Polarimetry of motional Stark effect and determination of current profiles in DIII-D (invited)

Abstract

Articles you may be interested inMeasurements of the internal magnetic field using the B-Stark motional Stark effect diagnostic on DIII-D (invited)a) Rev. Sci. Instrum. 81, 10D729 (2010); 10.1063/1.3491209Motional Stark effect diagnostic expansion on DIII-D for enhanced current and E r profile measurements Rev.Motional Stark effect polarimetry for a current profile diagnostic in DIIID (abstract) Rev. Sci.The motional electric field E= v x B, where v is the velocity and B is the tokamak magnetic field, produces a strong Stark effect in spectral lines emitted by hydrogenic neutral beams. The tilt angle of the magnetic field line, a quantity related directly to the distribution of the plasma toroidal current, is deduced from a measurement of the direction of polarization of the Stark components. In the DIII-D tokamak, the Balmer-cY line of deuterium emitted by one of the high-power heating beams is analyzed. A multichord polarimeter measures the magnetic field pitch angle at eight spatial locations covering -0.6 of the nominal plasma diameter at the midplane outboard side. The diagnostic offers 2-g-cm resolution in the major radius and 1-ms integration time. The accuracy of the measurement of the polarization direction necessary for an adequate reconstruction of the current profiles is obtained with the use of active polarizing elements which produce high-frequency intensity modulation with an amplitude related to the direction of linear polarization of the plasma radiation. The current profiles in highly shaped (noncircular) plasmas cannot be determined solely from the tilt angle measurements because they do not provide any information about the shape of magnetic surfaces. Thus, the polarization measurements are used in conjunction with a large set of external magnetic measurements (magnetic field and flux probes, diamagnetic loops, and Rogowski coils) by the magnetic field equilibrium code EFIT, and provide a constraint on the possible solutions for the current profile.

A data acquisition system for real-time magnetic equilibrium reconstruction on ASDEX Upgrade and its application to NTM stabilization experiments

“…(24) where c, ..., c9 are a set of coefficients for each channel relating the local components of electric field (E r and E z ), the radial, vertical and toroidal components of magnetic field (b r , b z and b φ ) and diagnostic beam velocity, vbeam [28,47,48]. The components of the poloidal magnetic field at the centre of the measurement volume are also evaluated by a matrix-vector multiplication using pre-calculated Green's functions.…”

Reduction in Neutral Beam Driven Current in a Tokamak by Tearing Modes

“…The noninductive current profiles have been measured during three times intervals (1.5 to 2.5 sec, 2.5 to 3.5 sec, 3.5 to 4.5 sec) for each shot in this study. Magnetic reconstructions of the MHD equilibria are found using the code EFIT, constrained by 16 internal measurements of the poloidal field radial profile by motional Stark effect spectroscopy and by external magnetic probes [10,11]. For each time interval the loop voltage profile is calculated from time derivatives of the poloidal flux, and a time-averaged current density profile is calculated [12].…”

Determination of the Noninductive Current Profile in Tokamak Plasmas

“…Central to this technique is the determination of the current density profile j(p) from measurements and equilibrium reconstructions [5] gives a measurement at a fixed point.…”