Sawtooth observations in ohmic discharges compared between hydrogen and helium plasmas on JT-60

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Abstract
Experimentally observed sawtooth activities have been investigated for ohmic discharges in divertor operation of the JT-60 Tokamak through a detailed analysis of soft X-ray emissions and line electron density. To characterize the sawtooth behavior, the sawtooth period and inversion radius were measured and compared between hydrogen and helium plasmas, including the scaling studies. For both plasmas, most of the observed sawteeth were roughly grouped into compound and single types of sawteeth without precursor oscillations prior to an internal disruption, but often associated with m=odd successor oscillations. A particular interest lies in the experimental observation that the hydrogen discharge is apt to give rise to compound sawteeth, but the helium discharge is not. Such different features are discussed on the basis of the resistive skin time and the characteristic time of the growing inversion radius. The results suggest that some reason besides skin current effect must be more incorporated into the interpretation of the different sawtooth activities.