Diffusion of trapped reversed magnetic field in a theta pinch in the presence of a probe

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Abstract
The trapped reversed flux a 26⋅kJ theta pinch with known initial conditions is measured in as a function of time with a radial multiple probe and compared with numerical calculations. The measured flux agrees with the numerical calculations during the implosion of the plasma but decreases rapidly after the maximum compression whereas the calculated flux remains almost constant because of the increasing plasma temperature. It is shown that radiation cooling by probe impurities can explain this enhanced field diffusion in the neighbourhood of the probe. The numerical calculations can be fitted to the experimental results by increasing the radiation loss due to hydrogenic continuum radiation by a few times 104. It is estimated that the heat flow to the probe, which is mainly carried by ions (with an energy according to the floating potential of the probe), liberates enough oxygen to account for this factor due to line radiation. Radial framing-camera photographs show a tearing mode resistive instability developing near the probe which seems to be started off by this radiation cooling. A limit on the reliability of probe measurements is given as a function of plasma density and temperature.