The magnetic-field-induced transition from an expanding plasma to a double layer containing expanding plasma

The magnetic-field-induced transition from a simple expansion to a double layer is experimentally investigated in an argon low pressure radio frequency helicon source plasma. When the magnetic field is increased from 30to140G in the plasma source, an abrupt increase in the plasma density and upstream potential is measured at 50G. In the downstream plasma, the plasma density and potential show a small decrease with increasing magnetic field and no abrupt change. When the upstream jump is measured, simultaneous measurements in the downstream plasma show an ion beam characteristic of a double layer near the source exit.