Charge-State-Resolve D Ion Energy Distributions In Aluminum Vacuum Arcs.pdf

Researchers at RWTH Aachen University and Lawrence Berkeley National Laboratory have studied and analyzed the charge-state-resolved ion energy distributions in a cathodic arc plasma of aluminum. Contrary to previous literature, they found that higher charged metal ions (Al1+, Al2+, Al3+) had lower energies. This observation was attributed to opposing effects: deceleration due to part of the discharge voltage and acceleration by pressure gradient, as well as electron-ion coupling.

Methodology:

- Vacuum arc plasma generated from a conical aluminum cathode.
- Powered by direct current (DC) arc supply with an arc current of 35 A.
- Measurements performed in oil-free vacuum at a base pressure of 1 x 10^-4 Pa, 45 cm away from the cathode.
- Ion energy distributions measured using a mass-energy analyzer and Langmuir probe for plasma characterization.

Key Findings:

- Higher charged ions (Al2+, Al3+) exhibited lower average energies (44.2 eV, 42.9 eV) compared to lower charged ions (Al1+).
- This result contradicts previous literature, which reported either increasing ion energy with charge state or approximately constant distributions.
- The observed trend cannot be explained by potential hump theory or gasdynamic model.
- A possible explanation involves the electric field due to the discharge voltage drop, which decelerates ions formed at different distances from the cathode surface.

Analysis:

- Ion energy distributions fitted with shifted Maxwellian distributions (SMD) for additional insights into plasma parameters.
- The SMD approach provided good fits to the measured distributions and offered information on plasma temperature and potential.