High source potential upstream of a current-free electric double layer

Large plasma potentials (up to ~80 V) are measured upstream of a current-free electric double layer generated in a helicon rf argon discharge for a constant rf power of 250 W (at 13.56 MHz) and for low operating pressures (<2 mTorr) using an electrostatic ion energy analyzer. The energy of the Ar+ ion beam formed by acceleration through the potential drop of the double layer is found to be strongly correlated with the large source potentials: both the source potential and the beam energy increase with decreasing pressure. The creation of the double layer depends on the magnetic field generated by the solenoid near the closed end (glass plate) of the helicon source.