Asymmetrical Normal-Zone Propagation Observed in the Aluminum-Stabilized Superconductor for the LHD Helical Coils

Sekiguchi

et al. 2004

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Abstract-A pair of helical coils of the Large Helical Device is the largest pool-cooled superconducting magnet. The first excitation test up to 1.5 T was carried out at the end of March, 1998. In the first trial to charge up to the design value of 3 T, wide propagation of a normal-zone was induced at 11.45 kA, and the quench detection system acted. It was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into the thick pure aluminum stabilizer. In these six years' operation, propagation of a normal-zone has been observed 17 times at almost the same current. The normal-zones were recovered within several seconds except for the wide propagation. By a novel detection system of the propagation with pick-up coils along the helical coils, it was disclosed that the recovered normal-zones propagated in only one side. In order to attain plasma experiments near 3 T, higher excitations were tried by the current grading method, in which the current of the innermost block is decreased and those of the other two blocks are increased. The average current of 11.67 kA was attained. The excitation tests up to the highest currents have been carried out after each cool-down. Degradation is not observed in the coil performance, and the stable operation has been demonstrated.

mentioning

confidence: 99%

Performance of the Helical Coils for the Large Helical Device in Six Years' Operation

Sekiguchi

et al. 2004

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“…3(a) and 3(b). At a current close to the minimum current to begin propagation, a normal-zone propagated to only the downstream side of the current [4]. At higher currents, a normal-zone propagated to both sides.…”

Section: Resultsmentioning

confidence: 99%

Improvement in Cryogenic Stability of the Model Coil of the LHD Helical Coil by Lowering the Temperature

Hamaguchi

et al. 2006

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Abstract-Helical coils of the Large Helical Device are poolcooled superconducting magnets, in which propagation of a normal-zone has been observed several times at about 86% of the nominal current of 13.0 kA. It is planned to improve the cryogenic stability by lowering the inlet temperature. In order to estimate the effect, the cryogenic stability of a model coil of the helical coil was examined in saturated and subcooled helium. Liquid helium is supplied from the bottom of the model coil, and it is exhausted through the winding to the current-leads tank. The inlet helium is subcooled by a pre-cooler. A normal zone was initiated by a heater on the conductor at the bottom of the coil. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to propagate over the next turn varies from 10.7 to 11.2 kA in the four cases that are without or with additional thermal shields, and before or after being subcooled. The difference is considered to be caused by the change of quality of saturated helium inside the winding or by the change of the wetted condition of the conductor surface. The minimum currents are higher at the lower temperatures in subcooled helium. It is raised up to 11.7 kA at 3.5 K of the temperature inside the winding. The propagation velocity at each minimum current is almost same. Namely, the propagation velocities at the same current are slower at the lower temperature in subcooled helium.

“…A finite normal-zone propagated with recovery from the upstream side. The direction is same as the vector of , where the is the transfer current from the superconducting strands to the aluminum stabilizer, and is the magnetic field [5].…”

Section: Resultsmentioning

confidence: 99%

Results of Stability Test in Subcooled Helium for the R&D Coil of the LHD Helical Coil

Hishinuma

et al. 2004

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Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.Index Terms-Aluminum stabilizer, dynamic stability, minimum propagating current, subcooled helium.