Measurement of internal magnetic field pitch using Li pellet injection on TFTR (invited)

Journal Article · Mon Oct 01 04:00:00 EDT 1990 · Review of Scientific Instruments; (USA)

DOI:https://doi.org/10.1063/1.1141775· OSTI ID:6135114

Terry, J L; Marmar, E S; Howell, R B ^[1]; Bell, M; Cavallo, A; Fredrickson, E; Ramsey, A; Schmidt, G L; Stratton, B; Taylor, G ^[2]; Mauel, M E ^[3]

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1. Plasma Fusion Center, Massachusetts Institute of Technology, Cambridge, MA (USA)
2. Princeton Plasma Physics Laboratory, Princeton, NJ (USA)
3. Department of Applied Physics, Columbia University, New York, NY (USA)

A diagnostic technique which measures the direction of the internal magnetic field pitch angle has been used successfully on TFTR. The technique requires the injection of high-speed Li pellets. The magnetic field direction is measured by observing the polarization direction of the intense visible line emission from Li{sup +} ({lambda}{approx}5485 A, 1{ital s}2{ital p} {sup 3}{ital P}{sub 0,1,2}{r arrow}1{ital s}2{ital s} {sup 3}{ital S}{sub 0}) in the pellet ablation cloud. The presence of the large (primarily toroidal) magnetic field causes the line to be split due to the Zeeman effect, and the unshifted {pi} component is polarized with its polarization direction parallel to the local magnetic field. In devices with sufficiently strong fields ({ital B}{approx gt}4.5 T), the Zeeman splitting of the line is large enough, relative to the linewidth of each Zeeman component, that enough residual polarization remains. Because the pellet moves about 1 cm before the Li{sup +} is ionized ({tau}{sub ionization}{approx lt}10 {mu}s), the time history of the polarization direction (as the pellet penetrates from the outside toward the plasma center) yields the {ital local} magnetic field direction. In the TFTR experiment, spatial resolution of the measurement is typically {approximately}7 cm, limited by the requirement that a large number of photons must be collected in order to make the measurement of the polarization angle. Typically, the pitch of the field is measured with an accuracy of {plus minus}0.01 rad, limited by the photon statistics. The measurements of the internal field pitch angle, combined with external magnetic measurements, have been used in a code which finds the solution of the Grad--Shafranov equation, yielding the equilibrium which is the best fit to the measured inputs.

• OSTI ID: 6135114
• Journal Information: Review of Scientific Instruments; (USA), Journal Name: Review of Scientific Instruments; (USA) Vol. 61:10; ISSN RSINA; ISSN 0034-6748
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
700102* -- Fusion Energy-- Plasma Research-- Diagnostics
ALKALI METALS
COMPUTER CODES
DIFFERENTIAL EQUATIONS
ELEMENTS
EMISSION SPECTRA
EQUATIONS
GRAD-SHAFRANOV EQUATION
LITHIUM
MAGNETIC FIELDS
METALS
PARTIAL DIFFERENTIAL EQUATIONS
PELLET INJECTION
PLASMA DIAGNOSTICS
POLARIZATION
SPECTRA
TFTR TOKAMAK
THERMONUCLEAR REACTORS
TOKAMAK TYPE REACTORS
ZEEMAN EFFECT