-D discharges indicate 1) B_pol evolves throughout the L phase until the L-H transition, 2) a reduction in B_pol occurs in the region just inside the pedestal as the pressure gradient builds, and 3) a clamping of B_pol profile after ELMing begins and the pressure gradient becomes more or less constant. These phenomena are consistent with a marked increase of edge current in the gradient region in H-mode. The effects of current ramps and density dependence of the current evolution will also be discussed.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- October 2003
- Bibcode:
- 2003APS..DPPGO1011T