The sawtooth activity of the decaying spheromak in the CTCC-I

In the CTCC-I experiment, gun-spheromak plasmas were produced in a flux conserver, (FC) of 0.75 m diameter and 0.4 m height successfully. Reduction of low Z impurity loss made spheromak with MHD activity that MHD instabilities were always observed during the resistive decay process. Magnetic field signals near the FC wall in the midplane showed stepwise drops which indicated the instabilities to occur intermittently as correlated with the relaxation phenomena. This phenomena has been investigated from the view point of energy and particle confinements. To investigate plasma behavior during the resistive decay process the authors made a spatially resolved soft X-ray measurement by using a 7 surface barrier diode (SBD) array, each is individually filtered by polypropylene films (high energy pass filter with cutoff at 50 eV). The SBD signals observed are considered to respond primarily to impurity radiations ranging from ultra soft X-ray to vacuum ultra violet (VUV). The SBD signals coming from the hotter plasma region (r=0.20-0.32 m) showed such a sawtooth waveform as observed in the OV line radiation. While the peripheral chord signals (r=0.17 and 0.35 m) showed an impulsive increase at the sawtooth fall time correspondingly. Simultaneous measurement of the sawtooth at different radii and at different locations in the toroidal direction showed that the sawtooth drop occurred at the same time everywhere except the surface region. The radial profile of soft X-ray radiations became a peaked one towards the magnetic axis in time and rapidly changed to a flattened one during the relaxation process