Spatially resolved energy analyzer measurements of an ion beam on the low potential side of a current-free double-layer

Spatially resolved measurements of a supersonic Ar⁺ ion beam are obtained using a movable retarding field energy analyzer positioned on the low potential side of a current-free helicon double-layer (DL). The DL is generated near the open end of the helicon source where the fairly "collisionless" argon plasma is expanding in a diverging magnetic field for a radio frequency power of 250 W and a pressure of about 0.35 mtorr. The potential drop of the DL induces ion acceleration and the formation of an ion beam which propagates downstream of the DL. An image showing the ion beam spatial properties 12 cm downstream of the DL is presented.