Measurement of the magnetic field distribution in ultra fast theta-pinch

In ultra fast theta-pinch which uses light gases such as H2, D2, He, etc. it is reported that the diffusion speed of a magnetic field is faster than that of magnetic acoustic waves reaching the central axis of the chamber. In the experiment, the magnetic field distribution in argon gas was observed, and the magnetic field distribution time was studied in relation to initial plasma density. In heavy argon gas, unlike in the lighter gases mentioned above, the diffusion time seemed to be decided by the Alfven velocity, and the dominance of classical collision in plasma was found. The Buneman instability and the ion acoustic instability did not seem to arise in view of the drift velocity and the thermal velocity of electrons. Furthermore, as the conditions of modified two-stream instability were satisfied, there is possibility that the instability occurs. Although the diffusion time of magnetic field could take a considerably small value caused by the anomalous resistance derived from the instability, the diffusion time was decided mostly by the Alfven velocity in this experiment. (Fujii, T.)