Time development of a current-free double-layer

Abstract

The time development of a supersonic (∼2.3Cs) argon ion beam generated by a current-free double-layer (DL) is obtained by pulsing a “helicon” discharge (13.56 MHz) and measuring the total ion current and the ion beam current during the first few milliseconds using a retarding field energy analyzer. The ion beam current is detected during the plasma breakdown phase (60–250 μs) and is stable thereafter (⩾250 μs). Temporal measurements of the floating potential upstream and downstream of the DL show evidence of wall charging in the plasma source during the first 250 μs of the discharge which appear to be related to the appearance of the double-layer. A comparison between the DL case and a somewhat higher pressure “non DL” case suggests that the double-layer is formed at ∼100 μs.