Abstract
Two types of oscillation phenomena are found just after hydrogen ice pellet injections in the Large Helical Device (LHD). Oscillation phenomena appear when the deposition profile of a hydrogen ice pellet is localized around the rotational transform ι = 1 rational surface. At first, damping oscillations (type-I) appear only in the soft X-ray (SX) emission. They are followed by the second type of oscillations (type-II) where the magnetic fluctuations and density fluctuations synchronized to the SX fluctuations are observed. Both oscillations have poloidal/toroidal mode number, m/n = 1/1. Since the type-II oscillations appear when the local pressure is large and/or the local magnetic Reynold's number is small, it is reasonable that type-II oscillations are caused by the resistive interchange modes. Because both types of oscillations appear simultaneously at slightly different locations and with slightly different frequencies, it is certain that type-I oscillations are different from type-II oscillations, which we believe is the MHD instability. It is possible that type-I oscillations are caused by the asymmetric concentration of the impurities. The type-I oscillations are similar to the impurity snake phenomena observed in tokamaks though type-I oscillations survive only several tens of milliseconds in LHD.
Highlights
“Snake” oscillation is one of the important but not well understood phenomena related to transports in fusion plasmas
Since the type of oscillations (type-II) oscillations appear when the local pressure is large and/or the local magnetic Reynold’s number is small, it is reasonable that type-II oscillations are caused by the resistive interchange modes. Because both types of oscillations appear simultaneously at slightly different locations and with slightly different frequencies, it is certain that type of damping oscillations (type-I) oscillations are different from type-II oscillations, which we believe is the MHD instability
The frequencies of type-I and type-II oscillations are approximately 680 Hz and 490 Hz in Fig. 9 (b). These observations suggest that the snake-like type-I oscillation are different from resistive interchange modes though snake-like type-I oscillation has the mode structure closed to m/n = 1/1. Since both types of oscillation appear simultaneously at slightly different locations and with slightly different frequencies, it is certain that type-I oscillations are different from type-II oscillations, which we believe are the MHD instabilities
Summary
“Snake” oscillation is one of the important but not well understood phenomena related to transports in fusion plasmas. Though snakes are localized on the q = 1 rational surface they survive the sawtooth crashes Several theoretical models, such as the generation of the magnetic island by the excitation of the tearing mode[2] or helical core equilibrium[3], have been proposed. Snakes having m/n = 1/1 structures induced by the impurity concentration are reported in several devices, such as the Doublet-III4, PBX5, or the Alcator-C mod device[6] Because these types of snake-like oscillation phenomena are reported in most tokamaks[1,7,8], RFPs9, and a spherical torus[10], it may be a common feature in magnetic confinement devices. Snakes are interesting phenomena, they are quite important phenomena to understand the mechanism of the formation since, in the present ITER experimental scenarios, the q
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