Negative ion production and beam extraction processes in a large ion source (invited).

Area of Science:

• Plasma Physics
• Ion Source Technology

Background:

• Understanding negative-ion-rich plasmas is crucial for applications like fusion energy and materials processing.
• Cesium (Cs) is often introduced to enhance negative ion production in plasma sources.

Purpose of the Study:

• To review recent research on negative-ion-rich plasmas in large negative ion sources.
• To investigate the spatial distributions of negative hydrogen ions (H-), positive hydrogen ions, and electrons.
• To analyze the effect of cesium on plasma potential and electric field penetration.

Main Methods:

• Utilized a 4-pin probe and Langmuir probe with photodetachment (PD) for diagnostics.
• Calibrated PD signals with scanning cavity ring down PD measurements to determine H- density.
• Analyzed plasma potential and electric field distributions under varying conditions, including Cs introduction and plasma grid bias.

Main Results:

• Introduced Cs alters plasma potential distribution based on plasma grid bias.
• High electron density H2 plasma shields bias potential, behaving like a metallic free electron gas.
• In Cs-seeded plasmas, electric fields penetrate even at similar electron densities, influencing electron transport differently than ions.

Conclusions:

• Cesium significantly modifies plasma properties, enabling electric field penetration in electronegative plasmas.
• Electron and ion transport mechanisms differ substantially in response to electric field penetration in Cs-seeded plasmas.
• These findings are vital for optimizing negative ion source performance.