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Author(s):
K. Yamazaki, O. Motojima, M. Asao, M. Fujiwara and A. Iiyoshi
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Title:
Design Scalings and Optimizations for Super-Conducting Large Helical Devices
Date of publication:
May 1990
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Key words:
helical system, superconducting device, system optimization analysis
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Abstract:
Scalings of plasma configurations and engineering device parameters for l=2 helical systems with continuous coils are derived from a wide range of computations. Using these scalings, typical optimized designs of heliotron/torsatron experimental devices with NbTi superconducting (SC) coils are obtained after maximizing a fusion product ntauT within both physics and engineering constraints. Optimization studies have been carried out for the next-generation Large Helical Device (LHD) having a major radius of ~ 4 m and magnetic field of ~ 4 Tesla, in which to demonstrate a divertor concept is one of key experiments. These studies clarified that higher gamma_c (helical coil pitch parameter) configurations (gamma_c is greater than or equal to1.25) with a larger plasma minor radius are not acceptable from the requirement of the clean divertor configuration. More compact lower-m systems (m is less than or equal to 8) are bounded by the equilibrium beta limit of the plasma and the stability limit of the SC coil current due to higher maximum magnetic field strength. Larger-aspect-ratio larger-m systems (m is greater than or equal 12, gamma_c ~1.2-1.3) with better neoclassical confinement properties are not effective because of a lower stability beta and a narrower clearance between the divertor layer and the wall. The divertor clearance becomes more severe in normal-conducting (NC) designs than in SC systems, and more than 5-second plasma operations are not possible. An l=2/m=10/gamma=1.2 SC system is found as one of optimized high ntauT configurations for 4m/4T next-generation experiments with respect to the high-beta requirement, the clean divertor installation, the SC coil engineering and the cost optimization.
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