Grid design and placement in a Cs-Ba tacitron

Abstract

The electron temperature, plasma density, and plasma potential in a Cs-Ba tacitron were measured using Langmuir probes. Experimental results showed that the voltage drop was lowest with the grid centered. Placing the grid off center resulted in higher electron temperature, larger sheath potential, and lower plasma density in the smaller region. The one arrangement with the emitter on the side of the grid opposite the Cs reservoir performed differently than all the others, suggesting that the Cs pressure was not the same on both sides of the grid. Plasma parameters in the grid hole were measured using a double grid with a probe placed between the grids. This measurement showed that between the grids, the potential was accelerating to electrons, the electron temperature increased, and the plasma density decreased relative to the emitter region. The plasma in the tacitron was modeled using a 2-D particle-in-cell plasma code. The model results were in good agreement with the measurements. Reducing the Cs pressure in the region opposite the grid brought all calculated parameters closer to the experimental values, confirming that the Cs pressure was non-uniform in the experiments. Calculations showed that current continuity through a high transparency grid was maintained by a reduction of the plasma density in the grid hole, while in low transparency grids no significant reduction in plasma density was required. The model also showed that the voltage drop across the grid increases with grid thickness.