Oscillatory instability development in extraction system of a negative ion source

Area of Science:

• Plasma Physics
• Ion Source Technology
• Beam Dynamics

Background:

• Negative hydrogen ion sources are crucial for fusion energy research.
• Secondary electrons in the extraction system can destabilize the ion beam.
• Parent gas leakage from the discharge chamber contributes to instability.

Purpose of the Study:

• Investigate conditions for oscillatory instability in negative hydrogen ion source extraction systems.
• Analyze the impact of secondary electrons on beam stability and current modulation.
• Determine the range of stable beam propagation and characteristics of the instability.

Main Methods:

• Analytical modeling to determine stable beam propagation ranges.
• Calculation of instability increment and oscillation group velocity.
• Numerical simulation using COMSOL Multiphysics for low-energy beam propagation.

Main Results:

• Identified secondary electrons as the cause of oscillations and potential beam current modulation.
• Derived the range for stable beam propagation.
• Found the instability increment to be relatively small.
• Observed maximum instability increment when beam velocity equals plasma electron thermal velocity.
• Confirmed oscillations are convective, with group velocity near beam velocity.
• Simulations showed beam current modulation during low-energy beam propagation.

Conclusions:

• Secondary electrons in the extraction system are a key factor in oscillatory instability.
• The instability is generally weak and convective, but can be amplified under specific conditions.
• Understanding these instabilities is crucial for optimizing negative hydrogen ion source performance and beam quality.