Large-volume, helicon-plasma source for simulation experiments of space plasmas
Résumé
A large-volume, helicon-plasma source (73.8 cm in diameter and 486 cm in axial length) has been developed to be used to investigate various wave phenomena observed in space plasmas, and its characteristics have been investigated. The discharge antenna used is a flat spiral type installed just outside the quartz-glass window at one end of the vacuum vessel, where the background magnetic field (B-field) is nonuniform. Antennae of two different sizes are used: 23 cm in diameter with 6 spiral turns and 43 cm in diameter with 4 spiral turns (the number of spiral turns is smaller for the latter so as not to increase the antenna inductance significantly). The uniform B-field in the central plasma region can be varied up to 2 kG. The plasma density after the density jump for Ar and He plasmas can exceed 10 12 cm-3 with approximately 600 W of the injected rf power at 7 MHz. The reason for this excellent discharge efficiency is discussed, considering the power balance between input and loss. It has been found that the radial density profile can be varied rather easily by applying the following two means: 1) by changing the B-field configurations near the antenna by adjusting the field strength of the auxiliary coil installed near the antenna and 2) by changing the antenna radiation-field patterns by using the different rf feeding points on the antenna. By changing the magnetic field configuration near the antenna, the threshold power and the degree of the density change in a density jump can be varied. The electron density reaches the maximum away from the antenna; then decays weakly along the axial direction.
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