Simulations of negative ion plasma sheaths

Area of Science:

• Plasma Physics
• Computational Physics
• Beam Dynamics

Background:

• Accurate modeling of negative ion beams is essential for high current sources.
• One-dimensional (1D) models are fundamental for understanding sheath/presheath physics and numerical solver stability, despite the need for finer simulation meshes (e.g., Debye length, $\lambda_D \approx 0.01$ mm).

Purpose of the Study:

• To investigate a 1D model for negative ion production from an extraction wall/grid.
• To analyze the influence of a driver plasma and bias voltage on plasma sheath structure and beam current.

Main Methods:

• A 1D model incorporating negative ion production from the extraction surface.
• Coupling to a driver plasma acting as an electron and proton reservoir with variable plasma potential.
• Particle motion modeled via diffusion equations (protons) and magnetized electrons; Poisson equation solved globally.

Main Results:

• The formation of an inverted sheath was observed.
• The forward current of negative hydrogen ions (H(-)) was found to be sensitive to the applied bias voltage between lateral walls and the extraction wall.

Conclusions:

• The 1D model provides fundamental insights into sheath physics and numerical stability for negative ion beam simulations.
• Bias voltage is a critical parameter affecting inverted sheath formation and H(-) current, impacting the design and operation of high current negative ion sources.