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The effects of downstream magnetic field on current-free double layers and beam formation in the Njord helicon plasma device

The Njord device was constructed with the aim of investigating instabilities and turbulence in plasmas with flows and beams, which are common also in space plasmas. Njord is an inductively coupled helicon plasma device with 13.56 MHz RF power inserted into the working gas by means of a saddle antenna through a 13.8 cm inner diameter Pyrex tube. The source plasma expands through a 7 cm long and 20 cm wide port into a 0.6 m diameter and 1.2 m long chamber. Two magnetic field coils around the source and one additional downstream coil produce a magnetic field of about 25 mT at maximum. We have characterized current-free double layers in argon plasmas, and we find that they are generated at the position where the plasma expands into the main chamber. Further, the effect of shaping the magnetic field from an expanding one to a mirror shaped field by means of the downstream coil has been investigated. The downstream density and the plasma potential increase significantly when all the magnetic field lines in the source are passing also through the port–dome intersection without intersecting the port walls. The ion beam disappears when the plasma potential increases up to a potential similar to that in the source.