Alternative model of space-charge-limited thermionic current flow through a plasma

Area of Science:

• Plasma Physics
• Electromagnetic Theory
• Materials Science

Background:

• Thermionic current flow in plasma is typically assumed to be limited by space-charge-limited (SCL) cathode sheaths.
• SCL sheaths accelerate ions into the cathode and consume the cathode's negative bias.

Purpose of the Study:

• To formulate and investigate a fundamentally different current-limited mode of plasma operation.
• To compare the properties of the proposed inverse mode with conventional SCL modes using simulations.

Main Methods:

• Formulation of the inverse mode theory where electrode potentials are above plasma potential, confining ions.
• Development of a continuum kinetic planar plasma diode simulation model.
• Comparison of SCL and inverse cathode sheath properties under various conditions, including charge-exchange collisions.

Main Results:

• The inverse mode limits current via the anode sheath and by returning thermoelectrons to the cathode.
• Plasma density varies linearly across the electrode gap in planar geometry.
• SCL modes require disabling charge-exchange collisions; with collisions, inverse modes become the necessary equilibrium.

Conclusions:

• Inverse operating modes are likely present or possible in many hot cathode plasma devices.
• The inverse mode presents opportunities for reduced sputtering and power consumption.
• Rethinking the assumption of SCL sheaths is crucial for exploring new plasma device functionalities.