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Understanding Three-dimensional Contributions to Eddy Currents and Their Effect on Start-up

Author: Laura F. Berzak
Requested Type: Poster Only
Submitted: 2009-12-04 10:32:16

Co-authors: A.D. Jones, R. Kaita, T. Kozub, R. Majeski, J. Menard, L. Zakharov

Contact Info:
Princeton Plasma Physics Laboratory
PO Box 451
Princeton, NJ   08543

Abstract Text:
The Lithium Tokamak eXperiment (LTX)* is modest-sized spherical torus (ST) designed to investigate the novel, low-recycling lithium wall operating regime for magnetically confined plasmas. LTX reaches this regime through a heated shell coated with liquid lithium internal to the vacuum vessel. This shell will provide an area of 5 m2 of liquid lithium, more than 90% coverage of plasma facing components. While the presence of the shell allows LTX to reach this new operating regime, the three-dimensional nature of the shell causes the fields and eddy currents to be three-dimensional as well. This added dimensionality presents a challenge to traditional diagnostics and simulations, a challenge which must be overcome for machine operation and detailed data analysis.
Furthermore, start-up is a complicated issue faced by magnetic confinement machines, including LTX. For an ohmically-driven ST like LTX, start-up entails producing a suitable vertical field null near the peak in loop voltage to allow formation of the initial plasma current channel. In order to design and generate a vertical field null at the appropriate time, it is necessary to understand the three-dimensional vacuum magnetic fields produced inside the machine. An extensive array of unique magnetic diagnostics has been engineered to withstand both high temperatures and incidental contact with liquid lithium. The magnetic sensors have been specifically located to account for the presence of the shell, a significant source of three-dimensional eddy currents. This dense magnetics array has been designed to allow for highly-constrained reconstructions of the plasma equilibrium and to permit more accurate evaluation of the global energy confinement time.
In conjunction with the magnetic diagnostics, a three-dimensional electromagnetic model of the vacuum vessel and shell has been developed. This model can be benchmarked using vacuum fields and then utilized to understand the three-dimensional field perturbations and subsequently include field contributions from these perturbations into equilibrium reconstructions of the plasma. Because of the choice of location and type of magnetic diagnostics, the array has proven valuable during the initial start-up phase of LTX. The magnetic diagnostics, coupled with two and three-dimensional modeling, have permitted detailed measurements of vacuum fields from the field-generating coils and eddy currents induced in LTX components. From analyzing changes in the sensor signals as the applied field was varied, a suitable field null was produced to permit the generation of plasma currents. This approach is widely applicable, and its details will be presented.

* Supported by US DOE contract #DE-AC02-09CH11466

Characterization: A4

Please group (1)R. Majeski, (2)L. Berzak, (3)D. Lundberg together

Princeton University

Innovative Confinement Concepts Workshop
February 16-19, 2010
Princeton, New Jersey

ICC 2010