Generation of a large swelling in the spatial magnetic field profile near the coil axis of a theta pinch

A large magnetic field swelling near the coil axis in a theta pinch with low discharge energy was investigated in detail by changing the gas-fill pressure, the kind of gas and the intensity of the applied magnetic field. Under the experimental conditions, the maximum magnetic field around the axis amounted to more than three times the applied field. It was experimentally clarified that the shifting of the plowed plasma to the back of the current sheath in the implosion phase is essential for the phenomenon and that the amplitude of the field around the axis can be explained by the flux trapped by the shifted plasma.