Excitation properties and cryogenic stability of helical coils for the LHD

Abstract

Abstract-The helical coils for the Large Helical Device are the world's largest pool-cooled superconducting coils in operation. These were expected to be cryustable up to 13.0 kA at 4.4 K on a basis of the measured recovery currents in all the shorl samples. However, a normal-zone was induced at higher than 11 kA repeatedly. It propagated to the finite length and recovered within several seconds except at 11.45 kA. Because of slow current diffusion into a pure aluminum stabilizer, a normal zone can propagate dynamically below the cold-end recovery current. The excitation tests have been carried out, and average 11.65 kA has been achieved by grading the current in the three blocks of the helical coil. The disturbance during excitations and the cryogenic stability of the coil are presented.

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

“…Since the quality of liquid helium may affect the cryostability, the charging rate was lowered from 0.1 T/min to 0.01 or 0.02 T/min at currents higher than 10.8 kA. In spite of that, the propagation and recovery have been observed repeatedly at almost the same currents [5]. From the measurement with pickup coils along the helical coils, it was revealed that the recovered normal-zones propagated to only one side.…”

Performance of cold compressors in a cooling system of an R&D superconducting coil cooled with subcooled helium

“…So far, plasma experiments have been carried out at the magnetic field lower than 3 T successfully [2]. However, the operating field has not reached the nominal field of 3 T [3]. So, it is considered that the helical coils of * Corresponding author.…”

Analysis on the cryogenic stability and mechanical properties of the LHD helical coils

“…1 shows an observed waveform of the H-I balance voltage when a transient normal-transition was observed in the H1-I block for about 4 s in the fourth cycle operation. As is discussed in [4], a resistive component can be extracted by subtracting the H-M balance voltage from the H-I balance voltage with an appropriate coefficient. Here it is noted that the H-M block serves as a cancellation coil to cancel out the inductive component.…”