Mixed Pierce-two-stream instability development in an extraction system of a negative ion source

Area of Science:

• Plasma physics
• Beam-plasma interactions

Background:

• Negative ion sources are crucial for fusion energy and industrial applications.
• Volume-produced plasmas in extraction systems can exhibit instabilities.
• Understanding these instabilities is key to optimizing ion beam quality.

Purpose of the Study:

• To investigate the occurrence and characteristics of mixed Pierce-two-stream instability in negative ion source extraction systems.
• To determine the conditions for unstable beam propagation and the instability threshold.
• To analyze the effect of inclined perturbations on instability behavior.

Main Methods:

• Analytical determination of unstable beam propagation conditions.
• Numerical simulations using COMSOL Multiphysics.
• Investigation of the influence of inclined perturbations.

Main Results:

• The study confirms the existence of a mixed Pierce-two-stream instability.
• The instability involves interactions between external circuit electrons and background positive ions.
• The calculated instability threshold is higher than that of pure Pierce instability.
• Inclined perturbations were found to influence the instability behavior.

Conclusions:

• Mixed Pierce-two-stream instability is a relevant phenomenon in negative ion source extraction.
• The presence of background ions and external circuit electrons contributes to this instability.
• The findings provide insights for mitigating instabilities and improving negative ion source performance.