N. "Yanagi

S." Imagawa

National Institute for Fusion Science (NIFS-Repository)

§ 2. Pulse Height Analysis of the SpikeSignals Observed on the BalanceVoltage of the LHD Helical Coilsduring a "1M" mode shutdown process with a dischargingtime constant of 300 s, so that the DC power suppliescould be practically disconnected from the coils. Theobtained signals are found to be less disturbed by noise asis shown in Fig. 2.Yanagi, N., Imagawa, S. 4000...------.---.,..--.....,..---.Fig. 1 Cumulative spike-voltage signals as a function ofthe excitation number in each operation cycle.Fig. 2 Waveforms of the balance voltage signals observedwith different frequency resolutions: 10 Hz (top), 400 Hz(second), 4 kHz (third) during a "1M" mode dischargingprocess. The decay curve of the magnetic field (measuredat the HC coil-can) is shown at the bottom.References1) Yanagi, N. et al.: in Proceedings ofICEC18 (Mumbai,2000) pp. 179-182.2) Yanagi, N. et al.: IEEE Trans. Appl. Supercond. 12(2002) pp.662-665.#1-0 mode80 =2.7 T3rd Cycleo4th,"".;(.5th. '\6th '\.OL-_--l__--:'-__o 1 2 3Excitation Number20001000Q) 3000io>Q)'0.ena;(5....>E-!J-- __ .i --- 10 1kHzal ! i Finer: 4 kHz111o 120 240 360 480TIme(s)The cryogenic stability and mechanical properties ofthe helical coils (HC) have been one of the most importantitems to be clarified in the series of excitation tests for thesuperconducting coil system of the Large Helical Device(LHD). For these studies, the balance voltage signalsmeasured between the corresponding pairs of the Heblocks have been intensively analyzed. The balancevoltage contains a number of spike signals during ramp-upand ramp-down processes of excitation. They might begenerated by rapid changes of the self-inductances of thecoil windings due to mechanical displacement (conductormotions) caused by electromagnetic force. Pulse heightanalysis (PHA) has been successfully applied to thesesignals and we found that the number of counts of spikesignals obey exponential distribution functions [1,2].The total intensity of spike signals measured in asingle excitation is an effective measure to investigate thechanges of mechanical properties of the coil windings. Ashas been found in the former experimental cycles, it wasagain found in the sixth cycle that the total intensitydrastically reduces from the second run with a sameexcitation condition as is shown in Fig. 1. This observationclearly indicates a favorable training effect of the windings.Moreover, it is observed that the total intensity obtained inthe first excitation in each experimental cycle shows acontinuous decrease, especially for the high-energycomponent. This suggests that the training effect is notfully lost after warm-up and re-cooling of the coils.From the fifth cycle, the balance voltage has beenmeasured using a new monitoring system with ahigh-frequency resolution ("'10kHz), in addition to theformer low-frequency ("'10Hz) monitors. Comparing thewaveforms observed by the two systems, it is found thatthe high-frequency measurement contains more signalsthan those observed by the low-frequency ones. This isdue to the fact that many signals have oscillatorywaveforms (frequency",1 kHz) and thus they are cancelledout through a low-pass filter. It was also confrrmed thatthese oscillatory signals coincide with those observed inthe balance voltage of the poloidal coils. Mechanicaldisturbances and subsequent vibrations of the poloidalcoils might be the cause of this kind of balance voltage.For the high-frequency signals, the electrical noisecould be an obstacle to obtaining good pulse heightanalysis. In this respect, the balance voltage was measured71

Pulse Height Analysis of the Spike Signals Observed on the Balance Voltage of the LHD Helical Coils

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