The plasma has the ability to shield itself from material walls. Plasma electrons are retained inside the plasma due to a potential difference between plasma and wall. The potential gradient is limited to the narrow sheath region. Plasma is lost on the slow rate of the ions only.
The laser-induced fluorescence (LIF) technique is used to measure the ion-velocity distribution in the plasma sheath. From the measured Doppler shift, the sheath-potential profile can be derived. First measurements were done in the double-plasma device. For the measurement shown in the figure, the observation optic was axially moved to get the sheath potential profile. Measurements will be extended to ion-waves and related phenomena as well as to electronegative plasmas. Detailed comparisons with PIC simulations are envisaged.