The perturbing effect of a Langmuir probe near a magnetised double layer

A sequence of colour photographs of an emissive probe moving through a luminous anode double layer is presented. The double layer is seen 'following' the probe for as much as 10 cm. This effect may result in erroneous measurements of both the double-layer width and its position. Recently, a number of experimental studies of doublelayer formation have been performed, particularly in magnetised plasmas (see, for example, TorvCn and Anderson 1979, TorvCn and Lindberg 1980, Anderson 1981, Stenzel et a1 1981, JovanoviC et a1 1982, Merlino et a1 1984, Cartier and Merlino 1987). The potential profiles associated with these double layers are usually obtained from measurements of the floating potential of an emissive probe (Smith et a1 1979) as the probe is moved through the double layer. In this Letter, we present visual observations of the effect that a Langmuir probe has when it is moved through a double layer. The double layers are of the 'anode-type' and were produced in the device shown schematically in figure 1. An argon plasma is generated by a discharge drawn between a conical spiral filament and a grounded anode Anode mesh Langmulr probe E h 1 "mesh in the source chamber. The plasma drifts into the main chamber where it is confined radially by an axial magnetic field of 3 4 kG. The neutral argon gas pressure is in the range of 10-4-10-3 Torr. Typical plasma parameters in the main chamber were ne = 109-1011 cm-3, T, = 1-3 eV, and Ti = 0.1 T,. Further details are given in Alport et a1 (1986) and in Cartier and Merlino (1987). The double layers were produced in the diverging magnetic field region within the end chamber by applying a positive bias to the 11 cm diameter anode plate. When the plate bias voltage, Vp, is increased to sufficiently high values that the potential drop in the sheath exceeds approximately the argon gas ionisation potential (15.8 eV), the electrons accelerated through the sheath can become sufficiently energetic to ionise the background gas and produce a 'new' plasma within the