Temporal evolution of the decaying spheromak in the CTCC-I experiment

In the CTCC-I spheromak experiment, after low-Z impurities have been suppressed by titanium coating on the inner surface of the flux conserver, MHD activity increases to the point that instabilities are present in every discharge at some time during the decay process. Most of the instabilities occur intermittently, as evidenced by the O V line radiation intensity which rises gradually and then drops abruptly. In some experiments, a conducting pole is inserted along the symmetry axis to increase the magnetic shear of the configuration. This conducting pole effectively suppresses a destructive instability, but other MHD instabilities are still present. Magnetic field measurements indicate that the occurrence of the instability is correlated with a relaxation process during which the peaked current profile at the magnetic axis is flattened by the MHD instability. An estimate of the energy loss during the subsequent relaxation process is given.