Potential structures in radio frequency (rf, 13.56 MHz) discharges con
taining negative ions (SF6 in He gas) have been clarified experimental
ly in an asymmetrical electrode system using an emissive probe. A doub
le layer type potential profile is picked up near an instantaneous ano
de unlike the almost flat profile in He plasma. A SF6 addition provide
s a spatial shift of the double layer position toward the electrode, l
owers the plasma potential, and decreases the sheath thickness. These
results are ascribed to the negative ion production which is confirmed
by an electrostatic analyzer measurement.