Beam and plasma properties downstream from positively biased source chamber

Abstract

We study the effect of changing the separation grid bias in a double plasma device from -50 V to the same bias as the anode in the source chamber, 115 V on the plasma and beam parameters downstream in the target chamber.

Two different probes were used to analyze the downstream plasma. We used an RFEA-probe to obtain information about the ions, such as the ion energy distribution and we constructed a Langmuir-probe in order to measure the temperatures, densities and plasma potentials in the new configuration.

We performed measurements with the RFEA-probe for both configurations and found that the beam density is reduced by 80% at the higher pressures and that the induced background plasma potentials increases from 42 V to 104 V with a corresponding reduction of beam energy. We also found that the background plasma density increased twofold as a result of the change.

The Langmuir plasma potentials closely matched the beam potentials detected with the RFEA probe, rather than the detected plasma potentials, and this discrepancy indicates that the Langmuir plasma potentials are not accurate for the new configuration. Further the Langmuir probe measurements provided electron temperature measurements in the range of 2-5 eV at radial positions within the grid aperture, and densities ranging from 5e+15/m^3 at the lowest pressure to 9e+15/m^3 highest pressure at the 0 cm radial position.

It was also found that it is possible to detect an electron beam when the separation grid was biased at the anode potential. This beam has a peak energy that approaches the anode-cathode potential difference as the pressure decreases and can possibly represent a means of detecting how effective the discharge current from the filaments in the source is at ionizing the neutral argon atoms in the source. These electron beams were only visible at the far edge of the grid aperture.

Uneven structures in the radial density distributions of the ions beams were detected, with the largest ion beam densities being found at the far edge of the aperture. These increased densities are thought to be caused by an uneven distribution of filaments in the source. This uneven distribution also serves as a possible explanation as to why we only found the electron beam on the far side of the grid.